Ivermectin resistant Haemonchus contortus in Louisiana lambs

Fifteen weaned crossbred (Suffolk x Louisiana Native) lambs were treated with albendazole (10 mg kg-1) to remove existing nematode infections. They were inoculated with Haemonchus contortus infective larvae from a residual population surviving treatment with the oral formulation of ivermectin (0.2 mg kg-1). One group of five lambs remained untreated, another group of five was treated with the injectable formulation of ivermectin (0.2 mg kg-1), and a third group of five was treated with the oral formulation of ivermectin (0.2 mg kg-1). Subsequent to the treatments, there was essentially no difference in mean fecal egg count or mean number of H. contortus recovered at necropsy between the three groups.

Resistance to benzimidazole anthelmintics by Haemonchus contortus in goats in peninsular Malaysia

A previous study had suggested that local strains of goat trichostrongyles, comprising largely Haemonchus contortus, might have developed resistance to benzimidazole anthelmintics. A trial involving 18 goats was conducted to confirm this. There was a significant (P < 0.01) reduction in worm burdens in goats given levamisole, but this was not so for those animals given albendazole, fenbendazole, oxfendazole and mebendazole (P > 0.05).

Multiple and multigeneric anthelmintic resistance on a sheep farm in Malaysia

The anthelmintic efficacy of benzimidazoles, levamisole, closantel, ivermectin and moxidectin was evaluated on an institutional farm in Malaysia using faecal egg count reduction tests, controlled slaughter trials and an in vitro egg hatch assay. The results of this study indicated simultaneous resistance of Haemonchus contortus against benzimidazoles and ivermectin and of Trichostrongylus colubriformis against benzimidazoles and levaminsole on the same farm. Moxidectin was effective against the ivermectin resistant H. contortus.

Serum antibody responses of Texel sheep experimentally infected with Haemonchus contortus

The primary and secondary serum antibody responses of Texel sheep to infective larvae (L3) and adult worms of Haemonchus contortus were studied. Ten-month-old sheep were infected with 20,000 H contortus L3, treated with ivermectin seven weeks later and, after four weeks, reinfected with 10,000 L3 once a week for six weeks. Faecal egg counts were significantly lower during the secondary infection than during the primary infection, but both infections induced antibody responses, as demonstrated by an enzyme-linked immunosorbent assay (ELISA). The primary antibody response developed rather slowly, but the secondary response developed more rapidly and the IgA responses against L3 antigens and the IgG1 and IgG2 responses against adult antigens were twice those observed during the primary infection. These accelerated and enhanced responses after the reinfection suggest an immunological memory for H contortus antigens.

Control of haemonchosis in Malaysian goats with closantel

The therapeutic and prophylactic effects of closantel on natural infections with Haemonchus contortus were studied in goats in Peninsular Malaysia. Closantel was highly effective against H. contortus, either at a subcutaneous (s.c.) injection of 5.0 mg kg-1 body weight (100%), or in an oral drench mixture with mebendazole at a dose of 10.0 mg kg-1 (99.2%), as indicated by faecal egg counts. H. contortus larvae were absent from faecal cultures for 5, 6 and 7 weeks following treatment with s.c. injections of closantel at doses of 2.5 mg kg-1, 5.0 mg kg-1 and 10.0 mg kg-1 respectively, and for 6 weeks after treatment with closantel at 10.0 mg kg-1, given orally. Through its sustained activity, closantel not only prevented reinfection with H. contortus but also caused a dramatic reduction in pasture contamination. The potential utility of closantel in the strategic control of haemonchosis in goats, and as an alternative treatment for benzimidazoles and levamisole resistant H. contortus strains, is discussed.

Anthelmintic resistance in Haemonchus contortus from sheep in Malaysia

Albendazole, oxfendazole, fenbendazole, levamisole, closantel, ivermectin and febantel were administered to sheep on four farms and their efficacy assessed by faecal egg count reduction test. High level of resistance of Haemonchus contortus was found to benzimidazoles (albendazole, oxfendazole, fenbendazole) on all farms and to febantel on the one farm where it was tested. No resistance to closantel and levamisole was observed. Resistance to ivermectin was absent on the three farms examined under this study, but has been reported on the fourth farm in earlier work. It is concluded that anthelmintic resistance to benzimidazoles and the probenzimidazole, febantel, is a serious and widespread problem in H. contortus in sheep in Malaysia.

Effect of subclinical Eimeria species infections in tropical goats subsequently challenged with caprine Haemonchus contortus

Fourteen goat kids of the local indigenous breed naturally infected with Eimeria species were divided into two equal groups. The first group was superinfected with 500,000 Eimeria species oocysts and the second group was treated with amprolium. Sixty days later both groups were infected with 5000 third-stage caprine Haemonchus contortus larvae. The goats experimentally superinfected with eimeria shed more H contortus eggs and gained weight more slowly.

