Animal Health Update, July 2004 BLISTER BEETLE POISONING IN HORSES Arizona horse owners and veterinarians should be aware that three horses in the Benson, Arizona area have died and two more are reported to be sick as a result of consuming a batch of locally-grown baled alfalfa hay found to be contaminated with blister beetles (Epicauta sp.). One of the three horses died in 24 hours and two more died the next day. All were fed from the same bale of a new lot of second-cutting alfalfa on Friday, (07/02/04) evening and were ill and refused to eat the next morning. Personnel in the AZVDL toxicology section subsequently examined the alfalfa hay consumed by the horses and found blister beetles. Clinical signs that were reported by the owner and submitting veterinarian were typical of blister beetle poisoning and included: depression, shaking, refusal to eat, "toxic" mucous membranes, diarrhea, colic with sweating, and immersion of the muzzle and playing with the water as though the horses were attempting to flush their mouths. Straining to urinate was also observed in some and one horse was observed to void bloody urine. The hay grower has voluntarily recalled all lots of hay from the cutting.
Blister beetle (cantharidin) poisoning occurs most often in horses but may also occur in cattle, sheep, and goats. Poisoning is associated with the consumption of alfalfa hay contaminated by blister beetles (Epicauta sp.). Swarms of blister beetles commonly congregate around hay fields to feed on pollen and plant nectar and to mate. Modern harvesting practices where hay fields are swathed and crimped in one pass create a greater risk by killing beetles infesting the hay, which then contaminate hay bales. Because blister beetles tend to swarm their concentrations in hay can be highly variable. Many bales may have no beetles present while only a few bales may have significant numbers. Dead beetles inadvertently consumed by a horse in its hay are extremely toxic. Accidental consumption of only a few dead beetles will poison a horse. The beetles contain the toxin cantharidin, which is thought to protect the insects from their natural predators. Cantharidin is a terpenoid compound that is a strong vesicant that is extremely irritating to skin and mucous membranes and is rapidly absorbed and eliminated in the urine. Severe damage to the oral and gastrointestinal mucous membranes, urinary tract, and other organs occurs and in severely poisoned horses shock and death can occur within a few hours. The severity of poisoning is dose-related and less-severely-affected animals can be treated successfully. There is some uncertainty whether residual effects occur in horses that survive poisoning; however, given the fact that the toxin has effects on many organ systems surviving horses should be monitored closely. A diagnosis of blister beetle poisoning is suggested by (1) compatible clinical signs and a history of alfalfa hay consumption and (2) finding blister beetles in the hay or in the bottom of the feed bunk. Confirmation of the diagnosis can be made by chemical analysis of urine, serum, or stomach contents for cantharidin. Clinical signs vary with the severity of poisoning but suggestive ones include: colic, sweating, feed refusal, salivation, congested oral mucous membranes, ulceration of the oral mucosa, washing the muzzle in water, straining to urinate, blood in the urine, elevated body temperature, diarrhea, abnormal heart rate and respiration, fever, and muscle stiffness with short-strided or "goose-stepping" gait. Horses showing neuromuscular signs may have abnormal serum calcium and magnesium levels, which will require correction. Laminitis has been reported to occur in some equine cases. Animals suspected of having Blister beetle poisoning should be seen on an emergency basis by an equine veterinarian. Severely affected animals may require hospitalization because of the intensive treatment required. There is no specific antidote for blister beetle poisoning but prompt treatment and reducing absorption of the toxin can sometimes salvage affected horses. Appropriate doses of activated charcoal followed by mineral oil two to three hours later have been recommended. Fluid therapy is usually necessary as are analgesics for pain control. Clinical signs and serum electrolytes should be monitored closely and treatment adjusted as necessary to correct any deficits and neuromuscular signs. Gastrointestinal protectants are also suggested in cases where there is persistent feed refusal and water playing. Broad-spectrum antibiotics are also recommended. However, amino glycoside-type drugs are contraindicated. The prognosis varies with the amount of toxin ingested but survival of horses for two to three days has been reported to be associated with a more favorable prognosis and outcome. A photo of crushed blister beetles contaminating hay in this case accompanies this Animal Health Alert. Readers should realize that the appearances of the many species of this insect vary. Hay containing beetles of uncertain identity should be considered suspect, withheld from use as feed, and the beetles submitted to the AZVDL for identification. T. H. Noon, DVM AZVDL Toxicology and Nutritional Testing Section S. Peder Cuneo , DVM, MS Extension Veterinarian
