US at risk from African Horse Sickness, say scientists

April 3, 2009

African Horse Sickness, the most lethal infectious disease of horses known, is a potential threat to the United States, according to scientists.

Two scientists with the United States Department of Agriculture, writing in the latest Gluck Equine Disease Quarterly, said a similar threat was posed to the US by the closely related Bluetongue virus, which badly affects sheep and deer.

Senior staff veterinarian Dr William White and national equine programme manager Dr Timothy Cordes said the spread of both diseases relied on certain species of Culicoides biting midges as a vector.

The pair pointed to the unexpected entry of Bluetongue into Northern Europe in August 2006 in an outbreak which continued into 2007, highlighting the potential threat to the US.

The feasible routes for entry of African Horse Sickness virus into the US included importation of infected animals and introduction of infected midges.

"The introduction of infected vectors (midges) is dependent upon several weather factors," they wrote.

Dispersion of midges over distances up to 700km over water and 150km over land have been postulated. However, the shortest distance from Africa to the United States is 4830km.

"To cover such long distances, transport would need to be at high altitude (3.5 miles, or 6000m), at which air temperature is far below 32°F (0°C), and Culicoides spp. would not survive," they said. "Temperatures at 80-86°F (27-30°C) are optimal for AHS virus transmission in the laboratory, while temperatures below 59°F (15°C) inhibit virus replication within the midge.

"As temperatures increase, midge infection rates increase and virus replication quickens, but midge survival rates decrease. At cooler temperatures, AHS virus within the Culicoides spp. vector becomes 'latent', but replication begins rapidly as temperatures warm."

Midges, they said, are most active around dusk and night with light wind speeds in areas of minimal to no rain and a relative humidity of 75%-85%. The midge can become desiccated at low relative humidity and oversaturated at high levels.

There are no references available describing the midges in cargo to the US, including imported flowers or plants. There is hardly any record of the presence of the midges on aircraft, the scientists said.

They said the US had multiple components that would support at least a localised outbreak of AHS: the presence of susceptible horses, areas with suitable weather conditions that would suit any introduced midges, and the presence of a capable vector already in the US - Culicoides sonorensis - which has proven to be a highly competent experimental laboratory vector for the virus.

"This vector has a wide US distribution - absent only from the northeastern states - and is the biological vector for Bluetongue virus.

"If a foreign midge vector were to successfully invade the C. sonorensis eco-niche and begin an African Horse Sickness virus epizootic, C. sonorensis would soon become infected and [become] the likely primary vector."

They said the only component missing for establishment of enzootic areas in the United States is a sufficiently large zebra population, or another yet unknown reservoir host, and the virus.

African Horse Sickness is currently on the list of diseases for which a response plan is soon to be written by federal agriculture authorities.