Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

The massive outbreak of infectious upper respiratory tract disease that hit Iceland four months ago is finally slowing down, and there is now a little more information available about what they've been seeing and what authorities believe has been going on.

There is a fairly clear description of the clinical signs seen in affected horses (as previously described), although signs vary from horse to horse.  Typically illness begins with a serous (clear) nasal discharge which then becomes mucopurulent (snotty).  There is sometimes mild conjunctivitis (e.g. red puffy eyes) and most commonly a dry cough.  Horses that are examined using endoscopy show signs of laryngitis.  Signs generally last 4-6 weeks, but this can vary.

There's also more information about the epidemiology of the disease (i.e. the pattern of spread).  Morbidity seems to be close to 100%, meaning almost every horse that is exposed gets sick.  Fortunately no mortality has been associated with the disease, meaning all the horses eventually recover.  The incubation period (the time from exposure to illness) is 1-4 weeks.  It seems to be spread mostly by direct contact, but indirect transmission is also possible, as even free-roaming herds with little contact with other horses or people have been affected.  Horses are not completely immune after recovering (meaning they can get sick again if they are re-exposed), but there is evidence that the "herd immunity" of the horse population in Iceland is increasing, and the outbreak seems to be dissipating.  It is still lingering in some horse groups where there are naive (unexposed) horses coming into contact with infected horses.

The way this disease has spread is very typical of a virus burning through a naive population which has never been previously exposed to or vaccinated against the disease.  Even the clinical signs are very typical of an upper respiratory virus.  However, extensive testing has been done using both PCR and antibody titres for a multitude of viruses including (but not limited to): equine herpesviruses 1, 2, 4 and 5, equine arteritis virus, equine influenza virus, equine reoviruses, and equine rhinitis viruses.  None of these viruses appear to be the culprit, despite early suspicions that the cause was in fact equine herpesvirus.  They are still testing for new/unknown viruses using viral culture, but this is a difficult and very time-consuming task.

What authorities have been able to find in affected horses is Streptococcus equi subsp. zooepidemicus, which is what is currently being blamed for the outbreak.  They say it has been cultured from almost all coughing horses and all horses with mucopurulent nasal discharge.  But that's not really surprising, as S. zooepidemicus is a very common commensal bacterium that is even carried by healthy horses in their upper respiratory tracts.  It's also a very common opportunistic pathogen in horses, meaning when the animal gets sick for another reason (for example, if a horse is infected with a respiratory virus), then S. zooepidemicus moves in and causes a secondary bacterial infection, which can make the clinical signs worse or last longer, and may lead to other complications.  In this case they are trying to make S. zooepidemicus out to be the primary pathogen, but it would have to be a very unusual strain of the bacterium - very virulent, highly infectious and totally novel to the horse population in Iceland - to cause an outbreak of this scale.  They are doing additional testing to characterize the strain they have isolated from infected horses and to compare it to other strains previously isolated from horses in Iceland.

The good news is that after the horse industry in Iceland was paralyzed by this outbreak for almost four months, equestrian activities are now resuming under the supervision of the Icelandic Food and Veterinary Authority.  That shouldn't pose a problem to the Icelandic horses, as there are very few naive horses left in the country that would still be highly susceptible to this still somewhat mysterious disease.  However, export of horses from Iceland is slated to recommence on September 15.  There are still quarantine requirements preceding export (horses must not be sick nor be in contact with a sick horse for 30-60 days prior, depending on what country they're going to), which always are and will remain very important for infectious disease control.  However, with the cause of the outbreak still being somewhat unclear, it is unknown if any horses may now be healthy carriers of the outbreak pathogen, be it an unusual strain of S. zooepidemicus or an unidentified virus.  If horses in other countries start getting sick a few weeks after exposure to exported Icelandic horses, strict control measures will have to be rapidly implemented to possibly prevent similar outbreaks from developing.

Photo credit: David Blaikie (source: http://commons.wikimedia.org)

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Despite initial declarations that known common causes had been ruled out, the investigation into the mysterious respiratory disease outbreak in Iceland is now focusing on the role of equine herpesvirus. Few details are available, but the country's Chief Veterinarian is reporting that equine herpesvirus, complicated by secondary streptococcal infection, is suspected.

That makes sense. As I wrote earlier, it's much more likely for an outbreak to be caused by an uncommon presentation of a common problem than by a completely new bug. More information about how they have diagnosed equine herpesvirus is important, however, to make sure that it's really the culprit. This virus is widely disseminated in the horse population and lives dormant inside a large percentage of healthy horses, so care must be taken to ensure that it's causing the outbreak, and they're not merely finding a virus that happened to be there all along. Outbreaks of herpesvirus-associated respiratory disease in older horses are quite unusual, so if this is the case, it's equally important to determine why this strange outbreak has occurred.

