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

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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Three barns at Calder Race Course in Florida have been quarantined after a horse was diagnosed with equine herpesvirus (EHV-1) infection. Horses from this barn are quarantined for 3 weeks and are not allowed to race. Other horses will not be allowed to enter the grounds for the next 2 weeks, but racing will continue with horses that are currently on the grounds and not under quarantine (1800 horses are present at the track). Track personnel believe the infection has been restricted to one horse but are taking these measures proactively.

Equine herpesvirus can cause a range of clinical signs, including fever and respiratory disease, severe neurological disease and abortion. The affected filly in this case had neurological disease and was euthanized. Outbreaks of abortions or neurological disease are the greatest concern, and a specific type of EHV-1 has been implicated as the main cause of neurological disease.

It's always hard to say what the most appropriate response is to a scenario like this. Equine herpesvirus is an endemic virus that is present throughout the world and lies dormant within a large percentage of healthy horses. Most infections are sporadic and only involve a single horse or small number of horses, but outbreaks can occur and that's why aggressive measures are sometimes taken. With only a single diagnosed case and no apparent evidence of transmission to other horses in this case, it's uncertain whether such an aggressive approach is required. However, it's worse to be too lax initially than too aggressive, and a logical approach is to implement aggressive measures, and then reassess them as more information becomes available. If no other horses develop signs consistent with EHV-1 infection, then loosening of the restrictions would be reasonable. If there is evidence of transmission and disease in other horses on the property, continuation with aggressive measures makes the most sense.

A great resource regarding EHV-1 is the ACVIM Consensus Statement on this subject, which is available on the equIDblog Resources page.

Image source: www.calderracecourse.com

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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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Dr. Bob Wright of the Ontario Ministry of Agriculture Food and Rural Affairs has compiled a document with various pieces of information about disease occurences in horses in Ontario and Canada. While there's nothing too earth-shattering in it, it does give a nice synopsis of certain diseases and shows how Ontario is a pretty good place to be in terms of some nasty diseases. The report can be viewed by clicking here.

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The American College of Veterinary Internal Medicine (ACVIM) has released a consensus statement on equine herpesvirus type 1 (EHV-1). The ACVIM is the specialty organization for veterinary internal medicine in North America, and every year it produces consensus statements on selected large and small animal topics. These are developed by a group of experts in the field, who review current knowledge (and knowledge gaps) and come up with a comprehensive overview of the subject. This consensus statement has detailed information on various aspects of EHV-1, including diagnosis, vaccination and infection control. It can be accessed through the ACVIM website, on the equIDblog Resources page, or by clicking here.

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Unlike dogs, which commonly have urinary tract disease, urinary tract infections (UTIs) are very uncommon in horses, but they can occur.

• Foals less than a month old are at risk of developing upper urinary tract infections (affecting the kidneys themselves) secondary to bloodstream infections (septicemia).
• Critically ill foals that are given broad spectrum antibiotics to treat serious bacterial infections can (rarely) develop fungal infection of the lower urinary tract (affecting the urethra and bladder) caused by Candida sp. These infections can also “ascend” the urinary tract, eventually traveling from the bladder up the ureters to the kidneys as well.
• UTIs in adults are very uncommon, but when they occur they usually affect the lower urinary tract. Mares are slightly more likely to develop UTIs than geldings and stallions because they have a much shorter urethra, so it’s easier for bacteria to reach the bladder (they don’t have as far to go!).

Upper UTIs are very serious because the kidneys themselves are affected (nephritis), which can ultimately lead to kidney failure. Animals with this kind of infection are typically very sick, depressed, and can have a fever. In general, lower UTIs are not as serious, but they do make it very uncomfortable for the animal to urinate (the same is true for people!). However, lower UTIs that result in or are the result of stones in the urinary tract can be life threatening if one of the stones becomes lodged in the urethra, preventing the horse from urinating altogether (this is similar to the situation with “blocked cats,” which occurs relatively commonly). Most of these infections are caused by the same common types of bacteria – E. coli, Proteus mirabilis, Klebsiella sp., Staphylococcus sp., Enterobacter sp., Corynebacterium sp., Pseudomonas aeruginosa – but sometimes fungi can be involved (such as Candida sp.) and there are also some parasites which rarely can affect the urinary tract of horses.

The important thing to remember, though, is that these infections are uncommon in general, and they are very rare in otherwise healthy horses. So if a horse has signs of a UTI (e.g. difficulty urinating, unusual behaviour when urinating, frequenting “squirting” without emptying the bladder completely, scalded skin from urine dribbling, blood in urine), there is often something else wrong. An important predisposing factor is bladder paralysis, when a horse loses control of the muscles in the bladder wall that make the bladder contract. When the bladder can’t contract, it never empties, and the urine just sits there, allowing bacteria to grow and sometimes even allowing infection to travel up to the kidneys (i.e. ascending infection). Bladder paralysis can result from diseases or certain toxins affecting the central nervous system (e.g. equine herpesvirus type 1 (EHV-1), Sorghum spp. toxicosis). In horses, it is often difficult to determine if stones (uroliths) are the cause of a UTI or the result of the UTI.

