K. Bell

Principles for the use of macrocyclic lactones to minimise selection for resistance

OBJECTIVE To provide principles for the appropriate use of avermectin/milbemycin or macrocyclic lactone (ML) anthelmintics in sheep, to ensure effective worm control and to minimise selection for ML resistance. STRATEGY The principles were based on an assessment of the information currently available. The MLs were categorised into three groups (ivermectin [IVM], abamectin [ABA] and moxidectin [MOX]) based on structural differences, persistence and efficacy against ML resistant strains. The reported order of activity or efficacy against ML resistant worm strains was IVM<ABA<MOX. General treatment schemes were considered for Australian conditions and were divided into the following situations: 1. quarantine treatment, 2. treatment on/to clean pasture, 3. treatment on/to safe pasture, 4. treatment on/to moderate/heavily contaminated pasture. For each of these situations a strategy was considered for farms where ML resistance was present or absent. It was assumed that resistance commonly occurs in some or all other broad spectrum anthelmintics, and even where ML resistance has been detected, the ML group remains the most effective. The guidelines provided are general and it is expected that state agencies and sheep/veterinary advisers would give specific advice to suit their environments and drug resistance/worm problems. CONCLUSIONS The primary recommendation is to use a mixture of effective drugs when treating sheep. However, unless the combination treatment is highly effective it is unlikely to delay selection for ML resistance if sheep are being treated and moved to a clean or safe pasture. Where possible, reliance on the ML anthelmintics should be reduced by not using them every year, not using them in low risk stock or by using narrow spectrum and low efficacy drugs such as naphthalophos when appropriate. Anthelmintic treatment should be given as part of a strategic worm control program. It is suggested that IVM-oral and IVM-capsules should not be used when ML resistance is present. In this situation MOX or ABA should be used in combination with other drugs, provided that the chosen ML is effective against the resistant parasite. It is essential to monitor the efficacy of ML and drug combinations by post-treatment worm egg counts, particularly when ML resistance has been detected.

Longitudinal investigation of protozoan parasites in meat lamb farms in southern Western Australia.

In this study, 96 faecal samples were collected from pregnant Merino ewes, at two broad-acre, commercial sheep farms in southern Western Australia, on two separate occasions (16 and 2 weeks prior to lambing). Following lambing, 111 (Farm A) and 124 (Farm B) female crossbred lambs (2-6 weeks old), were individually identified using ear tags (a numbered tag and a radio-frequency tag). A total of 1155 faecal samples were collected only from these individually identified lambs on five separate sampling occasions. All samples were screened using PCR to detect Cryptosporidium (18S rRNA and actin loci) and Giardia duodenalis (glutamate dehydrogenase and triosephosphate isomerise loci). The overall prevalences (lambs positive for a parasite on at least one of the five samplings) at Farm A and B were 81.3% and 71.4%, respectively for Cryptosporidium and similarly 67.3% and 60.5% for Giardia, respectively. Cryptosporidium and Giardia prevalences at individual samplings ranged between 18.5 and 42.6% in lambs and were <10% in the ewes. Cryptosporidium xiaoi was the most prevalent species detected at all five samplings and was also isolated from lamb dam water on Farm B. Cryptosporidium ubiquitum was most commonly detected in younger lambs and Cryptosporidium parvum was detected in lambs at all five samplings, typically in older lambs and as part of a mixed species infection with C. xiaoi. A novel, possibly new genotype (sheep genotype I), was identified in six Cryptosporidium isolates from Farm B. Giardia duodenalis assemblage E was the most common genotype detected at all five samplings, with greater proportions of assemblage A and mixed assemblage A and E infections identified in older lambs. This longitudinal study identified high overall prevalences of Cryptosporidium and Giardia in lambs grazed extensively on pastures, while reinforcing that sampling a random selection of animals from a flock/herd on one occasion (point prevalence), underestimates the overall prevalence of these parasites in the flock/herd across an extended time period. Based on these findings, grazing lambs were identified as a low risk source of zoonotic Cryptosporidium and Giardia species/genotypes, with these protozoa detected at all five samplings in some lambs, indicating that these individuals were either unable to clear the naturally acquired protozoan infections or were repeatedly re-infected from their environment or other flock members.

Associations between nematode larval challenge and gastrointestinal tract size that affect carcass productivity in sheep.

