Development and survival of free-living stages of equine strongyles under laboratory conditions

In a series of laboratory studies the optimum conditions for the development and survival of the free-living stages of strongyle parasites occurring in horses in tropical north Queensland were determined. No differences in behaviour were noted between the strongyle species. Development to the infective stage occurred only between 10 and 35°C. The rate was affected by temperature, taking 15–24 days and 3 days, respectively, at the lowest and highest temperatures for the developing stages to reach the infective third stage. Yields of infective larvae were very low outside the range 20–33°C, and were highest at 28°C. Survival of infective larvae was good between 20 and 33°C, and large numbers were recovered after 3 months in faeces incubated at 20–28°C. At 33 and 37°C larval survival was affected by the moisture content of the faeces, with infective larvae surviving better in dry than in moist faeces; even a residual moisture level of 40% significantly reduced the number of larvae recovered from faeces incubated at 37°C for 1 month. Moisture also affected larval development, especially at the higher temperatures of 25–39°C. When faecal moisture content fell to ⩽ 20% by 3 days, larvae which had not yet reached the infective stage were still pre-infective at 7 days, while all larvae in faeces with adequate moisture had reached the infective third stage. It was not possible to determine the critical faecal moisture level below which larval development ceased, however, 28°C (range 25–33°C) was found to be the optimum temperature. Larval development was very rapid and yields of infective larvae highest at this temperature.