Kinetics of expulsion of Haemonchus contortus from sheep and jirds after treatment with closantel

Experiments were conducted in sheep after intramuscular treatment with closantel and in jirds after oral treatment with closantel to determine when expulsion of established H. contortus commences. Expulsion starts at about 8 h in sheep and coincides with the onset of reduced motility in worms recovered from the abomasum. In jirds, expulsion starts by 2 h after treatment. Experiments also conducted in jirds showed that infective larvae are first killed by circulating closantel 3 days after infection, when blood feeding starts, and that by 8 days 80% of larvae are lost.

Acute or chronic administration of prolactin alters ovine infections of Haemonchus contortus

Prolactin has been implicated as a modulator in the phenomenon of periparturient egg rise during ovine nematodiasis. In the first experiment, lambs (4 months of age) with patent infections (10,000 infective larvae) of Haemonchus contortus were injected daily (Days 28-39 post-inoculation (PI)) with prolactin (20 or 200 IU) or saline. Fecal egg concentrations and total daily egg production were monitored for each lamb. After 12 days, lambs injected with the lower dose of prolactin had the highest daily egg production. Fewer adult worms were collected from prolactin-treated lambs, and the female worms were larger than those from the saline-injected controls. Worm size and number were not correlated. In the second experiment, lambs were injected daily with ovine prolactin (25 IU) or saline. Five days after initiation of treatments, each lamb was inoculated with 3000 infective larvae of H. contortus. After 42 days of injections (Day 37 PI), prolactin-treated lambs had larger and fewer worms, with higher fecundity, than saline-treated controls. Increases in endogenous circulating prolactin during late pregnancy and lactation in ewes, therefore, might contribute to periparturient egg rise irrespective of the developmental stage of the parasite when the hormone exposure occurs.

The efficacy and persistent anthelmintic effect of ivermectin in sheep

The direct efficacy and the long-term persistent anthelmintic effect of an oral suspension and an injectable formulation of ivermectin at a dose rate of 0.2 mg kg-1 was studied in sheep. Lambs were infected experimentally with Haemonchus contortus, Trichostrongylus vitrinus and Cooperia curticei. After 3 weeks they were treated with an oral suspension or with an injection, and reinfected with the same dose of larvae 3, 6 or 10 days after treatment. Post-mortem worm counts showed no persistent effect of the oral suspension. The injectable formulation showed an excellent persistent effect for up to 10 days against H. contortus. Reinfection with C. curticei 3 days after treatment resulted in a 64% reduction of the worm burden, but reinfection after 6 and 10 days was 100% reduced. The reduction of T. vitrinus was 46% after 3 days and 92% after 6 and 10 days.

Demonstration of co-resistance of Haemonchus contortus to ivermectin and moxidectin

Studies were conducted to determine whether organisms which are resistant to ivermectin are also resistant to moxidectin. The mechanisms of action of moxidectin and ivermectin were compared by measuring the changes in membrane conductance they induced in leg muscle fibres of the common shore crab (Pachygrapsus crassipes) by using standard micro-electrode techniques. Meriones unguiculatus (jirds) were infected with ivermectin-resistant or -susceptible strains of Haemonchus contortus, treated with moxidectin or ivermectin at a dose which routinely clears more than 98 per cent of the susceptible strain, and examined for parasite clearance. The results showed that moxidectin induced a rapid loss of membrane resistance in the muscle preparation, and that the effect was almost 50 per cent reversible with the chloride channel-blocker picrotoxinin; this pattern of activity is qualitatively similar to that of ivermectin. In the jird model, moxidectin achieved a clearance of only < or = 47.2 per cent against ivermectin-resistant H contortus at a dose which invariably clears > or = 98 per cent of an ivermectin-susceptible strain. These results indicate that moxidectin and ivermectin share a common mechanism of action, and that organisms that are resistant to ivermectin are also likely to be resistant to moxidectin.

An outbreak of haemonchosis associated with anthelmintic resistance in sheep

An outbreak of haemonchosis associated with anthelmintic resistance was recorded in a flock of 150 crossbred lambs. Closantel and ivermectin were 100% effective against Haemonchus contortus while fenbendazole and morantel reduced faecal egg counts by 87 and 29%, respectively. Levamisole showed an efficacy of 95%. At double the recommended dose (10 mg kg-1), fenbendazole had an efficacy of 99% against H. contortus infection. This appears to be the first report of an outbreak of haemonchosis associated with fenbendazole and morantel resistance in a strain of H. contortus in sheep.

Model predictions for anthelmintic resistance amongst Haemonchus contortus populations in southern Brazil

A computer model developed to study Ostertagia circumcincta resistance to anthelmintics in UK sheep flocks has been adapted for use with Haemonchus contortus under southern Brazilian conditions. The model simulates the effect of different anthelmintic control regimens on the year-to-year pattern of resistance in breeding ewes. The nematode control regimen most used by Brazilian sheep farmers was found to increase the frequency of genes which confer resistance from approximately 3% to 14% in an H. contortus population over a 20 year period. The effect of early versus late season anthelmintic treatment was investigated. This indicated that early season treatment would select for resistance rapidly, whereas late season treatments would not, owing to large numbers of untreated parasites accumulating at the beginning of the season. A model which can predict the development of anthelmintic resistance in parasites of ewes is a valuable tool in the understanding of the effect of different strategies on nematode control programmes and merits further consideration.

