Caused by the EIA virus, this exotic disease affects horses, donkeys and mules and is spread by biting insects. It is notifiable and legislation introduced in 2006 provides statutory powers to deal with outbreaks.
Swamp fever occurs sporadically in the UK. It was last seen in September 2010, when single horses were affected by separate occurrences in Devon and Northumberland.
Horses are most likely to become infected when abroad or following importation of infected horse(s). Contaminated biological products can also be a source. This was the case in the 2006 outbreak in Ireland, which spread to 38 cases and was believed to stem from hyperimmune plasma imported without a licence.
The disease is common in Western Europe, with outbreaks reported this year in France, Belgium, Germany and Austria, and is endemic in Italy and Romania. Risk posed by importation of horses from these countries is constantly monitored by Defra.
While the origin of the Cornwall infection is, as yet, unknown, the 2010 Northumberland case occurred in a horse that had recently travelled from the Netherlands. It is not clear how the horse in Devon was infected; it was imported from Belgium in 2008 but investigations suggested that it may have originated from Romania.
The EIA virus is largely transmitted between horses by large biting insects, such as horseflies and stable flies, while they are feeding on blood. These flies are active between May and September and this represents a peak period of risk.
Large flies like these rarely travel more than 200m and are not moved large distances by wind. Hence restrictions during an outbreak tend to be local rather than nationwide. EIA is not normally spread by small flying insects like midges and mosquitoes.
There is also potential for spread of infection via blood, milk or placental fluids, as well as through saliva, nasal secretions, faeces and semen. This is why stallions and mares are tested for EIA at the start of the breeding season. Proper disposal or cleaning of veterinary instruments and dental equipment is also important.
The incubation period of EIA is generally one to three weeks. Early signs of infection include dullness, weakness and lethargy, with recurring fever, anaemia and oedema under the chest or at the bottom of the limbs. Pregnant mares may abort.
Signs of EIA are non-specific and appear to similar to a number of common diseases, such as equine influenza and herpes virus, as well as the notifiable diseases African Horse Sickness and Equine Viral Arteritis.
There is no treatment and EIA is fatal in approximately 50% of cases. Animals that recover rarely thrive, lose weight and become emaciated. They are vulnerable to secondary infections and parasite infestations.
Early blood testing of suspicious cases is essential to diagnosing the disease and assessing its spread. It is important to test all animals in close proximity because some may show no or only very mild signs and will only be detected by blood tests.
A key point is that all horses infected by EIA remain carriers of the virus for the rest of their lives, regardless of how severely they might have been affected. There is no way of eliminating infection and there is no available vaccination.
While euthanasia may well be indicated for the welfare of affected horses, it is also the only way of preventing spread of EIA.
The UK legislation provides powers to slaughter horses affected with EIA as well as those that have positive blood tests. A separate order also limits compensation for animals slaughtered for EIA to just £1.
In an outbreak, all horses identified as being in-contact with EIA cases will be kept under restrictions and tested until Defra vets are satisfied that they are not affected.
As of October 4, 2012, test results for other horses on the premises of the case in Cornwall are negative. These animals will be monitored for a period of 90 days, during which time movement restrictions remain in place.
It is hoped that these measures will prevent escape of EIA into the wider equine population. They certainly limited the disease in Devon and Northumberland in September 2010 to single cases. The fact that activity of biting flies is low at this time of year is also of comfort.
The price of failing to control EIA is likely to be considerable. It's not just the testing and loss of animals; restrictions on movement and commercial activities would also prove costly.
As an example, the filly Danedream that won last year's Prix de l'Arc de Triomphe was unable to defend her title this year after being caught up in an EIA outbreak in Germany. She was stabled at Cologne racecourse where another horse tested positive for the disease and, along with 300 other horses, has been grounded for 90 days pending blood tests.