An evaluation of a modified preventive drenching programme on commercial sheep farms

The effectiveness of a commonly advocated five drench preventive drenching programme in lambs was assessed on nine farms in the Gisborne, Hawkes Bay and Wairarapa districts between December 1983 and March 1985. The productive performance of two groups of preventively drenched lambs was compared with lambs in which parasitism was suppressed by fortnightly drenching from weaning until mid-August 1984. One of the groups received the standard five drench programme commencing at weaning and finishing in March 1984. The other group received six additional 4-weekly drenches, with the last drench in mid-August 1984. All the other lambs on the farms were drenched according to the five drench programme. The availability of infective larvae on pasture and the build-up of parasitic infection in the lambs was monitored. The effects of the different drenching programmes on liveweight and faecal egg counts was followed until March 1985, when the animals were 18 months old. Between March and July 1984 on all nine farms there was a significant depression of growth rates of the standard five drench group when compared with the suppression-drenched group (P<0.01). The mean liveweight difference from December 1983 to July 1984 for all farms was 4.0 kg (P<0.05). Hogget wool weights were significantly depressed (mean difference 0.37 kg, P<0.05). Faecal egg counts rose steadily after the completion of the five drench programme in March, exceeding 1000 eggs per gram on seven farms and 4000 eggs per gram on three of these farms. Trichostrongylus spp. was the major constituent of the winter parasite increase except on one farm when Cooperia spp. succeeded a previous Haemonchus dominance. Two farms had major build-ups of Haemonchus spp., one in early May, the other in June. Six farms salvage-drenched the standard five drench groups. The lambs in the standard drench groups on these farms recovered about half the bodyweight difference between August and March. The mean weight gains to July and wool weights of the modified drench programme groups were reduced slightly compared with the suppression-drenched group (mean liveweight difference 1.4 kg; mean wool weight difference 0.15 kg). No major rises in faecal egg counts occurred. Herbage larval counts exceeded 1000 larvae/kg at some stage on seven farms and, of these, the counts exceeded 4000 larvae/kg on three farms.     

A survey of nematode control measures used by sheep farmers and of anthelmintic resistance on their farms Part 2: South Island excluding the Nelson region

Forty-three randomly selected farms located throughout South Island, excluding the Nelson region, were surveyed for anthelmintic usage and for sheep nematodes resistant to anthelmintics.

Most farmers had used both benzimidazole and non-benzimidazole broad-spectrum drenches on their properties in previous years.

Sheep were being drenched, on average, 5.6 times within their first year of life but much less frequently thereafter. Commercial interests played the dominant role in helping farmers formulate their drenching policies. On each farm 24 numbered ewe replacement lambs, born during the spring of 1980, were sampled for faeces at the beginning of the trial to provide material for egg counts and larval cultures. The lambs were weighed and divided into three groups of eight. One group received thiabendazole (TBZ) at 66 mg/kg, the second levamisole (LEV) at 8 mg/kg while the third remained untreated as controls. All were resampled 4 to 10 days later.

On 32 (88%) of the 40 farms where drenching trials were successfully carried out during autumn 1981, the faecal egg count depression (FECD) following treatment with either drench was 100%. On one farm TBZ was less than 100% effective as gauged by FECD. LEV proved to be less than 100% effective on 7 farms. On one farm the FECD was less than 80%, on 2 between 90% and 95% and on 4 between 95% and 99%.     

A survey of nematode control measures used by sheep farmers and of anthelmintic resistance on their farms. Part 2: South Island excluding the Nelson region

