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

The latest USDA report on equine encephalitis surveillance data gives a good picture of what has happened so far this summer with West Nile virus, eastern equine encephalitis (EEE) virus and western equine encephalitis (WEE) virus in the US. The data, last updated July 28, report no cases of WEE anywhere in the US. In contrast, EEE has been found in seven states (Florida (43 cases), Georgia (22), Mississippi (16), Alabama (6), North Carolina (4), Louisiana (3), Virginia (1)). This is a pretty typical distribution, with most cases being found in a small number of states, and even in certain regions within those states. West Nile virus cases are, not surprisingly, low so far. This disease does not tend to occur until late summer and fall. As of July 28, only six cases have been reported (Washington (2), one each in West Virginia, Texas, Montana and Missouri). The important question is what will happen with West Nile virus for the rest of the year? Stay tuned.

Image: Scanning electron micrograph of a mosquito (source: www.cbc.ca)

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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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If you get bitten by a mosquito carrying West Nile virus, one of three main things could happen. The best case scenario, which happens in about 80% of people that are infected, is nothing happens. Most of the other 20% people that get sick only develop mild disease, including fever, headache, body aches and similar flu-like symptoms. These cases are usually not diagnosed as West Nile virus infection because signs are vague, typically mild and often last only a few days. Approximately 1 in 150 people that are infected will develop neurological disease. This is the most serious and potentially fatal form of West Nile virus infection.

In horses, we really only recognize the two extremes - no disease or neurological disease. Does this mean the mild form (sometimes call West Nile fever) doesn't happen in horses? It probably does happen, it's just that we rarely notice or diagnose the problem. Mild flu-like disease is easily missed in horses, particularly horses that are housed outside. If these signs are noticed, West Nile virus doesn't often jump to mind and testing is not common. It's likely that many horses actually develop this mild form of disease, but the only way to know for sure would be to test more horses with mild, non-specific disease in areas where West Nile virus occurs during the typical West Nile virus season. This might be hard to justify, since horse owners would have to pay for the testing and the results would not change what's done with their particular horse. If horses that were infected and developed mild disease had life-long immunity to the virus after infection, then that would be a reason to test (because they would no longer be at risk of the more severe form), but we don't have enough evidence to say that's the case.

So, if your horse develops mild signs of illness such as fever, in a West Nile virus area, and during the right time of year, it certainly could be a mild case of West Nile virus infection - but it could also still be a lot of other things. Horses that develop fevers or other signs should be watched closely to make sure more severe disease is not developing, and getting a veterinarian to examine the horse for other (possibly more subtle) signs is never a bad idea. Testing may not change what you do, but it might help you and your veterinarian understand whether West Nile virus exposure is a problem in your area and explain why your horse got sick.

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Here's a recap of West Nile virus activity in Canada in 2008.

Humans: There were 36 people diagnosed with disease caused by West Nile virus. Five had neurological disease, 29 had non-neurological disease (i.e. West Nile fever) and in two cases the clinical syndrome was not defined. Most affected people were from Saskatchewan (17) and Manitoba (12), with three from Ontario and two from Quebec. There were single cases in Alberta and British Columbia, and both of those were thought to have been acquired outside of the respective provinces.

Horses: West Nile virus infection is an immediately notifable disease in Canada, whereby the Canadian Food Inspection Agency must be notified of all diagnosed cases in animals. Six positive horses were reported in Canada in 2008, including two in Ontario horses, both in eastern Ontario. Two other cases were reported in Saskatchewan and two more were reported in Quebec. Whether this low number of diagnosed cases is because the disease is now actually this uncommon in canadian horses, or because there is less testing for the disease is unclear. Based on anecdotal information from other veterinarians and horse owners, I suspect that West Nile virus infection in horses is truly rare in Ontario, but is more common in Saskatchewan. This makes sense considering the different types of mosquitoes in the two provinces, as well as the corresponding difference in the number of human cases. These questions highlight the need for proper diagnostic testing to be performed on all horses with neurological disease. The more confidence we have in the surveillance data, the better conclusions we can make about patterns of disease and the need for vaccination.