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Streptococcus zooepidemicus is an important cause of infections in horses. This bacterium can also be found in healthy horses. When you consider the large number of horses that are infected and the larger number of healthy horses that are carriers, along with the close contact that people have with horses, it's pretty obvious that people are regularly exposed to "Strep zoo". This bacterium is not well-adapted to survive in people and cause infections, so human infections are quite uncommon, but they can occur. There are periodic reports of S. zooepidemicus infections in people, with varying degrees of association with horse-contact.

An upcoming edition of journal Epidemiology and Infection contains a report of S. zooepidemicus meningitis in a 51-year-old woman (Minces et al, 2010). This person had a mild upper respiratory tract infection, then developed signs of meningitis (including fever, unresponsiveness, respiratory distress). A spinal tap was performed and S. zooepidemicus was isolated. The woman fortunately responded to treatment and recovered.

Upon initial questioning of the patient's mother, no animal contact or ingestion of unpasteurized dairy products (another risk factor) was reported. However, it was later revealed that the woman's daughter had started horseback riding at a friend's farm approximately one month earlier. The type of contact that the woman had with horses (if any) at the farm was not reported, nor was there any investigation of S. zooepidemicus shedding by horses on the farm.

Based on the fact that this is typically an equine-associated bacterium and the history of contact (albeit potentially limited or indirect) with horses, horse contact was blamed for the infection. It's a reasonable conclusion but it's far from certain because of the nature of the contact, the lack of any proof of the same strain of S. zooepidemicus in horses on the farm, and previous reports of infections occurring in people with no contact with horses.

Exposure to S. zooepidemicus is an inherent risk of having contact with horses. It's nothing to lose sleep over and is probably relatively low on the list of potential health problems associated with horse contact. The risk is probably greatest in people with compromised immune systems and other general risk factors for disease such as advancing age and pregnancy. Good general hygiene measures, avoiding contact with sick horses and close attention to hand hygiene probably minimize these already low risks.

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The outbreak of respiratory disease that was reported in Iceland a few weeks ago is apparently still undiagnosed and ongoing. Signs of the disease, characterized by weakness, nasal discharge, cough and occasionally fever, can apparently persist for up to six weeks. Icelandic authorities have stated that most known diseases have been ruled out, including influenza, and suspect that the illness is caused by a virus with possible secondary bacterial infection. (The outbreak pre-dated the recent icelandic volcanic eruption, so we can't blame that).

It will be interesting to see what they find, and whether they have really been able to completely exclude the most common offenders, like equine influenza. A general rule of medicine is "common things occur commonly." While that may seem like an obvious statement, the point is that you should look for common causes first.  Strange outbreaks are more often atypical outbreaks caused by a known infectious disease, rather than a completely new disease. New diseases certainly do occur, but they are rare and it's important to not be too quick to rule out the common causes.

If this is a new disease, intensive study is required to determine what it is. As well, if there is real suspicion of a new and highly contagious disease, there should be immediate and complete restriction of movement of horses to and from Iceland. It makes no sense to say that there might be a new infectious disease that is highly transmissible and take no measures to keep it from spreading. If it really is a new disease, it's quite fortunate that it's on an isolated island, where containment might be possible. If such a highly transmissible disease were to reach the horse population in Europe or elsewhere, containment will be very difficult.

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We've taken the plunge into (the time- and money-sucking abyss of) pony ownership for our 3 daughters. Foxie Casie, a 4-yr-old Welsh cross, arrived yesterday, much to the delight of the girls and mixed delight and trepidation to their parents. I figured I might as well discuss some of the infectious disease-related issues dealing with acquisition of a new pony.

Vaccinations

• She came vaccinated against rabies and tetanus, with the last round having been done this spring. I'll re-do rabies and tetanus next spring. She'll get a rabies vaccination every year, but I will probably stretch out the tetanus vaccine since yearly vaccination is likely not necessary (especially considering tetanus vaccination is labeled for every 5 years in Australia).
• I always debate about the necessity of West Nile virus (WNV) vaccination, since it's very rare now in Ontario and I'm not aware of any mammalian cases in the general vicinity. I'll probably err on the side of caution and vaccinate her, but will do so in late summer, in advance of the true high risk period here. Typically, any WNV cases that occur here develop late August through October, so vaccinating the pony in late July, to get peak immunity during the main risk period, makes sense.
• I will probably not vaccinate her against eastern equine encephalitis (EEE) since it's so rare and hasn't been found around here.
• She lives with no other horses and won't be traveling off the farm in the near future, so there's no need to vaccinate against things like influenza, equine herpesvirus or strangles.

Deworming

• She was dewormed about one month ago. I'll check a fecal egg count soon and go from there. Since she's the only horse on the farm (the sheep are low risk for passing parasites to her), if we get started right and keep her from contaminating the pasture, a pretty conservative parasite control program with periodic fecal egg counts and deworming as needed should suffice.

