Cerebellar abiotrophy and segmental axonopathy: two syndromes of progressive ataxia of Merino sheep.

Findings of a study of 39 sheep with progressive ataxia from 14 farms in the Yass district of New South Wales are described. Microscopic lesions in 25 sheep, 3.5 to 6 years of age, diagnosed as having clinical cerebellar disease, consisted of an apparent primary loss of cerebellar Purkinje neurons, and glial cell accumulation. It is suggested that this previously unreported disorder may be an hereditary cerebellar abiotrophy of Merino sheep. A further 14 sheep, 1 to 4 years of age, had distinguishable clinical signs referable to a spinal cord lesion with widespread segmental axonal ballooning, or "spheroids", in the white matter of the brain and spinal cord. It is suggested that these sheep have a unique form of neuroaxonal dystrophy, described here as segmental axonopathy, and that this is likely to be the same condition described previously as Murrurindi disease (Hartley and Loomis 1981).

Inhibited development of Ostertagia ostertagi in Australia related to survival over summer in the host or on pasture.

During autumn and winter, separate pasture plots were seeded with Ostertagia ostertagi eggs produced by worms which had persisted in the host over summer as inhibited larvae, or which had developed directly from larvae acquired from pasture in late summer. Successive groups of parasite-free calves grazed the plots in spring for 14 days at 4-week intervals and were killed for worm counts 14 days after removal from pasture. The proportion of inhibited early 4th-stage larvae was substantially greater in calves which grazed the plot carrying the progeny of previously inhibited worms. This finding is more easily reconciled with differential survival on pasture over spring and summer of inhibiting and non-inhibiting morphs, than with the concept of a physiological switch from inhibiting to non-inhibiting behaviour in individual larvae.

The activity of triclabendazole against immature and adult Fasciola hepatica infections in sheep.

The efficiency of a new benzimidazole anthelmintic, triclabendazole, was tested against cumulative infections with Fasciola hepatica aged 1 to 12 weeks in sheep and compared with that of rafoxanide. At 10 mg/kg, triclabendazole was 99% effective in eliminating both immature and adult flukes. At a lower dose rate of 5 mg/kg, triclabendazole was highly effective against adults and significantly reduced the number of early immature flukes with an 87% overall reduction of fluke burden. Rafoxanide at 7.5 mg/kg showed high efficiency against adult fluke, but its effect on immatures was not significant, and overall efficiency was 64%.

Inhibited development of Ostertagia ostertagi in relation to production systems for cattle.

On a coastal farm in New South Wales where beef and dairy cattle production was carried on side-by-side, separate pasture plots were contaminated with eggs of Ostertagia ostertagi by calves from each production system in autumn, winter or spring. Successive groups of parasite-free tracer calves grazed on the plots for 14 days at 4-week intervals and were then killed for worm counts 14 days after removal from pasture. On all plots, the proportion of inhibited early 4th-stage larvae in tracer calves reached a maximum in spring, and was consistently and very significantly higher in calves which grazed plots contaminated with O. ostertagi of beef cattle origin. Factors which may be responsible for this difference between beef and dairy cattle populations of O. ostertagi are discussed.

Inhibition of development of Ostertagia ostertagi--effect of temperature on the infective larval stage.

Populations of Ostertagia ostertagi established from fresh larvae and from larvae stored at 4 degrees C for up to 12 weeks contained about 10% inhibited early 4th-stage larvae. This value rose to 21% after 16 weeks, due to an increase in numbers of inhibited larvae while adult numbers remained unchanged. Storage at 15 degrees C had no effect. In the case of larvae stored at 4 degrees C for 8 weeks, increasing the larval dose to 90000 had no significant effect on the proportion of the dose which was inhibited. The response to low temperature storage was much weaker than in previously reported studies on British populations, which accords with our earlier conclusion that low temperature cannot be the effective stimulus for inhibition in Australia.

Effects on inhibition of development of the transfer of Ostertagia ostertagi between geographical regions of Australia.

Pasture plots in a warm coastal region were separately contaminated in the autumn, winter and spring with eggs of a coastal strain of Ostertagia ostertagi derived from dairy calves. At the same times, a parallel set of plots was contaminated by a strain transferred from within a beef cattle management system in a cooler tablelands region. A reciprocal transfer of the coastal strain was also made to the tablelands, where pasture plots were contaminated with eggs of both strains of O. ostertagi in winter. Successive groups of parasite-free calves wee grazed on the plots for 10-14 days at 4-week intervals and then killed for worm counts 14 days after their removal from pasture. In both environments, irrespective of the season of pasture contamination, inhibition of development reached a maximum in spring and the two strains differed only in the proportion of the population inhibited, which was significantly greater for the tablelands strain. There was no strain difference for either numbers of infective larvae on pasture or total O. ostertagi numbers. This suggested that the difference between coastal and tablelands populations in the extent of inhibition is genetically determined. If an environmental stimulus was implicated in the spring occurrence of inhibition, the stimulus was provided equally in coastal and tablelands environments and it cannot be exposure to low temperatures.

