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

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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Coincidentally, just after writing my post on the concerns about the potential effects of climate change on diseases like African horse sickness (AHS), ProMED-mail posted a report about equine encephalosis (EE) virus in horses in Israel. This virus was first identified in South Africa in the 1960's. The most common signs of illness associated with the EE virus are high fever, decreased appetite, swelling (edema) of the lips and eyelids, and high respiratory and heart rates. Neurological disease and abortion can also occur. Most horses recover uneventfully with supportive treatment (i.e. no antiviral medications) but fatal infections can occur.

Equine encephalosis virus is an orbivirus like the virus that causes AHS.  The EE and AHS viruses are transmitted by the same type of insects - midges of the Culicoides family. As discussed in my other recent post, there is growing concern that AHS will re-emerge in Europe because the closely related disease of ruminants, Bluetongue, is now common in some areas that were previously free of the disease. Since all three of these diseases are closely related and involve the same insect vectors, it is logical to conclude that there may be a risk of both AHS and EE viruses reaching Europe, potentially resulting in widespread outbreaks, and eventually establishing themselves as endemic diseases.

Image from http://www.nationalinsectweek.co.uk/gbbu.php

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Climate change is a pressing topic these days. There are many potential ramifications of climate change, including shifts in disease patterns. The climate affects the populations of insects that can transmit infections. This can result in expanding ranges of certain diseases, or movement of diseases into totally new regions.

Climate change has sparked significant concern in the UK about African horse sickness (AHS), a highly fatal disease that is transmitted by biting insects called midges (Culicoides species). Concern has been increased by the emergence and spread of Bluetongue virus in cattle and sheep in Europe. This virus is related to the AHS virus and is transmitted by the same insect species.  African horse sickness was identified in Europe (Spain and Portugal) in the late 1980s, but was eradicated from the region using aggressive programs including slaughter of infected animals, movement restrictions, vector (insect) control and horse vaccination. With the current insect populations and climate conditions, the concern is that if (or more likely when) the AHS virus reaches Europe again, it might be much more difficult to control.

Emergence of "foreign" infectious diseases can cause major problems. Almost no horses have antibodies against foreign diseases, because they've likely never been exposed to them.  This results in a large, highly susceptible population and an ideal scenario for a major outbreak. This was clearly demonstrated when West Nile virus found its way to North America, and when equine influenza virus reached Australia. Another problem is that few veterinarians know much about exotic diseases - most people don't spend much time studying or thinking about diseases that they don't think they are ever going to see. That can delay recognition of emerging problems.

Adequate response to foreign disease threats requires:

• Good communication between horse owners, veterinarians, specialists and government officials.  Something we don't have.
• Ready access to specialized diagnostic testing. This is variably available... but who pays for it?
• Skilled government officials with a mandate, interest and expertise in equine infectious diseases.  Such people can be hard to come by.  Horses often fall to the wayside, as regulatory agencies tend to focus on food animal diseases and zoonotic diseases like rabies.
  
• Willingness by horse owners and regulatory personnel to take early aggressive measures that may be required to contain a problem: I'm not convinced we have this, either.
  

It's impossible to be completely prepared for the unknown, but we know enough about foreign diseases and infection control to make a plan - to develop appropriate early intervention strategies to help contain outbreaks when they occur and try to limit their impact. Unfortunately, we don't have a good system to do this at the moment, and we are at high risk for problems should a foreign equine disease reach Canada. Unfortunately, it's a question of when, not if, this will catch up with us.

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