Studies on the abomasal parasites of cattle in subtropical and temperate climates

In this thesis the literature pertaining to abomasal parasitism of cattle is reviewed in relation to temperate, subtropical and tropical climates. This review is followed by the experimental sections in which factors affecting the ecology and epidemiology of abomasal parasitism are investigated in Paraguay and Scotland. In the ecology sections the free-living development of a bovine trichostrongyle Haemonchus contortus suited to tropical or subtropical climates and that of a bovine trichostrongyle Ostertagia ostertagi more suited to a temperate climate were studied under theoretically optimal conditions for each parasite. In the sub-tropics namely, Paraguay, development of H. contortus from egg to infective third stage larvae occurred throughout the year but was apparently most successful during the winter and spring. The numbers of larvae on the pasture at these times, although sufficient to cause significant infections in permanent tracer cattle under commercial farming conditions, were very low in terms of the actual yield of larvae from the numbers of eggs deposited in the faeces, never exceeding one per cent. Survival of the larval stages was closely linked to that of the intact faecal pat and mortality occurred rapidly once the faeces disintegrated. By contrast, survival of O. ostertagi in the temperate climate of West Scotland was not so closely linked to survival of the intact faecal pat and infective larval stages were capable of surviving in considerable numbers for several months beyond pat disintegration. Some of these larvae survived on the herbage and others in the soil, possibly associated with terrestrial transport hosts such as earthworms. Yields of O. ostertagi larvae from eggs deposited were also low, but not as markedly so as those of H. contortus and in two months of the year, namely, June and July, the percentage yield exceeded ten per cent. The ecology of another Ostertagia species, O. leptospicularis was studied for the first time. Generally, the results were similar to those of O. ostertagi and yields of infective larvae from eggs were again low except for the months of June and July. A major difference occurred in the ability of the free-living stages of the two species to survive the winter with only relatively low numbers of O. leptospicularis being present in the following spring. Confirmation that O. leptospicularis and O. ostertagi had similar requirements of temperature for their development but differed in those required for survival were obtained under laboratory experimental conditions, When inocula containing ten per cent of O. leptospicularis and 90 per cent of O. ostertagi were administered to parasite-naive calves and passaged five more times through other calves the more fecund O. leptospicularis become the dominant species present in the calves, reaching a level in excess of 80 per cent in the sixth calf. When the larval progeny of the Ostertagia worm burdens in that calf were stored at 4?C for a period of several months to simulate winter conditions, the O. leptospicularis larvae succumbed completely. It was proposed that the lack of fitness for survival at cold temperatures of O. leptospicularis explained why this very pathogenic species was only involved sporadically in outbreaks of bovine ostertagiasis. Where such outbreaks occurred the source of infection is likely to result from deer, the natural host of this parasite, having grazed and contaminated the fields prior to susceptible cattle. The abomasal worm burdens of the permanent tracer calves examined in Paraguay were a mixture of species, namely H. contortus. H. similis and T. axei. There was some evidence that the proportions of these present altered seasonally and gave rise to clinical outbreaks of parasitic gastritis in the late winter and spring. The T. axei burdens appeared to make a major contribution to these outbreaks and although several thousand Haemonchus spp. were also present the cattle were only moderately anaemic. While the seasonal nature of the worm burdens may reflect the differing ecological requirements of the free-living stages there was some indication that the Interactions of the one species on the other, possibly involving cross immunity, may also have been involved. In view of the interesting results obtained with the two Ostertagia species, experimental studies are recommended to evaluate the interactions of the two Haemonchus spp. and T. axei. Since recognition of larval stages at species level is a pre-requisite to such a programme a computer analysis comparing data on larval morphology and measurements from O. ostertagi and O. leptospicularis as a model was undertaken. Statistically different results were obtained which were unfortunately difficult to recognise visually