The influence of temperature on the development and survival of the pre-infective free-living stages of nematode parasites of sheep

Abstract

AIM: To develop a model for the development dynamics of the pre-infective stages of nematode parasites on pasture and to use it to investigate the importance of temperature fluctuations on the proportion of eggs developing to infective larvae. METHODS: Data from published literature was used to derive relationships for development and survival rates in response to temperature for Teladorsagia (=Ostertagia) circumcincta, Trichostrongylus colubriformis and Haemonchus contortus. A model describing development from the egg to infective third larval stage (L3) for each species under the influence of temperature was constructed. It was assumed that moisture is never limiting. The model was initially used to estimate the proportion of eggs developing to L3 under 12-hourly temperature oscillations of different magnitude around constant temperatures of 10, 20 and 30°C. Subsequently the model was run on hourly temperature data collected from the field, and on daily and monthly means calculated from the hourly temperatures. As each set of comparisons had the same mean temperature, differences in proportional development were the result of the cumulative effects of temperature fluctuations on the development and survival of the developing stages. RESULTS: Oscillating temperatures always resulted in a lower estimated proportion of eggs developing to L3 than the constant mean, and the larger the oscillations the greater the reduction in development. Similarly, development estimated on hourly temperatures measured in the field was always lower than those estimated on daily mean temperatures calculated from the hourly values, sometimes by a large amount. Monthly mean temperatures resulted in estimates for development which were sometimes higher and sometimes lower than those based on hourly or daily means. CONCLUSIONS: Consistent accurate estimation of the proportion of eggs developing to L3 in the field will require the integration of temperatures at relatively short time intervals. This will be necessary to capture the effect of temperatures fluctuating, even for short periods, to temperature values which are less than optimal for development. The use of daily mean temperatures, or means taken over longer periods, is likely to yield inaccurate estimates of development at least some of the time.