Forty-three randomly selected farms located throughout South Island, excluding the Nelson region, were surveyed for anthelmintic usage and for sheep nematodes resistant to anthelmintics. Most farmers had used both benzimidazole and non-benzimidazole broad-spectrum drenches on their properties in previous years. Sheep were being drenched, on average, 5.6 times within their first year of life but much less frequently thereafter. Commercial interests played the dominant role in helping farmers formulate their drenching policies. On each farm 24 numbered ewe replacement lambs, born during the spring of 1980, were sampled for faeces at the beginning of the trial to provide material for egg counts and larval cultures. The lambs were weighed and divided into three groups of eight. One group received thiabendazole (TBZ) at 66 mg/kg, the second levamisole (LEV) at 8 mg/kg while the third remained untreated as controls. All were resampled 4 to 10 days later. On 32 (88%) of the 40 farms where drenching trials were successfully carried out during autumn 1981, the faecal egg count depression (FECD) following treatment with either drench was 100%. On one farm TBZ was less than 100% effective as gauged by FECD. LEV proved to be less than 100% effective on 7 farms. On one farm the FECD was less than 80%, on 2 between 90% and 95% and on 4 between 95% and 99%.     

Some aspects of epididymitis of rams in New Zealand

Fifty four randomly selected farms, located throughout the North Island and the Nelson region of South Island, were surveyed for anthelmintic usage and for sheep nematodes resistant to anthelmintics.

Information on anthelmintic usage was recorded on a standardized questionnaire.

Most fanners had used both benzimidazole and non- benzimidazole broad spectrum drenches on their properties in previous years.

Sheep were being drenched, on average, 7 times within their first year of life but much less frequently thereafter. Commercial interests played the dominant role in helping farmers formulate their drenching policies.

On each farm 24 numbered ewe replacement lambs were sampled for faeces at the beginning of the trial to provide material for egg counts and larval cultures. The lambs were weighed and divided into three groups of eight. One group received thiabendazole (TBZ) at 66 mg/kg, the second levamisole (LEV) at 8mg/kg while the third remained untreated as controls. All were resampled 4 to 10 days later.

On 37 farms (74%) the faecal egg count depression (FECD) following treatment with either drench was 100%. On 7 farms TBZ was less than 100% effective as gauged by FECD. Of these 6 had an FECD above 90% and one below. LEV proved to be less than 100% effective on 8 farms. Of these 5 had FECDs greater than 90% and 3 FECDs less than 90%. Only two farms had lambs with an FECD below 100% for both LEV and TBZ.

Preliminary evidence based on larval cultures suggests that those surviving the TBZ treatment were chiefly Haemonchus and those surviving LEV Trichostrongylus spp. One population of Haemonchus resistant to TBZ has been identified by the egg-hatch technique.     

Production and immunological responses associated with controlled-release-capsule vs 5-drench preventive anthelmintic programmes for parasite control in lambs

AIMS: To determine whether: a) using a controlled-release anthelmintic capsule (CRC) instead of a programme of 5 oral drenches administered at 3-4 week intervals, would delay the development of anti-parasite immunity in lambs; b) the use of ivermectin instead of albendazole, administered either as a CRC or as a programme of 5 oral drenches, would delay the development of anti-parasite immunity in lambs; c) lambs treated with CRCs would have higher liveweight gains than lambs drenched orally 5 times at 3-4 week intervals, and; d) delayed onset of anti-parasite immunity is associated with reduced liveweight gains in the period following anthelmintic treatment. METHODS: Three field trials were conducted, 1 on a research farm and 2 on commercial sheep farms, in which groups of 30 lambs were treated with either a CRC containing albendazole, a CRC containing ivermectin, 5 oral drenches with albendazole, or 5 oral drenches with ivermectin, administered at 3-4 week intervals. Liveweights and faecal nematode egg counts (FECs) were recorded in all trials. Immunoglobulin-G (IgG) antibody levels to Ostertagia circumcincta and Trichostrongylus colubriformis adult and larval antigens were measured in Trials 1 and 3, and fleece weights and resistance of animals to nematode challenge infection were measured in Trial 1. RESULTS: CRC-treated lambs had higher levels of antibodies to O. circumcincta infective-stage larvae (L3) than orally drenched lambs in Trial 3, but no other immunological differences due to mechanism of delivery were detected. Antibody levels were lower in lambs treated with ivermectin than albendazole, as a CRC or oral drench in Trial 1, but this was not associated with any measurable effects on FEC or productivity. No significant differences (p>0.05) were detected between drench types (albendazole vs ivermectin) or delivery mechanisms (CRC vs oral drenching) in any of the production parameters measured, in any of the trials. Albendazole-CRCs failed to control FECs in all 3 trials. CONCLUSIONS: Although some differences between treatments in antibody levels were detected these were not associated with measurable differences in level of parasitism or productivity of lambs. CRC use did not appear to offer substantial gains in productivity over a structured programme of 5 oral drenches administered at 3-4 week intervals.     

