19 Sept 2016
Andy Durham and Shelly Hann discuss some exotic diseases that potentially pose a risk of incursion into the UK, including country of origin, various clinical signs and methods of treatment.
Some of the symptoms of African horse sickness. Image: David Mullins.
Notifiable diseases represent a group of primarily exotic diseases considered to pose a risk of incursion into the UK (Panel 1).
Many of these diseases have never been seen in the UK, or at least not recently. Historically, occurrences of equine notifiable diseases in the UK have been cases of contagious equine metritis and equine infectious anaemia.
Requirements of dealing with notifiable diseases are via numerous pieces of legislation, which essentially require that when a notifiable disease is suspected it is notified, without delay, to the divisional veterinary manager (DVM) of Defra or a police constable.
In reality, notification of a suspected notifiable disease to the APHA is acceptable and preferable. Telephone contact with the duty veterinary surgeon should be possible 24 hours a day1, although, in practice, experience suggests the process can be less than straightforward. Records indicate that, across species, around two or three notifiable disease reports are sent to the APHA each week, triggering further investigation and enquiries by APHA staff.
Establishment of a compulsory and legal basis for control of a disease outbreak facilitates a coordinated approach to disease eradication and increases the chances of successful eradication. Notifiable diseases generally pose the potential for widespread negative impact on animal welfare, although significant public health threat from certain zoonotic diseases may also be the basis for classification as a notifiable disease.
Thankfully, zoonotic notifiable diseases of horses are exceedingly rare and include anthrax, equine viral encephalomyelitis, glanders, farcy and rabies.
In addition to benefits domestically, notifiable disease control facilitates international trade. It is in the UK’s interest to declare disease-free status for several diseases that require statutory controls and verification. Health certificates for exportation of horses may require declaration of freedom from certain notifiable diseases, and the maintenance of trade between countries relies on international trust that such diseases are closely monitored.
If a notifiable disease is confirmed, action will be taken on the infected premises to reduce the risk of the disease spreading, including movement restrictions.
The following briefly outlines some key features of the most likely notifiable diseases encountered in the UK.
Equine viral arteritis (EVA) is a disease common in other countries, including the US and many European states. EVA is caused by equine arteritis virus, which targets vascular endothelial cells and macrophages.
Clinical signs of EVA are highly variable and include dullness, pyrexia, anorexia, respiratory signs, conjunctivitis and oedema of the ventrum, legs and scrotum. Abortion may occur several days after the onset of clinical signs. However, most commonly, infection is subclinical.
Many EVA-seropositive horses enter and reside in the UK, mainly following resolved infection earlier in life, and pose no significant threat to other horses. However, infection within the previous month poses a risk of contagion, as does breeding, teasing and artificial insemination from a chronically infected (shedding) carrier stallion.
Thus, EVA is a notifiable disease only under specific circumstances as defined by the Equine Viral Arteritis Order (1995), which states “anyone who owns, manages, inspects or examines a horse must notify the DVM of Defra when: they suspect the disease in a stallion, either on the basis of clinical signs or following blood or semen testing; they suspect disease, either on the basis of clinical signs or following blood testing, in a mare that has been mated or artificially inseminated in the past 14 days”.
It is recommended stallions and teasers are vaccinated against EVA. It is important to note vaccinated horses will become seropositive, indistinguishable from seropositivity due to infection. Thus, all horses should be tested before being vaccinated to prove they are free from disease.
It is important booster vaccination schedules are followed precisely for avoidance of any doubt that future seropositivity is solely related to vaccination rather than infection during periods of waning immunity. One inactivated vaccine is available in the UK. Following the primary course of two vaccinations three to six weeks apart, boosters are required every six months.
Contagious equine metritis (CEM) is a highly contagious venereal disease of mares and stallions. It is caused by Taylorella equigenitalis, a Gram-negative coccobacillus that solely infects the reproductive tract. Both mares and stallions can become infected and act as asymptomatic carriers.
Usually only mares will show clinical signs of metritis. Transmission occurs during mating, artificial insemination with infected semen or via contact with fomites, such as veterinary equipment. Control of the disease is based around ensuring horses are free from disease prior to mating and prevention of transmission during mating.