New horse infectious disease issues

• She didn't have much of a trailer ride and appeared healthy when I saw her the day before, so shipping associated diseases like pleuropneumonia ("shipping fever") are pretty unlikely.
• Diarrhea's always a concern. Risk factors for diarrhea include shipping, stress, diet change and antibiotic use. Well, she's been shipped, presumably stressed, had her diet changed and been introduced to pasture, so she has some risk factors. She's not exposed to other horses, which helps, and she has no need for antibiotics (at least at the moment). So, basically, I'll give her a good diet (good hay and not-very-lush pasture) and keep an eye on her. My diarrhea research background probably makes me look at her feces more than the average person, but so far, so good.

I'm sure there'll be more to come... hopefully not much from the actual infectious disease standpoint. 

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Eradication of infectious diseases is a great goal, but it's rarely practical. The best known (and perhaps only) example of infectious disease eradication is the elimination of smallpox. So, why is it so hard to do?

The following general criteria need to be in place to eradicate a disease:

• It must have a clearly defined host range and that range is ideally only one species. A disease that can affect multiple species is very hard to control.
• It must predictably cause disease in individuals that are infected.
• There must be no long-term carriage state. Once a person/animal gets over the illness, he/she/it must get rid of the infection completely in a defined and predictable period of time.
• A highly effective vaccine should be available.
• There must be a commitment to put in lots of time, money and effort everywhere the disease exists.
  

This isn't the case with most diseases, and equine herpesvirus (EHV) has many characteristics that make eradication impossible:

• Unpredictable disease: EHV infection doesn't always cause signs of disease. When it does cause disease the signs can be quite variable and difficult to easily differentiate from other infections.
• Longterm carriage: This is the biggest problem with herpesviruses. EHV is able to survive in a latent (dormant) state in the body after infection. It can lie dormant for a long period of time, but infected animals can always start shedding the virus again. A large percentage of horses are carrying EHV in their bodies and there's no way to get rid of it.
• Vaccine: Vaccines are available but they are by no means 100% effective at preventing infection.
• Time, effort, money and cooperation are terms that are not commonly associated with disease control in horses. Getting everyone to follow a standard recommendation (if one were able to control disease) would be difficult to impossible. The entire horse-owning population would not be willing to spend the money for broad control measures, and there's no real impetus for governments to do so. Even getting people to agree to follow basic vaccination and infection control recommendations is difficult.  If there is any negative impact on use of horses, ease of management or any other minor inconvenience, 100% compliance with any recommendation becomes impossible to obtain.

We have to live with EHV. It will always be a risk to horses. Good infection control measures and vaccination of certain groups (e.g. pregnant mares) can help control the impact of the virus.

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Rhodococcus equi is a bacterium that is an important cause of respiratory disease in foals. It typically causes numerous abscesses in the lungs, and it is an important cause of illness and death in foals from a few weeks to about six months of age. Infections of other parts of the body can occur, and these can cause serious problems because they can be hard to diagnose and hard to treat.

A recent study in the Journal of the American Veterinary Medical Association (Reuss et al 2009) described extra-pulmonary disorders (EPDs, disorders in parts of the body besides the respiratory tract). They studied 150 foals, and 74% had a least on EPD. These included:

• Immune-mediated synovitis (25%): This is a well-recognized problem where R. equi-infected foals get enlarged joints because of accumulation of excess joint (synovial) fluid. Fluid-distended joints can occur with joint infections, but with immune-mediated synovitis it's caused by the immune system alone. This typically does not result in long-term joint problems, although it's important to determine whether the foal has an infection which needs aggressive care, or just inflammation which is best left alone.
• Abdominal abscesses (17%): This is a very serious problem and abscesses in the abdomen can be very difficult to treat. These foals often die.
• Uveitis (11%): Inflammation of the eye, which can occur as a result of the immune response or infection. 
• Septic synovitis (9%): As opposed to immune-mediated synovitis, this is an infection in the joint. This is very serious and can cause permanent joint damage.
• Pyogranulomatous hepatitis (11%): This is inflammation of the liver with abscesses and pus.
• Pleural effusion (3%): Accumulation of fluid between the lungs and body wall. This can be the result of severe lung disease.
• Meningitis (3%): Infection of the layer covering the brain, which is never a good thing.

Foals that had EPDs were more likely to die than foals with infections only involving the lungs, which isn't particularly surprising. In particular, the presence of uveitis, bloodstream infection, joint infection (not immune-mediated synovitis) and abdominal abscessation were associated with failure to survive.

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Herd immunity is an important infectious disease concept. Basically, it involves trying to ensure that a high enough percentage of a population is resistant to an infectious disease so that the disease cannot be spread easily through the group. Ensuring that a large percentage of the population is vaccinated helps protect individuals that cannot be vaccinated (because of allergy, disease or other reasons) or that did not properly respond to vaccination (not all vaccines protect all vaccinated individuals).