Survival and development of ivermectin-resistant or susceptible strains of Haemonchus contortus under field and laboratory conditions

The free-living development of three strains of Haemonchus contortus was studied in two experiments. Day 21 faecal samples were collected from lambs infected with either a susceptible strain, a laboratory-selected ivermectin (IVM) resistant strain or a South African field strain showing multiple anthelmintic resistance, which included IVM. No eggs hatched in samples cultured at 4 or 10 degrees C. At 22 degrees C the laboratory-selected strain showed the highest rate of development while at 27 degrees C the susceptible strain produced the highest yield of third stage larvae (L3): at both temperatures the field strain showed the lowest percentage development to L3. The second experiment was a field study carried out in southern Brazil. Faeces containing either an IVM-susceptible or an IVM-resistant strain of H contortus were placed in two series of grass plots during each of three summer months. Soil subsequently yielded more larvae than did grass suggesting migration or mechanical transport into the soil. For plots contaminated during the first two months there was no significant difference in recovery rate between the two strains (P > 0.05). When contamination occurred during the third month, the IVM-resistant strain produced significantly higher recovery rates (P < 0.05) from both pasture and soil.

The pattern of faecal egg output in lambs infected with a multiple resistant strain of Haemonchus contortus after treatment with albendazole

Worm-free lambs (n = 6) were infected with 5,000 L3 of an ivermectin/benzimidazole/closantel resistant strain of Haemonchus contortus. On Day 21 post infection animals were allocated, according to worm egg counts, to 2 groups of 3 animals each and on Day 29 one group was treated with albendazole at 3.8 mg kg-1, while animals in the other group remained untreated as controls. All animals were slaughtered on Day 44 when the treated group was found to have 2,083 +/- 1,718 worms and the controls 2,783 +/- 633, a reduction of 25.2% in worm burdens. The pattern of egg output was monitored and it was found that 2 d after dosing, worm egg counts had been suppressed by 89% in the treated group; thereafter the egg counts increased, but never returned to the pretreatment levels and by Day 15 post treatment, they were still 41.1% lower in the treated group than in the controls.

Efficacy of moxidectin against a strain of Haemonchus contortus resistant to ivermectin, a benzimidazole and a salicylanilide

The efficacy of moxidectin at 0.2 mg kg-1 live body mass against the multiple anthelmintic resistant White River Krtz strain of H. contortus was determined and compared to that of ivermectin, a benzimidazole, a salicylanilide and levamisole. Moxidectin and levamisole were effective against this strain of H. contortus and reduced the arithmetic mean burdens of this parasite in the sheep by 99.98% and 99.59%, respectively, while the efficacy of ivermectin, albendazole and closantel was only 29.1%, 33.75% and 78.3%, respectively.

Controlled tests of activity of several antiparasitic compounds against natural infections of Haemonchus contortus and other helminths in lambs from a flock established in 1962

Antiparasitic activity of several compounds was evaluated over a long period (about 25 years) in the same flock of sheep. Haemonchus contortus was of special interest, including its relation to drug resistance, especially to thiabendazole and other benzimidazoles, in addition to phenothiazine. Eleven compounds were evaluated in 15 controlled tests, done between 1966 and 1989 in naturally infected lambs (n = 145) born and raised on the same pasture. Sheep were first placed on the pasture in 1962, and a few more were added thereafter. Internal parasites in these sheep were classified in 3 general categories: indeterminate exposure to parasiticides; H contortus, resistant to thiabendazole; and H contortus, resistant to phenothiazine. The parasitic infections probably became more homogeneous after several years because of few introductions of outside sheep after initial establishment of the flock. Activity against naturally acquired internal helminths was evaluated for cambendazole (CBZ: dosage, 20 mg/kg of body weight), fenbendazole (FBZ: 5 or 7.5 mg/kg), mebendazole (MBZ: 10 mg/kg); oxfendazole (OFZ: 3.5 or 10 mg/kg), oxibendazole (OBZ: 10 mg/kg); parbendazole (PBZ: 15 mg/kg), phenothiazine (PTZ: 550 mg/kg); pyrantel pamoate (PRT: 25 mg base/kg), tetramizole (TET: 15 mg/kg); thiabendazole (TBZ: 30 or 44 mg/kg), and trichlorfon (TCF: 100 mg/kg). Thiabendazole was used more often (9 tests) than the other compounds. Thiabendazole was more active against mature H contortus in later years than when first used in 1966, although it was never 100% effective. Efficacy against immature H contortus for TBZ did not exceed 86%. Activity against immature and mature stages of this parasite was good overall for the other benzimidazoles.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of closantel against fenbendazole and levamisole resistant Haemonchus contortus in small ruminants

Sheep infected with a fenbendazole resistant strain of Haemonchus contortus and goats with a levamisole resistant strain of the same parasite were used to evaluate the anthelmintic efficacy of closantel. Closantel at the rate of 10 mg/kg body weight was 100% effective against the 2 resistant strains of the parasite as indicated by faecal egg count and worm reduction. The efficacy of fenbendazole in sheep and of levamisole in goats was 56.68 and 76.90%, respectively.

