The exceptionally hot weather we experienced over a period of several months this summer may become more normal in the UK, according to scientists.
There are pros and cons to this potential change. One concern is that warmer summers may lead to an increased risk of illness in people, horses and other animals, caused by exotic diseases currently found in countries with warmer climates.
Warmer temperatures can enable insects that normally do not survive in a UK climate to become established — including certain types of midge and mosquito. Midges and mosquitoes are more than just a biting nuisance. As “vector” (carrier) insects, they also have the ability to spread disease.
It is well known that midges are responsible for the equine skin condition sweet itch. Some types of midge, however, can spread viruses that cause disease and even death — the most notable of which is African horse sickness (AHS). Mosquitoes can also transmit potentially fatal diseases, such as West Nile virus (WNV).
Research has shown that midges and mosquitoes are common on many horse premises in the UK. Worryingly, the midge and mosquito types present here are changing to include those more typically found in warmer parts of the world.
Should the likes of AHS or WNV ever reach our shores, these insects are the ones with potential to transmit these deadly diseases among our horse population.
Dramatic effects
In areas of the world where viruses spread by vectors are endemic (common), horse owners and vets are usually aware of the early signs of disease. Protection measures are generally in use or readily available, and some horses may even develop immunity to disease, making large outbreaks less common.
The most dramatic effects are seen when a new disease suddenly appears in a different area where horses don’t have immunity.
British horses have no natural immunity to a virus such as AHS, for example, so an outbreak among this “naïve” population could be catastrophic.
While the UK has never been hit by AHS, an outbreak in Spain, Portugal and Morocco in the 1980s caused widespread illness.
It is thought that the virus was introduced via a group of infected zebra imported from Africa and then spread by the local midge population.
The outbreak lasted for four years and caused the death of at least 1,000 horses. The disease hasn’t been seen since in northern Europe, but we should be aware that this situation could change.
A major concern is that owners and vets here may not be aware of the typical signs of these exotic diseases. The fact that an outbreak is developing may not be recognised until multiple horses are affected and the problem has spread further.
In addition, we are less well equipped to deal with these insects. Our main means of protecting our horses is to use insect repellents, but there is evidence that many have little effect against biting midges and mosquitoes.
In countries prone to vector-borne viruses, other methods are favoured. Stabling is typically located inside buildings that have been “vector-proofed” with mesh netting, which is often impregnated with insecticides. Horses are kept stabled during times of peak insect activity.
Vaccines form an important part of disease control and are available in some countries. Should an exotic disease arrive here, however, there may be a delay in getting the right type of vaccine and making it available in large numbers. In addition, there is evidence that horse owners might be unwilling to vaccinate their horses due to a perceived low risk of disease and concerns about vaccine side-effects.
Should we panic?
Continued hot summers could have real consequences for horse health in the UK, but are we right to panic?
Owners can be reassured that measures are in place to minimise the risk of disease occurring and to detect disease threats as soon as possible.
Vets and other scientists in the UK and Europe continually monitor for disease in all animals, including horses, and are on the alert for diseases new to these countries. This allows for early warning systems to be introduced should the risk that a particular condition occurs be higher than normal.
Strict rules govern the import of animals; this is important to reduce the chance that an infected horse, pony, donkey or mule is brought into the UK and bitten by a midge or mosquito which then passes disease on.
Simple management measures can also reduce risk. Stagnant water provides ideal conditions for mosquitoes and midges to breed and for larvae to develop, so items such as old tyres and plastic containers left lying around should be emptied frequently.
It is very important that UK vets and owners are aware of some of the signs of these exotic diseases. Should you have any concerns about unusual signs, particularly in a group of horses and especially in any recently imported into the UK, your vet is the first point of contact. More common conditions can then be ruled in or out and your vet can seek advice about further investigations, if necessary, to determine the cause of illness.
Vector-borne viruses show varying clinical signs, if any, and may affect different horses in different ways. Technology is already in place to allow real-time surveillance of diseases in small animals, using data from vet practices and diagnostic laboratories. It is likely that similar systems will be developed for horses.
Should a disease spread by midges or mosquitoes occur among UK equines, it is vital that the veterinary profession and government communicate quickly and effectively with horse owners. This would include providing information about clinical signs, advice on preventing the spread of the disease and addressing concerns about vaccination — all of which would play a key part in controlling and eradicating the disease in question.
Ref Horse & Hound; 18 October 2018
