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

International equine athletic events can create a lot of hassles from an infectious disease standpoint. Mixing animals from different areas creates a prime opportunity for transmitting infectious diseases between horses. An additional concern is when some horses might carrying infections that are considered "foreign" to horses from other areas.

During the 1996 Olympics in Atlanta, concerns about the potential for introduction of piroplasmosis (to the US) led to much angst, and restrictions such as only allowing horses carrying the disease to compete in indoor events. Piroplasmosis concerns have also played a role in planning the World Equestrian Games in Kentucky.

Piroplasmosis is caused by the bloodborne parasites Babesia caballi and Theileria equi, and is spread by ticks (as well as re-using needles and other modes of blood transmission). While it can be a fatal disease, some infected horses appear healthy, and these silent carriers can spread the disease to the ticks, which then spread it to other horses.

What to do regarding piroplasmosis was an important consideration for the World Equestrian Games. To even be awarded the games, a clear and fair plan was required to get approval from both the FEI and US Department of Agriculture. Balancing infectious disease risks with limited disruption of activities isn't easy, and it's not an exact science. Significant efforts have been taken to reduce the potential for piroplasmosis transmission. These include:

• Surveys of the park taken prior to the bid that showed minimal tick activity.
• Planning the event for late September and October because ticks are typically dormant by that time of the year (in Kentucky).
• Trapping small mammals at the park from 2006-2008 to study tick levels and types.

During the games:

• Known piroplasmosis carriers will be housed together in a separate barn that will be treated to eliminate ticks.
• Carriers will be inspected for ticks when they enter or leave the stable area.
• In advance of the event, all horses currently at the park have been inspected and treated for ticks.

You can never guarantee that an infectious disease won't slip through even the most strict measures, but that's the nature of infection control. This seems like a nice evidence-based, logical plan to reduce the risk of piroplasmosis dissemination during the Games.

Photo by Ronald Yochum, 2006 (click for source)

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TheHorse.com has just published a readers' poll that asked the question "What is your most important parasite concern?"

Here are the results, with my commentary:

Small strongyles (cyathostomes): 26.08% (103)

• A narrow victory over large strongyles but a worthy winner. Cyathostomiasis is definitely a problem and it can cause serious disease. It seems to be increasingly common, or at least increasingly diagnosed. Resistance to dewormers among this group of parasites is also becoming a concerning problem.  Cyathostomes are probably the most important overall parasite issue, in terms of real impact on horse health, in most areas.

 Large strongyles (bloodworms): 23.04% (91)

• Large strongyles, particularly Strongylus vulgaris, used to be a major problem, before the advent and widespread use of ivermectin. These worms migrate through blood vessels in the abdomen and can cause blood clots to form, compromising the blood supply to the colon. This used to be a leading (if not main) cause of surgical colics and killed a lot of horses. They are a very rare problem now because of their susceptibility to drugs in the ivermectin family and fenbendazole.

Tapeworms: 15.70% (62)

• It's hard to determine the importance of tapeworms. There have been conflicting studies about their relevance, with some studies indicating they play a role in certain types of colic (e.g. gas colic, ileocecal intussusceptions) and others saying they are not involved in disease. The risks probably vary by region and type of colic, and tapeworms probably play a role in some colics, but it is difficult to quantify just how big a role.

Bots: 11.90% (47)

• It's not surprising that this was a popular choice since people are (hyper)aware of the tiny bot eggs that can be deposited on the horse's haircoat, especially on the lower legs. However, while bot eggs are visible and the sight of adult bot worms attached to the stomach during a gastroscopy can freak people out, bots are pretty innocuous. Adult bots can cause small superficial errosions in the stomach wall where they attach, but this isn't of much relevance to the horse. The main issue with bots is how much the adult flies annoy horses when buzzing around and laying their eggs.

Other 10.13% (40)

• I'm not really sure what people meant when they chose "other." I can't think of any other parasites that would be contenders for "most important."

Ascarids (roundworms): 9.37% (37)

• I'm surprised this isn't higher. Ascarids are controllable but can and do cause disease, particularly in foals. The main issue is when foals are not dewormed properly early in life, and accumulate a large burden of worms. When these foals are eventually dewormed, the worms  die all at once and can cause an intestinal obstruction.

