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

Abstract

Desiccation of the 3rd stage larvae of Trichostrongylus 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 embryonation 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. Parasites which have free-living stages as part of their normal life cycles are frequently subjected to environmental conditions far from optimum for their survival. Although we do not understand how all parasites withstand such deleterious conditions as freezing or hot dry weather, we do have considerable information on how certain groups survive, at least in small numbers. Some parasites, for example, form stages highly resistant to environmental extremes, such as cysts, spores, or metacercariae. Certain parasitic arthropods overwinter in the egg stage, even though the adults are all killed by freezing weather. Some helminth parasites may be acquired by transport hosts, such as earthworms or arthropods, and yet remain infective for their respective definitive Received for publication 12 August 1967. * This investigation was supported by research grants AI-06197 from the NIAID, NIH, and CC-00037 from the Communicable Disease Center, U. S. Public Health Service, Department of Health, Education, and Welfare. t Present address: Department of Zoology and Entomology, Brigham Young University, Provo, Utah 84601. hosts if the transport hosts are subsequently ingested. Existence within the transport host, while not obligatory, nevertheless provides a favorable temporary environment for the parasite and also prevents death from desiccation as long as the transport host continues to live. The life cycles of most trichostrongylid nematodes which parasitize sheep and cattle are similar in that the eggs of the adult worms are passed to the outside with the feces. Under suitable conditions of temperature and moisture these eggs develop to 1st and 2nd stage larvae and finally to 3rd stage (infective) larvae. All stages occur within the host's feces after deposition on the ground, or on the vegetation near the ground surface. Many investigators (Monnig, 1930; Stewart and Douglas, 1938; Crofton, 1948; Gordon, 1948; Silverman and Campbell, 1959; Prasad, 1959; Rose, 1963; Kates, 1965; Andersen, Wang, and Levine, 1966; Williams and Mayhew, 1967) have found that embryonated eggs and infective larvae are the stages most resistant to the deleterious temperature extremes that would probably occur on pasture at different times of the year. It is not clear why these stages