According to Dobson et al., 1990a, Dobson et al., 1990b and Dobson et al., 1990c, the primary manifestation of host immunity against continuous infection for T. colubriformis is a reduction in the number of incoming larvae that became established, this is followed by arrested worm development, reduced worm fecundity and the eventual loss of the established worm population; this response is influenced by rate and duration of infection, and by host

age. Several studies have demonstrated the central role of the acquired immune response in the resistance against gastrointestinal nematode infections in sheep (Peña et al., 2004 and Shakya et al., 2009). This response has been associated with the activity of Th2CD4+ lymphocytes, eosinophilia

and increased number of inflammatory cells in the selleck chemical mucosa such as eosinophils, mast cells and globular leukocytes (Amarante and Amarante, 2003). As reported by Pernthaner et al. (2006), another immunological process was also observed, where high levels of specific immunoglobulins (IgG and IgA) against T. colubriformis larvae and adults were detected. In animals of the present study, an immune RG 7204 response of this type was evident and prevented most of the infective larvae from establishing themselves as adults. Paradoxically, the highest worm burden, 26,830 specimens (27.5% of the inoculum), was recorded in one animal that, at the end of the experiment, presented the highest antibody levels in the blood and mucus and the lowest eosinophils and mast cells counts in the mucosa of the small intestine. These findings emphasize the importance of the inflammatory

cells in the mucosa and also indicate that the efficiency of the immune response depends not only on the presence or quantity of the immunological components, but also on the interaction of these elements together. Starting at the ninth or 10th weeks post infection until the end of the trial, all Santa Ines lambs presented faeces with an altered aspect and consistency, however clinical signs of severe diarrhea were Non-specific serine/threonine protein kinase not observed in these animals. In the three Santa Ines lambs with the lowest worm burdens, in addition to the alterations in their faeces, they also presented clinical signs of apathy, weakness and discomfort, during the ninth or 10th week post infection. These alterations were probably caused by the severe immunopathological changes in the intestinal mucosa that occurred as a consequence of the constant contact with infective larvae, during their attempt to establish in the mucosa. In New Zealand, Morris et al. (2000) observed in Romney sheep, selected for low FEC (resistant) and infected naturally by Trichostrongylus spp. and Ostertagia spp., a productive performance that was lower than that displayed by sheep selected for high FEC.