"Herd immunity" usually refers to this concept when applied to herds of animals (such as horses or cows), but the "herd" can be a a small local population, a regional population, or broader, and it can be people or animals. For some human infectious diseases, it's been shown that vaccination of 75-95% of the population is required to prevent outbreaks. If vaccination rates start to slip, the chance of an outbreak increases. This is best seen in some areas where vaccination rates decline in certain groups of kids because parents are reluctant to have their children vaccinated (for one reason or another), and subsequently outbreaks of disease start occurring (or increasing).

Vaccination is an important (but not the only!) infection control tool. For diseases that are transmissible between horses (or dogs, or people, or whatever other "herd" is being considered), vaccination of a single horse helps protect that individual from disease, and also helps protect the rest of the population.

During a public health infectious disease course that I teach, a student showed a link to this interesting and amusing demonstration of herd immunity from the UK. Make sure you have your sound turned on. It's an entertaining description of the concept of herd immunity.

When you're deciding on your vaccination program, for both individual horses or a farm, be conscious of the herd immunity concept.

This equIDblog entry was originally posted on the Worms & Germs blog on 05-Oct-09.

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Rhodococcus equi is a very well recognized pathogen in horses – it is a common cause of pneumonia in foals between the ages of 1-6 months, and infection is also sometimes associated with other problems such as diarrhea, swollen joints and abscesses in other parts of the body. The infection can be very difficult to treat because the bacteria are able to live inside white blood cells, which helps protect them from the body’s immune system, and because they often cause abscesses to form, which are difficult for antibiotics to penetrate. Rhodococcus equi infection in foals has been studied extensively, but there’s still a lot we don’t know how the body defends itself against this organism.  These are a few things we do know:

• Almost all foals are exposed to R. equi as neonates, but most of them never develop signs of infection.
• Giving newborn foals hyperimmune plasma (plasma with extra antibodies against R. equi) may have some beneficial effects on farms where the infection is a recurrent problem, but this practice is still controversial.
• Adult horses are essentially immune to the infection.
• In almost all cases if clinical disease in foals, the R. equi strain involved carries a special gene called vapA.
• Mortality rates in foals vary considerably from 0% to 30%.
• So far, efforts to develop a vaccine to help protect foals have been unsuccessful, but research in this area is ongoing.

People can also be infected with R. equi, and as in foals, pyogranulomatous pneumonia (infection of the lungs which results in the formation of many abscesses) is one of the most common conditions caused by this organism. However, there are a few important differences between infection in people and infection in horses:

• 85% to 90% of people with R. equi infection are immunocompromised, meaning their immune system is weakened or suppressed for some reason, e.g. HIV infection, or immunosuppressive drugs taken by organ transplant or cancer patients.
• Among people infected with R. equi who have normal immune systems (i.e. immunocompetent), about half of the infections are localized, meaning they only affect one small part of the body. Many of these are associated with wound infections.
• Only 20% to 25% of the R. equi isolates in people carry the vapA gene.
• Infection in immunocompetent people can be fatal in approximately 11% of cases, but among HIV-infected patients the mortality rate from R. equi infection can be as high as 50% to 55%.

Rhodococcus equi is actually a soil organism, and this is likely the most common source of the organism for both horses and people. Only approximately 1/3 of humans infected with R. equi report that they have had contact with horses or pigs (pigs can also carry the bacterium). So we don't know how much of a risk an infected foal is to a person.  However, it is prudent for people, particularly those with weakened immune systems, to take precautions to avoid potential transmission of R. equi from horses.

• Try to reduce dust levels on the farm. Because R. equi most often lives in the soil, it can get stirred up into the air in dusty areas, which can then lead to inhalation by animals and people. Doing things like planting grass or other vegetation, installing windbreaks in high-traffic areas, or wetting down dusty stalls or paddocks can help reduce dust levels in the air.
• Keep open wounds and other broken skin covered when working around animals.
• Always wash your hands after handling a foal (or any horse)
• If you have a foal that develops signs of R. equi infection, make sure you have your veterinarian examine it as soon as possible so the diagnosis can be determined and the foal can be treated properly as soon as possible. Some foals with R. equi may develop severe pneumonia very quickly, so it’s important that they are examined right away.

This equIDblog entry was originally posted on the Worms & Germs blog on 09-May-09.

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Rhodococcus equi is a common pathogen in foals between the ages of 1 and 6 months of age that is most infamous for its ability to cause pneumonia. Classic R. equi infection results in the formation of large abscesses throughout the lungs of young foals (see picture left), which can be especially difficult to treat because the bacteria are able to hide from the body’s immune system by living within white blood cells. However, this organism’s bag of tricks doesn’t end with lung abscesses – it can also travel to other parts of the body and cause all sorts of trouble. These kinds of infections may occur with or without the classic lung infection, and are referred to as extrapulmonary disorders (EPDs).