Multiple species of nematodes resistant to ivermectin and a benzimidazole-levamisole combination on a sheep farm in New Zealand

AIM: To confirm the presence of multiple anthelmintic resistance on a sheep farm in New Zealand. METHODS: Three groups of 10 weaned Romney-cross lambs were treated either with an oral dose of ivermectin (0.2 mg/kg), or a benzimidazole/levamisole (BZ/LEV) combination (4.75 albendazole and 7.5 mg/kg levamisole), or were left untreated. Ten days later, animals were necropsied, and adult worms recovered and identified from the abomasa and small intestines. Pre- and post-treatment faecal nematode egg counts (FEC) were recorded, and larval cultures were performed. RESULTS: In a faecal egg count reduction test (FECRT), adjusted to reflect pre- and post-treatment larval culture results, ivermectin resistance was detected in Teladorsagia (Ostertagia), Trichostrongylus and Haemonchus spp, while BZ/LEV combination- resistant Teladorsagia and Trichostrongylus spp were also present. Adult worm counts confirmed these results, and identified the species involved as Teladorsagia circumcincta, Trichostrongylus colubriformis and H. contortus. CONCLUSION: Multiple, multi-generic anthelmintic resistance was confirmed on a sheep property in New Zealand. This included the first confirmed case of ivermectin resistance in T. colubriformis from sheep in New Zealand.     

A preliminary undifferentiated faecal egg count reduction test survey in the Caledon area

During October 1998 a study was performed in the Caledon area to determine the presence of anthelmintic resistance in the southern Western Cape, which falls within the winter rainfall region of South Africa. The study took the form of an undifferentiated faecal egg count reduction test (FECRt) survey. The predominant worm genera of the region are Teladorsagia and Trichostrongylus, but Haemonchus causes occasional outbreaks in sheep. No resistance data for any area in the Western Cape Province are available at present. Resistance (< 90% FECR) was recorded on 73% of the farms included in the study, with 46% of these involving resistance to 1 drench, 36% to 2 drenches and 18% to 3 drenches. No moxidectin resistance was detected when it was administered at the therapeutic dosage of 0.2 mg/kg. The results of this study show clearly that resistance of nematodes to other anthelmintics occurs in the Caledon area.     

Anthelmintic resistance in nematode parasites of goats

Cases of anthelmintic resistance on 3 goat farms in Gippsland were investigated. On the first farm Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia (Teladorsagia) circumcincta were found to be resistant to fenbendazole, but the first 2 species were fully susceptible to levamisole. On the second farm a population of T. colubriformis, resistant to concurrent full doses of levamisole and a benzimidazole, was found to retain this resistance when transferred to sheep. On the third farm, heavy mortality due to Ostertagia and Trichostrongylus spp burdens was arrested only by the use of concurrent full doses of levamisole and oxfendazole; no single anthelmintic was found to be effective. A disturbing rise in the incidence of clinical helminthiosis, often accompanied by anthelmintic resistance, occurred on goat farms in Gippsland early in 1984.     

Efficacy of oxfendazole against naturally acquired gastro-intestinal nematode infestations in buffaloes in Egypt

The benzimidazole anthelmintic, oxfendazole, was used as an oral suspension at 4.5 and 2.8 mg/kg to treat naturally infested indigenous buffaloes in Egypt. Worms of the genera Haemonchus, Cooperia, Trichostrongylus, Ostertagia, Chabertia and Oesophagostomum were identified in the animals prior to treatment. A 100% reduction in faecal egg counts was obtained at the 4.5 mg/kg level when compared to untreated control animals. Reductions in faecal egg counts greater than 90% were recorded at 2.8 mg/kg and these exceeded reductions obtained by drenching with the older benzimidazole anthelmintic, thiabendazole, at 67 and 111 mg/kg.     

