Effect of Desiccation on Survival of the Free-Living Stages of Trichostrongylus colubriformis

Abstract

Desiccation of the 3rd stage larvae of T. colubriformis was beneficial to their survival at temperatures below freezing and at 35 to 50 C, but was of indifferent effect at intermediate temperatures of 20 and 25 C. After 128 days, half the clean larvae which had been desiccated and stored at -95 C were still alive, as compared with none of those that had not been desiccated; however only 7% of the larvae which had been desiccated in fecal pellets were still alive. After 128 days at -20 C, 5% of the larvae that had been desiccated in fecal pellets were alive, and at -10 C, 12%. At the optimum survival temperature (4 C), the best survival for any treatment occurred in the control larvae kept in water; 92% were still alive after 128 days, as compared with 60% in moist fecal pellets, 65% of the larvae desiccated within fecal pellets, and 17% of the clean desiccated larvae. At 20 C, survival was not as good as at 4 C, and at 25 C it was still worse; desiccation was generally neither beneficial nor harmful at these temperatures. At 35 C, 10% of the larvae desiccated in pellets were still alive after 128 days, and less than 1% of the undesiccated ones in pellets. At 45 and 50 C, larvae desiccated in fecal pellets survived as long as 64 days as compared with 12 days in moist fecal pellets. Larvae which had been stored in water for approximately 1 month or longer at 4 C were not as resistant to desiccation as younger larvae. Eggs in the advanced stage of embryona-tion were approximately as resistant to desiccation as infective larvae. Unembryonated eggs were only slightly resistant to desiccation. First and 2nd stage larvae removed from the fecal pellets were highly susceptible to drying, but small numbers survived at least 8 days in dried fecal pellets at 30 C and 65 to 75% RH. It appears that there is generally sufficient moisture within fecal pellets deposited on pasture for the eggs to develop to the resistant embryonated egg stage before the moisture evaporates. Additional external moisture is then probably required before development to the next resistant (3rd larval) stage can occur. After the infective larvae migrate from the pellets onto the herbage, they probably desiccate again and remain so until rehydrated with rain or heavy dew, or until eaten by grazing hosts.