As in any other animal, the best way to definitively diagnose a UTI is to culture bacteria from a urine sample that has been collected as aseptically as possible (usually by passing a catheter, as in the photo above). It’s important to collect a urine sample for culture before starting any antibiotic treatment. If you suspect your horse may have a UTI, it should be examined by your veterinarian as soon as possible.

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While it's a little early in the foaling season, it never hurts to think about vaccination plans for foals. Vaccination is an important part of the disease prevention program - but it's only one part... general infection control practices are as, or often even more, important. Vaccination of foals is not the same as vaccination of adult horses. Foals may be more or less susceptible to certain infections. They may have greater or lower risks of exposure, and they may respond differently to vaccines.

There is no standard vaccination program for foals. Specific vaccination programs need to be designed for each horse on each farm based on risk of exposure and infection, cost and many other factors. Your veterinarian can help you determine what your animals should be vaccinated against. When it comes to foals, here are some important things to considered:

• Foals typically need to be vaccinated several times to get the desired immune response. There are at least a couple of reasons for this. First, if a foal has high antibody levels from colostrum, it may not respond properly to the vaccine until those antibodies are used up. These antibody levels drop over time, but the rate and timing of the drop is different for each foal. Vaccinating the foal several times (at appropriate intervals) helps reduce the risk of vaccine failure from colostral antibodies. Second, foals are born with a fully functional immune system, but the immune response to an infectious agent or vaccine is slow and low the first time an individual encounters it. The first dose of vaccine may produce some response, but a much higher response is generated with boosters.  This is actually true of adult horses as well.
• Over-vaccination can be a problem. Vaccination too early and/or too frequently could actually result in a decreased immune response or complete vaccine failure, a phenomenon known as immune tolerance. So, giving many doses of vaccine starting at a very young age may actually be counter-productive. This is probably a bigger concern with influenza vaccination compared to other vaccines.

Talk to your veterinarian about vaccination. Over-, under- and improper vaccination can lead to increased disease risks, plus wasted time and money.

Guidelines for vaccination of foals have been developed by the American Association of Equine Practitioners. These are also available from the link on the equIDblog Resources page.

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The latest version of the DEFRA/AHT/BEVA Equine Quarterly Disease Report has been released. This is a joint venture of three British organizations, and often contains some interesting infectious disease information. This edition contains brief information about diagnoses of selected diseases such as equine herpesvirus (EHV) -1 abortion, EHV-1 neurological disease and equine influenza. There is also a brief review of the eradication of equine infectious anemia (EIA) following the 2006 outbreak in Ireland, which was also discussed in a previous equIDblog post.

Another useful piece of information in this report is the data regarding strangles (Streptococcus equi subsp. equi) diagnoses. They report 280 positive S. equi culture results and 167 positive S. equi PCR test results during the last quarter. This shows once again that strangles is still a rather common disease. This is important to remember when considering routine infection control practices, because the risk of strangles exposure is ever-present, it is not just concern during outbreaks.   Also, people shouldn’t overreact to single cases of strangles (as they often do), since this disease is far from rare.

The full Equine Quarterly Disease Report can be downloaded by clicking here, or through the Animal Health Trust’s website.
 

Image from http://snapshot.parade.com/mainemb.php?g2_itemId=783542.

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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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The December 2008 newsletter from the University of Guelph's Animal Health Laboratory contains an article about diagnoses of equine viral infections from 1998-2008. Equine herpesvirus type 1 (EHV-1) was the most commonly detected virus and the most commonly diagnosed infectious cause of abortion. Three cases of neuropathic EHV-1 (the strain that was recently identified as being an important cause of the neurological form of this disease) have been diagnosed since specific testing for this strain started late last year. Small numbers of influenza cases were confirmed and, as expected, they all involved the H3N8 type virus. Eastern equine encephalitis (EEE) is a rare disease in Ontario, but an outbreak occurred in 2008, with 8 cases identified. Equine rhinitis viruses A and B (formerly equine rhinovirus 1 and 2) were found in a few horses with fever, respiratory tract disease and swelling (edema) of the limbs and abdomen.

This newsletter, along with previous versions, can be found here. (Note: at the time of writing, the December newlsetter was not yet available online. If you look and it's not there, check back again later because it should be available soon).

Image: TEM of equine herpesvirus (source: Gluck Equine Research Center)

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A few years ago, I investigated an equine herpesvirus (EHV) outbreak associated with a yearling sale. We found a lot of sick horses, both from the sale and horses that were infected when purchased horses were brought home. A lot of actively racing horses got sick and missed races, which cost people even more money.  Part of the investigation was asking people what percentage of horses from sales they expected to get sick right after the sale. The average answer was 80%, and many people said 100%. When you think about it, it's absolutely astounding that people will spend what is often a great deal of money to buy a horse that they expect will get sick, and then (more often than not) put it in the same barn as their other horses, thereby putting all the animals at risk of infection.

