STUDIES ON THE SEASONAL INCIDENCE OF GASTRO-INTESTINAL NEMATODES IN SHEEP
Yang Chunfu;Zhou Yuanchang (Department of Veterinary Medicine)
The seasonal incidence of the gastro-intestinal nematodes of sheep was studied in Zhaodong county, Heilongjiang province. Results showed that the egg outputs or worm burdens of gastro-intestinal nematodes in adults rise highly from April to June in a Spring Rise, while another rise occurs in August and September. However, in summer and winter, egg counts and worm burdens are very low. Infective larvae were first found in lamb faeces incubated in early July and the peak of the egg outputs appeared in late August From then on, only a few larvae were found in the faeces. The percentage of the inhibited larvae of Haemoncus contortus decreased gradually from July to December gradually. In late December, up to 99.73% of the H. contortus population consisted of early fourth stage larvae (EL_4). The percentage of the inhibited larvae of Trichostrongylus spp. was much lower than that of H. contortus and its highest inhibited rate did not exceed 20%, as found in early October. Larva examination showed that infective larvae on herbage reached their peak in late September, and even in March and April of the next year a few infective larvae of Trichostrongylus spp. were still found. It was also found that eggs of H. contortus, Bunostomun sp., Oesophagostomun spp. and Trichostrongylus spp, can not overwinter on pasture whereas the infective larvae of Trichostrongylus spp. can. The results of these trials were discussed in detail and it was suggested that the optimal time of anthelmintic treatment be late December or early January.