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Of vectors and viruses...

by
01 November 2013, at 12:00am

The British Equine Veterinary Association (BEVA) held its 52nd annual congress in Manchester from 11th to 14th September. 

Gina Pinchbeck of the University of Liverpool chaired Thursday morning’s session on “Infectious Diseases”, which was sponsored by the Horse Trust. David Sutton of the University of Glasgow gave his views on “It’s a small world: infectious threats to the UK horse industry”. 

Infected horses are the most likely means of importing disease. With something like 600,000 movements of thoroughbreds and 26,000 movements of horses competing under FEI rules each year, international movements of horses are on the grand scale. 

Within Europe there are no border inspections and there is a tripartite agreement which allows free movement between the UK, Ireland and France. Problems arise where a horse can be imported from outside the EU and moved on by means of an inaccurate passport. 

Trade in biological products is also increasing with inherent risk. For example, the 2006 outbreak of Equine Infectious Anaemia (EIA) in Ireland originated in frozen plasma.

Increasing range 

African Horse Sickness (AHS) is regarded as “low risk” for the UK but the two species of midges, Culicoides imicola and C. bolitinos, which are associated with the endemic infection in sub-Saharan Africa may have the potential to increase their range in the face of global warming. Alternatively, other species of midge may become vectors. 

Like AHSV, bluetongue virus is an Orbivirus, and in the UK outbreak it spread by wind-borne midges from continental Europe where new species of resident midges proved capable of maintaining the infection. 

While horses show an acute form of ASH (especially where the disease is not endemic) the Namibian zebra that started the outbreak on the Iberian peninsula which lasted from 1987 to 1990 had inapparent infection. 

AHSV causes a viremia which lasts four to eight days, then there is progressive microvascular endothelial cell damage which shows as pleural and pericardial effusions, pulmonary oedema and myocardial damage. 

The course of the disease is peracute and mortality can reach 95% in a naïve population. The species of midge (C. pulicaris and C. obsoletus) from which AHSV-4 was isolated in this outbreak were vectors in the Northern European bluetongue outbreak, so there is the potential that they could maintain AHS should it get here. 

Multiple serotypes of AHSV have spread into North Africa with the potential to spread around the Mediterranean basin. The most likely routes of import of AHS to the UK are inapparently infected animals or infected midges spread by the wind.  

The other big risk disease is EIA which is caused by a retrovirus of the Lentivirus group. In its acute form it causes extreme pyrexia, limb oedema, petechiae and death within two to three weeks. This clinical syndrome needs to be differentiated from severe endotoxaemia. 

Spread can occur through biological products, instruments such as a stomach tube, or needles or aerosolised blood from an affected horse. 

Large biting insects are the main means of spread which occurs most readily from an acutely viraemic horse to susceptible individuals where there is a dense population of biting flies. 

EIA is endemic in Romania and North Italy where many horses will have a quiescent phase where the disease is inapparent. 

The Coggins AGIDT is used as a screening test where horses are imported from affected countries. The test is repeated at 90-day intervals and again 28 days after import but seroconversion can take up to 157 days post-exposure, rather than the expected 45 days. The EIA ELISA gives better sensitivity and more rapid diagnosis. 

The three outbreaks confirmed in the UK since 2010 all came through Romania. EIAV can undergo antigenic variations which allow it to evade the immune system. Returning competition horses may appear healthy but have some virus which develops into illness weeks or months later. 

Detailed clinical examination with an awareness of the potential for exotic disease is the first line of defence in the effort to keep the UK free from these diseases and it is also important to be aware of disease alerts for conditions such as West Nile Virus and Dourine.

Being prepared 

Daniel Horton, of the Wildlife Zoonoses and Vector-Borne Diseases Research Group at the Animal Health and Veterinary Laboratories Agency (AHVLA), described “Preparing for vector-borne diseases in the UK”.   

The state of vector-borne viruses is one of continual change: viruses undergo genetic change, evolving and adapting, the vector distribution changes in response to climate and weather events and human activity plays its part too. 

For example, waste tyres proved an important breeding ground for mosquitoes in the USA outbreak of WNV. Surveillance of this complex situation is challenging but essential. 

West Nile Virus (WNV) is a Flavivirus (as are Bovine Virus Diarrhoea virus, yellow fever and classical swine fever) which maintains its life-cycle between birds and mosquitoes with the occasional spillover into horses and humans. 

WNV has a simple genome and it is possible to track its evolution and spread around the world. It is now the world’s most widely distributed arbovirus, established in Europe with an increasing number of outbreaks in recent years. 

The big outbreak which started in 1999 in the USA was under -reported. It started in Florida in 2001 and had spread to the Western seaboard by 2002. It involved over 1,500 reported cases with a human fatality rate up to 1%.  In the UK surveillance is directed at humans, horses and wild birds. 

There is no reliable evidence of WNV here yet, but there is a team of about 50 people who can get working within 30 minutes if an outbreak is confirmed. 

In Europe it was found that surveillance of horses, birds and mosquitoes gave a three-week interval between finding the virus and the onset of human cases. 

As with AHS, WNV detection rests with thorough clinical examination and appropriate response by clinicians in the field while surveillance gives an ongoing assessment of risk which forms the foundation for the design of control strategies and government policy.