Blister Beetles Pose Deadly
Problem For Horses In Hay

By Donald Stotts
Oklahoma State University

STILLWATER, Okla. — It sounds like a plot from a dubious direct-to-video movie: Insect poisoning in animals results in sickness and possible death, leading to emotional and monetary duress on the part of animal owners.

"Unfortunately, blister beetle poisoning in horses is quite real," says David Freeman, Oklahoma State University Cooperative Extension equine specialist. "The actual toxin is cantharidin, a chemical that blister beetles carry in their hemolymph (insect blood)."

The most common blister beetle species involved in equine cantharidiasis in Oklahoma is the black- and yellowstriped blister beetle. The striped blister beetle has been reported in most of the United States, ranging from the Eastern Seaboard to as far west as Arizona and Colorado. Other species of striped and non-striped blister beetles also can induce toxicosis in horses.

"Estimates on levels of beetle ingestion that cause poisoning in horses vary, mainly because species, gender and mating status affect the amount of cantharidin in blister beetles," Freeman says.

Some research estimates 100 to 150 beetles are lethal. However, it is known that lower levels of ingestion can cause a variety of illnesses, especially those related to digestive disorders.

"Blister beetles are known to swarm and feed in large numbers," Freeman explains. "Thus, small amounts of forage can have large doses of cantharidin."

Alfalfa hay is the forage usually associated with blister beetle poisoning in Oklahoma, mainly because it is the main food source of the blister beetle that is also used as hay by horse owners.

"Although not as common, processed alfalfa products such as alfalfa pellets also can contain cantharidin, because the compound is stable to processing," Freeman adds.

He says the lack of documented cantharidin poisoning with processed products may be caused by the dilution effect of mixing beetle-contaminated hay with large amounts of other alfalfa during the feed manufacturing process.

Additionally, beetles are known to feed on many other plants as well.

"Reducing the incidence of blister beetle poisoning problems involves a number of practices, such as timing harvest, scouting for presence of beetles in fields, and using insecticide and harvesting methods that decrease the chance of dead beetles remaining in the hay during baling," Freeman says.

OSU entomologists have not collected adult striped beetles before mid-May. Usually, the adult blister beetle does not become active until mid-June in Oklahoma.

"In a typical year, the first cutting of alfalfa can be beetle-free if it is cut before this time," Freeman advises. "Similarly, hay harvested late in the season generally is free of beetles, as insect activity ceases in early fall."

However, El Niño already has been shown to have altered weather conditions in the state, and the changes in weather conditions may be enough to speed up or put off beetle appearance in alfalfa fields.

Freeman says scouting programs prior to cutting can be helpful. Blister beetles tend to congregate in large numbers in small areas.

"Usually, beetles do not migrate long distances, so the outer areas of the fields are more likely to have beetles than are inner areas," Freeman notes. "Spraying fields with a short residual insecticide just prlor to harvesting to kill beetles also can be an effective tool to reduce the presence of beetles." Regardless of scouting or spraying measures, most hay contaminated with blister beetles is the result of the beetle being crushed during the harvesting process.

"This usually happens when the swather goes through a swarm of beetles," Freeman says. "The crimper-roller kills the beetles, and the remains are left in the hay."

As such, cutting without using crimpers decreases the chance of equine lngestion of beetles. It is common for many of the live beetles to leave the area shortly after cutting.

Clinically, horses display symptoms that are associated with the irritant effect of the toxin on the mucosal membranes of the digestive and urinary tracts. "Colic symptoms are common," Freeman says. "Additionally, changes in the blood chemistry are potential signs."

Other clinical signs range from changes in the gait, kidney dysfunction, oral ulcerations, diarrhea, elevations in heart and respiration rates, and dehydration.

Equine cantharidiasis can result in sudden death, or horses can go into shock and die within a few hours.

Freeman says the diagnosis of cantharidin in live horses is determined by veterinarians, who take a urine sample from the horse and submit it to a laboratory such as the Oklahoma Animal Disease Diagnostic Laboratory at Stillwater.

"Stomach and cecal contents are examined post-mortem in horses suspected to have died from blister beetle poisoning," he says.

There exists no known antidote for cantharidin. Treatment is aimed at removal of the source of toxin, reduction of toxin absorption, protection of the digestive tract mucosa, control of pain, and supportive measures to maintain the horse's fluid and electrolyte balance.

"Although it has long been reported that prognosis is poor and mortality is high, prognosis can be improved if the toxicosis is recognized early and appropriate therapy is applied," Freeman says. "As such, immediate veterinary intervention is the key to reducing damage."

Prognosis is good when horses survive the first several days, although long-term studies on problems post-poisoning are virtually nonexistent.