African horse sickness virus (AHSV) and bluetongue virus (BTV) are both members of the genus Orbivirus of the family Reoviridae. Both cause serious, non-contagious but infectious, arthropod-borne diseases in equids and ruminants, respectively. AHSV infects all equids, causing asymptomatic infection in zebra and African donkeys, but it is the most lethal infectious disease of horses known, with mortality as high as 95%. BTV is thought to infect all known species of ruminants; however, severe disease usually occurs only in certain breeds of sheep and some species of deer. Zebra are thought to be the reservoir host of AHSV in equines and of BTV in bovines.
The distribution of both diseases reflects the presence of their infected arthropod vectors, which are certain species of Culicoides biting midges, the temperature required for viral replication in these vectors, and transmission by these vectors. BTV unexpectedly entered Northern Europe in August of 2006, creating a rapidly spreading bluetongue epizootic in the Netherlands, Belgium, Germany, France, and Luxembourg, with over 2,000 cases. The virus overwintered by an unknown mechanism, although the 2006-2007 winter was the second mildest winter in Northern Europe on record. The epizootic continued into 2007, resulting in 45,000 cases. Because of the recent dramatic change in epidemiologic status of BTV and its Culicoides sp. vectors in Europe, both the equine and ruminant industries have become concerned about the potential for entry for these diseases into the United States.
The feasible routes for entry of AHSV into the United States include importation of infected animals and introduction of infected vectors.
Federal regulations exist for the legal importation of domestic and wild equidae from countries that the USDA -APHIS considers to be affected with AHS. Further regulations are available on the USDA Web site about the minimum 60-day quarantine for all equines originating in AHS-affected countries. (Go to http://www.aphis.usda.gov/import_export/animals/animal_import/equine/equine_import60day.shtml).
These animals can only be imported through the New York Animal Import Center. At this facility, only 16 horses and no zebra entered from AHS-affected countries during the last three years. Zebras are not generally imported because of the expense and presence of successful breeding programs in the United States.
The introduction of infected vectors is dependent upon several weather factors. Dispersion of Culicoides spp. over distances up to 400 miles (700 km) over water and 90 miles (150 km) over land has been postulated. However, the shortest distance from Africa to the United States is 3,000 miles (4,830 km). To cover such long distances, transport would need to be at high altitude (3.5 miles; 6,000 m), at which air temperature is far below 32° F (0° C), and Culicoides spp. would not survive.
Temperatures at 80-86° F (27-30° C) are optimal for AHSV 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, AHSV within the Culicoides spp. vector becomes "latent," but replication begins rapidly as temperatures warm. Midges are most active around dusk and night with light wind speeds in areas of minimal to no precipitation and a relative humidity (RH) of 75-85%. The midge can become desiccated at low RH and oversaturated at high levels.
There are no references available describing Culicoides spp. in cargo, including imported flowers or plants. There are almost no data recording the presence of Culicoides spp. on aircraft.
Outbreak Scenario in the United States
The United States has multiple components that would support at least a focal outbreak of AHS: the presence of susceptible horses, areas with suitable weather conditions that would encourage viability of an introduced vector, and the presence of a capable vector. A highly competent experimental laboratory vector for AHSV is Culicoides sonorensis. This vector has a wide U.S. distribution (absent only from the northeastern states) and is the biological vector for BTV. If a foreign midge vector were to successfully invade the C. sonorensis eco-niche and begin an AHSV epizootic, C. sonorensis would soon become infected and the likely primary vector.
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.
AHS is currently on the list of diseases for which a response plan is soon to be written by USDA-APHIS VS National Center for Animal Health Emergency Management. This plan will replace the outdated AHS Red Book previously published by the USDA.
Dr. William R. White
Senior Staff Veterinarian, USDA-APHIS-VS-NVSL-FADDL
Dr. Timothy R. Cordes
National Equine Program Manager, USDA-APHIS-VS-NCAHP-ASEP
Published at Equine Disease Quarterly newsletter (Volume 18, Number 2)
Maxwell H.Gluck Equine Research Center
Department of Veterinary Science, University of Kentucky