Development and survival of infective larvae of nematode parasites of sheep on pasture in a cool tropical environment
Introduction
In many parts of the tropics the number of free-living stages of parasitic helminth parasites found on the pasture follows seasonal fluctuations. Investigations carried out on nematode larvae ecology in parts of sub-Saharan Africa (Dinnik and Dinnik, 1961; Chiejina and Fakae, 1984; Onyali et al., 1990; Chartier, 1991; Ndamukong and Ngone, 1996) have shown that the rate of development and the longevity of eggs and larvae vary with temperature, rainfall and humidity in different geo-ecological regions. However, none of the studies include the seasonal availability and the abundance of the free-living stages of sheep nematode parasites on the pasture in the cool highland tropics. Such epidemiological information is essential in the formulation of successful helminth control programs.
The present study was conducted to determine the annual patterns of development and survival of H. contortus, T. colubriformis and L. elongata eggs to infective larvae (L3) on the pasture in the cool tropical environment of the Ethiopian highlands.
Section snippets
Site
The study was carried out at the ILRI Debre Berhan Research Station located in the central highlands of Ethiopia about 120 km north-east of Addis Ababa at an altitude of 2780 m above sea level at longitude 39°E and latitude 10°N. The climate is characterized by a long rainy season (June–September accounting for 75% of the total rainfall), a short rainy season (February/March to April/May) and a dry season (October–January).
Experimental design
The experiment was conducted on a piece of land at the station which had
Results
The average annual rainfall during the study period was 920 mm. The average monthly minimum air temperature at 0.5 m ranges from 2°C in November to 8°C in August. The corresponding monthly maximum air temperature ranges from 18°C in September to 23°C in June. Average mean air temperatures and rainfall for the period of the study and for a 15-year period (1977–1992) at Debre Berhan Research Station are shown in Fig. 1. The natural pasture is dominated by Andropogon grasses (Andropogon longipes)
Discussion
Development of eggs into infective stages took place from June to October and survival time varied between 1 to 7 weeks. A similar study carried out in a mid to high altitude area of Cameroon at an altitude of 1500–2000 m above sea level (Ndamukong and Ngone, 1996) showed that development of H. contortus and Trichostrongylus sp. eggs to infective larvae during the rainy season (April to October) took place within a week with a survival period on pasture of 7 to 13 weeks.
In the present study the
Acknowledgements
The author wishes to thank the staff at the ILRI Parasitology Laboratory in Addis Ababa and at the ILRI Debre Berhan Research Station for their valuable technical assistance and thanks Dr. R.L. Baker, Professor A. Lahlou-Kassi and Dr. J.E.O Rege for encouragement and continuous support throughout the planning and execution of this experiment and Ian Barger for advice on the procedures for counting infective larvae from the pasture. Comments on an early draft of this paper by Professor T.M.
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