At the 2008 Forum of the American Association of Equine Practitioners (AAEP), researchers at Texas A&M presented the results of a study they recently completed looking at EPDs in 150 foals with R. equi infection over a 20 year period. Here are some of the highlights:

• 74% of the foals had an EPD associated with their R. equi infection. On average foals had two EPDs each and up to as many as nine EPDs in a single animal.
• Many of the EPDs did not cause separate, detectable clinical signs. These were therefore most often found on necropsy in foals that died.
• The most common EPD was diarrhea, which occurred in 50 foals (33%).
• Immune-mediated polysynovitis (inflammation and swelling of the joints without infection of the joints themselves) was the second most common EPD
• 31 foals (21%) had ulcers and inflammation somewhere in their intestine (ulcerative enterotyphlocolitis), all of which were diagnosed at necropsy
• 25 foals (17%) had abscesses in the abdomen, 71% of which could be detected by ultrasonography

Thirty-nine different EPDs were identified in the group.  Other EPDs included uveitis (inflammation of the eye), hepatitis (inflammation of the liver), septic arthritis (joint infection), lymphadenopathy (enlarged lymph nodes), peritonitis (inflammation of the lining of the abdomen) and septicaemia (bloodstream infection).

• Among the foals with EPDs, 43% survived hospitalization, whereas 82% of foals without EPDs survived.  However it is very important to remember that many EPDs were only detected at necropsy, therefore they were more often found in foals that died, but it is unknown if they actually occurred more commonly in one group or the other.
• Risk factors for foals developing EPDs included longer time from onset of clinical illness to referral (e.g. foals that were sick at home for longer before being sent to the hospital were more likely to have an EPD), higher heart rate on admission and a higher white blood cell count.

It’s important to remember that even though an infectious pathogen may usually affect an animal in a certain way, “the bugs don’t read the textbooks” (as we often say), and they can cause problems in other ways. That’s just one of the reasons it’s so important to have your veterinarian perform a full physical exam of your animal if it is sick - even if it looks similar to something you’ve seen before - in case the pathogen causing the problem starts to affect other parts of the body, which may require more or different kinds of treatment.

Photo credit: M. Anderson

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Infectious diseases are a major problem in young foals. Diseases, including diarrhea, pneumonia, meningitis, umbilical infections and joint infections can range from mild to rapidly fatal.  Even in foals that survive the initial infection, these conditions can sometimes result in permanent problems. A study published in a recent edition of the Equine Veterinary Journal (Wohlfender et al, 2009) looked at infectious diseases in the first 30 days of life in 1031 foals on 36 breeding farms in the Newmarket, UK area. This very large study provided some interesting information:

• The overall incidence of infectious diseases was 8.3%, meaning around 1 in 12 foals developed some type of infectious disease during the first 30 days of life.
• The most common infectious problem was diarrhea, which occurred in 5.9% of foals. Foals with diarrhea that didn't have other signs of illness were not included.  This is because most young foals with diarrhea and no other abnormalities likely have foal heat diarrhea, which is not an infectious disease.
• Umbilical infections developed in 0.7% of foals.
• Joint infections occurred in 0.4% of foals.
• Bone infections occurred in 0.2% of foals.
• Respiratory infections were uncommon, occurring in only 0.6% of foals.
• There was no difference in the incidence of infections in foals that were treated routinely with antibiotics to "prevent" infections. Whether or not this practice is effective has been a controversial issue for a long time.  The best way to answer the question would be to have a study that randomly assigned foals to receive antibiotics or no antibiotics, and then monitored both groups for infections.  This study didn't do that, so there could be some "bias" to this particular finding, but the results are still useful. I don't find the results surprising - there has never been good evidence that antibiotics are routinely needed in newborn foals. This study may help control routine (and in my opinion unnecessary) antibiotic treatment in healthy newborn foals.

This study provided more information confirming that infectious diseases are a significant problem in foals, and that measures are needed to reduce the incidence of infections. It also provided evidence that routine antibiotic use is NOT an effective means of doing this. While there is a lack of good, objective evidence, routine hygiene, sound management and every-day infection control practices are probably critical factors for infectious disease prevention in these young animals.

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Following an influenza outbreak that sickened at approximately 70 horses last year, the Crawford Country Fair in Pennsylvania is requiring influenza vaccination for all equine participants this year. This year, all horses must be vaccinated between 6 months and 15 days prior to the fair. Vaccination against such an infectious disease is a sound recommendation to be sure, although the six-month window might be a little long. I'd prefer to see the horses vaccinated closer to the time of the fair, in order to maximize the protective effect during the highest-risk period.

The fair board is also requiring a certificate of veterinary inspection for each horse at the fair, issued within 30 days prior to opening day. I'm not sure that this kind of inspection will do much in terms of preventing sick horses from participating in the fair, as most infectious diseases (including influenza infection) don't last for 30 days.