Survey of anthelmintic resistance in Western Australian sheep flocks 2. Relationship with sheep management and parasite control practices

Owners of 116 farms, whose flocks had been tested for anthelmintic resistance, were interviewed to determine their use of various sheep management and parasite control practices and their knowledge and adoption of recommended procedures for the prevention and control of resistance. Farmers knowledge of current recommendations related mainly to changing drenches and drench groups. Other aspects of the recommended program including reduction of drenching frequency and the use of alternative management strategies were not considered as important by farmers. For most questions a high proportion of farmers (greater than 20%) had no opinion. Associations between various strategies for nematode control and resistance of Trichostrongylus and Teladorsagia to thiabendazole and levamisole were examined. These relationships differed between anthelmintics and nematode genera. A number of factors were related to resistance of one or both nematode genera to one or both anthelmintic groups. These factors included flock size, percentage of ewes in the flock, cattle number, main sheep production activity, grazing strategy, frequency of drenching, changes in the frequency of drenching, number of summer drenches and the method of estimating dose rates. It was concluded that the methods employed to control anthelmintic resistance may vary with the nematode, its resistance status and the anthelmintic to which it is exposed. Modifications to the previously recommended program have been proposed which incorporate selection of the anthelmintic to be used following a test for anthelmintic resistance.     

The geographic distribution of carcinomas of the small intestine in New Zealand sheep

A sample survey of anthelmintic usage and farmers' drenching policies was conducted on 614 sheep farms, in the North and South Islands, selected from among those carrying 1500 or more breeding ewes. The survey was based on information for the 1978/79 and 1979/80 farming years recorded on questionnaires completed during personal interviews between Livestock Officers and the farmers concerned.

Results show that there were no significant between-Island differences in mean drenching frequencies for any age class of sheep. In 1979/80, the overall New Zealand drenching frequencies were 6.3 for lambs, 1.8 for 1-2-year-olds and 1.2 for sheep older than 2 years. Of the farmers surveyed, 20% did not drench 1-2-year-old sheep and 29% did not drench sheep older than 2 years.

There were also no between-Island differences in timing of drenches and alternation of drench brands. Sixty-nine per cent of the farmers followed a pre-determined drenching programme whereas 27% stated that they drenched if and when necessary. For lambs, some drenching was carried out in all months of the year but fewer farmers drenched during the mid-winter to mid-spring period (July-October). Older stock were drenched in all months also but there was a general acceptance by farmers of the practices of pre-tupping and pre- or post-lambing drenching.

At the time of the survey, 48% of the farmers were using three or more brands of drench with a maximum of nine. Analysis of data relating to brands of drench used shows that whereas 39% of the farmers were alternating drench families [i.e. benzimidazoles (BZ) c.f. non-benzimidazoles (NBZ)] within a single farming year, only 3% were practising a strict alternation of drench families between years.

General comments by the farmers surveyed indicate a disturbing degree of misunderstanding and misconception regarding the properties and limitations of current anthelmintics. The survey also revealed a lack of appreciation of the rationale of prophylactic (‘preventive’) parasite control. These aspects are discussed in relation to currently recommended control strategies and the problem of anthelmintic resistance.     

A survey of anthelmintic resistance on ten goat farms in the Manawatu region in 1988

A survey of anthelmintic resistance on ten goat farms was carried out. The criterion for resistance was a faecal egg count reduction (FECR) of <90 percent based on arithmetic means of 11 to 19 goats per group. It was found that nine of the ten farms harboured nematodes resistant to oxfendazole when used at 5mg/kg although on two of these, the FECR's were 89.2 percent and 89.8 percent. Post treatment larval cultures indicated Haemonchus, Ostertagia and Trichostrongylus were the resistant genera involved. On none of the ten farms was resistance to oral ivermectin at 0.2mg/kg detected.     