Pinworms: 3.80% (15)

• It's good to see that there's not much concern, but no one should be most concerned about pinworms. Pinworms are, at best, a minor annoyance that can cause tail rubbing.

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Strangles (Streptococcus equi infection) is suspected to be the cause of death of at least six horses in an Australian outbreak.  At least 30 suspected cases has been identified and affected stables are under some form of quarantine. Local horse and pony club events have been canceled to help limit the spread.

Little information about the clinical signs is available, but usually it's pretty easy to establish a high index of suspicion based on the typical signs of strangles, particularly during an outbreak. Confirmation of strangles in an outbreak is also usually pretty easy, since isolation of the bacterium is quite straightforward. Results are expected soon.

Strangles outbreaks are not uncommon and this disease is constantly circulating in most horse populations at some level. Some references say that approximately 10% of affected horses die, a number that I think is excessive, but the key point is that strangles can be fatal to a small but not inconsequential percentage of affected horses. The potential severity of disease plus the high infectivity are big reasons why good control measures are needed.

One common problem with strangles control is described by horse trainer Joe Byrne, who said "the most damaging thing with strangles is the attitude of "ignorance and secrecy". Owners with infected stables "straight away clam up, don't tell anyone" and remain operating like nothing has happened and that is very problematic."  Indeed it is.

Photo: A horse with strangles (S. equi infection) showing the characteristic severe swelling of the submandibular lymph nodes (click image for source).

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Following a repeating trend that has been ongoing over the past couple of years, piroplasmosis has been identified in more US horses with no clear source. The latest cases were in North Carolina, which has reported 11 infections in four different locations.

This is quite concerning because it now seems to be a regular event to get a report of piroplasmosis in US horses from different areas and with no clear link to other outbreaks. The disease, caused by the blood parasites Theileri equi and Babesia caballi, can produce a wide range of clinical signs, and kills approximately 20% of infected horses. Some horses can become healthy "silent" carriers, complicating recognition and control. The bloodborne parasites are naturally transmitted by ticks, but can also be spread by other forms of bloodborne transmission, most notably the reuse of needles. Often, poor needle handling practices are blamed by officials for spread of piroplasmosis among groups of horses on a farm, but that doesn't address the most important question - where did it come from in the first place?

PIroplasmosis is considered a foreign disease in the US, but the repeated cases of unknown origin in at least 19 US states suggests that the disease is actually endemic (established) in the US. Any explanation of where the causative parasites keep coming from is pretty much always missing from US piroplasmosis reports. It certainly seems possible, if not likely, that there is an unidentified pool of infected horses and tick species in the affected areas that are able to transmit the infection.

If the US is going to get piroplasmosis under control, a serious effort needs to be undertaken to understand how widespread the infection is and how it's being spread. Tracking known cases and their contacts is important, but a more concerted effort is required. That takes time and money, and often governments are reluctant to spend much of either when it comes to horses (as opposed to food animals), but considering the potential implications of endemic piroplasmosis and the often overlooked economic importance of the horse industry, it's a worthy investment.

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A horse from Missouri has been diagnosed with equine infectious anemia (EIA) as part of a routine Coggin's test. As is normal, the infected horse was euthanized and the farm was immediately quarantined so that all other horses could be tested.

EIA, which is sometimes also called swamp fever, is a caused by the un-originally named equine infectious anemia virus. It is a retrovirus (the same type of virus that causes HIV/AIDS in people and feline leukemia in cats) that can cause serious infection but also long-term carriage by apparently healthy horses. The ability of this virus to cause lifelong infection in some horses is a major problem and complicates its control. The infection is not treatable and there is no vaccine. For these reasons, aggressive control measure are in place in most regions. The main component of this is routine testing.

EIA testing is a standard requirement in many areas of the world for horses that are traveling, competing or being sold. Traditionally, the Coggins test has been used for EIA testing. More recently, another test (a cELISA test) has become available. This test is thought to be more sensitive and takes less time to perform, but positive results still need to be confirmed with a standard Coggins test.

