D.M. West

Resistance of field isolates of Trichostrongylus colubriformis and Ostertagia circumcincta to ivermectin.

Twelve Romney lambs and 10 Angora goats were infected with 7000 infective third-stage larvae (89% Trichostrongylus, 11% Ostertagia) collected from goats suspected of harbouring ivermectin-resistant nematodes. On 28 days p.i., the lambs and goats were divided into treatment and control groups of six and five animals, respectively. The animals in the treatment groups were treated with ivermectin (0.2 mg/kg) and necropsied 35 days p.i. Faecal egg counts were estimated on days 28 and 35 p.i. and larval development assays (LDAs) were conducted on 22, 24, 26, 28, 30, 32, 34 and 35 days p.i. The ivermectin treatment reduced Trichostronglus colubriformis burdens by 39% and 13% and Ostertagia circumcincta by 33% and 0% in lambs and goats, respectively. When compared with a susceptible strain, the LDAs indicated a resistance factor before treatment in lambs for T. colubriformis of 2.6 and 1.5 with ivermectin and avermectin B2, respectively, which rose to 3.4 and 2.0 after treatment. The LD50 values of the two control groups were relatively constant throughout the experiment. Prior to ivermectin treatment the LD50 values of the treated groups were similar (P > 0.05) to the control groups but following ivermectin treatment their LD50 values increased steadily until the animals were killed on 35 days p.i. The LD50 values for ivermectin and avermectin B2 of sheep were always slightly higher and significantly different (P < 0.01) than those of goats indicating a host effect on this parameter. The greater reduction in worm counts in goats suggests a difference in the efficacy of ivermectin between lambs and goats. This is the first confirmed report of ivermectin resistance in a field strain of T. colubriformis.

Inefficacy of moxidectin and doramectin against ivermectin-resistant Cooperia spp. of cattle in New Zealand

Two cases are described in which strains of Cooperia spp. of cattle were found to be resistant to either the oral or the injectable formulations of ivermectin. Injectable moxidectin was not effective against the latter strain and doramectin was not effective against either of the ivermectin-resistant Cooperia isolates. In both cases, the anthelmintic efficacy of oral levamisole was 100%.

Nematode worm egg output by ewes.

The results from routine monitoring of parasite burdens in ewe flocks from 1980 to 1991 by the Massey University Veterinary Clinic were analysed. Faecal strongylate nematode egg counts from 401 flock samples were analysed according to ewe age (two-tooth, 16-23 month-old vs mixed-age, greater than 2-year-old ewes) and month of the year. Each flock sample contained faeces from ten ewes and nematode egg counts were carried out on each faecal sample. ln the mixed-age ewes, the overall mean faecal egg count per gram of faeces (epg) was lowest during January (82 epg) and rose to a peak during October (539 epg). The overall mean faecal egg count in two-tooth ewes remained constant from January (240 epg) to June (294 epg) and dropped during July (156 epg). Overall, 8% of the mixed-age ewes and 12% of the two-tooth ewe submissions had a mean egg count equal to or greater than 500 epg. It is possible that these data over-estimate the pasture contamination by ewes as farmers are more likely to submit faecal samples and to seek veterinary assistance when parasitism is thought to be a problem. The role of ewes as a source of pasture contamination should be considered when developing parasite control regimes on farms.

Ineffikacy of ivermectin against Cooperia spp. infection in cattle

Abstract Ivermectin, either as an injectable or topical formulation, is used widely to control parasites in New Zealand cattle. These formulations have been well documented as having high efficacy against the major nematode species which infect tattle(1)(2)(3)(4), although the efficacy against adult Cooperia oncophora is reported to be 95%(5). This is considered to be one of the doselimiting species. There have been two reports of lower than expected efficacy of the topical formulation against Cooperia spp. in cattle. In a slaughter trial, Bisset et al. (6) found that ivermectin applied topically at the recommended dose rate of 500 𝛍g/kg failed to achieve a statistically significant reduction in the burden of Cooperia spp. worms compared with untreated calves. The geometric mean of adult Cooperia spp. worms present in the treated calves was 5788 compared with 7524 present in the untreated calves. This reduced efficacy was not attributed to acquired anthelmintic resistance because ivermectin had not been u...

Observations on three outbreaks of Leptospira interrogans serovar pomona infection in lambs

Sickness and deaths associated with leptospirosis were reported in three lamb flocks in the Manawatu region. The clinical, pathological and serological features of Leptospiru interrogans serovar pomona infection are described. The mucous membranes of affected animals were pale and jaundiced. Haemoglobinuria, haemoglobinaemia and centrilobular hepatocellular necrosis were also consistent findings. All clinically sick lambs had very high titres in the microscopic agglutination test for L. pomona. Circumstantial evidence of pigs being the source of infection was found in one of the three cases.

