Posted on April 6, 2009 by Scott Weese

African Horse Sickness: US Outbreak Scenario

The latest edition of Equine Disease Quarterly, a newsletter produced by the Gluck Equine Research Center, has a couple of interesting articles. Coincidentally, one involves African horse sickness (AHS), a disease foreign to North America that I've mentioned in the past week. The article is about the potential threat of AHS being introduced into the US, and  gives a relatively brief but excellent overview of the disease and why there is concern. The authors discuss a hypothetical outbreak scenario in the US, which is certainly not beyond the realm of possibility - many of the required pieces to the puzzle are already in place when in comes to the potential for this virus to spread in the US:

  • The US has a large and often concentrated pool of susceptible horses. Very few horses have any immunity against the AHS virus.
  • Many areas of the country have weather conditions that are suitable for survival of the insect vectors (Culicoides midges) that might be introduced carrying the virus.
  • A different midge species, Culicoides sonorensis, is widespread in the US (except for the northeastern part of the country) and has been shown experimentally to be a highly capable vector for the AHS virus.

So what is needed for an outbreak to occur?

First, the virus needs to enter the country. This could occur by 1) inadvertent introduction of an infected midge (e.g. a "hitchhiker" on a plane would be one way for this to happen) or 2) introduction of an infected reservoir host. The reservoir host for this virus is zebras - fortunately  few zebras get imported into the US because of successful captive breeding programs, so this is probably less likely than the first. The other possibility, which certainly can't be ignored, is 3) intentional introduction of the virus: bioterrorism.

For an outbreak to maintain itself, a large population of reservoir hosts is needed. In Africa, the virus resides in zebras. Midges get infected by feeding on infected zebras, then infect horses by biting them. The lack of a large zebra population in North America could be the difference between a minor introduction and a major outbreak, unless there is another species that can act as a reservoir host in North America. This is one of the crucial unknown factors in the equation, but we shouldn't count on the lack of zebras as our main line of defense against this devastating disease.

The April 2009 edition of Equine Disease Quarterly can be downloaded here.

Image from: http://www.vet.uga.edu/vpp/fad/horse/vector.htm

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Posted on April 2, 2009 by Scott Weese

Climate Change and Equine Diseases

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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