Seasonal availability of nematode larvae on pastures grazed by cattle in New South Wales.

Trends in the numbers of infective menatode larvae on pasture plots contaminated by cattle at different seasons of the year were defined in 3 different climatic regions. The main nematodes were Ostertagia ostertagi, Trichostrongylus spp, Haemonchus spp and Cooperia spp. On the North Coast of New South Wales with a sub-tropical climate, the numbers of infective larvae of all 4 nematodes rose rapidly to peak levels soon after each seasonal period of contamination began, then fell quickly within a few months. On the Central Coast of New South Wales, the trends were similar to those on the North Coast, except that the larvae persisted on the pasture for a much longer time. On the Northern Tablelands of New South Wales, where temperatures were much colder than on the coast, larval development was slower and major peaks of larval availability did not occur until early spring. These different seasonal trends in each region were considered to be related to the climatic differences between the regions. On pastures which were contaminated continuously, larval numbers reached maximum levels in mid-winter on the Central Coast and in early spring on the Northern Tablelands. It was concluded that the majority of these larvae were derived from the contamination of pastures in autumn and winter. Subsequently in summer, a rapid dying out of larvae was observed in all the regions, probably due to the effect of hotter weather. The studies suggest that a reduction in the contamination of pasture with nematode eggs in autumn and winter could result in pastures carrying fewer larvae and thus form the basis of effective worm control programs for cattle.

Seasonal changes in the structure of nematode populations of cattle in New South Wales in relation to inhibited larval development.

Pasture plots in 3 climatic regions were contaminated with worm eggs of Ostertagia ostertagi, Trichostrongylus axei, Haemonchus spp and Cooperia spp in the autumn, winter and spring. Successive pairs of parasite-free calves were grazed on the plots for 7 to 10 days at 4-week intervals and then killed for worm counts 14 days after their removal from pasture. On the Northern Tablelands of New South Wales, irrespective of the season of pasture contamination, the degree of inhibition of O. ostertagi was low in winter and highest in spring. T. axei showed similar trends while Cooperia spp showed negligible inhibition. On the North Coast of New South Wales, inhibited larvae accounted for a very small proportion of the O. ostertagi burdens, while in comparison T. axei showed a much greater degree of inhibition. Larval inhibition of Haemonchus spp occurred in autumn and early winter after which it did not occur. There was negliglible inhibition in Cooperia spp. On the Central Coast of New South Wales, there was little inhibition of O. ostertagi and none in T. axei. For Haemonchus spp, inhibited larvae were found mainly in autumn and winter. The numbers of inhibited Cooperia larvae were also highest in autumn and winter and were associated with large worm burdens. The marked difference between the tablelands and coastal regions in the seasonal trends of inhibition of O. ostertagi was considered to be due to a difference in strains between the geographical regions. The possible effect of climatic factors on the inhibition-proneness of infective larvae on pasture is discussed for Ostertagia and other nematodes. The roles of host resistance and density-dependence are also discussed.

Studies on nematode infections of beef cattle in New South Wales.

The occurrence and seasonal trends of nematode parasite infections in beef cattle on the Tablelands and North Coast regions of New South Wales are described, based on worm counts from 627 spring-born steers slaughtered at 2-monthly intervals from 6 to 24 months of age. The predominant parasites were Ostertagia ostertagi, Trichostrongylus axei and Cooperia spp. Large burdens of adult Ostertagia were seen in 12-month-old animals in late winter and early spring, and later in 18- to 20-month-old cattle in the following late summer and autumn. These infections often resulted in outbreaks of clinical parasitism. Massive numbers of inhibited early fourth stage larvae (EL4) also accumulated in the yearlings during their first spring, reached peaks in mid-summer and then declined. The possibility of their resumption of development to mature worms in the late summer and autumn period is discussed. Adult T. axei showed a similar seasonal trend to Ostertagia and may have increased the severity of outbreaks of clinical disease. Cooperia populations of EL4 and adult worms were highest in weaners during winter, but lower numbers thereafter indicated a strong resistance to re-infection. In addition, Haemonchus placei occurred frequently on the North Coast both as EL4 and adults in cattle of all ages up to 20 months. Of the other cattle nematodes, Oesophagostomum radiatum and Trichuris spp occurred in low numbers, mainly in weaners. Bunstomum phlebotomum and H. contortus occurred sporadically on the North Coast and Tablelands respectively. Intestinal Trichostronglyus spp, Oesophagostomum venulosum and Dictyocaulus viviparus were seen occasionally and Nematodirus spp were not seen. There was a poor relationship between worm counts and faecal egg counts. Cooperia spp dominated the egg counts, while those for other genera were generally low and did not reflect the relative abundance or seasonal changes in worm numbers.