As midges spread a deadly equine virus through Thailand, Professor Debra Archer MRCVS discusses the risk of a UK outbreak
The strange, sudden death of a racehorse in Thailand in February was the first sign of an unfolding crisis about to hit the country’s horse industry.
Within weeks, multiple cases of sudden equine death and of horses showing abnormal clinical signs were being reported by vets – prompting a full-scale investigation by the Thai animal health authorities. After laboratory testing of blood samples, African horse sickness (AHS) virus was confirmed.
This notifiable disease must be reported to the global authorities, so control measures were immediately put into place. While AHS is common in Africa, it rarely occurs elsewhere. When it does, it has the potential to cause a deadly outbreak – such as that seen in the late 1980s in Spain, Portugal and Morocco. This outbreak is the first in this part of Asia.
AHS is a virus spread by biting midges (Culicoides imicola). Zebra are the main reservoirs of infection in Africa and their implication in the Thai outbreak is under ongoing investigation. While zebra don’t show any signs of disease when infected, they can carry the virus in their bloodstream for up to 40 days.
The disease has a fatality rate as high as 70 to 95% among horses and ponies who are not immune. Donkeys show only mild signs, or none at all; fewer than 10% of them will die.
Swellings around the eyes (known as perioribital oedema) are most characteristic of the clinical signs, which can also include high temperature, loss of appetite, difficulty in breathing and bleeding around the eyes. Horses may die suddenly, however, showing few signs.
A shot of hope
A standardised protocol is in place for control of AHS, and has been implemented in Thailand. This comprises movement restrictions of all equids and a ban on their import or export, along with midge sampling and control, testing of horses and vaccination.
It is vital that vulnerable animals are protected from midge bites. Owners are using insect repellents, avoiding turnout, particularly when midges are most active, and stabling and exercising their horses in midge-proof buildings. Mosquito netting is ineffective, so an even finer mesh must be used with the tiniest of gaps sealed.
At-risk horses in areas close to the outbreak are receiving a vaccine imported from South Africa. This “live” vaccine is not ideal, being a weakened version of the virus itself, but it is immediately available and provides protection from this particular strain of the AHS virus.
Horses must be quarantined immediately afterwards, in case vaccination causes a mild infection, and microchipped so they can be tracked following the outbreak. More than 500 horses in at least six provinces in Thailand have so far died due to AHS.
The outbreak has had a huge impact on the Thai equestrian industry, in addition to the challenges already brought about by the Covid-19 pandemic.
Working ponies and their owners, who rely on tourism, have been particularly affected. The Lampang Pony Welfare Foundation is helping to supply feed and netting to protect these ponies and some of the country’s endangered native breeds.
To donate, visit: lampangponywelfare.org/special-giving
How worried should we be?
The enormity of the Covid-19 pandemic and the outbreak of AHS in Thailand are a warning to us all. New infectious diseases have the potential to appear at any time, and existing diseases may suddenly develop in previously unaffected countries – a risk increased by globalisation and climate change.
While the threat to equids in Europe from AHS remains low, the situation is being monitored by multiple authorities. There are midges here that could spread AHS.
Current EU regulations minimise the chances of an infected equid being imported, but it will be critical that these safeguards remain in place beyond Brexit. Any AHS outbreak in Europe would be a cause for concern, since bluetongue, a similar virus that occurs in cattle and sheep, was spread to the UK by wind-borne midges.
Research into vaccines is ongoing, but the challenge is being able to produce these quickly for the correct AHS virus type. A global effort, creating vaccine banks, may be economically viable if the level of risk increases.
Case study: “They lived in a netted world”
Thai endurance rider Wipawan Pawitayalarp has seen nearly half of the 55 horses at Prutirat R Serireongrith’s The Horses Endurance Stable lose their lives to AHS. Among the fatalities was top prospect Konfirm MI.
“We noticed Konfirm had a very high temperature, of 39.3ºC, so we isolated him,” she says. “He stood for four days, but finally passed away at the end of March. He was a true athlete and a brave warrior – no words can explain the loss.”
After vaccination in April and blood tests to check AHS immunity, the remaining horses recently emerged from their “netted world” to enjoy the fresh air. Wipawan’s 2014 World Equestrian Games ride, Mulawa Angelus, remains in good health.
“We had to keep Angelus in a tight-netted stall until he and the others could leave quarantine,” explains Wipawan. “Very luckily for him, he is one of the survivors.”
Ref Horse & Hound; 18 June 2020