If you buy alfalfa hay, be aware of blister beetles. Small but sinister, blister beetles contain cantharidin, a deadly horse toxin. In small amounts, blister beetles can cause colic. In large amounts, they can be fatal. Cantharidin is very stable and persistent—dried beetle bodies remain toxic, as does the hay after the beetle bodies have been removed.
Blister beetles are most common in dry western states but can be found all over the US. They are usually 1/2 to 1 inch long. Their narrow, cylindrical bodies can be black, gray, brown or orange-striped. 
To reduce your horse's risk, buy first-cutting or late fall cutting alfalfa hay and buy your hay from reliable growers.
Check your hay carefully for blister beetle bodies and discard any hay that is suspicious.
Cantharidin can cause severe skin inflammation and blisters in humans and livestock. When eaten, blister beetle toxin is absorbed through the intestine and causes symptoms such as colic, elevated body temperature, increased heart and respiratory rates, sweating, dehydration and diarrhea.
If you suspect blister beetle poisoning, contact your veterinarian immediately.