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The Government of Ontario has stopped funding surveillance for West Nile virus in dead birds. Since the virus first arrived in the province in 2002, large numbers of dead birds from across Ontario were tested for West Nile virus every year to monitor the distribution of the disease. Birds were tested because finding the virus in certain species is a good indicator of West Nile activity, and affected birds are found well before disease occurs in people or horses every year.

Will this have any negative effects?  Probably not. Bird surveillance was very important when West Nile virus was new to the province, to see where it was, how it spread and when it occurred in different regions. Since the virus has been around for many years now and the patterns of disease in birds don't seem to be changing, bird surveillance results likely wouldn't change much, and therefore wouldn't impact the provincial government's recommendations (e.g. wearing insect repellent) and preventative measures (e.g. larvicidal treatment of mosquito breeding grounds).  In a perfect world, surveillance would continue because you never know for certain what you are going to find, and active surveillance networks are useful for identifying new problems (e.g. new diseases).  However, at this point the benefits are likely limited, which makes it difficult to justify the cost of such a program.

If you are in a region where West Nile virus has been found in the past, assume that it's going to be back again this year, at about the same level and around the same time of year. Take precautions to reduce mosquito exposure.  For more information, see our previous post and video about "Preventing West Nile Virus Infection" and our West Nile virus archives.

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In response to cases of West Nile virus (WNV) in northern Italy, the UK's Department for Environment, Food and Rural Affairs (DEFRA) has performed an assessment of the risk of WNV emerging in the UK. 

Among their conclusions (with my comments in italics) are:

• There is a continuous low risk of WNV introduction from migratory wild birds.
• Introduction of WNV from legal trade in horses is very low. Really, it's zero since horses can't spread the virus.
• WNV would cause "some impact" on the UK horse industry because of illness and death, and  movement of horses from affected holdings would be subject to controls. "Some impact" is presumably an understatement, considering what has happened in North America. Restricting movement of horses doesn't make sense because infected horses can't transmit the virus, so there would be no reason to restrict any horse movement.
• The potential for WNV becoming established in the UK is "very low". I don't know the details about mosquito populations in the UK, but I suspect this is understating the issue. When WNV first emerged in North America, it was often stated that it wouldn't survive the winter and would not be a problem in the next year. That turned out to be (very) wrong. 

If we've learned one thing from WNV (and other emerging infectious diseases), it's that prediction of risks is very difficult. An understanding of the biology of the organism, its vectors and its hosts can be used to make logical assumptions, but we get fooled time and again by diseases that don't do what they're "supposed to do". Will WNV emerge as a problem in the UK? Probably. Given the potential for expansion of the range of this virus in Europe, potential effects of climate change and the inherent unpredictability of infectious diseases, it's more likely than not that WNV will reach the UK sometime in the future.

Image from http://www.ci.berkeley.ca.us/ContentDisplay.aspx?id=16144

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Scott and I are having a difference of opinion. Speaking from experience, it’s certainly difficult to win a debate with him most days (that might be a huge understatement), but I’ll weigh in with my two cents on this one. If Scott were to vaccinate a horse for West Nile virus (WNV), he’s stated that he wouldn’t do so until July, based on when the peak incidence of disease and therefore presumably the period of highest risk is. I would still vaccinate a southern-Ontario horse in April. Here’s why:

West Nile vaccines are labeled to provide protection for 12 months. Before they can be marketed with such a label, these vaccines have to be tested to prove that they still offer some protection for the animal for at least that long. For some vaccines, like rabies, protection likely lasts much longer than the label claim, but until recently no one’s bothered to study most vaccines beyond one year. I have no doubt that the protective immunity does decrease with time – the protective effects of vaccine are likely highest (as Scott said) about 30 days post vaccination, and lowest at the end of the 12 months. But there is no evidence that the immunity drops off so fast that after 4-6 months the vaccine would require a booster to be adequately, if not maximally, effective. There are vaccines, like herpesvirus and influenza, for which we recommend boosters for horses semi-annually, but this is for animals that are at ongoing high-risk for exposure to these diseases, which are very common. Six months after mid-April is mid-October, and in this part of the world there are very few mosquitoes still flying around at that point.

