Epidemiology of parasitic bronchitis in sheep
Al-Samarrae, Saadi Ahmed Ganawi
The epidemiology of parasitic bronchitis in sheep has been studied on two lowland farms in .Midlothian, Dryden and Easter Bush, during 1972-1975 and also in controlled experimental infections in housed sheep. The main techniques used to follow the infections were faecal larval counts, faecal egg counts, pasture larval counts, eosinophil counts and other haematological estimations, live weight gains, and wori: counts at autopsy in permanently pastured and tracer lambs.The main techniques used to follow the infections were faecal larval counts, faecal egg counts, pasture larval counts, eosinophil counts and other haematological estimations, live weight gains, and worm counts at autopsy in permanently pastured and tracer lambs.In the field studies Dictyrocaulus filaria, iuellerius capillaris and Pratostronvylus rufescens larvae were identified, but the latter were seen in small numbers only in the faeces from lambs at Easter Bush, while L. capillaris were seen In the faeces from ewes at Dryden . D. filaria larvae were found mainly in lambs' faeces, with only very low numbers being occasionally found in ewes' faeces. The prevalence of D. filaria in lambs increased slowly during the summer to reach a pe..1, in autumn, thereafter the lambs appeared to become resistant to the infection. The peripheral eosinophil counts tended to increase gradually in these animals to reach a high peak at about the same time as the numbers of larvae in the faeces dropped to a very low level.The studies on pasture samples showed that the number of D. filaria larvae on the herbage remained low throughout the year, although there was a small rise in early autumn. The observations on the tracer lambs also suggested that the highest levels of infection with D. filaria were acquired at this time.Soar lambs appeared to be more susceptible to D. filaria infection than Blackface lambs running on the same pasture , an a male Blackface lambs appeared to be more susceptible to this parasite than female lambs. However, no difference was demonstrated between Blackface lambs, Merino lambs and Southdown lambs running on the same pasture. April-born Suffolk lambs tended to show higher larval counts than Suffolk-cross lambs born about the same time, whereas January-born Suffolk lambs showed rather lower counts during their first year of life. In all the breeds some of the lambs, or yearlings in the case of the January-born Suffolks, showed a further small increase in the numbers of D. filaria larvae in their faeces during the early spring.Several controlled experiments with D. filaria infections were carried out in housed experimental lambs and goats. Single infections confirmed earlier findings that even young lambs and kids became resistant to re-infection and eliminated their burdens within 3-0 weeks of patency.An attempt to simulate the natural pattern of infection with D. filaria in lambs by the administration of a slowly escalating :infection, ave rise to a prolonged infection similar to that seen under field conditions. When these animals were challenged with 40 third-stage D. filaria larvae por Kg bodyweight 28 weeks after initial infection, only the control lambs developed a patent infection with a marked increase in their peripheral eosinophil counts. The eosinophilia was not soon in the previously infected and challenged group and these lambs continued to show only the residual faecal larval counts.