Haemonchus contortus resistance to ivermectin and netobimin in Brazilian sheep

Suffolk, Texel, Hampshire Down and Ile de France sheep from the municipalities of Porto Amazonas, Piraquara and Araucaria in the State of Paraná, and Bagé in the State of Rio Grande do Sul were brought to Sobral, State of Ceará, to be used in a cross-breeding project. On arrival they had clinical signs of nematode parasitosis, and one Suffolk female died. The animals were treated orally with ivermectin (0.2 mg kg-1) and fifteen days later with netobimin (20.0 mg kg-1). Neither drug reduced the egg counts (measured in eggs per gram, EPG) significantly, and this suggested that the nematodes in the sheep were resistant to the anthelmintics used. Haemonchus contortus was the species involved. The egg counts were reduced after oral treatment with trichlorfon (100.0 mg kg-1). Haemonchus contortus larvae obtained from these animals before trichlorfon treatment and passaged through two nematode-free sheep were used in a further experiment. Twenty 6- to 9-month-old nematode-free lambs were infected with the H. contortus larvae (10,000 per animal) and after the infection was confirmed, were randomly divided into four groups of five animals. Group I was orally treated with ivermectin at 0.2 mg kg-1, Group II with oral netobimin at 20.0 mg kg-1, Group III with oral trichlorfon at 100.0 mg kg-1 and Group IV was a non-treated control. Egg counts and faecal cultures were taken before dosing on the day of treatment and seven days later when all animals were necropsied and the nematodes were collected from the abomasa and counted.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of moxidectin against gastrointestinal nematodes of cattle

Three groups of 11 naturally infected crossbred beef calves were injected subcutaneously with moxidectin 1 per cent injectable at 0.2 or 0.3 mg moxidectin/kg bodyweight or with the unmedicated vehicle. Nematode infections had been acquired during grazing from December to April. Based on the faecal egg counts and total worm counts of the control calves at necropsy (11 to 13 days after treatment) most of the calves had heavy parasitic burdens. Ostertagia ostertagi was predominant and the mean numbers of adults, developing fourth stage larvae (L4) and inhibited early L4 were 45,906, 10,061 and 68,918, respectively. Haemonchus placei and Trichostrongylus axei were also present in the abomasa. Three species of Cooperia, Oesophagostomum radiatum L4 and T colubriformis adults were found in the intestinal tract. Both dosages of moxidectin were equally effective (P < 0.05) against all the abomasal nematodes (99.9 to 100 per cent) and the intestinal tract nematodes (99.4 to 100 per cent). No adverse reactions to the moxidectin treatment were observed. Abomasal pathology characteristic of heavy O ostertagi infection was observed in the control calves, but not in the treated calves.

Response to ivermectin treatment of parasitic stages of Haemonchus contortus resistant or susceptible to ivermectin

Two groups of 33 helminth-naive lambs were infected with 5,000 L3 of an ivermectin-resistant or -susceptible strain of Haemonchus contortus (groups R and S). On days 6, 10, 16, and 21 postinfection, 5 animals from each group were chosen at random and orally treated with 0.2 mg/kg of ivermectin. On each occasion, 2 randomly selected lambs from each group were also killed to determine the number and stage of development of the worms present at the time of treatment. These necropsies revealed that by day 6 early and late fourth-stage larvae were present, whereas on day 10 the early fifth stage had been reached; by days 16 and 21 all worms had reached the adult stage. Necropsies on day 28 postinfection revealed that although animals treated at day 6 had 26.3% fewer worms than the controls, there was no significant difference (P greater than 0.05) between worm burdens from any of the animals infected with the R strain and treated at different times after infection when compared with the untreated controls. With ivermectin significant reductions were obtained in the worm burdens of the animals infected with the susceptible strain; these were reduced by 96% when treatment was given on day 6 against fourth-stage larvae and 98.9% when the drug was given on day 21 against adult stages. From these results it is clear that resistance to ivermectin in this strain of H. contortus is present as early as the fourth larval stage.

