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Dr. Robert Stenbom, DVM, senior equine professional services veterinarian with Boehringer Ingelheim Animal Health, responded to a question posed by an audience member at his 2018 Midwest Horse Fair program about equine health issues caused by mosquitoes and mayflies.

MADISON — Some of the smallest animals on earth can be the deadliest. Insects such as ticks, mosquitoes and mayflies are instrumental in causing illness and death in humans and horses.

“Mosquitoes are responsible for more human deaths than any other animal,” said Dr. Robert Stenbom, DVM, senior equine professional services veterinarian at Boehringer Ingelheim Animal Health, which sponsored the presentation.

Stenbom presented a program titled, “Killer Mosquitoes and Mayflies” at the 2018 Midwest Horse Fair.

Giving a condensed history of the mosquito’s evolution, Stenbom said the insect originated in South Africa and was well established in the Jurassic era.

“Mosquitoes were three times bigger then than today’s mosquitoes,” he said. “There are 3,000 species and subspecies of mosquitoes in the world and 150 in the United States.”

The veterinarian asserted the insects kill or infect other animals by transmitting viruses they unintentionally carry.

“The virus has adapted to have the mosquito do its work,” Stenbom said. “It takes more than a mosquito to spread viruses. The viruses prefer birds and will reproduce in birds. People and horses are incidental hosts.”

Birds are common carriers of virus because they migrate to warm climates where they are exposed to mosquitoes year-round.

Mosquitoes are likely to attack moving targets and can fly 1.5 mph. Only the female mosquitoes extract blood from their victims. They need the protein in the blood to nourish their eggs. As the mosquito feeds, it transfers any viruses it carries to its blood-host.

“Mosquitoes are very good at doing the ‘surgery,’” Stenbom said. “Most people don’t know they’re doing it until the mosquitoes’ saliva is injected. The saliva has anticoagulant properties and is the vehicle for the disease the mosquito carries.”

Among the diseases mosquitoes will carry and transmit is encephalitis, an inflammation of the brain and spinal cord. Eastern and western are two strains of equine encephalitis of most concern in the United States, but there is also a Venezuelan strain. Symptoms associated with the disease are fever, depression, loss of appetite, weakness, irritability, aggressive blindness, excitability and sensitivity to light and sound.

“The disease is difficult to treat,” Stenbom said. “It has 75 to 90 percent mortality. Even if the horse pulls through, the animal is neurologically impaired.”

Equine encephalitis is a legally reportable disease and 11 confirmed cases were reported last year in the state between the end of July and Sept. 11.

Stenbom advised vaccinating in the spring so the vaccine can be the most effective when the virus is prevalent in the northern part of the country in the mid to late summer and during the fall.

“The vaccines work and they work very, very well,” he said. “It’s very unusual for vaccinated horses to succumb (to the disease).”

Mosquitoes can also pass along West Nile virus, a type of encephalitis, and again, birds often serve as vectors in the life cycle of the virus.

Horses infected with WNV will show symptoms of difficulty walking, uncoordinated movements, head tilt, muscle tremors and the inability to rise.

Although mayflies don’t bite and extract a victim’s blood, they have been identified as a vector for Potomac horse fever. Horses become infected when they inadvertently eat dead mayflies, infected flukes from snails or other infected insects while drinking or grazing in areas with high numbers of the dead insects. In addition to the mayflies, other insect hosts include caddisflies, damselflies, dragonflies and stoneflies. Bats, birds and amphibians are also suspected of being carriers.

The symptoms of PHF include depression, fever, colic, watery diarrhea, abortion, laminitis and low white blood cell counts. Stenbom advised vaccinations can reduce the severity of the symptoms if horses do contract the disease.

“Potomac is a mild to life-threatening disease,” Stenbom said. “This disease is an insult to the horse, not a fun disease at all, but not as deadly as others. The prognosis is good if the horse doesn’t get laminitis.”