For the Health of It
Blister Beetle Poisoning
Reprinted with permission from the October 1999 issue of Equine Disease Quarterly
Blister beetle (Epicauta spp.) poisoning occurs in
horses that consume alfalfa hay contaminated with
these insects. Over 200 species of blister beetles can
be found in the United States. Blister beetle larvae
feed on grasshopper eggs while the adults feed on
pollen or nectar of flowering plants, such as alfalfa. The adults swarm and mate in summer and fall
when alfalfa is in bloom. Hay is especially vulnerable to contamination when mowing is done during
this swarming period.
Adult blister beetles have narrow bodies that are
0.5 - 1.25 inches long. The various species can range
in color patterns from striped or spotted to solid
colors such as black or gray. The
toxin present in the beetles is cantharidin, which causes irritation
of the mucosa and hypocalcemia.
Cantharidin content can range
from 0.1-12.7% of dry weight,
depending on the species and sex
of beetle. The minimum lethal
dose of cantharidin for horses
may be less than 1 mg/kg of body
weight. Therefore, as little as 4-6
grams of dried beetles could be
fatal to a horse.
Symptoms are most often
related to abdominal pain caused
by the irritating effects of cantharidin: colic, depression, and
decreased appetite. Irritation of the urinary tract
can result in frequent urination or straining to
urinate. Other possible signs are fever, diarrhea, dehydration, increased respiratory rate, and increased
heart rate.
Signs associated with hypocalcemia include
muscle fasciculations, synchronous diaphragmatic
flutter, stiff gait, and abnormal behavior such as
aggressiveness, head pressing, swaying, or disorientation. Sudden death with no signs of struggle has
also been reported.
Abnormal laboratory findings can include low
levels of calcium, magnesium, and protein in the
blood as well as impaired kidney function and
blood in the urine (hematuria). Necropsy findings,
when present, are reddening or ulcers of the gas-
12 • Walking On
trointestinal tract (most often in the esophagus and
stomach) and of the urinary tract (bladder, ureters,
renal pelvis). Some horses will have patchy necrotic
areas in the heart. Diagnosis can be confirmed by
testing the urine and gastrointestinal contents for
cantharidin.
No antidote is available for blister beetle poisoning. Suspect alfalfa hay, cubes, or pellets should be
removed from the animal’s diet. Activated charcoal
and mineral oil will decrease the amount of toxin
that is absorbed into the body.
Symptomatic care includes analgesics, fluids,
electrolyte replacement, and possibly antibiotics. Prognosis depends on the amount of blister
beetles eaten as well as the speed and aggressiveness
of therapy.
While there is no way to completely eliminate
blister beetle poisoning, many management options
are available to reduce the risk.
• Learn to identify blister beetles. Because the
beetles swarm, it is common to find a massive
number of beetles in a small area of the hay
while the rest of the hay is not contaminated. Pull individual blocks of each bale apart
and inspect before feeding.
• The beetles swarm and mate in late summer.
Therefore, first cutting hay is safer to feed than
hay cut later in the year.
• Cut alfalfa before it reaches the full bloom
stage. Also reduce the number of flowering
weeds in the hay field. Remember that blister
beetles are attracted to flowers.
• Modify your method of harvesting hay. Crimping traps the beetles in the hay, whereas mowing
without crimping allows the beetles to escape
from the cut hay before baling. Use a method
that allows the hay swath to be straddled by the
tractor so that the beetles aren’t crushed and
then baled into the hay.
• Consider using insecticides to control the blister
beetle population as well as the grasshoppers
that the larvae feed on.
• If you do not harvest your own hay, get to know
your hay suppliers and their management
practices.