Foaling season is a time to be concerned about intestinal threadworms, since they have a two-week lifecycle and can quickly become a problem for infected foals. Threadworms cause diarrhea and weight loss, and can inhibit a foal's growth and ability to thrive.
 
Threadworms (Strongyloides westeri) are tiny, about 1 cm long and quite slender. They can be picked up by the foal orally either through nursing or from the pasture or stall. Larvae can also penetrate the foal's skin from the environment and migrate to the lungs, resulting in respiratory difficulties. Intestinal threadworms are different than neckworms (Onchocerca cervicalis), which infect the skin and ligaments of the neck from withers to poll and can result in 'sweet itch' and dermatitis.

While horses develop a natural immunity around six months of age, preventing threadworms is important for a foal's health and development until that immunity kicks in.
 
It's best to treat mares for threadworms, both before foaling and within a few days after foaling, than to deworm the foals. Threadworms aren't present in colostrum, but can be passed to the foal through the mare's milk. Ivermectin is the most commonly used dewormer for threadworms; if you're concerned about possible intestinal threadworms in your foals, be sure to talk with your veterinarian in advance of your mare's anticipated foaling date.