Positive results are immediately reported to the appropriate government agency (in Canada, it's the Canadian Food Inspection Agency). Immediate euthanasia of positive horses is the typical response, although keeping the horse in permanent quarantine, with insect control and a minimum distance from any other horse (or donkey or mule) is also an option in some cases. It may seem draconian to euthanize an apparently healthy horse and quarantine the farm, but this is an important disease that is hard to control if it gets established in an area.

The most important EIA control measure is regular testing, which allows for early detection of cases and implementation of appropriate (albeit unfortunate for the horse) measures. Other measures that help reduce the risk of transmission if the virus is in the area include control of biting insects and ensuring that there is no re-use of needles or other equipment or biologics that can transfer blood. 

There's no indication as of yet where the infection originated from in the Missouri horse. That's a very important factor, because the horse had to get it from somewhere, and that means another infected horse, donkey or mule. Finding one positive animal means that there must have been another positive animal somewhere in the vicinity to infect the insect (the most likely vector) that infected this horse. Hopefully, the horse did not pick it up from another animal in the US, but careful tracking of this horse's travel history and detailed surveillance of equids in the area will need to be performed to figure this out.

Image: A black horse fly (Tabanus atratus); members of this genus are capable to transmitting EIA between horses when they bite.  (Photo credit: Mark Cassino, source: USDA Systematic Entomology Laboratory)

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The US economic meltdown and slow recovery are being blamed, in part, for the increase in eastern equine encephalitis (EEE) cases in some parts of the US. The premise is that as the economy weakens, people cut down on discretionary spending. One area of discretionary spending is veterinary care, particularly non-emergent care like vaccination. It may be an even greater issue for a rare disease like EEE, as even people who choose to vaccinate their horses may pare down the vaccines they use, by limiting vaccination to core vaccines such as rabies and tetanus.

“The economy is very much a factor” stated veterinarian Steven Halstead of the Michigan Department of Agriculture. Similarly, veterinarian James Connell said "It’s flat out the economy. People can’t afford to feed them, how can they afford to vaccinate them?” Dr. Connell reports a decrease in vaccination rates in his practice of approximately 15% per year over the past 3 years, something that has also been reported by other Michigan vets.

It's always tough to say, with confidence, why disease trends occur. EEE is a sporadic disease in many regions, like Michigan, and there can be no cases some years and clusters other years. So, there's no guarantee that vaccination is the problem. However, it makes sense that it could be involved since decreasing vaccination rates increase the number of susceptible horses.

The only saving grace with regard to decreasing EEE vaccination rates is that it only hurts the horses that are not vaccinated. With some diseases, namely those that are spread horse-to-horse, maintaining a high overall vaccination rate in the horse population is critical to reduce the chance of disease transmission. This is the concept of "herd immunity."  With those diseases, when people stop vaccinating their horses, they also in part increase the risk to other horses in the area. With a disease like EEE that is spread by mosquitoes which cannot be infected by other horses, herd immunity doesn't apply. The percentage of horses in the region that are vaccinated has no impact on the likelihood that any single horse will be exposed.

(click image for source)

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A 4-year-old Thoroughbred at Keeneland's training center on Rice Road developed Potomac Horse Fever (PHF) last week and was euthanized. Potomac Horse Fever is caused by the bacterium Neorickettsia risticii and typically causes diarrhea and laminitis. Sometimes, severe laminitis may be the primary problem with  mild or no diarrhea. It's usually treatable if caught early, but severe infections can be fatal or have long-term repercussion, usually as a result of the laminitis. 

While a fatal infection at a facility with a lot of horses always (rightfully) raises concerns, this isn't anything for people at Keeneland to be worried about.

• Potomac Horse Fever is not transmitted between horses. Horses are infected by ingesting aquatic insects (e.g. caddis flies, mayflies) that are infected with the causative bacterium. Horses that graze around rivers or creeks can inadvertently ingest these insects while eating or drinking. There is no evidence of horse-to-horse transmission.
• The horse presumably acquired the infection elsewhere. The horse was only at Keeneland for a few days prior to becoming ill. Given what we know about the incubation and normal progression of the disease, it's most likely that the horse was already infected when it arrived but simply was not yet showing signs of illness.

