African Horse Sickness: Fighting a Foreign Foe
- May 1, 2005
If you're a horse owner in the United States, you've probably never worried about African horse sickness (AHS), let alone seen a case of it. Sounds pretty exotic, doesn't it? But then again, so did West Nile virus six years ago.
The bad news? AHS is nearly always fatal in horses; there's no cure. The good news? Officials and veterinarians say AHS is contained to the African continent and it's highly unlikely for the disease to make it to North America.
But AHS plagued its home continent with several outbreaks in the Western Cape of South Africa in early 2004, putting it on the radar for veterinarians and researchers who study foreign diseases and how to contain them. In this article, Corrie Brown, DVM, PhD, Dipl. ACVP, and Kathleen Connell, DVM, share their knowledge about AHS and how owners and managers can protect horse populations from this deadly disease.
Brown has worked as the head of pathology for the USDA at Plum Island, where she specialized in the diagnosis and pathogenesis of foreign animal diseases. In the late 1980s, she observed AHS in the field during an outbreak in Spain. She's currently at the University of Georgia at Atlanta and has lectured about AHS to the American Association of Equine Practitioners.
As a consultant and past state veterinarian for Washington, Connell lectures to horse owners, veterinarians, and veterinary students about preventing the spread of infectious and contagious diseases.
Read on to learn more about AHS, its causes and clinical signs, and how you could help stop an outbreak if this disease made its way to North America.
Cause--AHS is an infectious virus that afflicts horses, zebras, and other equids, as well as dogs and camels. Although not contagious from animal to animal, AHS is spread by the blood-feeding insect Culicoides, typically C. bolitinos. A biting variety of midge, Culicoides are tiny, two-winged flies related to mosquitoes and are the most commonly recognized vector of the virus.
"The virus is inoculated by the bite of one of these Culicoides," Brown says. "Horses, when they're infected, develop a viremia, which means the virus is present in the blood at high enough levels for the Culicoides to pick it up. So the horses have this viremia for a period of a week before they get sick and die. Now, with zebras, when the Culicoides feed on them, the zebras get a viremia, so they're infectious, but they don't get sick. The viremia can last for as long as six weeks."
Horses are most susceptible to AHS, with a 75-90% mortality rate. The disease also has a mortality rate of 50% in mules and 10% in donkeys. Zebras are least susceptible to AHS and get a subclinical form of the disease. "Horses are most severely affected; zebras the least," Brown says. "In between, donkeys will suffer a slight illness and recover, and mules will get a little sicker than the donkeys."
While the impact of AHS on other carnivores is unknown, dogs reportedly can pick up the disease by eating the meat of an infected carcass. AHS is fatal to the canines that contract the disease through contaminated meat. "What we don't know is whether or not dogs can be infected by the Culicoides," Brown says. Humans are not affected by the disease.
Once the virus-carrying vector feeds on a horse, "the virus replicates, then takes up residence in the cells lining the vascular blood vessels in the lungs and the heart," Brown says. "The animal ends up dying either from heart failure or overwhelming fluid outpouring in the lungs."
Clinical Signs--According to the International Association of Equine Professionals, AHS can manifest in pulmonary, cardiac, mixed, and mild forms. Clinical signs are severe in the first three forms of the disease. "Often the first thing you see is a dead horse," Brown says. "The animals are depressed, then they're dead."
Pulmonary (Peracute) Form--The pulmonary, or respiratory, form of AHS usually includes a fever of 104-107ºF, profuse sweating, severe respiratory distress, air hunger (gasping for air) and coughing, and foaming from the nares (nostrils) due to pulmonary edema (fluid lung swelling). Death comes quickly for horses with the pulmonary form of AHS. Dogs that contract the disease usually suffer from the pulmonary form.
Cardiac (Subacute) Form--Heart failure occurs in horses with the cardiac form of AHS. Clinical signs include swelling around the eyes, neck, shoulders, thorax, and intermandibular space. Terminal signs include petechiae (spotting on the skin or mucous membranes caused by minute hemorrhaging) around the eye and under the tongue. Death usually occurs within seven days.
Mixed (Acute) Form--Horses with the mixed form of AHS suffer from both lung and heart failure. In these cases, cardiac signs are usually subclinical and followed by respiratory distress and death.
Mild Form--Mild cases of AHS, sometimes referred to as horse sickness fever, can go undetected in mules and donkeys and account for the 10-25% survival rate in horses. Mild cases include an elevated temperature for three to eight days and increased heart and respiratory rates.
Other diseases that can mimic AHS include anthrax, equine infectious anemia, and equine viral arteritis, Connell says. AHS can also be confused with colic. "Because vasculature and other parts of the body are affected, sometimes blood vessels supplying the gut will also be impacted, so animals will look like they're colicky and a lot of cases have been misdiagnosed. It's kind of a great pretender."
Necropsy and laboratory confirmation are required to diagnose the disease.
As its name implies, AHS is endemic to Africa, especially in the tropical regions near the equator, and it regularly spreads to northern and southern Africa. When Europeans began to move south and colonize Africa in the 19th Century, they met a formidable foe as they came closer to the equator, with AHS infecting and killing many of the European mounts and work animals.
"It was one of the major diseases that slowed the European colonization of Africa," Brown says. "As they took horses into the interior, the horses died."
The disease made its first move out of Africa during the middle of the 20th Century, when a major epidemic spread from the Near East to India from 1950 to 1960. "It went up the Nile Valley, up into the Middle East, and all the way to Iran," Brown says. "Also, that same year, it went up North Africa and across the Strait of Gibraltar." A related outbreak also took place in northeast Africa in 1966.
