Ever-increasing global international horse traffic to and from the UK, combined with climate change and variations in arthropod vector behaviour, movement and distribution all increase the threat of entry of an exotic equine infectious disease to our shores. Although surveillance is markedly heightened compared with even ten years ago, and international reporting of infectious disease outbreaks around the globe is rapid and effective, not all the risk factors for disease introduction are within our control. Outbreaks of serious infectious and contagious diseases will occur in the UK, and it is probably just a matter of time before the index case of a new outbreak of an exotic infectious disease is presented to an equine practitioner in the field.
African Horse Sickness (AHS) is classified as an OIE (World Organisation for Animal Health) listed disease (List A). It is caused by the AHSvirus (AHSV), which is part of the Orbivirus genus of the Reoviridae family. AHSV has nine serotypes and is a variant of the Bluetongue virus. Therein lies the potential concern for anyone who has read the veterinary press or has had dealings with any aspect of ruminant livestock production since Bluetongue spread to the UK in 2007 and Schmallenberg virus in 2011. AHS is infectious but not contagious, and is spread by certain Culicoides spp. insect vectors when taking a blood meal. It is endemic in sub-Saharan Africa with outbreaks in the Iberian Peninsula over the period 1987-1990 which involved the incursion of serotype 4, most famously associated with import of sub-clinically affected Zebra imported to a Madrid zoo. Historically, epidemics have also been seen in northern Africa and the Middle East.
Four forms of disease are recognised. The viral vasculitis can lead to a peracute pulmonary form (Dunkop in Afrikaans) with fever, cough, dyspnoea and pulmonary oedema leading to a 95 per cent mortality rate. The subacute cardiac form (Dikkop in Afrikaans) results in fever and diffuse peripheral oedema of the face, neck and head with a mortality rate of over 50 per cent. The mixed cardiopulmonary form is the most common presentation in naïve animals including foals, resulting in 70-80 per cent mortality rate. Finally the mildest or often sub-clinical form, Horse Sickness Fever, if detected, is seen in donkeys, zebra and partly immune horses. Viraemia in infected animals may vary from a few days in horses or up to 40 days in zebra.
In sub-Saharan Africa the insect vectors are mostly C. imicola & C. bolitinos, while in Spain C. obsoletus and C. pulicaris were implicated. Mortality ranges from 0 to 95 per cent depending on the equid species involved. There is no treatment. Horses, donkeys, mules and zebra can all be affected, with the latter thought to be the reservoir hosts. Camels, elephants and rhinos appear to be able to be infected, but are not epidemiologically significant in disease transmission. Interestingly, dogs can also die with a peracute fulminating infection after eating infected horsemeat.
Should equine vets in the UK be concerned? The short answer is yes. The catastrophic effect on the horse population and associated industries of an outbreak of African Horse Sickness in the UK does not bear thinking about. But it is important to note that any threat is not imminent. Vigilance will enable the UK to keep the threat at a distance, and it is key that OVs effectively advise clients whose horses regularly travel the globe for competition, breeding or owner relocation.
The UK is, of course, in the privileged position of being an island nation. However, and most importantly, multiple national and international government and equestrian agencies work extremely hard to formulate and enforce policies on international trade and movement of all types of equids with the sole aim of minimising the risk of transmission of infectious diseases. Although strict regulations can cause friction, the UK horse population – and all those who rely upon equestrian industries for their livelihoods – in turn rely on careful control of international horse movement that is debated, negotiated, created and enforced by the likes of the OIE, FEI and Defra.
Every year there are more than 500,000 international thoroughbred racehorse movements to facilitate racing engagements, and some 25-30,000 FEI-registered sport horses travel internationally each year. Although the numbers of horses transported for slaughter has been steadily falling over the last 20 years, tens of thousands continue to move between EU member states. Border post inspections are not required for equids travelling within the EU. The movement of horses under the Tripartite agreement, originally introduced to facilitate unfettered thoroughbred movements for racing and breeding between UK, the Republic of Ireland and France, with no veterinary checks required prior to travel, improved greatly in 2014 with the announcement that only Higher Health Status horses were to be covered by the agreement.
Although in general equids are key to the maintenance of African Horse Sickness within a national population, as a vector-borne disease we cannot entirely eliminate the risk of AHSV introduction via its insect vector, otherwise known as the midge. In endemic regions AHS has a seasonal (late summer/autumn) and weather-related (drought followed by heavy rain) cyclical occurrence, clearly conditions favouring vector population expansion and distribution. The incubation period after vector transmission is typically five to seven days but can be as short as two days or as long as 14 days. The most recent outbreak was reported in South Africa in April of this year.
The arrival of AHS in the UK would require importation of an undetected incubating, infected equid or inadvertent importation of infected Culicoides spp. along with imported animals.
Although AHS is categorised by the APHA as low risk, introduction of AHSV at a time of favourable climatic conditions for resident relevant midge species could result in introduction and spread of virus. It should be remembered that between 1998 and 2005 Bluetongue virus spread across 12 countries and 800km further north in Europe than has previously been reported. This was probably driven by changes in European climate allowing increased virus persistence during winter, the northward expansion of Culicoides imicola and beyond its vector’s range and transmission by indigenous European Culicoides spp. Ecoclimatic modelling has suggested that C. imicola will be widespread in England by 2070. Also the recognised AHSV vector C. obsoletus has been identified in Newmarket feeding on horses.
An outbreak would be managed according to the African Horse Sickness (England) Regulations adopted in 2012 with immediate slaughter of affected equids, movement restrictions, a 50km or larger Surveillance Zone, a 100km Protection Zone and advice for vector avoidance with housing and insect-proof screening. Horse gatherings would cease and extensive programme of horse testing and vector surveillance would begin.
Currently there are no AHS vaccines licensed in Europe. Tri- and quadrivalent live attenuated vaccines are commercially available in South Africa but these are not considered safe outside an endemic area, and development of canarypox vector vaccine has been reported. Most recently the Pirbright Institute have developed a recombinant vaccine using a modified Vaccinia Ankara virus that has been shown to be protective against six of the nine AHSV serotypes in mice with ongoing trials in horses. Further funding is being sought to complete this research.
Although vets in UK practice are unlikely to stumble across a case of AHS in the near very future, they should remain vigilant as we move further into the increasingly globalised 21st century.