Potomac Horse Fever is one disease for which you need to know what is going on in your area, and what the specific risks are for your farm. If you live in an area where cases are diagnosed every year and your horses have potential access to aquatic insects, then the risk is higher. If you are in an area where the disease is rare and/or your horses don't have access to rivers or creeks (or pasture in close proximity to them) the risk is pretty low.

Knowing the risk is important for ensuring prompt diagnosis. Unfortunately, there aren' t any great preventive measures for PHF, apart from restricting access to areas where aquatic insects might be encountered. A vaccine is available but there's not much evidence that it's effective (it might decrease the severity of disease, but there's no evidence that it prevents disease). The most important thing is recognizing early signs of disease, which may be subtle (e.g. transient mild fever, decreased appetite, general malaise). These vague signs don't necessarily mean the horse has PHF and that it should be treated right away, but getting the horse evaluated and tested ASAP, with close monitoring to see if other signs of of the disease develop, can lead to earlier and better treatment, and likely a better outcome.

Photo credit: David Dew (click for source)

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An infant from Florida has died of eastern equine encephalitis (EEE). There have now been three human deaths in Florida this year from EEE, a number that is high compared to most years. Despite being called "equine," this virus can infect people if they are bitten by a mosquito that acquired the virus from an infected bird. Human infections are rare, with only 5-10 cases in the US each year, but they are often devastating, and kill approximately 30% of those who are infected.

It's concerning that there have been three deaths in Florida so far this year, considering we're still not even far into the typical EEE season. It is suspected that the higher number of cases is the result of a greater percentage of infected mosquitoes (not an increase in total number of mosquitoes).

There is a vaccine for horses, and it is recommended for all horses in areas where EEE virus is expected to be active. There is no vaccine for humans, so the focus is on decreasing mosquito populations and reducing mosquito bites. EEE virus cannot be transmitted from horses to people, and horses cannot pass the virus back to mosquitoes, so they play no role in human infections. The main role of horses in terms of human health is as an indicator that EEE virus is active in the area, since equine cases tend to occur earlier and be more common than human cases. If an infected horse is identified in a particular area, it's an indication that the virus is active and people are at increased risk.

(click image for source)

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While it seems straightforward, disinfection is pretty easy to screw up. It's not uncommon for me to find illogical or completely ineffective "disinfection" practices when I do farm investigations. In general, people don't really understand much about disinfectants and disinfection. That's not too surprising because it's not exactly a common or enthralling topic (and I didn't really know much about it until I became focused on infection control).

Good routine cleaning and disinfection practices are important for all barns, but the specific needs vary greatly between facilities. Proper disinfection practices are particularly important in certain situations, such as in foaling stalls, stalls housing sick horses and isolation areas. Because problems are so common, everyone should think about their disinfection program and the disinfectant(s) they use. When in comes to the disinfectant, here are some questions everyone should ask themselves:

Is it actually a disinfectant?

• Some people mistakenly use cleaners as disinfectants. Cleaners help remove debris and make the surface look good (i.e. clean), while disinfectants kill microorganisms through their chemical activity. You need a clean site for disinfectants to work, but using a cleaner in place of a disinfectant won't get rid of the microorganisms that remain after the visible dirt has been removed.

What kind of disinfectant is it?

• Disinfectants are a little like antibiotics. They have a "spectrum of activity" which tells you what types of microorganisms that they kill. Some disinfectants kill a wide range of microorganisms while the activity of others is more limited. Some work well in the presence of some dirt and debris, while others are readily inactivated and essentially won't work at all unless the surface is very clean first. Some are quite toxic and some are pretty innocuous.

What concentration should be used?

• There should be specific instructions on the product. Some are "ready-to-use" and don't need dilution. Most come as concentrates that need to be properly diluted. Excessive dilution will reduce the chance of of the disinfectant working. Inadequately diluting the concentrate ends up costing a lot more because you waste the product, and high concentrations of some disinfectants can be harmful. "Add two splashes to a bucket" is not proper dilution. You need to measure.

What contact time is needed?

• Disinfectants don't work instantly. They need time. Usually, it's anywhere from five to thirty minutes, depending on the disinfectant.

Common disinfection errors or misconceptions

A nice smell means it's clean.

• Clean actually has no smell at all. Products that leave a residual smell do so for aesthetic purposes, not for any real effect.

