Parasite Life Cycles

Eggs shed by female small strongyles hatch and develop into various larval stages depending on climatic conditions. Temperature and moisture play an important role in the rate at which the egg can hatch and develop. Strongyle eggs hatch when environmental temperatures range from 45-85°F. As a general rule, the infective stage of small strongyle larvae die off during hot summers (85 degrees Fahrenheit and above). In cold winters (45 F and below) there are smaller numbers of infective larvae. Cold temperatures don’t kill the larvae, but they do prevent earlier stages of larvae from molting into the stage where they are infective to the horse.

Small strongyles have a first, second, third, and fourth larval stage (often designated L1, L2, L3, and L4). Eggs hatch faster at higher temperatures, however the resulting first stage larvae will die rapidly in the extreme heat. At moderate temperatures, the L1 stage of larvae feed on bacteria and other organic material present in feces, and eventually molt into second and third stage larvae.

The rate at which strongyle eggs hatch and larvae develop from L1 to L3 is directly proportional to the environmental temperature. In warm weather, eggs can hatch and yield infective larvae in as little as three days, however it can take several weeks to become infective during the cooler months.

Third-stage larvae are very different from the first two stages. They are tightly ensheathed in a tough membrane without the ability to feed. They rely on their energy reserves to survive until ingested by a grazing horse. The warmer the temperatures, the faster they metabolize their energy reserves and die. However, in cooler conditions they use hardly any energy and can stick around for months.

Once infective larvae are ingested, they migrate to the cecum or colon where they have two options: Become hypobiotic and encyst themselves in the horse’s mucosa where they will stay inactive for a prolonged period of time -or- develop into mature worms.

Unlike large strongyles, small strongyles do not migrate beyond the mucus membrane of the cecum and colon, so damage is usually less dramatic. However, infection with large number of small strongyles (cyathostomins) can cause larval cyathostominosis which occurs as a result of mass emergence of arrested stages of larvae and has a high fatality rate despite the best standard of care.