Hot weather and infectious diseases
Hot weather brings an increased risk of infectious diseases that involve arthropods transmission to horses. Two important diseases in this category are West Nile virus (WNV) and Potomac Horse Fever (PHF). WNV causes neurologic signs and muscle trembling, killing almost a third of the horses that develop signs. Mosquito numbers often soar in late summer, as larvae hatch from warm water pools. These mosquitoes feed on wild birds, spreading WNV between them, and occasionally transmitting the virus to horses and people.
Horse owners can do 3 things to lower the risk of WNV infection: eliminate or treat all standing water in their horse's environment to discourage mosquito hatching, minimize the likelihood of mosquito bites by keeping their horses indoors at prime mosquito feeding times (dawn and dusk) and protected with repellants, and ensure their horses are well vaccinated against WNV. This may include a late summer booster vaccination, in addition to a vaccination in the spring.
PHF cases are also more frequent in late summer, and are characterized by fever, laminitis and often, diarrhea. The causative organism, has been shown to infect flukes that parasitize aquatic snails. In warm water, the bacteria pass out of the snails, and are then picked up by aquatic insects. Horses can be infected by drinking contaminated water, or by ingesting feed that has been contaminated by the insects from the aquatic environments. Many strains of the organism have been identified, but only one strain is protected against in the vaccine. Vaccination does not prevent the disease but may lessen the severity of the signs. Early treatment with the antibiotic oxytetracycline is more cost effective than vaccination. PHF resembles several other diseases, such as salmonellosis, which is contagious to humans and other animals, so immediate veterinary care and diagnostic testing are strongly recommended.