A code of practice formulated by the Horserace Betting Levy Board2 is well known to both stud owners and veterinary surgeons, and has been very effective in limiting disease occurrences. However, as occasional cases are reported intermittently, continued strict adherence to the code is important.
Equine infectious anaemia (EIA) is a widespread disease with frequent outbreaks throughout Europe in recent years.
The last recorded case in the UK was in Cornwall in 2012. Three horses were identified with the disease as part of routine testing and subsequently euthanised. The horses in this outbreak had been imported, but other outbreaks in Europe have been traced to equine biological products, such as blood plasma.
Routine testing is of paramount importance in preventing EIA, as most horses infected are subclinical, but act as virus reservoirs for flies. The EIA virus, a blood-borne lentivirus, infects the host cell and causes lifelong persistent infection. Generally, most infected horses become inapparent carriers.
During the acute viraemic stage of EIA, horses present with pyrexia, lethargy and inappetence. Petechiae and ecchymoses may be present on mucous membranes. During the chronic stage of disease, horses will have episodes of viraemia and clinical disease interspersed with periods of low viraemia and no clinical signs. However, at any stage, periods of stress may precipitate clinical disease.
Traditionally, transmission is thought to be only via biting flies. However, during an outbreak in Ireland in 2006, it was hypothesised some transmission occurred due to aerosolisation of the virus from cleaning with pressure washers.
The Coggins test remains the gold standard test for diagnosis of EIA and is the only test recognised officially for the international movement of horses. An ELISA test is also available and is widely used as a screening tool as it is highly sensitive and provides results more quickly and economically than the Coggins test. However, due to the high sensitivity, false positives can occur and as such, any positive EIA ELISA should be checked with a Coggins test. Importantly, unlike many other infectious diseases, seropositivity is also an indication of current infection with EIA.
West Nile virus (WNV) is an ornithophilic vector-transmitted virus and is a potential cause of neurological disease in horses.
It has been found repeatedly in areas of northern Italy and southern France during the past 15 years, but has not been confirmed in horses any closer to the UK (although infected birds are known to be present in the UK). However, given outbreaks of disease in Canada and Russia, our climate and latitude are certainly not preventive factors for it arriving in the UK.
Following the mysterious and devastating arrival of WNV in the US in 1999, reported equine case numbers peaked at more than 15,000 per annum in 2002 and have declined to around 100 to 200 annually. The virus normally cycles between mosquito vectors and an avian reservoir. Infection of mammals occurs due to a bite from a WNV-infected mosquito that feeds on birds and mammals. The UK is already home to several suitable insect vectors (such as Culex territans, Cigaritis modestus, Culiseta morsitans and Coquillettidia richardii).
Although many non-avian animal species may be infected, the main affected species are humans and horses. Thus, equine infection is important both for its consequences for the horses themselves, but also as a sentinel that potential human threat exists in the area. It is important to note horses are unlikely to develop a viraemia high enough to pose a threat of transmission to other horses and humans and are regarded as a dead-end host.
Seasonal occurrence of WNV from July to October is linked to peak mosquito activity. The virus is capable of overwintering in the ovary of its mosquito host and possibly via latent infection of avian hosts.
The presentation of disease has been extremely varied in different outbreaks. Of horses affected, generally only 10% will show neurological signs. In people, the number is 1%. Neurological signs most commonly include muzzle twitching, hindlimb ataxia, weakness, obtundation, recumbency and hypersensitivity, although a range of signs may be seen. Mortality rates typically range from 20% to 40%.
If WNV is on the differential list for a case, a serum sample should be submitted to the APHA. A plaque reduction neutralisation test (PRNT90) is carried out. This test is specific for WNV; however, vaccinated horses will also be positive.
Following a positive test, the local DVM will arrange further investigation of the case, although no direct risk exists to other horses from an index case. Treatment is generally supportive, with nursing care being especially important. Often, signs will stabilise in two to four days, although a full recovery typically takes a few weeks.
Vaccination is highly effective in reducing infection, disease and death. Two commercially available vaccines for horses exist – a killed vaccine and a live canarypox vectored vaccine. Both require an initial course of two doses, four weeks apart followed by an annual booster. Ideally, the initial course should be completed at least one month before anticipated WNV activity (for example, May to June).