Resistance to ivermectin by Haemonchus contortus in goats and calves

Efficacy of ivermectin on susceptible or resistant populations of the parasitic nematode Haemonchus contortus was determined in cattle and goats held in a barn. Goats were each infected with 3000 infective, ivermectin-susceptible or -resistant H. contortus larvae on day 0 and reinfected with 2000 infective larvae on day 24. Goats were treated orally with 600 micrograms kg-1 ivermectin on day 31. No significant differences were detected in blood packed cell volume (PCV) or total protein (TP), prepatent period, or epg among the four groups of goats that were each infected with one of four parasite strains (one susceptible, three resistant). There were no differences among the four parasite strains in the numbers of infective larvae that developed to the third larval stage from fecal cultures or in the viability of cultured infective larvae when held in the laboratory at 27 +/- 1 degrees C for 14 weeks. After treatment with ivermectin, there were significant differences among the parasite strains in PCV, TP, and epg. Total worm counts were reduced by 94 to 97% with three times the recommended dose. Immature and adult Skrjabinema ovis were also present in two treated goats. In a second test, one goat infected once with 10,000 infective larvae of a resistant strain of H. contortus and then treated with nine doses of ivermectin, increasing from 500 to 2000 micrograms kg-1 over a period of 133 days, had 35 adult worms at necropsy. In a third test, three calves were readily infected with an ivermectin-resistant strain of H. contortus from goats.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of ivermectin against experimental infections of Haemonchus contortus and Trichostrongylus colubriformis in goats

Fourteen nematode parasite-naïve kids were given a mixed larval inoculum (approximately 17,250 infective larvae) via stomach tube. Twenty-two days after infection, the kids were randomly assigned to control and treatment groups of seven animals each. Mean eggs per gram for control and treatment groups were 4304 and 5315, respectively. Kids in the treatment group were given ivermectin (0.2 mg kg-1) administered by subcutaneous injection. At necropsy, the mean numbers of Haemonchus contortus and Trichostrongylus colubriformis for control and treatment groups were 2259 and 0, and 3033 and 773, respectively. This reduction was significant (P less than 0.05) for both species, resulting in an efficacy of 100.0% and 74.5%, respectively. The lower efficacy against T. colubriformis may be a sign of resistance, a reduced effectiveness due to route of administration, or a higher dose may be required with subcutaneous administration, as has been observed previously.

Efficacy of moxidectin, nemadectin and ivermectin against an ivermectin-resistant strain of Haemonchus contortus in sheep

The efficacies of ivermectin, nemadectin and moxidectin were evaluated when administered orally to lambs infected with either a susceptible laboratory strain of Haemonchus contortus or a strain reported to be resistant to ivermectin. Groups of 24 Dorset cross Cheviot cross Suffolk lambs were infected with either the susceptible or resistant strain of H contortus and allocated to treatment groups according to their faecal egg counts 27 days after infection. One day later the lambs were dosed orally with one of the three anthelmintics at 0.2 mg/kg bodyweight, and they were killed and surviving worms were recovered 13 or 14 days after treatment. Against the ivermectin resistant strain, ivermectin did not significantly reduce the egg count or the numbers of adult H contortus; however, both nemadectin and moxidectin reduced the nematode egg counts and the numbers of H contortus by 99 and 100 per cent, respectively. Against the susceptible strain, all the anthelmintics reduced the egg counts by 100 per cent as early as four days after treatment and reduced the numbers of susceptible H contortus by 100 per cent. No adverse reactions to any of the drugs were observed.

Effects of dietary molybdenum on nematode and host during Haemonchus contortus infection in lambs

The addition of molybdenum (0.05 mmol kg-1 dry-matter) to the diet of lambs given a trickle infection of Haemonchus contortus larvae (500 third stage larvae d-1 over six weeks) reduced mean faecal egg counts (epg) from 3952 to 2312 +/- 402 by 32 days (P less than 0.02) and greatly reduced the mean number of worms recovered from the abomasum 14 days after infection ceased (907 compared with 4167: P less than 0.01). Infection reduced haemoglobin concentrations less in lambs given molybdenum although the difference was small relative to the reduction in worm burden. Lambs not given molybdenum had low intraepithelial mast cell counts in the abomasal mucosa and less abomasal hypertrophy than expected from abomasal parasitism. Molybdenum did not consistently reduce the copper status of the host or the parasite. Previous exposure to molybdenum greatly reduced protein but not proteinase activity in, or secreted by, adult worms cultured for eight hours. It is suggested that molybdenum either increased the inflammatory response which preceded worm rejection or that it indirectly enhanced that reaction by reducing the effectiveness of copper-dependent, anti-inflammatory enzymes in the gastrointestinal mucosa.