Scott pointed out that the first part of the summer is likely lower risk in terms of WNV transmission, at least for horses. But there is a time of year when the risk is even lower – October to April, when (as I just said) there are virtually no mosquitoes. If you vaccinate a horse in July, its immunity will be lowest over the first three months of the summer, when there are still birds and mosquitoes around that are carrying WNV. If you vaccinate a horse in April, its immunity is lowest in the late winter, when there’s almost no risk of transmission, so it doesn’t matter! I would rather have a horse protected for the entire mosquito season (May-September), and in the vast majority of cases the animal’s immunity will still be quite adequate come peak season in August, even without an extra booster in July. (Although we want to protect our animals from infectious disease, we also don’t want to give them any more vaccines than we have to.)

I don’t think there’s a right or wrong answer in this case. The best thing to do is talk to your veterinarian about the pros and cons of doing things either way, while taking into consideration the conditions in your specific region in terms of vector populations, disease prevalence, and the health management priorities for your own animals.

Photo: Transmission electron micrograph (TEM) of the West Nile virus (WNV). (Credit: Cynthia Goldsmith, CDC Public Health Image Library ID#10701)
 

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As spring approaches (slowly... at least here in Ontario!), people once again start thinking about vaccination programs for their horses. A question that comes up every year is when is the best time to vaccinate against mosquito-borne diseases? Depending on your region, the pathogens of concern may include West Nile virus (WNV), as well as Eastern/Western or Venezuelan equine encephalitis virus (EEE, WEE and VEE, respectively). Often people get the generic response of "30 days before mosquito season," which isn't always very helpful.

Why? It comes down to some basic mosquito biology and timing.

All mosquitoes are not alike. There are two main groups of mosquitoes when it comes to  transmission of diseases like WNV that circulate primarily in the bird population. One group is called "amplifiying vectors." These mosquitoes feed on multiple birds, thereby spreading WNV and increasing the number of WNV infected birds. However, these mosquitoes rarely feed on other types of animals, so they are not very important in transmission of WNV to horses and people. The other group of mosquitoes is the "bridging vectors." These mosquitoes feed on many different types of animals, not just birds. They can pick up WNV by feeding on infected birds, and then transmit the virus when they subsequently feed on horses or people. This is the most important type of mosquito in terms of disease transmission. Areas that have larger populations of more efficient bridging vectors tend to have greater problems with WNV.

The "30-day" part of the recommendation comes from the concept that we want to vaccinate animals about 30 days prior to the risk of exposure, so that the vaccine has ample time to work. The hard part is deciding when to start counting. One of the important things to consider is when do cases typically occur? If, as in many areas, WNV cases are not seen until late August or September at the earliest, vaccinating in April is probably not optimal. Your veterinarian and/or public health department may be able to tell you when West Nile virus activity is typically highest in your area. While there's no guarantee that things will be the same every year, it's unlikely that the WNV season would start well in advance of earlier years. Here in Ontario, disease due to West Nile is rarely reported in horses any more, but the cases that do occur still tend to appear in late August, at the earliest. Therefore, I'd be hesitant to vaccinate for WNV in the spring. If a horse is vaccinated in late April, it will be four months between vaccination and the time of peak risk. The period of peak protection would occur over a few months when there is little evidence of risk of exposure. So, if I was going to vaccinate against WNV, I'd start in late July. If a horse had been vaccinated in the spring, I'd consider a booster in the fall to make sure that the periods of peak immunity and peak risk conincide.

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In Ontario, and many other regions, mid-August is the beginning of the high risk period for West Nile virus infection in people and animals such as horses. The Ontario Veterinary College has published an informational video on YouTube. This video has information about measures you can take to reduce the risk of West Nile virus exposure and disease, for both people and horses. Click on the image to watch the video.

This equIDblog entry was originally posted on the Worms & Germs blog on 19-Aug-08.

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