Equine infectious anaemia (EIA) is a disease of horses, mules and donkeys. Also called swamp fever, this disease has been present since the early 1800s, and has been reported worldwide. It remains a significant cause of mortality and morbidity in endemic areas to this day and the disease carries significant economic considerations.
The disease is caused by a virus of the lenti-retrovirus group. The viruses attack macrophages and lymph nodes. The typical pattern is for the virus particles to invade these structures and then to become deactivated. Later activation causes release of the virus particles and the invasion of more body tissues. These recurring cycles of activity and deactivation create the typically persistent undulating clinical syndrome of an infected horse; horses suffering from equine infectious anaemia are ill and then recover to appear clinically normal. Sequential reactivations of the virus are antigenically different due to changes in the virus protein structure, which makes it extremely difficult for the body to develop a successful immune response. In this way, it is capable of avoiding humoral immune processes.
The ability by the EIA virus to change its genetic structure has also proven to be a major obstacle in the development of a successful vaccine. No effective vaccine has so far been produced. Because EIA is in the same family as the human acquired immune deficiency syndrome (AIDS) virus and because both viruses behave similarly, there is significant interest in using EIA virus as a model for the human condition. This interest may include development of an effective vaccine.
EIA is transmitted through infected blood, so vectors such as insects, contaminated syringes, needles, surgical or even dental equipment can spread the disease between horses. Insects must first feed on an infected horse and then transmit the virus to a non-infected horse by subsequent feeding. There must be substantial blood transmission and the virus must be passed from an infected to non-infected horse within a fairly short time period.
Because of these restrictions, large bloodsucking insects such as the horse fly, deer flies (Tabanids), stable flies (Stomoxys spp) and mosquitoes are thought to be the main vectors. These flies can transfer adequate blood volumes and their frequently interrupted feeding patterns allow multiple horses to be bitten during a single feeding period. In this way the likelihood of transmission is increased significantly.
This virus has also been shown to cross the placental barrier and infected stallions can transmit the disease during breeding – the virus is a significant cause of testicular inflammation (orchitis) and is harboured within the testicle.
The disease is not currently present in the UK.
Three clinical syndromes of EIA are recognized:
This is a typical outcome of an initial infection. Following initial infection, most horses exhibit clinical or subclinical fever within 7-30 days after infection. The majority of these horses are not dected as “ill”. The fever may be transient or mild and/or persistent or more clinically obvious. Affected horses will appear to recover and then begin a pattern of relapsing/recurring fever that leads to a chronically infected state.
Chronic (long-standing) infection
Recurrent episodes of fever with progressively more obvious clinical signs of depression, weight loss, ventral oedema (accumulation of fluid in the abdomen) and lethargy occur over the months following the introduction of infection. Other signs include progressive anaemia, pallor and increasing respiratory and heart rates. Less common symptoms may include lack of appetite, diarrhoea, frequent urination, jaundice of the eyes, pinpoint bleeding under the tongue and abortions in mares. A moderate orchitis (inflammation of the testis) may be detected in stallions. Many of these signs are associated with stressors such as changes in workload, hot or cold temperatures or other concurrent disease/infection.
Non-clinical carrier state
Unapparent carrier horses also exist; these horses are clinically normal but can shed the virus. These animals can only be identified as carriers through serological testing. These horses show positive results on serological examination, but are usually clinically normal. The virus is continually active without inducing signs so these cases are a serious threat to other horses for the rest of their lives. Interestingly, these cases seldom become clinical even in the face of significant stress.
The virus can be transmitted across the placenta and is shed into milk so around 10% of foals born to affected mares are positive. The prognosis for foals infected from birth is poor at best.
The lack of any possible therapeutic measures: the fact that affected horses remain at threat for life and its ease of transmission makes this a widely feared condition. EIA is a notifiable disease, i.e. it must be reported to the relevant government authority.
Owners of horses identified as positive face a difficult choice because of the lifetime threat of transmission and the fact that many of these chronically infected horses are often ill and in poor condition; euthanasia is often the most humane decision.
Prevention is the only means of controlling EIA in endemic areas. Current recommendations include:
- Use of sterile disposable needles and syringes for all vaccinations and medications.
- Scrupulous cleaning and sterilization of dental and surgical equipment.
- Aggressive fly control measures including sprays, wipes, masks and sheets where needed.
- Manage manure and other factors so as to reduce fly populations.
- Test all horses for EIA at least yearly.
- New horses should be quarantined for 45 days and monitored for signs of illness.