Efficacy of moxidectin against an ivermectin-resistant strain of Haemonchus contortus in sheep

The efficacy of moxidectin was determined against ivermectin-susceptible and resistant strains of Haemonchus contortus. At the onset of the trial, 40 lambs were each infected with 5000 third stage larvae of one of two strains of Haemonchus contortus. The lambs were randomly sorted into eight treatment groups 28 days post-infection and were treated as follows: Group 1, susceptible strain with no treatment; Group 2, resistant strain with no treatment; Group 3, susceptible strain treated with 0.2 mg moxidectin kg-1 body weight; Group 4, resistant strain treated with 0.2 mg moxidectin kg-1; Group 5, resistant strain treated with 0.4 mg moxidectin kg-1; Group 6, susceptible strain treated with 0.2 mg ivermectin kg-1; Group 7, resistant strain treated with 0.4 mg ivermectin kg-1; Group 8, resistant strain treated with 0.8 mg ivermectin kg-1. The lambs were killed 1 week post-treatment. Comparisons were made among groups based on the number of eggs per gram of feces on the day of treatment and the numbers of worms recovered from each lamb. Both moxidectin and ivermectin were effective in removing susceptible Haemonchus with efficacies of 100% and 99.7%, respectively. The efficacy of moxidectin against the resistant strain was 99.9% and 100% at 0.2 mg kg-1 and 0.4 mg kg-1, respectively, whereas there were only 38.8% and 53.1% efficacies in the lambs treated with 0.4 mg ivermectin kg-1 and 0.8 mg kg-1 body weight, respectively.

Controlled tests on activity of contemporary parasiticides on natural infections of helminths in lambs, with emphasis on strains of Haemonchus contortus isolated in 1955

Ten controlled tests were done between 1972 and 1989, in lambs on pasture, evaluating activity of fenbendazole (FBZ; 5 mg/kg of body weight), oxfendazole (OFZ; 3.5 and 10 mg/kg), oxibendazole (OBZ; 10 mg/kg), pyrantel pamoate (PRT; 25 mg of base/kg), and thiabendazole (TBZ; 44 and 50 mg/kg) against natural infections of helminths, with emphasis on 2 strains (A and B) of Haemonchus contortus. Strain A was phenothiazine-susceptible and strain B was phenothiazine-resistant when isolated in 1955. For approximately 10 years prior to these tests, sheep infected with both strains had been treated periodically each year with several compounds, including thiabendazole, which was used many more times than the other drugs. For this study, 4 (FBZ, OFZ, OBZ, and PRT) of the 5 compounds were evaluated in either 1 or 2 controlled tests. The fifth compound, TBZ, was used for 5 tests. Strain A H contortus was resistant to TBZ for all years tested, but more susceptible to FBZ, OFZ, OBZ, and PRT. Overall, strain B was susceptible to TBZ (with a few exceptions), and also to FBZ, OFZ, OBZ (activity less on immature forms), and PRT. Other abomasal parasites (2 species of Ostertagia and 3 of Trichostrongylus) were found in low numbers, but removal overall was good for the compounds tested. Trichostrongylus axei, found in higher numbers than species of Ostertagia and other species of Trichostrongylus, were effectively removed by all compounds in most cases. Activities of TBZ and PRT were also evaluated against several species of intestinal helminths, most of which were found in low numbers. Cooperia curticei were inconsistently removed by TBZ, but activity of PRT was effective.(ABSTRACT TRUNCATED AT 250 WORDS)

[The threat of wide dissemination of anthelmintic resistance by veld ram performance testing units]

A veld ram performance testing unit consists of an association of stud or commercial ram breeders, who compare the performance of selected rams under field conditions on common pastures. The best performers are subsequently auctioned at public sales. To date, at least 10 Merino veld ram clubs have been established in various centres in South Africa. A strain of Haemonchus contortus, which was isolated from the pastures of one of these performance testing units, was found to be resistant to ivermectin, oxfendazole and rafoxanide. Closantel, levamisole and disophenol were more than 99% effective. The wide dispersal of the rams after testing, constitutes a potential danger for dissemination of helminths with resistant genes. Precautionary methods should include careful routine monitoring of drug susceptibility on the communal pastures, on the farms of individual members and thorough deworming of rams before they leave the testing unit. The prevention and control of resistance in such testing units are discussed, but it is emphasised that this is difficult, particularly if the worm strains on the communal grazing have become resistant to some of the anthelmintic groups.

Disposition of oxfendazole in goats and efficacy compared with sheep

The disposition of intraruminally administered oxfendazole (OFZ) in goats was studied at 5, 10 and 20 mg kg-1. The area under the plasma concentration with time curve (AUC) increased with increasing dose but at a declining rate. AUC was lower after intra-abomasal compared with intraruminal administration. OFZ was less effective against drug resistant Trichostrongylus colubriformis in goats than in sheep but was of similar efficacy against drug resistant Haemonchus contortus in both host species. In the same experiment peak plasma levels of OFZ in goats were about half those in sheep given the same dose. Of 70 goats tested in the field, total rumen bypass occurred in 12 per cent and partial bypass in 67 per cent. Lower systemic availability due to bypass would be expected to reduce further anthelmintic efficacy in goats. From the results of these experiments a dose rate of 10 mg kg-1 is recommended for goats. When given at this rate as a divided dose at 12 hourly intervals over 24 hours, OFZ was significantly more effective than a single dose in reducing egg counts.

