J. W. A. Larsen

Minimising the development of anthelmintic resistance, and optimising the use of the novel anthelmintic monepantel, for the sustainable control of nematode parasites in Australian sheep grazing systems

OBJECTIVE To compare the risk of different treatment scenarios on selecting for anthelmintic resistance on Australian sheep farms. DESIGN A computer simulation model predicted populations of Trichostrongylus colubriformis, Haemonchus contortus or Teladorsagia (Ostertagia) circumcincta, and the frequency of anthelmintic resistance genes. METHOD Nematode populations and the progression of drug resistance for a variety of treatment options and management practices in sheep-rearing areas of Western Australia (WA), Victoria (VIC) and New South Wales (NSW) were simulated. A scoring system was devised to measure the success of each option in delaying resistance to each anthelmintic and in controlling nematode populations. RESULTS The best option at all sites was combining the new anthelmintic (monepantel) with a triple mixture of benzimidazole, levamisole and abamectin (COM). The next best option was: in NSW, rotation at each treatment between monepantel, moxidectin and COM; in VIC, rotation at each treatment between monepantel and COM; and in WA, rotation at each treatment between monepantel (used in winter) and COM or moxidectin (used in summer-autumn). In WA, rapid selection for resistance occurred as a consequence of summer-autumn treatments; however, if a small percentage of adult stock were left untreated then this selection could be greatly reduced. Despite purposely assuming relatively high resistance to benzimidazole and levamisole, COM was still effective in controlling worms and delaying resistance. CONCLUSIONS Because of cost constraints, it may not be feasible or profitable for producers to always use the combination of all drugs. However, the second- and third-best options still considerably slowed the development of anthelmintic resistance.

A multi‐species model to assess the effect of refugia on worm control and anthelmintic resistance in sheep grazing systems

OBJECTIVE Develop a computer simulation model that uses daily meteorological data and farm management practices to predict populations of Trichostrongylus colubriformis, Haemonchus contortus and Teladorsagia (Ostertagia) circumcincta and the evolution of anthelmintic resistance within a sheep flock. Use the model to explore if increased refugia, provided by leaving some adult sheep untreated, would delay development of anthelmintic resistance without compromising nematode control. PROCEDURES Compare model predictions with field observations from a breeding flock in Armidale, NSW. Simulate the impact of leaving 1-10% of adult sheep untreated in diverse sheep-grazing systems. RESULTS Predicted populations of Tr. colubriformis and T. circumcincta were less than those observed in the field, attributed to nutritional stress experienced by the sheep during drought and not accounted for by the model. Observed variation in faecal egg counts explained by the model (R(2) ) for these species was 40-50%. The H. contortus populations and R(2) were both low. Leaving some sheep untreated worked best in situations where animals were already grazing or were moved onto pastures with low populations of infective larvae. In those cases, anthelmintic resistance was delayed and nematode control was maintained when 1-4% of adult stock remained untreated. CONCLUSIONS In general, the model predicted that leaving more than 4% of adults untreated did not sufficiently delay the development of anthelmintic resistance to justify the increased production risk from such a strategy. The choice of a drug rotation strategy had an equal or larger effect on nematode control, and selection for resistance, than leaving 1-10% of adults untreated.

Outbreaks of diarrhoea (‘winter scours’) in weaned Merino sheep in south-eastern Australia

OBJECTIVE Identify the cause of outbreaks of diarrhoea during winter that are not attributable to gastrointestinal nematodes in weaned Merino sheep in the high rainfall regions of south-eastern Australia and determine the efficacy of antimicrobials used to treat this syndrome. METHODS We investigated 45 outbreaks on 24 farms. Faecal samples from affected animals were cultured for Yersinia, Campylobacter and Salmonella spp. Risk factors, including rainfall, temperature and worm egg count (WEC), were assessed. Yersinia spp. were identified with molecular tests and susceptibility to four antimicrobials was determined. RESULTS Yersinia pseudotuberculosis serotype III and virulent Y. enterocolitica were most frequently isolated. The frequency and severity of disease varied between region, farm and year. Yersinia pseudotuberculosis was detected only during winter, but Y. enterocolitica was present in all seasons. Pathogenic Yersinia species were more often isolated when WECs exceeded 500 eggs/g. A high proportion of Y. enterocolitica and Y. pseudotuberculosis were resistant to sulfafurazole (64% and 86.9%, respectively). CONCLUSIONS A bacterial enteritis caused by pathogenic Yersinia was the cause of the winter scours syndrome in the 24 flocks investigated. The use of molecular testing increased the sensitivity of detection and identification of Yersinia spp. No clear association between weather, WEC and disease was established, suggesting complex interactions between risk factors are more important than any single factor. Sulfonamides should not be routinely used to treat this syndrome. Rather, during an outbreak the targeted use of an effective antimicrobial, such as oxytetracycline, should be integrated with grazing management strategies, including moving affected mobs onto lower risk pastures and decreasing the stocking rate.

The effect of annual shearing time on wool production by a spring-lambing Merino flock in south-eastern Australia

Time of shearing affects many aspects of wool production and sheep health but no study has examined these factors concurrently in a spring-lambing, self-replacing Merino flock in southern Australia. A 5-year field experiment compared wool production in spring-lambing Merino ewes and their progeny shorn at different times and managed under commercial conditions in south-eastern Australia. Groups of 200 adult ewes were shorn in December, March or May, and their progeny were shorn in December or October, March or June, or May or July, respectively. There was no consistent association between time of shearing and the staple strength of wool (P = 0.73). December-shorn ewes produced significantly lighter and finer fleeces (average 19.1 μm, 3.0 kg clean weight) than did March-shorn ewes (19.4 μm, 3.1 kg). Fleeces from ewes shorn in May were of similar weight to those from March-shorn ewes (3.1 kg), but they were of significantly broader diameter (19.7 μm). In young sheep, desirable changes in some wool characteristics for each shearing group were offset by undesirable changes in others. On the basis of median historical (1991–2006) wool prices, shearing ewes in March and their progeny first in June produced the greatest total value of wool over a sheep’s lifetime (

A longitudinal study of faecal shedding of Yersinia enterocolitica and Yersinia pseudotuberculosis by Merino lambs in south-eastern Australia

161/head). The wool values per head for other shearing times were as follows: March (weaners)–March (ewes):

Financial effect of bovine Johne's disease in beef cattle herds in Australia

158; October–December:

Assessment of the efficacy of an autogenous vaccine against Yersinia pseudotuberculosis in young Merino sheep

157; December–December:

Diarrhoea in Merino ewes during winter: association with trichostrongylid larvae

153; July–May:

Production losses in Merino ewes and financial penalties caused by trichostrongylid infections during winter and spring

151; and May–May:

The pathogenesis and control of diarrhoea and breech soiling in adult Merino sheep

148. Thus, December and March shearing were appropriate alternatives for maximising wool value produced from a self-replacing Merino flock in south-eastern Australia, whereas May was a less preferable shearing time because it always produced wool of lesser value.