The coronavirus pandemic has been a stark reminder of the potential for the spread of human disease in modern times – and our staggering lack of preparedness for it.

As veterinarians, we know that preventive measures to limit the international spread of many animal pathogens have long been in place and many are highly effective.

Despite this, the potential for changes in the distribution of diseases associated with global travel and climate change mean we must remain vigilant.

Looking specifically at vector-borne diseases, where the distribution of various insect species is critical to disease, it has become clear that long-held concerns are fast becoming a reality. Of particular interest in recent years have been Zika virus disease, West Nile fever, bluetongue disease and African horse sickness (AHS).

What is AHS and why does it matter?

AHS is a viral disease of equids (horses, zebras and donkeys) that is transmitted between horses during feeding by the vector – species of Culicoides midges.

Disease is associated with mortality rates as high as 95%, especially in naive populations (Long and Guthrie, 2014).

Only non-specific supportive treatments are available. The disease is not considered zoonotic under normal conditions.

AHS is endemic in sub-Saharan Africa, and sporadic disease outbreaks have been reported in the Middle East, western Asia, India, north Africa, the Iberian Peninsula and, recently, south-east Asia. The disease has never persisted in any of these regions for more than a few years.

The World Organisation for Animal Health (OIE) classifies AHS as a listed disease based on its severity, outbreak implications and ability to sporadically occur outside the endemic region. Disease-free status can be obtained from the OIE based on several requirements.

The UK is classified as AHS disease-free, but an outbreak would have devastating effects on animal welfare and major economic consequences for the equine industry (Robin et al, 2016). It has been estimated that a widespread outbreak of AHS in the UK would wipe out about 50% of the economic contribution – currently around £7 billion – of the equine industry to the country (Allison et al, 2009).

In Spain, where an outbreak occurred in 1987-90, 110 horses died as a direct result of the disease, with a further 900 slaughtered as part of control measures (Rodriguez et al, 1992).

In March 2020, AHS was reported for the first time in Thailand, a country then officially classified as disease-free (King et al, 2020). Subsequently, the virus was detected in neighbouring Malaysia (which was also considered disease-free) in September 2020 and the OIE has since suspended the disease‑free status of Myanmar (OIE, 2021). Early reports of sudden death indicate severity of the outbreak in Thailand may be typical for a naive population; however, complete data are currently unavailable (King et al, 2020).

How likely is it that AHS will occur in the UK, and do we even need to be prepared?

The lack of appropriate vectors and climatic conditions for disease spread, and the physical advantage of being an island mean that UK vets have traditionally had little cause to worry about many vector-borne diseases, including AHS.

The furthest north AHS has been reported is the Iberian Peninsula, where the principal vector – Culicoides imicola – is found. Importantly, this species is not found in the UK; however, it is possible other Culicoides species indigenous to the country may be able to transmit AHS virus if it was to reach the country.

Support for this idea is provided by comparisons with the UK bluetongue outbreak in 2006-7. AHS virus and bluetongue virus are very similar, and share vector species, including C imicola. During the bluetongue outbreak in northern Europe and the UK, disease occurred in regions where C imicola is absent – indigenous Culicoides species must, therefore, have acted as vectors (Mellor and Wittmann, 2002). Theoretically, the same could be true for AHS.

Assuming competent vectors are present, the virus must first reach the country (in a horse or a midge) for an outbreak of AHS to occur. Current OIE regulations make the risk of virus entry within a legally transported equid negligible, although the virus clearly managed to get to Thailand in March 2020 (the source of the outbreak was still officially unknown at the time of writing).

Importantly, both Thailand and Malaysia were on the approved list for export of equids to the EU at the start of the outbreak.

One advantage of the high case-fatality rate in naive populations of horses is that an infected horse exported at the start of the outbreak would almost certainly be quickly identified and the risk of spread, even at this time, is still considered very low (Pacey et al, 2020). Certainly, no cases were reported in any horses exported at the start of the outbreak.

Virus introduction to the UK within an infected midge transported in cargo is possible, but considered very unlikely (Napp et al, 2013). Wind dispersal of infected Culicoides has been implicated as the cause of the overseas spread of AHS virus from Morocco to Spain in the past, and bluetongue virus from mainland Europe to the UK, although this would only become important if AHS virus was circulating in Europe (Gloster et al, 2008; Pedgley and Tucker, 1977).

Overall, while the risk of AHS outbreak in the UK is considered extremely low, it is possible that the appropriate Culicoides species exist to support an outbreak if the virus were to arrive.

Are we prepared?

AHS is a notifiable disease in the UK and suspicion must be reported immediately to the APHA.

An extensive outbreak control plan was published in 2012 and details are available elsewhere (Defra, 2012; Robin et al, 2016). The plan is similar to that for most notifiable diseases and includes the implementation of extensive restrictions on equid movement, culling of suspect horses during the initial stages of an outbreak (once the virus spreads to the vector population, this is not considered effective), prevention of vector‑host interaction (we all know how difficult it can be to stop midges from biting horses), disease modelling and the controlled use of vaccination.

Vaccination is not permitted in disease‑free regions, but would almost certainly be a key control method in the event of an outbreak. The use of multivalent live‑attenuated AHS virus vaccines is the mainstay of controlling AHS in endemic areas – they were used successfully to control the 1987-90 outbreak in the Iberian Peninsula and are being used in south-east Asia (Rodriguez et al, 1992).

Due to reports of vaccine‑associated disease outbreaks, a significant drive has been made to promote investment in the development of alternative vaccine types (Weyer et al, 2016). A vaccine that would allow differentiation of infected from vaccinated animals would be hugely useful for outbreak control, but is not currently available.

Work is ongoing on the development of recombinant vaccines that use viral vectors, or are based on reverse genetics or virus‑like particle technologies (Dennis et al, 2019). Concerns also exist over the large-scale availability of currently available vaccines, and previously suggested European vaccine banks have failed to materialise.

Conclusion

Overall, while current stringent import regulations and a lack of proven vectors make the chance of an outbreak of AHS occurring in the UK very low, the welfare and economic consequences would be extremely severe.

An extensive control plan is available and written into UK law, although questions over our ability to rapidly implement parts of it – especially vaccination – remain.