Concurrent infections with the ruminant nematodes Haemonchus contortus and Trichostrongylus colubriformis in jirds, Meriones unguiculatus, and use of this model for anthelmintic studies

Haemonchus contortus- and Trichostrongylus colubriformis-infected jirds (Meriones unguiculatus) are useful for anthelmintic studies. With concurrent infections of these parasites established in the jird, questions of not only anthelmintic activity, but to some extent spectrum, could be assessed in a single model system. This report outlines a model using immunosuppressed (0.02% hydrocortisone in feed) jirds concurrently infected with H. contortus and T. colubriformis. Immunosuppressed jirds were inoculated with approximately 1,000 exsheathed infective larvae of each species, treated per os on day 10 postinoculation (PI), and killed on day 13 PI. Stomachs and small intestines were removed, opened longitudinally, incubated in distilled water at 37 C for 5 hr, fixed in formaldehyde solution, and stored for subsequent examination. Contents of both organs were examined using a stereomicroscope (15-45 x). Various standard anthelmintics were evaluated in the model; modern broad-spectrum ruminant anthelmintics (benzimidazoles, febantel, ivermectin, levamisole hydrochloride, and milbemycin D) are active uniformly and in most cases at doses comparable to those required for efficacy against these parasites in ruminants. This model, using worms of 2 genera living in distinct sites, allows preliminary evaluation of anthelmintic activity and spectrum for experimental compounds in a single cost- and resource-efficient experiment.

Resistance to thiabendazole, fenbendazole and levamisole in Haemonchus and Trichostrongylus species in sheep on a Kenyan farm

Resistance to thiabendazole (TBZ), fenbendazole (FBZ) and levamisole (LVM) in naturally acquired gastrointestinal nematode parasites in sheep was investigated on a farm where anthelmintic resistance was suspected. This was measured by both the in vitro egg hatch assay, and reductions in faecal egg and worm counts in treated animals. In the egg hatch assay, nematode eggs were incubated in various concentrations of either TBZ or LVM. The level of resistance was expressed as the drug concentration inhibiting 50% of the eggs from hatching (LC50). The nematode population had LC50 values of 0.26 microgram ml-1 TBZ and 3.12 micrograms ml-1 LVM. In the faecal egg and worm count reduction test, naturally infected sheep were treated with either TBZ (88 mg kg-1), FBZ (10 mg kg-1) or LVM (15 mg kg-1). Faecal egg and total worm counts from these sheep were then compared with counts from untreated sheep. TBZ, FBZ and LVM failed to reduce the faecal egg counts and total worm counts by more than 90%. Based on the identification of larvae from faecal cultures, the most predominant nematode species in the resistant population were Haemonchus (62%) and Trichostrongylus (28%). TBZ reduced faecal egg counts for both species by less than 90%. FBZ and LVM also reduced Haemonchus spp. eggs by less than 90%. Other nematode species numbers did not satisfy criteria for the determination of efficacy.

Efficacy of some anthelmintics on an ivermectin-resistant strain of Haemonchus contortus in sheep

Following evidence of reduced efficacy of ivermectin in a field population of Haemonchus contortus in Brazil, this strain of the parasite was submitted to a controlled anthelmintic test. Eighty worm-free lambs were randomly split into two groups of 40. Each lamb in the first group was infected with 5000 third stage larvae (L3) of the ivermectin-resistant strain; the remaining 40 lambs were each infected with 5000 L3 of a H. contortus strain of known susceptibility to the major groups of anthelmintic compounds used in sheep. On Day 28 post-infection, each group was subdivided according to egg counts and at random into four sub-groups of ten lambs, each of which was treated with albendazole (ABZ) at 3.8 mg kg-1, levamisole (LEV) at 7.5 mg kg-1 or ivermectin (IVM) at 0.2 mg kg-1, or was left as untreated control. At slaughter, 7 days later, all the anthelmintics reduced worm burdens in animals infected with the susceptible strain (ABZ 98.9%, LEV and IVM 100%). By contrast, in the lambs infected with the ivermectin-resistant strain, LEV was 99.8% effective, but ABZ reduced worm counts by only 14.7% and IVM by only 10.4%. Interestingly at necropsy on Day 7 post-treatment, the egg counts in the resistant strain treated with ABZ had been reduced by 92.5%, although worm counts were reduced by only 14.7%.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ivermectin and fenbendazole in strategic treatment of gastrointestinal nematode infections in cattle

