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
