Vector-borne viral diseases in horses: climate, vectors and animal movement reshape equine risk

Mosquitoes, biting midges and horseflies transmit several of the most important viral diseases affecting horses worldwide. A new review in The Thai Journal of Veterinary Medicine highlights how climate change, environmental disruption and international horse movement are modifying the geographical distribution of these infections and increasing the importance of integrated surveillance.

The review focuses on the major vector-borne viral diseases of horses, including African horse sickness, equine infectious anaemia, Eastern, Western and Venezuelan equine encephalomyelitis, West Nile virus and Japanese encephalitis. These infections differ considerably in virology and epidemiology, but all depend on arthropod vectors or, in the case of equine infectious anaemia, mechanical transmission by blood-feeding insects.

Mosquitoes dominate the encephalitis group

Several major equine encephalitides are mosquito-borne. West Nile virus is now one of the most widely distributed arboviruses affecting horses. Birds act as the principal amplifying hosts, while horses and humans are generally considered dead-end hosts. Infected horses may develop fever, ataxia, weakness, tremors or other neurological signs, and severe cases can progress to recumbency or death.

Eastern, Western and Venezuelan equine encephalomyelitis viruses also rely heavily on mosquito transmission but differ in their geographical distribution, reservoir systems and zoonotic importance.

Japanese encephalitis virus, another mosquito-borne flavivirus, circulates mainly in Asia and the western Pacific. Its ecology involves mosquitoes, birds and pigs, with horses and humans becoming infected incidentally.

The review emphasises that the geographical limits of these infections are closely linked to the distribution and seasonal activity of their vectors.

African horse sickness depends on Culicoides

African horse sickness represents a different model of vector-borne transmission.

The disease is caused by African horse sickness virus and is transmitted mainly by biting midges of the genus Culicoides. Clinical disease can be severe, with pulmonary, cardiac or mixed forms and potentially very high mortality in susceptible horses.

Historically associated mainly with sub-Saharan Africa, the disease has periodically spread beyond its endemic range. The presence of competent Culicoides populations in many regions means that introduction of an infected equid can potentially lead to local transmission when environmental conditions are favourable.

This makes movement control and vector surveillance particularly important for international equine trade.

Equine infectious anaemia has a different transmission pattern

Equine infectious anaemia virus is also vector-associated, but transmission differs fundamentally from that of mosquito- or midge-borne arboviruses.

Horseflies and other biting flies transmit the virus mechanically, through contaminated mouthparts, rather than after viral replication inside the insect.

The disease can cause recurrent fever, anaemia, thrombocytopenia and weight loss, while some horses remain persistently infected without obvious signs. Because infected horses remain carriers for life, diagnosis and movement control are central to disease management.

Climate and environmental change alter vector distributions

One of the review's main themes is that vector-borne equine diseases cannot be considered geographically static. Rising temperatures can lengthen vector activity seasons and allow mosquitoes or biting midges to survive in areas where climatic conditions were previously less favourable. Changes in rainfall, wetlands, land use and wildlife distribution can also alter vector abundance and contact with horses.

At the same time, increasing international movement of horses for sport, breeding and trade creates opportunities for viruses to reach regions where competent vectors are already present. The review links these ecological and movement factors to the emergence of vector-borne diseases in previously unaffected areas.

Vaccination is available for some diseases, not all

Prevention varies markedly between pathogens. Vaccines are available against diseases such as West Nile virus, African horse sickness and several equine encephalitis viruses in regions where their use is authorised or recommended. For other infections, prevention depends more heavily on reducing exposure to vectors, testing animals and controlling movement.

Vector management can include stable management, insect repellents, elimination of mosquito breeding sites and reducing exposure during periods of peak vector activity. For internationally moved horses, laboratory testing and certification can be equally important because clinical examination alone may fail to identify incubating or subclinical infections.

Surveillance needs to combine horses, vectors and environment

The review argues that effective control requires coordination between veterinary surveillance, vector monitoring and environmental data rather than treating equine cases in isolation.

A neurological case in a horse may provide the first indication of West Nile or another arbovirus in a region. Detection of infected mosquitoes can provide an earlier warning. Environmental and climatic data can help identify periods when vector transmission is becoming more likely.

That combined approach is increasingly relevant as competent vectors expand their range and horse movements continue to connect distant regions.

For equine health, the key change is therefore not the emergence of a single new virus, but the growing overlap between mobile horse populations, expanding vector distributions and changing environmental conditions.