Periparturient Relaxation Of Immunity Research Articles

Resistance of nonlambing exotic and domestic ewes to naturally acquired gastrointestinal nematodes

No breed differences in Haemonchus contortus burdens were found when nonlacfating exotic (St. Croix and Barbados Blackbelly), domestic (Finn-Dorset × Rambouillet) and 1 2 exotic- 1 2 domestic ewes grazed fall pasture, but all ewes had significantly ( p < 0.05) lower worm burdens at necropsy than tracer lambs grazing the same pasture. Florida Native ewes grazing the same pasture showed significantly ( p < 0.05) less packed cell volume (PCV) decrease and significantly ( p < 0.05) lower fecal egg counts than the other ewe breeds but were not available for necropsy. Tracer lambs had a significantly ( p < 0.05) greater PCV decrease and significantly ( p < 0.05) higher fecal egg counts than ewes. The results of this study suggest that breed differences in the periparturient rise in fecal egg counts of exotic and domestic ewes observed in an earlier study may have been caused by breed differences in the periparturient relaxation of immunity rather than breed differences in the ability to acquire immunity to worms.

The epidemiology of helminth disease in farm animals

In this paper the factors which precipitate production loss due to helminth disease in farm animals are classified within accepted epidemiological principles. In endemic areas, outbreaks of helminth disease occur for three basic reasons. The first and most frequent is where there is an increase in the infecting mass which usually occurs seasonally and after at least one parasitic generation. The factors which influence such an increase are those affecting contamination of the environment with eggs or larvae and those controlling the development, dissemination, survival and availability of the free-living larval stages, i.e. translation. Important contamination aspects include biotic potential of the helminth, management practices such as dates of turning out to grazing of housed animals and density of stocking, immune status of the host and hypobiosis. Translation depends primarily on suitable temperature and humidity levels but is also influenced by general factors such as soil structure, vegetation type and drainage. Certain rotational grazing practices can also favour translation. The second is where the susceptibility of existing stock is altered either to the effects of an existing infection or to the acquisition of a new infection. The former may occur following a change in diet, particularly if the nutritional value is reduced or where deficiency of certain minerals or trace elements occur, e.g. phosphorus or cobalt, while the stress of pregnancy and lactation can exacerbate production loss without further infection occurring. The acquisition of new infections is also enhanced during late pregnancy and lactation due to the peri-parturient relaxation in immunity, and by intercurrent infection with other agents, drug therapy with steroids and occasionally anthelmintics. Sometimes hypersenstive reactions to reinfection of immune animals also result in impaired productivity. The third is where susceptible stock are introduced into an infected area. Helminthnaive juveniles are particularly susceptible although the absence of a significant age immunity to many helminths ensures the continuing susceptibility of helminth-naive adults. Genetic factors such as breed, sex, haemoglobin type and hereditary susceptibility of certain breeding lines may exacerbate this category. The longevity of helminth infective stages also contributes by maintaining infection between successive batches of livestock. Finally, helminthiases may occur in a non-endemic area due to the introduction of infected stock. This situation is less common and usually occurs where the recipient environment is suitable for the free-living development of the helminth introduced. Infection may also be introduced by the application of manure products such as slurry and by the introduction or the movement of vectors.