Effects of gastrointestinal parasitism on sheep productivity are usually described using live weight change, however carcass productivity is more accurately described using dressing percentage (carcass weight as a proportion of live weight). This experiment had a 2x2x2 factorial design whereby 10-month-old Merino wethers were fed lucerne (Medicago sativa) diets (fresh lucerne or lucerne chaff) with 2 levels of carboxymethycellulose (CMC) inclusion (0% or 8% CMC) and nematode larval challenge (no larval challenge or 10,000 Teladorsagia circumcincta and 10,000 Trichostrongylus colubriformis per week). Sheep were weighed and euthanased 50 or 51 days after larval challenge and CMC supplementation commenced. Weight of the carcass (hot standard carcass weight) and gastrointestinal organs (full and empty) were recorded and expressed as a proportion of live weight. Larval challenged sheep had a worm egg count (mean+/-standard error) of 173+/-38 eggs per gram of faeces and total worm count of 30,237+/-2013 at slaughter. Larval challenged sheep had 1.3% lower dressing percentage (p=0.048), and 2% heavier full (p=0.007) and 1.2% heavier empty gastrointestinal tracts (p=0.012) compared to unchallenged sheep. There was no effect of CMC inclusion or lucerne type (fresh or chaff) on gastrointestinal tract weight or dressing percentage. Larval challenged sheep had 1.1% heavier full (p<0.001) and 0.6% heavier empty (p<0.001) small intestines, and 0.6% heavier full (p=0.005) and 0.3% heavier empty (p=0.026) large intestines compared to unchallenged sheep. Use of live weight change or other measures based on live weight (e.g. feed conversion efficiency) to assess the impact of nematode challenge in sheep may underestimate carcass productivity losses associated with larval challenge in sheep even at moderate levels of larval intake and without overt clinical signs of parasitism. Measurement of carcass weight and/or lean meat yield may better reflect the true economic effects of parasitism in sheep.

Associations between trichostrongylid worm egg count and productivity measures in Dorper lambs.

Two flocks of pure bred Dorper lambs were managed separately according to sex (283 ewe lambs and 212 ram lambs) in southern Western Australia. Faecal sample collection, weighing and body condition assessments were performed for each lamb on 2 occasions, specifically pre-weaning (approximately 14 weeks of age) and post-weaning (approximately 9 months of age). Body condition score (BCS) was assessed using a scale of 1 (very thin, emaciated) to 5 (excessively fat). Faecal worm egg counts (WECs) were measured using a modified McMaster technique and larval cultures were performed to identify trichostrongylid nematode genera present. Eye muscle and c-site fat depths were measured using ultrasound at post-weaning. Lambs received an abamectin anthelmintic treatment at weaning (18 weeks of age). Worm egg count data was assessed for normality of data distribution and homogeneity of variance. This data was transformed using log(10)(WEC+25) to stabilise variances between groups prior to statistical analyses and general linear models were used to assess relationships between WEC and productivity measures. Mean WECs were 564 eggs per gram of faeces (EPG) and 514 EPG at pre- and post-weaning in the ewe flock and 552 EPG and 480 EPG at pre- and post-weaning in the ram flock. Teladorsagia (Ostertagia) circumcincta, Trichostrongylus spp. and Oesophagostomum spp. larvae were identified. No lambs with WEC<500 EPG pre-weaning had WEC>1000 EPG post-weaning. Ewe and ram lambs with WEC>1000 EPG at pre-weaning were 42 (12-145 95% CI) and 37 (9-153) times more likely to have WEC>1000 EPG at post-weaning than lambs with WEC 501-1000 EPG at pre-weaning. There were no significant relationships between WEC and live weight in the ram flock, while relationships between WEC and live weight were inconsistent in the ewe lamb flock. There was no relationship between WEC and eye muscle or c-site fat depth. Significant negative relationships between WEC and BCS were identified at pre- and post-weaning for both flocks. Lambs with WEC<500 EPG had 0.19-0.61 higher mean BCS than lambs with WEC>1000 EPG at pre- and post-weaning. In conclusion, high WEC was associated with lower body condition in Dorper lambs, however the relationship between WEC and live weight was inconsistent and there was no effect on eye muscle depth.

Summer-autumn rainfall effects on wool staple strength and position of break. II. Rainfall simulations, with or without wind, on sheep on days of different ambient temperatures

Previous small-scale field work failed to show any influence on staple strength from various different rainfall simulations, other than those where sheep grazed germinating green pasture following rainfall. The present study was designed to incorporate variables such as ambient temperature and the influence of wind into rainfall simulations onto Merino sheep. Different treatments did not show any effect on staple strength despite sheep being subjected to simulated rainfall events on warm and cool days, with or without additional wind. This lack of effect on wool quality was despite significant treatment effects on sheep rectal and skin temperatures.