Development and survival of the free-living stages during Phase 3 depends on prevailing environmental (temperature and moisture) and nutritional (oxygen and energy) conditions. In most worm species, the first stage larvae initially develop in the egg and then hatch into first-stage larvae. Development and survival to second-stage and finally third-stage (infective) larvae occurs within the fecal mass. The first- and second-stage larvae are unprotected and need oxygen and energy to grow. They feed on bacteria within the fecal pellet. However, the third-stage infective larvae emerges from the fecal pellet. It is enclosed in a protective sheath and does not feed.
Temperatures conductive for normal development and survival vary between worm species. Barber pole worm (Haemonchus contortus) prefers temperatures between 65-85˚F(18 – 30˚C). The lower or higher the temperature gets, development and survival are reduced. Barber pole first-stage larvae take 3 to 5 days to hatch at 77 to 79˚F but can take as long as 15 to 30 days to hatch if temperature stays constant at 50 to 52˚F. Moisture is also crucial for development and survival. Because the initial development and survival occurs within feces, moisture is usually adequate to complete development to the infective larvae; however, if the feces dries out quickly, due to high temperatures and/or physical disruption, the first- and second-stage larvae are susceptible to dessication and will die. Direct sunlight can heat the fecal pellet to 155˚F and sterilize it. This is why some farmers mow their pastures very short immediately after removing a herd from a grazing paddock especially if very hot, sunny and dry temperatures are predicted for the next few days.
If feces remain intact, retain some moisture and do not get too hot or too cold, infective larvae may remain alive for months. A moisture medium such as rain or dew is necessary for the infective larvae to migrate out of feces, and they are relatively resistant to unfavorable environmental conditions due to their protective sheath. Temperature is usually the only factor that may adversely affect the infective larvae. Generally, infective larvae can survive very low temperatures but may die off during hard freezes. The ability of worms to withstand Northeast winters is dependant on the worm species and on the insulating effect of snow cover. Temperatures above 95˚F (35˚C) are lethal to most worm species. However, the moisture condition at ground level under forage cover (tall grass, tree leaves, etc.) is usually adequate for infective larvae to survive. Since they don’t feed, their length of survival depends on how fast they use up their energy reserves. The hotter it is, the faster their metabolism, and the quicker they use up energy stores and die.
Infective third stage larvae move up and down the forage using a moisture medium such as advancing and receding dew or rain. For the most part, infective larvae cannot move more than about 12-24 inches from the feces they hatched from or more than 2-3 inches up on forage. So, the lower your goats graze and the closer they graze to feces, the more likely they are to consume infective larvae.