Why do we put up with a system where we accept this degree of illness? Is this expected and accepted rate of disease really acceptable? More specifically, are there practical (and really just common sense) measures that can be used to reduce the risk of horses getting infected at sales and/or transmitting disease to other horses once they reach the home farm?

Specific thoughts and facts about how to reduce disease transmission on farms, tracks, sales and everywhere else horses gather will come in other posts, but infection control is not rocket science. Basic measures can greatly reduce the risk of disease transmission. However, the first step is to change people's attitudes - it should never be considered "normal" for such a high percentage of animals to be sick.  If 80% of the horses get sick, we're doing something wrong.

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As of this morning, no new cases of equine herpesvirus (EHV) infection have been identified at Laurel Park in Maryland. (See our previous reports regarding this situation.) More than 50 horses on the premises have tested  negative for the virus, and no new horses have developed signs of disease. While they are not yet at the end of the period during which horses infected by the sick filly could develop disease, the fact that no new cases have been found so far is very encouraging. Horses at Laurel Park will remain under close monitoring for new cases for 21 days, but if no cases are found in the first 7-14 days, then it is unlikely that additional cases will develop.

Unfortunately, the affected filly had to be euthanized on Saturday. The track will still be under quarantine until around December 6.  This will be 21 days after the last evidence of clinical disease in any horse (in this case November 15 when the filly was euthanized), which represents the last chance of exposure of other horses to the virus.

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Equine herpesvirus type 1 (EHV-1) infection was confirmed in a filly at Laurel Park in Maryland (see yesterday's post). As a result, the track is now under strict quarantine.  Horses are not allowed to be shipped onto the track (except from an associated training facility) and horses already at those facilities may not leave. This is being done to reduce the risk of transmission of the virus to horses on other properties. As discussed yesterday, EHV is tough to control because it can be found in a dormant state in so many healthy horses, and we really don't understand enough about how the neurological form of disease is transmitted. It's possible that these quarantine restrictions are over-kill, but since we don't have enough information to say with certainty what is or is not necessary, and because large outbreaks of EHV-1 have occurred in the past, using a very cautious approach is understandable. Only time will tell whether there was transmission of this virus before the restrictions were implemented.

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A two-year-old filly at a racetrack in Maryland has been tentatively diagnosed with equine herpesvirus I (EHV-1) neurological disease. This virus can cause several types of disease, most notably neurological disease and abortion. The virus itself is very common in horses, but the neurological form of disease (also known as equine herpesvirus encephalomyelopathy) is uncommon. Certain types of EHV-1 are thought to be more likely to cause neurological disease than others.

Horses that develop neurological signs from EHV-1 infection typically first have a fever.  The neurological problems tend to be more obvious in the hind limbs, but a very wide range of abnormalities may be seen in different horses.  For example, the filly in this case is apparently unable to stand, but is still able to sit up and eat. "Downer" horses like this can sometimes be placed in a sling to help them stand (see picture), in order to help prevent damage to their muscles due to the pressure of their own weight.  The prognosis for horses that cannot stand is very poor, and gets worse the longer they are down.  There is a reasonable chance of recovery in horses that remain standing, even if they need some assistance to do so.

In the past, it was often assumed that horses with neurological EHV-1 were not infectious to other horses.  Most of the time the disease occured as single, sporadic cases. However, over the past 10-15 years, a large number of outbreaks have been reported, and the approach to these cases has changed dramatically. In the case of the Maryland filly, the barn the animal is in is currently under quarantine, although no other horses are showing signs of illness. Some other horses in the barn are currently being tested. Presumably, other horses at the track are also being closely monitored. The potential for the virus to spread (i.e. to other horses, barns, or even other facilites), as has occurred in other situations, is a major concern.

The best measures to control EHV-1 are currently unclear. One major hurdle its control is that the virus is present in a "dormant" state in a large percentage of healthy horses (similar to the "dormant" herepsvirus in people that sometimes causes cold sores).  Carriers of EHV-1 make it extremely difficult to eliminate the virus from a group of horses, compared to a pathogen like influenza virus which horses do not carry for long periods of time. Carriers usually don't shed the virus, and therefore normally are not a problem.  However, in certain situations the virus may become reactivated, and the carrier may start shedding it again, potentially infect other horses through direct contact, contaminated surfaces, or over short distances in the air. Isolation of infected horses and use of good infection control practices is critical. Close monitoring of those animals in contact with the affected horse(s) is important, so other infected horses can be identified, isolated and treated as quickly as possible. Vaccination has not been shown to be effective against EHV-1 neurological disease. However, vaccination of most or all horses in a group can be useful in that it can decrease the likelihood that the virus will be circulating in that population. Vaccination during an outbreak is generally frowned upon.

In a situation like this where a case has just been diagnosed, there is a very stressful period during which the best control program possible is implimented, but everyone has to wait to see if other horses have already been infected. The next 1-2 weeks is the critical period that will answer the question of whether this is just a single sporadic case, or the first case in an outbreak.

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