Some aspects of the population dynamics of Cooperia oncophora : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University, Palmerston North, New Zealand

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Massey University
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To develop better control strategies for Cooperia oncophora a detailed understanding of the population dynamics is desirable. To achieve this, aspects of the life cycle have been studied in a series of experiments, including the free-living and the parasitic phases. To investigate the development from egg to third stage larvae, faeces containing C. oncophora eggs gathered from different donor animals were incubated at different constant and variable temperatures as well as under natural conditions in the field. To examine the survival of C. oncophora third stage larvae they were kept under similar conditions as for the development experiments. Based on the results at constant temperatures, parameters were calculated for a model to simulate the effect of temperature during the free-living phase of this nematode. A further experiment was conducted over an 11 month period to investigate the effect of host age and previous exposure on the establishment rate of third stage larvae in 3 groups of young calves. Two groups received a high or low dose of trickle-infection and the third remained as an uninfected control group. At lower temperatures the development rate and success from egg-third stage larvae were both low but increased with higher temperatures. At 8°C 50% development was reached in 56 days with a success rate of 5.5% compared to 5 days and 26.4% respectively at 32°C. The highest development success rate of 37.4% was observed at 28°C. For larval survival, the median survival was 512.2 days at 8°C and decreased to 6.4 days at 37°C. Both development and survival were significantly (p<0.05) influenced by the host animal from which the faeces were sourced. Utilising these parameters in a model provides a useful tool to further understand the effect of temperature on the free-living stages. The establishment rate of C. oncophora in the trickle-infected groups declined rapidly compared to the control group but was not significantly different (p>0.05) to the control group if the existing worm burden was removed before challenge. A decline in establishment rate over the 11 month period of the experiment in the control animals was due to the age of the larvae.
Cooperia oncophora, Cooperia life cycle, Cattle parasites, Nematodes