Control Of Gastrointestinal Nematode Infections Research Articles

Gastrointestinal nematode infections of first-season grazing calves in Belgium: general patterns and the effect of chemoprophylaxis

Comparative analyses of the patterns of gastrointestinal nematode infections of first-grazing season cattle in Belgium are presented. The analysis involves 17 studies covering a 10 year period on 13 different farms in Flanders, Belgium. In all studies the calves were divided into an untreated control group, and one or two groups treated with chemoprophylactic systems. Two general infection levels emerged—‘sub-clinical’ (14 studies) and ‘clinical’ (three studies). The ‘sub-clinical’ infections were characterised by no clinical signs of parasitic gastroenteritis in the untreated control groups. Mean faecal egg counts remained low (less than 200), maximum pepsinogen levels only reached about 3500 mU tyrosine, and very small reductions in overall daily weight gain were observed compared with calves given chemoprophylaxis (less than 40 g day −1). Based on these results, on these ‘sub-clinical’ farms, chemoprophylaxis may not have been needed. In contrast, multiple salvage treatments of the control calf groups were required in the ‘clinical’ infections. Even with these salvage treatments mean faecal egg counts were high (more than 300), maximum pepsinogen levels were over 5500 mU tyrosine and there was a very large reduction in overall daily weight gain (more than 300 g day −1). However, it was not possible to predict either at turnout, or during the first month afterwards whether an infection on a particular farm would develop into a ‘clinical’ infestation. With the present data this prediction was possible from 8 weeks (Day 56) onwards, based on faecal egg counts and pasture larval contamination. It was also possible to predict using serum pepsinogen levels on Day 84. Therefore, one possible strategy for the effective control of gastrointestinal nematode infections of calves in temperate regions would be to evaluate faecal egg counts 2 months after turnout, and then only start treatment (i.e. metaphylaxis) if required.

Utilisation of the persistent anthelmintic activity of ivermectin in a preventive drenching programme for dairy calves

A study was undertaken to assess whether the persistent anthelmintic effect of ivermectin in cattle would allow an extension of the drenching interval in the currently recommended preventive drenching programme for the control of gastrointestinal nematode infection in dairy calves. A comparison was made of the parasitological and production responses of treatment groups of calves, grazing replicated plots, receiving either six drenches of oxfendazole at four-weekly intervals or four subcutaneous treatments with ivermectin at six-weekly intervals. Compared with the levels of infective larvae on pasture grazed by untreated control calves, mean larval numbers on pasture grazed by ivermectin and oxfendazole treated calves were 94.3% and 98.3% lower, respectively, during the period of maximum larval availability (March-May). Mean liveweight gains (December-August) of the treated groups (101.4kg and 110.2kg respectively) were not significantly different, but both were highly significantly different from that of the untreated controls (57.4kg). Mean plasma pepsinogen levels for the ivermectin, oxfendazole and control groups over the period of maximum values (June-August) were 1.92 i.u., 1.72 i.u. and 5.53 i.u., respectively. The difference between the treated groups was not statistically significant but both were highly significant different from the control group. The present results indicate that four treatments with ivermectin (subcutaneously) at six-weekly intervals achieved a similar level of prophylactic control to that effected by six treatments with oxfendazole at four-weekly intervals.

A field study of nematode parasite populations in the lactating ewe.

A typical postparturient rise of parasite egg output occurred in a group of lactating ewes. At parturition these animals carried few inhibited fourth-stage larvae and the rise was associated with a net increase in parasite populations, which did not occur in non-lactating ewes. Lactating ewes whose established parasite populations were effectively removed just prior to parturition showed a similar rise derived entirely from infection acquired during lactation, which was of much greater magnitude than that seen in similarly treated non-lactating animals. Circumstantial evidence of increased fecundity of T. colubriformis females in lactating ewes was also obtained.It was concluded that the postparturient rise of egg output may derive from an increase in adult parasite populations by an increased rate of establishment of newly ingested infective larvae which develop to the adult stage without inhibition, as well as by the resumption of development of previously inhibited fourth-stage larvae. In addition, both newly acquired and established adult female parasites may show increased fecundity.Ewes whose lambs were removed at birth or after a period of lactation showed a rapid fall in faecal egg counts associated with rejection of part of their existing worm burden and a pronounced increase in resistance to new infection.All of the differences observed between lactating and non-lactating ewes in the behaviour of their parasite populations were found to be qualitatively the same as those that have been frequently reported in susceptible and resistant non-reproductive sheep. The hypothesis is suggested, therefore, that the postparturient rise derives from a pronounced but temporary relaxation of immunological control of gastro-intestinal nematode infection associated with the endocrine status of the lactating ewe.