All disinfectants are created alike.

• Disinfectants are quite variable, in terms of the different organisms they can kill, how well they work in the presence of dirt and debris, how quickly they work, how toxic they are, and other factors.

The label says it kills 99.99% of microorganisms so it must be great.

• This type of advertising is misleading. Killing 99.99% of selected organisms in a lab environment is different than killing the entire range of possible microorganisms encountered in a barn, and in the presence of debris and difficult-to-disinfect surfaces.

If the area is dirty, just use more disinfectant

• Cleaning removes 80-95% of microorganisms, and is the most important step of the disinfection process. If the area isn't cleaned properly and has a lot of dirt and debris, the disinfectant will not be very effective, regardless of how much is used.  There's no substitute for good old fashioned "elbow grease" when it comes to getting a barn clean.

One disinfectant is good, so two must be better

• A single disinfectant with a good disinfection protocol is adequate. There should be no need for multiple products, unless you want to have a higher level disinfectant available for certain situations (e.g. a stall for a horse with diarrhea). However, you should never mix disinfectants, because highly toxic by-products can be produced.

I'm not having any infectious disease problems, so I don't need to worry about my disinfection practices.

• Everyone that has an outbreak can say, right before the outbreak, that they had no infectious disease problems. Just because things are going well today doesn't mean that things will be fine tomorrow. It's a common perception, however, and often we don't find out about peoples' inadequate disinfection (or other infection control) practices until we are dealing with an outbreak. There's no guarantee that a better disinfection program would have prevented the outbreak, but it certainly wouldn't have hurt and may have helped reduce the problem.  It's much easier to review things and make improvements before there are problems.
  

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In humans, infection with West Nile virus have a few different possible outcomes:

• Nothing happens: This is probably the most common outcome. People get exposed to the virus by being bitten by an infected mosquito, but their immune system fights it off. They may develop antibodies against the virus but they don't get sick.
• West Nile fever (also called West Nile non-neuroinvasive disease): In this scenario, the infected person develops a fever, often along with a headache and joint or muscle pain. This accounts for most of the people that get sick after being infected.
• West Nile neuroinvasive disease: This is the big scary problem, though it fortunately only occurs in a small percentage of infected people. This form is characterized by neurological abnormalities such as meningitis, encephalitis and/or paralysis.

US data indicate that there were 720 cases of West Nile virus disease reported in 2009, 54% of which were non-neuroinvasive and 46% of which were neuroinvasive.  While that may not seem to support my statement above that only a small percentage of infected people get neurological disease, you have to remember that if you don't get sick, you won't get tested. Further, if you just have a headache and muscle or joint aches, it's pretty unlikely you're going to get tested. Therefore, while a large percentage of reported cases were neurological, a very large percentage of non-neurological cases were presumably not identified because they were mild and testing wasn't performed.

Back to horses...

In horses, we pretty much only identify horses that are healthy but have been exposed (based on the presence of antibodies against the virus) and those with neuroinvasive disease. However, that doesn't mean that West Nile fever does not exist in horses. It probably does, we just don't diagnose it.

Horses that develop mild disease of short duration aren't always even examined by a veterinarian. If a veterinarian is called and the horse is just a little "off" with a fever and vague signs of illness, testing for West Nile virus will not be very high on the list of priorities (especially if the horse owner has to pay for testing). Most of these horses, if they are examined, probably get treated empirically with anti-inflammatories and rest, and they get better. So, we really have no idea whether this form of the disease exists or how common it is. In a lot of ways, it doesn't really matter. The main reasons that it would be nice to know whether horses with mild, non-specific disease have West Nile is to know that there is active West Nile virus transmission to horses going on in the area, and for a better general understanding of West Nile virus in horses.

If you have a horse that develops vague signs of disease during the typical West Nile season, especially if there are no major other risk factors for other diseases that typically cause fever and vague illness (e.g. going to shows, new horses in the barn), you should at least consider the possibility that West Nile virus is present. That doesn't necessarily mean you have to get the horse tested or do anything different, but it never hurts to have a better idea of to what infectious diseases your horses might be exposed.

Image source: www.lapublichealth.org (click image to enlarge)

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