The effect of electro-ejaculation on aversive behaviour and plasma cortisol concentration in rams

Electro-ejaculation is used in rams to collect semen for fertility and bacteriological evaluation. Concern has been expressed about animal welfare implications of EE. Two experiments were carried out to compare the aversiveness of electro-ejaculation, part shearing and free movement, and to determine the changes in plasma cortisol concentration produced by electro-ejaculation, shearing and lateral restraint. In the aversion test, transit time was greater (p<0.003) for rams subjected to part shearing than free movement. There were no major differences in the transit time between the rams subjected to electro-ejaculation and those subjected to part shearing or free movement. In addition, there was no significant difference in the effort required to move rams from the three treatment groups into or through the experimental route. These results suggest that electro-ejaculation is no more aversive than part shearing. There were no significant differences in the cortisol response of the rams subjected to electro-ejaculation, shearing or lateral restraint, indicating that there were no significant differences in the distress caused by the three procedures.

Multigeneric resistance to benzimidazole anthelmintics in four sheep flocks.

Clinical parasitism associated with multigeneric resistance to benzimidazole anthelmintics was investigated in four sheep flocks during February and March 1989. The genera considered resistant were Haemonchus and Nematodirus spp. on all four farms and Trichostrongylus and Ostertagia spp. on three of the four farms. It is suggested that when investigating cases of suspected drench resistance, post-drenching worm counts are useful to help identify the resistant worm species present.

Serological and necropsy findings for rams infected with Brucella ovis which were not identified by the complement fixation test

The eradication of Brucella ovis from a commercial flock of 36 Romney rams was complicated by four infected rams remaining undetected despite four successive flock examinations using the complement fixation test. These four rams were subsequently tested using an enzyme-linked immunosorbent assay and a gel diffusion test and shown to be infected by semen culture. All four rams could have been identified as infected at the initial test if the enzyme-linked immunosorbent assay had been used in addition to the complement fixation test. Although gross evidence of epididymitis was found in only one ram at necropsy, three had histological lesions of epididymitis and all four had a seminal vesiculitis.

The effects of age, weaning, drench volume and yarding on ruminoreticulum bypass in sheep, with reference to the anthelmintic efficacy of benzimidazole drenches

AIMSnTo determine the incidence of ruminoreticulum bypass in lambs before and after weaning, following drenching with anthelmintic-equivalent volumes of fluid; to investigate the influence of withholding feed for 24 hours before drenching on the incidence of ruminoreticulum bypass; and to investigate the hypothesis that ruminoreticulum bypass occurs more frequently following drenching with large than with low volumes. The relationship between these factors and the efficacy of benzimidazole anthelmintic drenches has been widely promoted in New Zealand and Australia.nnnMETHODSnVideo-taped fluoroscopy, using barium sulphate suspension with a specific gravity of 1.38 as a contrast medium, was used to study closure of the reticular groove and ruminoreticulum bypass in sheep.nnnRESULTSnWhen lambs were drenched with 3 ml of barium sulphate suspension/10 kg liveweight soon after removal from pasture, ruminoreticulum bypass only occurred before weaning and during the first 8 weeks after weaning. However, when a low volume drench (1 ml/10 kg) was used in lambs soon after removal from pasture during the same period, ruminoreticulum bypass was not observed. When recently weaned lambs were yarded for 24 hours before drenching with barium sulphate suspension, substantial (> or = 20%) ruminoreticulum bypass occurred in 25-58% of the animals. When 8-9 month-old hoggets were yarded for 24 hours before drenching, substantial ruminoreticulum bypass occurred in 35% of the animals. Ruminoreticulum bypass was not observed in the same group of hoggets when drenched soon after removal from pasture, in a reversed crossover experiment. This effect of withholding feed for 24 hours on ruminoreticulum bypass was significant (p < 0.0001). In hoggets which had been yarded for 24 hours before drenching with barium sulphate suspension, the incidence of ruminoreticulum bypass was significantly lower (p = 0.0026) following the use of a low volume (1 ml/10 kg) when compared to high volume (3 ml/ 10 kg) of drench. Thus, the effect of withholding feed for 24 hours on ruminoreticulum bypass was largely overcome when the low volume drench was used.nnnCONCLUSIONSnThese results support the use of low volume formulations of benzimidazole drenches for New Zealand pasture-fed lambs less than 6 months old. The results indicate that, in order to obtain the optimum efficacy of benzimidazole anthelmintics in New Zealand pasture-fed sheep, which have been yarded for 24 hours, low volume drench formulations should be used.

A strain of Haemonchus contortus resistant to thiophanate.

The occurrence of a field strain of Haemonchus contortus resistant to thiophanate is reported for the first time in New Zealand. In a controlled anthelmintic trial with experimentally infected animals, thiophanate at 50-100 mg/kg had no appreciable effect on the Haemonchus burden. Albendazole reduced faecal strongylate egg counts by 95% in animals with naturally acquired 59 infections.