• If a horse is clinically healthy 30 days before the fair, it certainly doesn't mean the animal won't be infectious at the time of the fair if it is exposed to a virus (or other pathogen) in the interim.
• If a horse has influenza 30 days before the fair, there's not much chance it will still be shedding the virus by the time of the fair.

While I wholeheartedly agree with the sentiment that regular veterinary examination is important for infectious disease control, but the time frame in this case just won't do much for control of influenza. If the fair board really wanted to reduce the risk of sick horses attending, they'd have to require inspection much closer to the date of the fair.  However, such a requirement could turn into a logistical nightmare, since a large number of horses would need to be evaluated by a small number of veterinarians over a short period of time. Furthermore, if someone competed at many fairs during the year and they all had similar requirements, a horse might have to undergo a weekly exam before each event, which could be cost-prohibitive.

Despite the limitations, this is still a good, proactive step on the part of the fair board, and similar precautions ought to be considered elsewhere. We need to stop simply accepting that some horses will get sick following fairs and that outbreaks will happen, and realize that there are practical measures that we can do the reduce (although not eliminate) the risks. One of the best things that could be done would be to require examination of all horses upon arrival at a show or event. The examination could be done by a veterinarian or veterinary technician.  At a minimum,  requirements could include that horses have no signs of respiratory disease, diarrhea or fever. Horses with these problems could then be immediately send home, greatly reducing the risk of disease transmission to other animals at the event. Yes, it would take some effort and money, but it really wouldn't be that hard to do, and could be a very useful preventive measure.  Compared to the cost and headache of quarantining and treating sick animals when an outbreak does occur, the idea really sells itself.

I'm not trying to be negative about the approach taken by this particular fair board. I think their awareness of the need to do things to reduce the risk of infectious diseases at their event is great. 

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Transportation, particularly over long distances, is likely the single most important predisposing factor for pleuropneumonia (aka "shipping fever") in horses. Pleuropneumonia is a very serious condition in horses that occurs when an infection in the lungs (pneumonia, typically bacterial pneumonia) spreads to the outer surface of the lungs (the pleura) and contaminates the space between the lungs and the chest wall (the pleural cavity).  The inflammation of the pleura (which is also very painful) usually results in accumulation of infected fluid around the lungs (photo right: drainage of pleural fluid in a horse using a chest tube).  The disease can be difficult to treat, and affected horses can end up with a lot of scarred lung and adhesions between their lungs and chest wall, as well as lung abscesses and other problems.

The main reason this condition is so often associated with transportation (which is why the illness is often called shipping fever) is primarily due to the fact that most horses are transported with their heads tied.  This prevents the animals from lowering their heads, a motion that regularly allows drainage of the normal respiratory secretions which naturally accumulate in the lower trachea (windpipe) and further down the respiratory tract.  The accumulation of this fluid, including the many bacteria from the throat and upper trachea that it normally contains, results in contamination of the lower airways and lungs, which are normally almost sterile.  In many cases, the animal’s natural defensive mechanisms are still able to prevent disease from developing, but in others this large amount of contamination results in bacterial pneumonia or pleuropneumonia after transport. Illness is more likely to occur in horses with one or more other predisposing factors, such as viral respiratory infection (which can be acquired through exposure to other horses during shipping), corticosteroid therapy, other illness, or animals that are exercised immediately following transport. The same phenomenon occurs if horses are restrained with their heads in an elevated position for a long period of time, even without the stress of transportation. Horses should be transported in such a way that they can freely lower their heads, whenever this can be done safely.

Because pleuropneumonia is most often caused by the bacteria that are naturally found in a horse's own respiratory tract, it is not considered transmissible between horses.  However, viral respiratory infections can predispose horses to developing more severe disease such as pleuropneumonia, and these infections certainly are transmissible.  A few basic infectious disease control measures to help prevent these infections include:

• Ensure that horses are appropriately vaccinated for viruses such as equine influenza and equine herpesvirus (types 1 and 4) prior to transportation.
• Don't ship sick horses or those that have been exposed to infectious diseases. This increases the risk to the individual horse and all others it encounters.
• Keep new horses and horses returning from shows and competitions isolated from other horses in the facility so they do not spread any viruses to which they were exposed to the other horses.
• Immediately isolate any horse that develops early signs of respiratory disease, and have it examined by your veterinarian.  Follow your veterinarian's instructions regarding treatment and the need for ongoing isolation.  Do NOT treat the horse with antibiotics (even if it has a fever) unless advised to do so by a veterinarian.

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The American Association of Equine Practitioners (AAEP) is currently having their annual forum in San Diego, California. Veterinarians from all over the world come to this conference to learn about the latest advances and techniques in veterinary medicine, with the goal of promoting and improving the health and well-being of horses everywhere.

