Approaches for Epidemiological Insights into the Iodine Nutrition of Ruminant Livestock under Natural Conditions in Zimbabwe.

Abstract

This thesis is based on work founded on an hypothesis that Zimbabwe is endemic for hypothyroidism as previously indicated in human studies, therefore a case for hypothyroidism for livestock was presumed.

The studies conducted for this thesis sought to establish population diagnostic systems for thyroid status, hence iodine status, that could be used in cattle and goats while establishing some reference values and epidemiological factors that could be used in herd health decision-making.

The approaches used for the epidemiological studies included a population cross-sectional study using cattle and goat sera, testing of pasture materials for iodide levels, a station supplementation experimental study and a farmer participatory field trial testing an iodised feed supplement. The diagnostic methods tested included radiommunoassay (RIA) for thyroid hormones, the iodide ion selective electrode (ISE) method for feed materials and the enzyme linked immunosorbent assay (ELISA) for serum thyroid stimulating hormone (TSH).

The RIA was able to detect serum free triiodothyronine and free tetraiodothyronine in cattle and goats. The results are presented confirming the effect of factors such as animal age, body weight, sex, lactation status and natural agro-ecological region in cattle and goats. The range for cattle was 0.089 to 1.900ng/dl with a median of 0.46ng/dl; while for goats the range and median were 0.430 to 1.49 ng/dl and 0.933ng/dl, respectively. An attempt to validate the RIA findings in cattle against levels of thyrotropin did not succeed because the ELISA method attempted suffered from cross-reaction with leutenising hormone (LH) and follicle stimulating hormone (FSH).

Despite that observations of goitre in goat kids were made in an area previously recorded as highly endemic for human hypothyroidism, a field trial involving iodine supplementation to dairy cattle in the same area could not substantiate hypothyroidism in lactating animals. On the contrary, the iodine containing supplement appeared to depress milk yield. Similarly, a controlled station experiment with weaner calves could not indicate a performance advantage resulting from iodine supplementation.

The studies have therefore established the RIA and the ISE as methods which can be used to study the iodine status of cattle and goats and some data to assist further investigations into supplementation regimes aimed at improving productivity in cattle and other livestock.