A survey of anthelmintic usage for sheep: a time for change

A sample survey of anthelmintic usage and farmers; drenching policies was conducted on 614 sheep farms, in the North and South Islands, selected from among those carrying 1500 or more breeding ewes. The survey was based on information for the 1978179 and 1979/80 farming years recorded on questionnaires completed during personal interviews between Livestock Officers and the farmers concerned. Results show that there were no significant between-Island differences in mean drenching frequencies for any age class of sheep. In 1979/80, the overall New Zealand drenching frequencies were 6.3 for lambs, 1.8 for 1-2-year-olds and 1.2 for sheep older than 2 years. Of the farmers surveyed, 20% did not drench 1-2-year-old sheep and 29% did not drench sheep older than 2 years. There were also no between-Island differences in timing of drenches and alternation of drench brands. Sixty-nine per cent of the farmers followed a pre-determined drenching programme whereas 27% stated that they drenched if and when necessary. For lambs, some drenching was carried out in all months of the year hut fewer farmers drenched during the mid-winter to mid-spring period (July-October). Older stock were drenched in all months also but there was a general acceptance by farmers of the practices of pre-tupping and pre or post-lambing drenching. At the time of the survey, 48% of the farmers were using three or more brands of drench with a maximum of nine. Analysis of data relating to brands of drench used shows that whereas 39% of the farmers w,ere alternating drench families [i.e. benzimidazoles (BZ) c.f. non-benzimidazoles (NBZ)] within a single farming year, only 3% were practising a strict alternation of drench families between years. General comments by the farmers surveyed indicate a disturbing degree of misunderstanding and misconception regarding the properties and limitations of current anthelmintics. The survey also revealed a lack of appreciation of the rationale of prophylactic (;preventive;) parasite control. These aspects are discussed in relation to currently recommended control strategies and the problem of anthelmintic resistance.     

The identity of nematode genera involved in cases of ovine anthelmintic resistance in the southern North Island of New Zealand

Larval cultures and faecal egg count reductions were used to determine the identities and frequencies of occurrence of nematode genera involved in cases of ovine anthelmintic resistance submitted to the Batchelar Animal Health Laboratory between 1992 and 1994. Based on an examination of 102 cases, it was found that, while there were some differences relating to the method of assessment, the genera most frequently involved were Trichostrongylus, Ostertagia and Nematodirus. Also implicated, although less commonly, were infections of Oesophagostomum/Chabertia, Haemonchus and Cooperia. The majority of cases involved resistance in two or more genera, with resistance in a single nematode genus occurring in 45% of cases. Most such monogeneric resistances comprised infections of Trichostrongylus (52%), Ostertagia (17%) or Nematodirus (11%). Anthelmintic resistance involving Haemonchus alone was found in only three cases. These results suggest that the potential role of narrow-spectrum drenches in the management and control of anthelmintic-resistant worms in New Zealand sheep is likely to be strictly limited.     

Anthelmintic resistance in the field: Changes in resistance status of parasitic populations in response to anthelmintic treatment

Changes in anthelmintic resistance in nematode parasites were monitored in sheep grazing on 2 separate farms, but with the same anthelmintic treatment program, over 16 years. High levels of benzimidazole resistance emerged in Ostertagia and Trichostrongylus spp populations on both farms following 9 years of continuous use of this class of drug. Subsequently, variations in the levels of resistance occurred for the same species between farms and between species on the same farm. A change to levamisole for 2 years resulted in a significant reversion towards benzimidazole susceptibility, but a concomitant rise in levamisole resistance, in Ostertagia on one farm. However, benzimidazole resistance increased rapidly following the re-introduction of oxfendazole into the anthelmintic treatment program. Results from both farms illustrate the pitfalls of using one anthelmintic class for an extended period and provide indirect support for the alternation of anthelmintic classes at approximately yearly intervals.     

