Dynamics of epidemiological intelligence for exploitation in effective worm management in sheep - A Rajasthan experience

Abstract

Being of ubiquitous nature and omnipresence of the gastrointestinal (GI) parasites, their management presents a difficult challenge following wide-spread emergence of anthelmintic resistance. Anthelmintics have a definite role to play in worm management but their frequency of use should be minimised to the extent possible and the available resources of anthelmintics could be used judiciously by exploiting the knowledge of parasite epidemiology. The present communication discusses different aspects of worm biology and their interaction with environment, host and managemental practices. The bioclimatographs suggested existence of suitable conditions for translation of exogenous stages of Haemonchus contortus from late May to late September in semi-arid region and from mid June to late August / September in arid region. Based on real-time observations, different strategies were developed and tested for efficient management of GI nematodes in sheep flocks of Rajasthan. The agro-climatic conditions and lambing pattern do not favour the development of strongyle larvae during December-June, resulting in absence of typical peri-parturient rise in faecal egg counts (FECs). However, the epidemiology of strongyle worms showed the possibility of hypobiosis. An initial rising trend in FECs in June could be probably due to resumption of development of hypobiotic worms within the host, giving peak of infection in July and providing the source of pasture contamination during monsoon. The decline in FECs in the following months may be due to spontaneous occurrence of "self-cure" phenomenon. The GI parasite populations in small ruminants are highly aggregated and over-dispersed, with around 80% of the worms found in only 20-30% of the host. Thus, requirement for treatment is a reflection of the genetic basis of host variation in either innate or acquired resistance to parasites or resilience. The over-dispersion phenomenon was used in the form of targeted selective treatment and for breeding for resistance / resilience to parasites ultimately with the aim to increase the size of refugia, maintain anthelmintic efficacy and cost-effective worm management.