Susceptibility of the small East African breed of goats, from different localities, to trypanosoma congolense, with emphasis on residual fertility

Abstract

In order to assess if there are any differences on the susceptibility and changes in the reproductive function of various varieties of the Small East African goats to tryapanosomiasis, 10 adult normocyclic females aged between 2-3 years from tsetse-free and tsetse-endemic areas of East Africa (MorOgoro, Arusha, Lambwe Valley and Imbo) were experimentally infected with Trypanosoma congolense strain (EATRO-1753). When the study was terminated 24 weeks later, distinct variety susceptibilities were noted. Morogoro goats were the most tolerant, followed by Arusha, Lambwe Valley and Imbo goats in that order. The Imbo goats, from a tsetse-free area, had the highest parasitaemia, most severe anaemia, marked emaciation and 70% mortality. The Morogoro goats from a tsetse-endemic area, had milder infection with lower parasitaemia, smaller weight losses, less severe anaemia and lower (30%) mortality. The Lambwe and Arusha goats (from tsetse-endemic areas) were intermediate in susceptibility to the Morogoro (resistant) and Imbo (susceptible) goats. Although changes in the reproductive function of the infected goats followed the same pattern for the first 3 months, this difference disappeared later, as all the goats became acyclic. Irregular and predominantly shorter (P<O.05) oestrous cycles were observed in all the infected goats before cessation at the second cycle in Imbo goats and fourth cycle in Morogoro and Lambwe goats. A significant decrease (P<O.Ol) in the mean monthly plasma progesterone and oestradiol-17S, peak luteal progesterone, as well as in preovulatory oestradiol-17S values were observed within the second, third and fourth cycles post infection, respectively. Decline in hormonal values was, however, lower in the Morogoro than in other infected goats at least for the first 2 months of infection (P<O.05). Remarkable increase in connective tissue in the ovaries and decrease in the number of primordial and primary follicles were observed in all the infected goats, and those developing became atretic at the tertiary stage. This resulted in the lack of corpora luteaformation. The pituitary gland of the infected goats revealed reduced degranulation of the basophils and little acini secretions, and slight hypertrophy of the acidophils. In the adrenals, the zona fasciculata were markedly hypertrophied in some areas and slightly atrophied in others. Extensive atrophy and degeneration of the thyroid gland was evident in most infected goats and plasma thyroxine (T4) levels were concomitantly reduced in all the infected goats throughout the experimental period. It is concluded that, there are differences in susceptibility to trypanosomiasis in different varieties of Small East African goats. The higher resistance found in the tsetse fly endemic areas may have evolved as a genetic selection following prolonged exposure to various strains of trypanosomes. Though this increased resistance may influence the degree of initial damage of the reproductive and other endocrine glands, prolonged presence of trypanosomes even at a low level in the blood stream may exacerbate any underlying diseases. The evidence obtained in this study indicates that, the hypothalamus may be the initial target of trypanosomiasis, reducing its liberins and thereby impairing the pituitary gland. This results in panhypopituitarism which in turn impairs the function of target glands including cessation of reproductive functions and damage of the thyroid and adrenal glands. It was also evident that, clinical tolerance is correlated with residual fertility; that is the greater the tolerance, the greater the retention of residual fertility and vice versa.