A very popular session at the AAEP convention is the Kester News Hour, which covers some of the most recent reports in the veterinary literature. One of the topics that was discussed this year was pulmonary fibrosis in horses. This is a very severe condition that tends to occur in older horses (average 14.5 years old) but can occur in horses as young as four years old. Affected animals may have a severe cough and a lot of trouble breathing, and they can also have a fever and an increased number of white blood cells in their bloodstream. These signs can also be seen in horses with pneumonia, so it’s important for a veterinarian to carefully examine the horse, the horse’s medical history, and the results of diagnostic tests in order to tell the difference. In horses with pulmonary fibrosis, tough fibrous connective tissue forms throughout the lungs, which interferes with the ability of the lungs to expand and collapse with normal breathing. Pulmonary fibrosis is a devastating condition that has a very poor prognosis for recovery.

There are several known causes of pulmonary fibrosis, including a condition known as silicosis, and injection of certain immunomodulatory drugs. However, the cause in the majority cases still remains unknown. Through the persistent and meticulous work of some dedicated researchers (Williams et al. 2007), it has been found that a virus, equine herpesvirus type 5 (EHV-5), may be involved in many of these cases. In their study, the virus was found in 19/24 affected horses, and only 2/23 control horses (horses without pulmonary fibrosis). There is not yet enough information to be able to say that EHV-5 infection is a cause of pulmonary fibrosis, but there does appear to be an association. More work will certainly be done to determine what roll EHV-5 may play in this terrible disease. If there is an infectious component, then preventing transmission of pathogens through basic infectious control practices that we always talk about (e.g. preventing nose-to-nose contact between horses, washing your hands between handling different horses, isolating “stressed”/sick horses that may be shedding virus, etc.) may be beneficial for preventing pulmonary fibrosis as well. Keep watching for more information from the conference!
 

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I'm trying not to sound like a broken record when it comes to antibiotics, but it's a very important topic so you'll see many posts on the subject. When you consider that antibiotic-associated diarrhea is a potentially life-threatening problem in horses, and that antibiotic-resistant bacteria are becoming more common in general, it should be obvious that we need to limit the use antibiotics to when they are actually needed, and potentially effective.  Horses with respiratory disease are commonly treated with antibiotics.  However, as discussed in a previous post ("Snotty-Nosed Horses: What To Do?"), there are many causes of respiratory disease in horses, and most of them are not bacterial.

A common cause of nasal discharge and coughing in horses is inflammatory airway disease (IAD, which is somewhat similar to asthma in people). This condition is different from heaves, which typically occurs in older horses and causes more severe clinical signs.  However, horses with IAD  may cough and/or have clear nasal discharge. They may also have decreased athletic performance. They do no have a fever, decreased appetite or other signs associated with an infection. However, antibiotics are (unfortunately) commonly used to "treat" these horses, which makes very little sense.  A few years ago a study was done looking at antibiotic treatment history (i.e. prior to diagnosis) in 55 horses diagnosed with non-septic inflammatory airway disease at the Ontario Veterinary College. Here are the highlights:

• 69% of horses were given an antibiotic for treatment of their respiratory disease. 52%  received more than one antibiotic, and 19% received 3 different antibiotics.
• Horses were treated with antibiotics for extended periods of time, with an average of 9.9 days.
• No testing was performed to detect bacterial infection in any horse prior to treatment.
• 21% of the horses that received antibiotics did not have any abnormal clinical signs beyond poor performance - not even nasal discharge.
• Horses with a history of coughing were more likely to have received an antibiotic, but cough is a very non-specific clinical sign that does not necessarily indicate an infection, let alone a bacterial infection.

Antibiotics are important drugs, but we overuse them (in people and in animals). We need to be careful with when and how we use to ensure they have the maximum beneficial effect and to reduce the risks of side effects or increasing bacterial resistance.

More information on inflammatory airway disease is available in the consensus statement on IAD in horses from the American College of Veterinary Internal Medicine (ACVIM).

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Respiratory tract disease is a very common problem in horses that has a wide range of causes, including bacterial and viral infections, as well as non-infectious conditions. Nasal discharge (i.e. a "snotty nose") is often one of the first signs of respiratory disease noticed by owners. Unfortunately, the nasal discharge itself cannot tell you what the problem is, nor how to deal with it.  If your horse develops nasal discharge, you should contact your veterinarian to determine the best approach to diagnose the cause and the best way to treat the animal.  Here are just a few of the factors that need to be considered:

• Does the nasal discharge come from one or both nostrils? Discharge that predominantly comes from just one nostril is more likely due to a problem in a sinus, or possibly one of the guttural pouches (but guttural pouch infection can also cause discharge from both nostrils, even if only one side is affected).
• Does the horse have a fever? Bacterial or viral infections usually (but not always) also cause a fever.
• Are there any other signs of illness, such as decreased appetite or decreased energy. If so, an infection is more likely.
• Is there more than one horse affected? This may indicate that a transmissible pathogen (virus or bacterium) is involved.
• Have new horses been introduced to the barn, or has the horse encountered other horses while off the farm? If so, this increases the likelihood of a viral infection.
• What does the discharge look like? Thick, yellow discharge is more often (but not always) caused by bacterial infection.  Clear, watery discharge may be caused by viral infection, or sometimes non-infectious causes such as heaves. Bloody discharge can indicate serious problems.
• Does the horse cough, and if so, when?  Is coughing more common during exercise, when the horse is inside, or while the horse is eating hay?  These signs may be associated with a non-infectious condition such as heaves.

