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September 01, 2020

After deaths of Thailand horses, researchers warn of risk in US

Biting midges could be vectors for African horse sickness virus
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As a viral disease outbreak killed horses in Thailand, entomologists at one university warned that midges in the U.S. could also potentially spread the disease.

Agriculture authorities in Thailand confirmed an outbreak of African horse sickness March 27, according to the World Organisation for Animal Health (OIE). By July 3, it had killed 562 of the 604 horses with known infections, OIE reports state.

Culicoides sonorensis midge
A one-sixteenth inch long Culicoides sonorensis midge drinks blood through an artificial membrane. Such midges have transmitted African horse sickness under laboratory conditions. (Courtesy of USDA Agricultural Research Service)

Researchers from the Texas A&M University Department of Entomology said biting midges present in the U.S. likely would be effective vectors for the AHS virus, which causes respiratory and circulatory impairment. In an article published on the department’s site, they cited study results that showed, under laboratory conditions, some midges present in Texas transmitted African horse sickness virus.

Pete Teel, PhD, who is a research entomologist and regents professor at Texas A&M, told JAVMA News the U.S. has a history of incursions involving arthropods and arthropod-borne diseases, despite controls at borders and ports. Examples of vectors that have been introduced include cattle fever ticks and Asian longhorn ticks; recent pathogen introductions include West Nile and Zika viruses.

When his department’s entomologists see news of African horse sickness spreading beyond Africa, “Our antennae, so to speak, go up,” he said.

Specialists in Texas A&M’s entomology department study mosquitoes, ticks, midges, methods to control vector-borne diseases, and ways to suppress vector populations, Dr. Teel said. He described a combination of interdisciplinary research and international monitoring.

African horse sickness is prevalent in sub-Saharan Africa and parts of the Middle East, according to the U.S. Department of Agriculture Animal and Plant Health Inspection Service. It affects all equids, although zebras and African donkeys typically develop mild or subclinical disease.

C sonorensis, which is native to North America and a vector of bluetongue virus, is a competent vector for AHSV in the laboratory.

From the etiology and ecology section of the U.S. Department of Agriculture Animal and Plant Health Inspection Service draft document “African horse sickness standard operating procedures

The African horse sickness virus is in the genus Orbivirus, as are the bluetongue virus that causes disease in ruminants in the Southern and Western U.S. and the epizootic hemorrhagic disease virus that circulates in the Southern U.S. and sometimes causes severe outbreaks among white-tailed deer in the Northern U.S., according to information from the USDA and Cornell University. All three viruses are known to spread through midges in the genus Culicoides, which includes 1,500 species worldwide and about 30 that are capable of transmitting orbiviruses, according to a draft APHIS manual for responding to African horse sickness.

C sonorensis, which is native to North America and a vector of bluetongue virus, is a competent vector for AHSV in the laboratory,” the document states.

Mosquitoes and ticks also can spread African horse sickness, though less often than midges, agency information states.

Horses can develop severe pulmonary and respiratory signs, and death rates can range from 50%-95%, the draft manual states. Mules tend to have death rates closer to 50%, whereas European and Asian donkeys have 5%-10% death rates and African donkeys and zebras seldom die from infections.

Equids may develop fever, shortness of breath, coughing, dilated and runny nostrils, conjunctivitis, fluid-filled swellings, and death from anorexia or cardiac arrest within a week.

Imported zebras were implicated in spread of the disease to Thailand, according to the Texas A&M article.

Thailand’s livestock department started vaccination efforts April 19, starting with horses raised to produce immunoglobulins to treat rabies or bites from venomous snakes and followed by all horses within 50 kilometers from disease sites in seven provinces. Reuters reported that month that, without the mass vaccination campaign, the disease threatened to wipe out the country’s approximately 11,800 horses.

The Texas A&M article indicates vaccines against African horse sickness are effective. But they contain live pathogens that may sicken horses or, by increasing virus replication, increase the risk of accidentally breeding genetic variants.

In regions with biting midges, horse owners can protect their animals by keeping them in stalls and using insecticides.

“It’s easier, obviously, to deal with an animal that is stabled,” Dr. Teel said. “It’s another matter to deal with animals that have to be pastured or are free-ranging.”

Dr. Teel said his department is monitoring other diseases worldwide and vulnerabilities in the U.S. For example, African swine fever devastated China’s swine herds over the past two years through pig-to-pig transmission, but the ASF virus spreads in its native range between African warthogs and soft ticks, he said.

He learned about four years ago that African warthogs tunneled out of captivity in southern Texas, and they since established a range in several counties. The state already had a competent soft tick vector, he said.


The American Association of Equine Practitioners recently published guidelines on identifying, diagnosing, and controlling African horse sickness (PDF).