If there’s one thing we’re learning about climate change’s effects on vector-borne diseases, it’s that the diseases will do something different and we can’t predict quite what that is going to be.
Lesions and signs of vesicular stomatitis in horses.
Vesicular stomatitis virus is zoonotic, so humans can get infected from handling livestock with lesions.
© 2023 by New Mexico State University
When vesicular stomatitis reappeared in the United States in 2023 after a three-year hiatus, the fact that a new incursion occurred was not a surprise; rather, everything else about the outbreak was out of character, from the geographic locations affected, to the species of animals that developed lesions, to the natural disasters that altered the trajectory of disease occurrence.
Vesicular stomatitis virus (VSV), spread by black flies, sand flies and biting midges, causes blister-like lesions on the muzzle, tongue and coronary bands of affected livestock.
The virus circulates year-round in southern Mexico and only occasionally moves into the U.S. when climate factors support expansion of the infected vectors northward.
Equids are most commonly infected, but cattle cases also occur. While other livestock species are susceptible, such as llamas, alpacas, pigs, sheep and goats, only a handful of cases in those species are diagnosed.
The painful lesions heal on their own in a couple of weeks and most affected animals only need supportive care, but the trade ramifications and movement restrictions for livestock during an outbreak are significant.
International movement of susceptible livestock may be halted from affected to test animals prior to export. Shows, events and county fairs may be canceled, and non- affected states restrict movement of livestock from affected states.
The outbreak usually lasts for months through the vector season (summer to fall), and often continues into winter as vectors die off or go dormant. Meanwhile, horse owners struggle to meet requirements to move to their shows and events and, as cases spread, they may suddenly find themselves in a newly-affected area and cannot return home. spray to horses and implementing aggressive fly mitigation strategies on farms to prevent their animals from contracting the disease.
Vesicular stomatitis virus is zoonotic, so humans can get infected from handling livestock with lesions, (yet another flu-like illness to worry about!) The 2023 VSV outbreak that produced 319 infected premises (311 had infected equids), ran from May 17, 2023, to Jan. 18, 2024, and primarily mpacted California, a state which had never previously recorded a natural incursion of the disease from Mexico.
For decades, Southern California had been protected from incursions by extreme drought conditions, but the Winter of 2022 saw heavy snow and Spring of 2023 brought buckets of rain. Lush vegetation abounded; the flies and midges thrived and expanded their range over the border carrying the virus. The only previously recorded VSV cases in California were caused by human error, not a natural incursion, and occurred in the 1982-83 outbreak when infected dairy cattle were moved from Colorado.
As the case counts increased quickly in horses in Southern California in Summer 2023, another new development occurred when a wildlife park in the region discovered multiple VSV-lesioned rhinoceros in their herd. Rhinos are a species that had never been identified as susceptible, and, ultimately, all 26 of the park’s rhinos became affected and struggled with lesion healing.
Some of the rhinos sloughed their foot pads and became badly lame, in addition to not wanting to eat or drink with sloughing tongue mucosa. Rhinoceros share ancient genetic lineage with horses, so perhaps we should have guessed their susceptibility.
Climate change wasn’t done surprising us as Hurricane Hillary formed in August and took shockingly rare aim at Southern California. The resulting flooding seemed to flush the vector eggs and larvae out to the Pacific, which stopped the outbreak in Southern California. Cases continued, however, to move northward from the Central Valley and eventually as far as Sacramento County before the cold snaps in early January 2024 finally ended the outbreak.
VSV incursion years are often followed by overwintering of the virus, and expansion year outbreaks, so it remains to be seen if this will occur in such a novel outbreak region like California. As a previously drought-riddled region becomes verdant (a new lake has formed in Death Valley!), climate change continues to keep us guessing about what new tricks an old vector-borne disease can perform.
A multi-disciplinary research team including USDA, state, laboratory and university collaborators has begun studying the impacts of climate parameters on VSV occurrence hoping to better predict what the virus has in store for us in the future.
Press release published by Equine Disease Quarterly - Angela Pelzel-McCluskey, DVM, MS Angela.M.Pelzel-McCluskey - USDA-APHIS-Veterinary Services Fort Collins, Colorado