Alfalfa, Blister Beetles and Cantharidin – everything you wanted to know about this poisonous insect and the threat it poses to your horse.
This is the second in a two-part series. Need to review Part 1?
Though sheep and cattle can be affected by the dangerous poison (cantharidin) in blister beetles, horses are most susceptible. Symptoms include colic, diarrhea, bloody stool and urine, and even shedding of the intestinal lining.“It’s a blistering agent. It basically rots through the tissue,” explains David Buntin, a professor of entomology at the University of Georgia.
Sometimes a horse poisoned by cantharidin will soak or splash its muzzle in water without drinking, and act anxious or depressed. Shock, lowered calcium levels resulting in body tremors and periodic jerking contractions of the diaphragm can also be indications of poisoning. There also can be frequent discharge of small amounts of blood-tinted urine. Poisoned horses might also suffer from increased pulse rate, dehydration and elevated temperature.
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Pregnant mares are especially sensitive. Abortion can occur within 24 hours of eating poisoned hay with a lower dose than the amount needed to cause mortality. Death could result in all horses exposed to contaminated hay.
There are a number of unknowns concerning the blister beetle. What many people want to know is how many it takes to kill a horse. Because the toxicity levels of cantharidin to horses have not been clearly determined, there is no real answer. In general, it is estimated that the minimum lethal dose is 0.5 to 1 milligram per kilogram of body weight. Regardless, it only takes relatively few to cause colic and other symptoms that could eventually lead to serious illness or death.
“People think that one beetle is going to kill their horse – that’s not true,” David says. “Generally, it takes a number of beetles to have that effect. The problem is when you cut with a crimper and the beetles are crushed in the hay, thousands and thousands of those beetles end up in three or four bales. That’s the reason they’re such a problem.”
Unfortunately, there is no answer to the threat that blister beetles pose. As horse owners, all we can do is be educated and vigilant. Buy hay from a reputable dealer or grower and build a working relationship with that dealer. If possible, try to obtain bales of alfalfa that come from fields that are scouted for blister beetles before the haymaking process begins.
Many hay producers nationwide attempt to make the first spring cut of alfalfa blister beetle-free. While seasons and harvest times vary, early cuts are less likely to have blister beetles in them. There is no guarantee, though.
“The question I always get is, ‘How do I produce certified blister beetle-free hay?’ ” David says. “Well, the answer is, ‘You can’t.’ You can reduce the risk, but you can’t totally eliminate it.”
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When horse owners flake hay, they should inspect it for blister beetles. Haying equipment can crush the insects into fragmented pieces. Inspection might seem like looking for the proverbial needle in the haystack, but it could reduce the danger of poisoning. If infested hay is discovered, destroy it by burning or burying it.
Resources are available to keep yourself informed. Talk to your hay provider. Find out what types of beetles are in your area and when they are most prevalent. Most importantly, watch your horse. If you suspect cantharidin ingestion, contact your veterinarian immediately.
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Quick Tips for Reducing Risk
- Concentrate scouting for blister beetles along field borders; immediately cut and spray infested areas if found.
- Avoid harvesting field borders.
- Spray border areas when spraying fields for control of other insects to reduce beetle migration.
- Cut hay without using crimpers and avoid wheel traffic on freshly cut alfalfa.
- Purchase alfalfa hay harvested before May or after September.