Nasal discharge is a sign that something abnormal is going on. Some causes of nasal discharge are very minor, but others are very serious. Some are not infectious, but others can be easily transmitted to other horses. Good communication with your veterinarian is important.  Do NOT give your horse antibiotics unless your veterinarian prescribes them.  Nonetheless, if there are any signs that suggest an infection (viral or bacterial) might be present, you should isolate your horse right away, at least until it has been examined by a veterinarian, in order to prevent possible infection of other horses.

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Recently, I wrote about a large disease outbreak involving horses in Israel that resulted in a country-wide quarantine. Equine viral arteritis (EVA) has now been identified as the cause of the outbreak. This viral disease can cause a wide range of clinical signs in horses. It is often associated with abortion, but flu-like disease is common in adult horses, and is consistent with what was reported in Israel. The EVA virus is spread mainly by the respiratory route - contact with infectious nasal discharge is a prime means of transmission between horses.

It is currently unclear where the Israeli outbreak originated, or how it was propagated.  Transportation of horses to shows, races and other events with subsequent mixing of horses is a great way to rapidly spread such a virus over a large area. The World Organization for Animal Health (OIE) reports that this virus has never been identified in Israel before. If this is indeed true, it could explain why the outbreak was so rapid and widespread - when a virus encounters a population of horses who don't have any immunity because they have not been exposed or vaccinated, more animals are likely to become sick and spread the virus further.  The massive influenza outbreak in Australia last year was a great example of that.

It will be interesting to see what happens with this outbreak. The implementation of a nationwide quarantine, while certainly disruptive to many people, may have been an important step in limiting further transmission of the virus.  A vaccine for EVA (which cannot be used in pregnant mares) is available, and could be used as part of outbreak control. However, good infection control practices are more important - vaccination will not be effective if it is the only measure taken.

Click here for more information on equine viral arteritis (EVA).

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Infectious disease risks increase whenever you mix large numbers of horses together, especially horses from many different farms. The risk is further increased if horses are allowed to have direct contact with each other, and indirect contact through people that touch many different horses. Add a little stress from shipping and you have a prime situation for infectious disease transmission.  Unfortunately, this very same situation is something that happens countless times every week, at horse shows and other events. Diseases associated with horses being at shows are very common. Some people think that horses getting sick is an unavoidable aspect of competing. While it is true that you can never completely eliminate the risk of infection, I think that our traditionally "accepted" degree of illness in show horses should not be considered "acceptable," and that we should be working harder to decrease the risks of disease transmission in these animals.

Here are some general recommendations to help do just that:

• Avoid direct contact between your horse and other horse - don’t let horses touch each other. 
• Make an extra effort to avoid going near any horses that appear to be sick, such as those that are coughing, or have nasal discharge or diarrhea.
• Don’t share items like water buckets, feed bins and hay nets between horses.
• Wash your hands or use an alcohol-based hand sanitizer after touching another horse.
• Don’t take a sick horse, or a horse that might have been exposed to a horse with an infection, to an event. You don’t want your horse to get sick at a show, and you also shouldn't put other peoples’ horses at risk of infection.
• Make sure your horse is appropriately vaccinated. Horses that go to shows or other events are at higher risk of exposure to certain infectious diseases. Your veterinarian should design a vaccination program that is appropriate for your horse.
• Make sure events that you attend have strict rules for keeping sick horses off the property. If they don’t, or if those rules aren’t being followed, let them know that you think it is unacceptable.

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An outbreak of an unknown infectious disease has occurred in Israel, sickening hundreds of horses and possibly causing death in a few. The disease is characterized by fever, lethargy and loss of appetite, yet the cause has not been identified. At least 30 different farms have been affected, and the spread of disease has prompted the Ministry of Agriculture to implement a nation-wide quarantine of horses. All horse events in Israel have been canceled.  Similar precautions were taken when the outbreak of  equine influenza hit Australia last year.

The described clinical signs in affected horses are very non-specific, so it's hard to speculate about the causative pathogen. There are a variety of potential culprits, including equine influenza virus (EIV), but so far testing has not detected EIV or any other known pathogens.  This "mystery" disease does not appear to affect other animals or people (but without knowing what is causing it, this is difficult to say for certain).  It is possible that this is a "new" disease, but the vast majority of infectious disease outbreaks are caused by pathogens that we already know about. Specialists from Britain have been called in to help with the investigation. Hopefully more information will be available soon.

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