Development and Survival of Trichostrongylus colubriformis on Pasture

A study was made of the development and survival of Trichostrongylus colubriformis on a grass pasture at Urbana, Illinois, during 1965 and 1966. A total of 78 experiments was done; in each, sheep feces containing unembryonated eggs were placed on the pasture and the vegetation, pellets, and soil were examined periodically for infective larvae. Few larvae were recovered from the soil. Fecal pellets persisted on pasture for 0 to 98 (mean 24.7) days. In general (but not invariably) the lower the temperature and the fewer the number of heavy rains, the longer the pellets persisted. If egg embryonation took place at all, it was completed in less than 7 days. There was essentially no embryonation when the mean weekly maximum temperature at the ground surface under 7 to 10 cm vegetation was 6.9 ± 5.2 (SD) C, and the unembryonated eggs died. Optimum embryonation (completed in less than 1 day) occurred at 36.8 ± 2.8 C. At intermediate temperatures, the higher the temperature, the faster the development. There was no great difference in survival between 1.0 and 0.1% of the eggs. Some embryonated but unhatched eggs were present on pasture practically all the time the pellets persisted; they would be a source of larvae when conditions were appropriate for them to hatch. Larval recovery (i.e., development) was essentially nil below 10 C, poor at 10.0 to 14.9 C, fair at 15.0 to 29.9 C, and moderate at 30.0 to 34.9 and 4.0 to 44.9 C; it was optimum at 35.0 to 39.9 C. If larvae developed at all, they tended to survive a month or more regardless of the temperature, at least up to the highest weekly mean maximum temperature recorded, 41.6 C. Relative transmission potentials (the number of days that infective larvae were recovered from the vegetation times the percentage of eggs that were recovered as infective larvae) were determined for each experiment and then plotted against temperature and soil moisture. Soil moisture at Urbana never went below 12% and was not an important factor. Transmission potential was essentially nil in winter, and was best from late June to mid-August 1965 and from mid-May to mid-October 1966. Good pasture transmission conditions should occur at Urbana when the total monthly precipitation is more than 25 mm and the mean monthly maximum temperature at the soil surface beneath 7 to 10 cm vegetation is above about 16 C. Seasonal trends for larvae and eggs are opposite; cold weather preserves infective larvae whereas it prevents eggs from developing