A total of 864 sheep sold for slaughter throughout South Australia were examined for infections with Sarcocystis spp using macroscopic, microscopic and immunoserological methods of detection. Macroscopic cysts were found in 6.7% of the sheep ranging in intensity from 1 to 64 cysts per carcase. Morphometric studies detected 2 types of macroscopic cysts which differed in their size and cyst wall morphology. Large ovoid cysts with thick primary cyst walls were identified as S. gigantea (syn. S. ovifelis) whereas small slender cysts with thin cyst walls were identified as S. medusiformis. The prevalence of each species was 4.5% and 3.1% respectively. In comparison, microscopic cysts were detected in 93.2% of the sheep and stereological calculations of the intensities of infection ranged from 20 to 32,700 cysts per unit volume of muscle. Two types of microscopic cysts were also identified by differences in their cyst wall morphology. Cysts with smooth thin walls were detected in 88.1% of the sheep whereas cysts with radially-striated thick walls were found in 74.7%. Although these 2 types appeared to conform to the original descriptions of S. tenella and S. ovicanis respectively, both were classified as S. tenella (syn. S. ovicanis) pending further taxonomic studies. Specific antibody was detected in 93.7% of the sheep by the complement fixation test and in 96.9% by the indirect fluorescent-antibody test (IFAT). A positive correlation was also found between the prevalence of IFAT antibody and the prevalence of infections by microscopic cysts.

Three years after the start of an experiment to assess the merits of thiabendazole (TBZ) treatment of trichostrongylid parasites in weaner sheep, field isolates of Ostertagia spp and Trichostrongylus spp were made from weaner sheep treated under one of three treatment schemes. Treatment frequencies were "nil", "planned" (5 or 6/year) and "regular" (every 3 weeks). In addition an isolate was taken from a group of "tracer" sheep drenched with TBZ every 10 days. Resistance to TBZ was assessed using an in vitro egg hatch assay, pre- and post-treatment faecal egg counts and a controlled anthelmintic efficiency test. Pre- and post-treatment egg counts revealed the presence of TBZ-resistance in field isolates of mixed species. Egg hatch assays indicated a level of resistance for Ostertagia spp which was proportional to the frequency of TBZ treatment. The "planned", "regular" and "tracer" strains of Ostertagia spp had resistance ratios for eggs of 4, 13 and 15 respectively when compared to the "nil" strain. In the anthelmintic efficiency assay treatment with 44 mg kg-1 and 88 mg kg-1 of TBZ removed 82 and 96% respectively of the total Ostertagia burden (adults and larvae) from the "nil" strain and 30 and 75% respectively from the "planned" strain. The same dose rates against the "regular" and "tracer" strains and additional rates of 132 or 176 mg kg-1 against the "tracer" strain failed to reduce the Ostertagia burden significantly. Intestinal Trichostrongylus spp from all isolates were fully susceptible to TBZ at 44 mg kg-1. Levamisole at 7.0 mg kg-1 was highly effective (99% reduction) against the "tracer" strain of Ostertagia.

The roles of six carnivores as potential sources to sheep of the sarcosporidial parasites causing cysts in meat were compared in a series of experiments carried out between 1973 and 1980. The research was concomitant with other studies that confirmed the prey-predator-prey-predator cycle of transmission. Infected carnivores act as vectors, excreting in their faeces coccidial sporocysts infective to the meat animal. For 60 experimental infections, sheep meat containing sarcocysts or sarcocysts removed from sheep meat were fed to experimental carnivores. Faecal samples were examined for sporocysts over 60 days post infection. Neither macroscopic (visible) sarcocysts nor microscopic sarcocysts from sheep carcasses were transmissible by humans or by ferrets (domestic polecats). Microscopic sarcocysts were readily infective to dogs, both domestic and dingo, as well as to foxes, but not to cats. Large numbers of sporocysts that could contaminate the environment were excreted. Both fat and thin visible sarcocysts were transmitted exclusively to cats. Although cats responded with relatively low levels of sporocysts, these were considered adequate to provide sufficient pasture contamination for the life cycle to be perpetuated. It is concluded that, while domestic dogs may be the greatest source of infection for sheep with microscopic sarcocysts, foxes as vectors also pose a threat to sheep production. Similarly, due to their widespread presence, feral cats play a role as well as domestic or semi-domestic cats in the spread of sarcocysts causing visible carcass lesions.

The species of trichostrongyloid nematodes present in 376 sheep from different agricultural regions of the state of South Australia were determined. Parasites encountered in the abomasum were Teladorsagia circumcincta, T. trifurcata, T. davtiani, Camelostrongylus mentulatus, Haemonchus contortus and Trichostrongylus axei. T. circumcincta was the species most commonly encountered, but C. mentulatus was widespread in arid areas. H. contortus was present in a small number of sheep only, mainly in southern areas. In the small intestine, Trichostrongylus vitrinus predominated in the wetter, southern districts, and T. rugatus in dry, northern areas. T. colubriformis was common, but was rarely dominant. T. probolurus was found in one sheep. Nematodirus filicollis was restricted to wetter, southern areas, while N. spathiger and N. abnormalis were widespread and predominated in dry areas. N. helvetianus was found once. Cooperia species were uncommon; species encountered were C. oncophora, C. surnabada and C. pectinata.

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