By the end of the outbreak, 300,000 equids had been destroyed, says Connell.
AHS hit outside of Africa again in 1987, this time in central Spain near Madrid. Working for the USDA at the time, Brown traveled to Spain to observe the country's efforts to contain the epidemic.
"It was imported into Spain by zebras," Brown explains. "They were taken to a safari park on the outskirts of Madrid. The zebras came from Namibia." Namibia is located south of the equator on the Atlantic coast of Africa.
"The zebras were probably infected by the Culicoides in Namibia, got on the boat, came all the way to Madrid, and were still viremic," Brown surmises. "Culicoides fed on them, then took (AHS) to horses in the surrounding areas, and by the time people in Spain really knew they had a problem, they had an extensive infection of the insects, which made it hard to control. I think they had 30 or 40 horses die before they realized they were dealing with African horse sickness."
AHS's move into Spain cost the country an estimated $20 million, with $1.9 million going to vaccinate 170,000 horses. A year after the outbreak, Oct. 1, 1988, Spain declared itself free of AHS. "Two days later, there was a case in southern Spain," Brown says. "It stayed in southern Spain for three years. Each fall, there was a major outbreak. Somehow it was over wintering. Was it in dormant Culicoides or in donkeys? Nobody really knows."
There's never been a case of AHS in the Western Hemisphere, Brown says.
No treatment is known for AHS. Horses that do survive horse sickness fever recover slowly. Mules and donkeys recover more quickly.
In AHS-endemic areas, the disease is prevented through insect control measures, including insecticides. Vaccinations against AHS are also available and recommended as part of a regular vaccination schedule in African countries. Control measures also include slaughtering of afflicted horses and proper handling and disposal of carcasses.
"Each animal, as it is infected, basically becomes a virus factory for the insects to pick (the AHS virus) up and spread it," Brown says.
For those living in endemic areas, stabling horses from dusk to dawn can protect animals from the vector population, because Culicoides are most active during evening and morning hours.
An outbreak of AHS in the United States could have a devastating economic impact on the horse industry, Brown says. "There'd definitely be panic," she says. Any foreign disease, including AHS, could halt horse transportation, shows, sales, breeding, and racing in the country.
"We're at a lot greater risk of foreign diseases coming in than ever before because of globalization and the tremendous increase in international traffic of people, animals, and animal products," Brown says.
But, Brown and Connell agree, while an AHS outbreak in North America is possible, it's not likely. "We in the United States and Canada have an excellent animal-health infrastructure," Connell says. "One way to keep animals safe is to prevent the disease from coming (into North America), your state, county, or stable in the first place."
A two-month quarantine period for horses imported from Africa helps protect the United States' equine population. Due to the nature of AHS, it's unlikely the disease would make it through the import/export quarantine. "We know that, with the way the disease works, the horses that are bitten by the Culicoides either get sick and die or recover within 10 days," Brown says. "The quarantine period protects us."
Aircraft and ship traffic, however, is a concern for transporting the disease. "We certainly worry about an infected Culicoides hitchhiking in a cargo vessel or in someone's clothing," Browns adds. She points to instances of foreign mosquitoes catching rides on planes and creating malaria outbreaks near airports.
"It's called 'baggage malaria,' " she says. "The same thing could happen with Culicoides, but there usually aren't a whole lot of horses near airports." Also, biting midges are usually poor fliers, unable to cover large distances. An American Mosquito Control Association study in 1985 released adult midges near Yankeetown, Fla., and found the midges covered an average distance of 2 kilometers, or 1.24 miles, over four days.
"Nobody knows if the North American Culicoides would be a capable vector of African horse sickness," Brown says. "But everyone presumes that they would be."
Another concern for contamination, says Brown, is, "if it got into a country with less rigid import requirements than we have. Let's say (AHS) got into a country in South America, and then we import a polo pony from that country that happens to have it." The disease could also be transported out of Africa through the illegal importation of horsemeat, horses, other equids, or dogs.
If AHS does make it to the United States, most likely the infected horses would be destroyed and a "ring" of vaccinations would take place in the area surrounding the outbreak, Brown says. Prevention would also include insect control measures.
"If an owner suspects any unusual, serious disease, they are encouraged to first contact their private veterinarian," says Connell. "If a foreign animal disease or other serious disease is suspected, the private veterinarian will contact the state veterinarian or the USDA veterinarian. The animal will probably be restricted to the premises and tests conducted to confirm the presence of any serious disease."
The best protection from foreign diseases besides governmental restrictions, Connell says, is an educated horse public and owners who pay close attention to their horses. "People don't have to memorize names of diseases and the viruses that cause them," she says. "What's important is to know your animal (so you can tell when he's not feeling well)."
She adds that it's important for horse owners to participate in government actions to contain the spread of infectious diseases, such as AHS, if they do enter the country.
The good news? By working together, members of the U.S. horse community might never see a case of AHS on American soil.
About the Author
Michelle Anderson serves as The Horse's digital managing editor. In her role, she produces content for our web site and hosts our live events, including Ask the Vet Live. A lifelong horse owner, Anderson competes in dressage and enjoys trail riding. She's a Washington State University graduate (Go Cougs!) and holds a bachelor's degree in communications with a minor in business administration and extensive coursework in animal sciences. She has worked in equine publishing since 1998. She currently lives with her husband on a small horse property in Central Oregon.
POLL: Mosquito-Borne Diseases