During an AAEP Roundtable presentation sponsored by CareCredit, Sally DeNotta, DVM, PhD, DACVIM, from the University of Florida’s College of Veterinary Medicine, and Angela Pelzel-McCluskey, DVM, MS, equine epidemiologist for the USDA-APHIS Veterinary Services, discussed the current and future impact of climate change on infectious diseases.
Local climates are changing in many regions, with more natural disasters occurring and in larger geographic scopes. This is having a significant impact on vector-borne disease transmission. Vector insects are flourishing with warmer temperatures and greater precipitation, exotic vectors are entering new areas, and reservoir hosts are expanding. DeNotta and Pelzel-McCluskey said this is happening all over the country, increasing the incidence of diseases covered by AAEP-recommended core vaccines as well as risk-based diseases.
DeNotta gave some examples of amplified risks:
- Increased risk of wildlife interactions increases rabies risk.
- Structural damage and debris during tornadoes and hurricanes increase the risk of projectile injuries and tetanus in wounds.
- Standing water immersion increases the risk of noninfectious diseases of the skin and hooves and potential sepsis.
- Standing water provides breeding grounds for arbovirus vectors to increase incidence of West Nile virus (WNV) and Eastern equine encephalitis (EEE).
- Flooding creates situations of congestion and crowding of horses into smaller areas on a farm or at evacuation centers, potentially increasing the incidence of risk-based diseases.
Eastern Equine Encephalitis (EEE) and West Nile Virus (WNV)
The highest human case count of EEE occurred in 2019, said Pelzel-McCluskey. Compared to previous years of seven or fewer cases, there were 38 human cases in 10 states with a 50% fatality rate. Similarly, horse infections with EEE ticked up markedly, resulting in 184 reported cases in 24 states. EEE appeared in alternate species and wildlife, and WNV cases doubled.
Another change occurred in 2020, with the highest annual infection rates of EEE occurring in horses in the upper Midwest rather than Florida or East Coast states. Eastern equine encephalitis showed up as far west as North Dakota and, in 2023, Arizona reported an EEE equine case.
Vesicular Stomatitis Virus (VSV)
Vesicular stomatitis virus (VSV) has also shifted in the last few years, reported Pelzel-McCluskey. This disease is endemic in southern Mexico, circulating between black flies, sand flies, and Culicoides biting midges. She explained how climate and ecological factors create epidemic transmission into northern Mexico in winter months. The vectors then travel north into the U.S. with VSV cases developing in Texas, New Mexico, Arizona, and then shooting up the waterways into the Rocky Mountain region.
The first year of northward movement of a vector/virus is called an incursion year. The following year (or years) is an expansion year due to overwintering of vectors and virus. Climate factors contribute about one-third to incursion and two-thirds or more to expansion events. An example is the 2019 VSV outbreak, the largest in 30 years, with 1,144 quarantined premises in eight states, mostly in horses. While 2020 was expected to be an expansion year, only 326 premises were affected in eight states. The lack of expansion likely occurred because of an extreme drought that didn’t allow virus-carrying vectors to flourish. Interestingly, in 2020 VSV occurred in south Texas, Kansas, Oklahoma, and Missouri because of standing water from flooding that allowed Culicoides vectors to propagate for longer durations.
Pelzel-McCluskey advised that movement of animals with VSV lesions can bring vectors to an area. She described the natural incursion of VSV-infected vectors into California from Mexico for the first time ever. Historically, extreme drought in California has protected against such an incursion, but in the winter of 2022-2023, heavy snow and spring rains caused flooding and lush vegetation that created a vector haven.
Vesicular stomatitis virus also spread to 26 rhinoceros in a safari park in 2023. Previously, it was unknown if this was a susceptible species, but rhinos have an ancient genetic connection to horses, a normally susceptible species.
Another surprising event occurred in August 2023 after Hurricane Hilary hit Southern California with flash flooding, washing insect vectors, larvae, and eggs into the ocean. Subsequently, no more VSV cases materialized.
Ticks and Tick-Borne Disease
The Asian longhorned tick from northeast Asia was first identified in the U.S. in 2017. However, it is confirmed to have been here since 2010. As a parthenogenic tick, a female reproduces in the absence of a male to create massive, explosive mini populations. They are extremely aggressive with no chance of eradication. These ticks prey not only on mammals but also on birds and raptors, which carry them further distances. An infestation on an animal creates stress, reduced growth and production, anemia, weight loss, and severe blood loss.
These ticks also carry many infectious diseases that affect horses: Anaplasma, Borrelia burgdorferi (Lyme disease), equine piroplasmosis from Babesia caballi or Theileria equi, and Powassan virus, to name a few. When faced with an equine case with fever, anemia, and neurologic or nonspecific illness, consider the tick vector as a cause, especially if an owner pulled ticks off the horse. Tick identification is available at www.veterinaryentomology.org/vetpestx. Submission to a lab also provides information about disease risk and identification.
Other Risks
Smoke inhalation from wildfires can create significant lung inflammation and damage, subjecting a horse to greater risk of viral respiratory infections if not adequately immunized. Such a situation arose at the BLM wild horse facility in Canon City, Colorado, in 2022. Over two to three days, 57 horses died from equine influenza virus, and 145 total horses died out of 2,550 head on site. It is noteworthy that live viruses can travel at least a kilometer on the wind and infect highly susceptible individuals.
Another unexpected disease outbreak occurred with Type C botulism introduced to drought-stricken Florida from hay purchased in the western U.S. that was contaminated with dead animals. Hay imported from elsewhere makes it difficult to recognize a problem if it is not typically endemic to an area.
Yet another infectious disease is migrating northward from vampire bats in Mexico: rabies. These bats feed at night and in flocks that attack whole herds simultaneously. While their historic range is Central America and southern Mexico, they’re shifting toward Texas. Vaccination of horses against rabies is preventive.
Preventive Suggestions
Pelzel-McCluskey stressed the importance of horse owners recognizing that even animals that never leave the farm can be exposed to infectious disease at an evacuation center during a natural disaster. She advises owners to vaccinate against both AAEP-described core infectious diseases and highly contagious respiratory diseases such as influenza, herpesvirus, and strangles if living in a natural-disaster-prone area. Many evacuated horses in group housing come from areas of unknown vaccine status, while others might carry disease but not show illness.
Good biosecurity practices are important to implement at evacuation centers to avoid transfer of pathogens between horses from different living arrangements. Isolation areas help contain sick horses before disease has an opportunity to spread.
Another useful suggestion DeNotta and Pelzel-McCluskey offered is when faced with a natural disaster, spray paint a phone number on the side of each horse or use a wax marker, especially if a horse isn’t microchipped.
As more natural disasters occur and the climate becomes more amenable to flourishing insect populations, equine husbandry and management tactics must accommodate these changes. Veterinarians and owners can accomplish this through surveillance, collaboration with animal and public health resources, proactive strategies, core and risk-based vaccinations of all horses in natural-disaster-prone areas, and preparation.
Related Reading
- Disease Du Jour: Equine Disasters
- Disease Du Jour: West Nile Virus Refresher
- Disease Du Jour: Lyme Disease in Horses
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