Production and metabolism studies on bovine trichostrongylosis

Abstract

In this thesis the sequential development of naturally occurring bovine trichostrongylosis in the same animals was studied, for the first time, over a two year period. The species of gastro-intestinal trichostrongyle most frequently present were primarily Ostertagia ostertagi and Cooperia oncophora; a few Trichostrongylus axei were also recorded. In a control group of 16 calves, overwintered larvae of O. ostertagi and C. oncophora were acquired in the spring of their first grazing season. The eggs resulting from these infections gave rise to another generation of larvae on the pastures by August, which was sufficient in number to cause the type I disease, characterised by loss of appetite, diarrhoea and weight loss. One animal was destroyed in extremis just prior to the type I disease, clinical signs of parasitic bronchitis occurred namely, increased respiratory rate and coughing necessitating treatment with diethylcarbamazine and then levamisole. At housing in October, five calves were slaughtered and the post-mortem worm burdens consisted almost entirely of early 4th stage larvae of O. ostertagi and C. oncophora, which were arrested in development. Following housing in October, the animals were clinically normal during the so-called pre-type II phase, until late February and early March when some reduction in appetite was noted which was followed by clinical diarrhoea and weight loss in April and May in the so called type II disease. During the second grazing season, the animals acquired a solid immunity to C. oncophora and a good immunity to O. ostertagi and this was reflected in the low numbers of eggs in the faeces, relatively low numbers of larvae an the pasture and low worm burdens at final slaughter two months after housing in October. A second group of 16 calves, which were grazed on immediately adjacent but separate fields, received a sustained release device containing the anthelmintic morantel tartrate, which was introduced by a special dosing gun into the rumen of each calf prior to grazing in the spring of each year. The boli were designed to release the drug over a 90-day period and their introduction prevented the build-up of larval infection on the pasture and the occurrence of the type I and the type II disease. The advantage in live weight gain alone over the two year period amounted to a mean of 33 kg over the controls. Furthermore, the introduction of the boli did not interfere with the acquisition of immunity in the second grazing season. Several biochemical parameters were monitored of which two, serum pepsinogen and gastrin levels, proved particularly interesting. All values of these parameters became markedly elevated when large numbers of parasites were actively developing to the adult stage i.e. during the type I and type II disease, but not during the pre-type II phase when the worm populations consisted mainly of arrested larval stages, or when the animals become immune. The single and combined linear relationship between pepsinogen, gastrin and numbers of developing and adult Ostertagia parasites was very highly significant. Although further information is necessary to define normal bovine plasma gastrin levels and the affecting factors, the results in this study suggest that the evaluation of plasma gastrin could be a useful adjunct to plasma pepsinogen as a combination diagnostic test for ostertagiasis, particularly when pepsinogen values may be only moderately elevated in Immune cattle under larval challenge. (Abstract shortened by ProQuest.)