The prevalence of anthelmintic resistance in sheep nematodes on the central tablelands of New South Wales

A survey was undertaken to assess the prevalence of anthelmintic resistance in sheep nematode populations on 40 commercial farms distributed throughout the central tablelands of New South Wales. Representatives of the 2 major groups of broad spectrum anthelmintics with different modes of action (thiabendazole and levamisole) were used at the manufacturer's recommended dose rates. Efficacy was assessed on the basis of the reduction in faecal strongyle egg counts 7 days after treatment. An efficacy of less than 90% using both anthelmintics was obtained on 4 farms. Thiabendazole had an efficacy of less than 90% on a further 21 farms and levamisole had an efficacy of less than 90% on an additional 4 farms. There was no evidence of anthelmintic resistance on 8 farms, while the remaining 3 had insignificant parasite burdens. Based on larval cultures from faeces, Ostertagia and Trichostrongylus were the most significant species in resistant populations. Haemonchus burdens were sporadic and levels of resistance relatively low. Nematodirus burdens were widespread but no evidence of resistance was detected.     

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.     

Resistance to macrocyclic lactone anthelmintics and associated risk factors on sheep farms in the lower North Island of New Zealand

AIM: To estimate the prevalence of resistance to macrocyclic lactone (ML) anthelmintics over a 3-year period on sheep farms in the Taihape area, and to determine associations between the presence of ML resistance and farm management practices. METHODS: All farmers with >1,000 lambing ewes served by one veterinary practice in the Taihape area of the central North Island of New Zealand were invited to take part in the study (n=157); respondents were selected on their willingness to participate. Resistance to ML was measured during 2004-2006, using a standard faecal egg count reduction test (FECRT) and a half dose of oral ivermectin (IVM; 0.1 mg/kg; n=84). Additional FECRTs were conducted separately on some farms, using full doses of IVM (n=47), levamisole (LEV; n=49), a benzimidazole (BZ; n=76), and a BZ/LEV combination (n=42). Resistance was defined as <95% faecal nematode egg count reduction (FECR) 7-10 days post-treatment. Larval cultures were performed on pooled faecal samples from all pre- and post-treatment groups for which any positive faecal nematode egg counts (FEC) were recorded. To ascertain whether particular management or drenching strategies were associated with the presence of resistance, a questionnaire comprising 50 questions was developed, and each farmer interviewed individually. RESULTS: Of all farmers invited, 84/157 (54%) participated in this study. Based on undifferentiated FEC, ML resistance to the half-dose IVM was evident on 46/84 (55%) participating farms. The majority of these cases (71%) involved Ostertagia (Teladorsagia) circumcincta. Resistance was also evident to full doses of IVM on 22/47 (47%), BZ on 41/76 (54%), LEV on 18/49 (37%), and BZ/LEV on 2/42 (5%) farms. Positive associations (p<0.05) were evident between the presence of ML resistance and management practices, such as: weaning over half of the lambs onto paddocks not grazed by lambing ewes since June; not always returning lambs to the same paddock after drenching; and the use of visual signs to assess 'worminess'. Measuring drench efficacy in January to March rather than later in the year, and presence of resistance to BZ or LEV were associated with a higher apparent prevalence of ML resistance (p<0.05). CONCLUSIONS: Anthelmintic resistance, and particularly ML resistance, was widespread on sheep farms in the Taihape area, and mostly involved O. circumcincta. The prevalence of resistance was lower on farms on which management practices, particularly those involving lambs, maintained refugia of unselected nematodes. Further investigation is required to develop control strategies which minimise selection for resistant worms, and how these may vary with climate.     

Prevalence of benzimidazole resistance of nematodes in sheep in The Netherlands

A survey was carried out in The Netherlands to determine the prevalence of benzimidazole resistance in sheep nematodes. It was shown that 47 per cent of the 59 investigated flocks harboured trichostrongylid populations with resistance to benzimidazoles, as demonstrated by an LD50 value of 0.12 micrograms ml-1 thiabendazole or higher in an in vitro egg hatch assay. On these farms Haemonchus contortus was found to be the dominant species. In two flocks resistant Ostertagia and, or, Trichostrongylus species populations were found. Farms with resistance were revisited to determine the anthelmintic efficacy in vivo by means of treatment and egg count. In only four of the 19 flocks which showed resistance in vitro was an efficacy of less than 95 per cent in vivo achieved. This means that there is a discrepancy between the in vivo and in vitro results. Resistance was found more frequently on pure sheep farms than on mixed farms. It is suggested that mixed farms offer more refuge for susceptible H contortus populations.