Prompt reporting of cases vital in containing African horse sickness in Thailand – report

Adult female blood sucking midge, Culicoides imicola.
Adult female blood-sucking midge, Culicoides imicola. © Alan R Walker / CC BY-SA

Widespread surveillance and immediate reporting of cases will be vital in ensuring that the outbreak of African horse sickness in Thailand does not spread beyond its borders, researchers say.

Researchers with The Pirbright Institute in Britain, in a disease alert in the journal Transboundary and Emerging Diseases, gave the background to the outbreak, which was confirmed in March.

Samples from four horses on a farm in the Nakhon Ratchasima province were sent to the institute by a private veterinarian. The horses were showing clinical signs of African horse sickness, including eye swelling, fever and a frothy discharge from the mouth/nostrils. The horses died within 12 to 24 hours of symptoms appearing.

The samples tested positive for the virus, with further work revealing they were serotype 1. The virus appeared most closely related to two field strains originating from the Republic of South Africa.

Simon King and his colleagues noted in their report that the primary African and European vector of the disease, a biting midge known as Culicoides imicola, has been identified in Thailand and surrounding countries.

“The wide geographic distribution of Culicoides in the region and potential year‐round seasonal activity as adults poses an inherent risk of African horse sickness virus spreading to neighbouring countries.”

They summarised containment efforts in Thailand, which include inoculation of susceptible animals with live attenuated vaccines, and the use of stabling and treatment of horses with insecticides and repellents to keep the midges at bay.

“Widespread surveillance and immediate reporting of cases will be vital in ensuring that the outbreak does not spread into further provinces or neighbouring countries,” the authors wrote.

They noted that while live attenuated vaccines remain the only commercially available option for long‐term control of the disease, issues including reversion to virulence and reassortment with field strains have been recorded by researchers.

“While a wide range of alternative vaccination strategies have been developed, the market for these vaccines has not to date been sufficient to drive the extremely rapid commercial availability and deployment required for use in areas experiencing African horse sickness epidemics.

“This will only be resolved through partnerships between the public and private sector and an alternative product that is both cost‐effective and safe.”

Thailand is home to about 6000 horses. The source of the outbreak has yet to be confirmed, although the nation has since imposed tight controls on zebra importations.

The authors say the economic impact and disruption caused by the outbreak could potentially be felt on many levels, and its spread to other countries would have a similarly major impact.

Meanwhile, Thai authorities, in their latest weekly report to the World Organisation for Animal Health (OIE), report no new cases. The last confirmed cases described in the reports were three weeks ago, with two animals confirmed with fatal infections.

The Ministry of Agriculture and Cooperatives reports that the outbreak has been resolved in seven Thai provinces.

In other news, the American Association of Equine Practitioners (AAEP) has released its infectious disease guidelines for African Horse Sickness, written by Peter Timoney.

Timoney describes four potential routes of virus entry into a country:

  • Importation of infected equids from countries in which the disease is endemic or occurs regularly. The greatest risk is associated with zebras or donkeys, or horses with partial immunity.
  • Importation of infective animal products such as equine serum or semen from affected countries.
  • Introduction of infected vectors, specifically Culicoides species, via aircraft and ships.
  • Windborne carriage of infected vectors.

The guidelines can be read here.

Outbreak of African horse sickness in Thailand, 2020
Simon King, Paulina Rajko‐Nenow, Martin Ashby, Lorraine Frost, Simon Carpenter, Carrie Batten.
Transboundary and Emerging Diseases, June 27, 2020,

The outbreak alert, published under a Creative Commons License, can be read here

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