Four groups of 18 beef calves each were used to evaluate effects of different treatments on parasite control and weight gains. The investigation extended from November 1986 (weaning) to October 1987. Group-1 calves were treated with ivermectin (200 micrograms/kg of body weight, SC) at approximately 6-week intervals for a total of 8 treatments; group-2 calves were given the same dosage of ivermectin by the same route of administration as group-1 calves in November, March, and July; group-3 calves were given fenbendazole paste (5 mg/kg, PO) at the same times as group-2 calves; and group-4 calves served as untreated controls with provision for ivermectin salvage treatment. All groups grazed on individual pairs of larval-contaminated, 1.6-ha pastures. Highest (P less than 0.05) initial worm counts in fall tracer calves were found in group 3 (Ostertagia ostertagi and Trichostrongylus axei adults) and group 4 (O ostertagi and Haemonchus adults). Fecal egg counts of group-1 calves were low throughout the experiment and pasture larval counts remained negligible after July. Egg counts and larval counts of other groups remained higher into summer. Worm counts, including O ostertagi inhibited early fourth-stage larvae (EL4), were highest (P less than 0.05) in groups-3 and -4 spring tracer calves; numbers of O ostertagi EL4 were similarly high in groups 2, 3, and 4; and T axei counts were highest (P less than 0.05) in groups-3 and -4 yearlings slaughtered in spring. Liveweights of group-1 calves were greater (P less than 0.05) than in other groups from March 2 to October, and by July 2, group-2 calves had a liveweight advantage over group-4 calves. Group-3 calves had the lowest rate of gain from March to July and mean liveweight of the group was less (P less than 0.05) than in all other groups from April to October. Only minimal worm numbers were recovered from groups-1 or -2 calves in October. Large numbers of O ostertagi and T axei were recovered from group-4 calves and O ostertagi from group-3 calves. A few calves in groups 3 and 4, but particularly in group 4, were affected by type-II disease (chronic to acute gastritis caused by maturation and emergence of previously inhibited larvae) from August to October. Final mean liveweights in descending order were 365 kg in group 1, 328 kg in group 2, 316 kg in group 4, and 281 kg in group 3.

The efficacy of netobimin against a benzimidazole susceptible and a resistant strain of Haemonchus contortus in sheep in The Netherlands

Netobimin was tested for efficacy against Haemonchus contortus using 7 groups of 5 parasite-free lambs of six months age. The lambs in group 1 and 2 were infected with 10,000 larvae of a benzimidazole susceptible strain and those in groups 3-7 with the same dose of a resistant strain. The following treatment scheme was applied 21 days after infection: lambs in groups 2 and 4-7.5 mg kg-1 netobimin, in group 5-20 mg kg-1 netobimin, in group 6-5 mg kg-1 oxfendazole and in group 7--3.8 mg kg-1 albendazole. The lambs in groups 1 and 3 remained untreated. All lambs were slaughtered 28 days after infection. Egg counts decreased in all lambs after treatment, but increased again in lambs in groups 4, 6 and 7. There was a slight increase in lambs in group 5, while those in group 2 remained negative. Post-mortem worm counts showed a reduction of 99.8 per cent in lambs in group 2 compared to those in group 1. In lambs in group 4-7 the reduction of worm counts was respectively 40.9, 89.5, 24.7 and 40.7 per cent compared to those in group 3. Egg development assays carried out 20 days after infection showed an average LD50 of 0.46 mg ml-1 thiabendazole for the resistant strain. After treatment (day 27) the LD50 was 0.53, 0.48, 0.58, 0.56 and 0.47 in lambs in the groups 3-7. It is concluded that netobimin and other (pro)-benzimidazoles should not be used in cases of benzimidazole resistance and that levamisole, pyrantel tartrate or ivermectin are preferable.

Efficacy of ivermectin against benzimidazole-resistant nematodes of sheep

Two trials involving a total of 36 Dorset horn lambs were conducted to assess the anthelmintic efficacy of ivermectin against experimental infections of benzimidazole-resistant strains of Haemonchus contortus and Ostertagia circumcincta. Two resistant strains of each of the two species were used and in each trial the lambs were allocated to three groups. One group was given 200 micrograms ivermectin/kg bodyweight orally, the second group was given 5 mg oxfendazole/kg bodyweight orally and the third group remained untreated as controls. Fourteen days after treatment the lambs were necropsied. Ivermectin was found to be more than 99 per cent to 100 per cent effective against all four benzimidazole-resistant strains, whereas oxfendazole was 78.6 per cent and 83.8 per cent effective against the H contortus strains, and 25.6 per cent and 39.8 per cent effective against the O circumcincta strains.

Critical controlled test to evaluate resistance of field strains of Haemonchus contortus to thiophanate

Following reports of suspected gastrointestinal strongylid nematode resistance to thiophanate in sheep and goats at Sokoine University of Agriculture (SUA), Tanzania, field strains of Haemonchus contortus, one from SUA and another from Kimamba, Tanzania, were selected for a critical controlled test to evaluate the efficacy of thiophanate. The SUA H. contortus strain in goats showed a marked resistance to thiophanate at an oral dose of 50 mg kg-1 liveweight (efficacy = 24.1%), while the Kimamba H. contortus strain showed very marked susceptibility (efficacy = 99.7%) to thiophanate at the same oral dose. This is the first report from East Africa of a field strain of H. contortus resistant to thiophanate. The incidence of resistant strains of strongyloid parasites to benzimidazole anthelmintics in sheep and goats in East Africa is on the increase. The magnitude of this problem should be evaluated and remedial solutions sought.

Effect of selection pressure for thiabendazole resistance on fitness of Haemonchus contortus in sheep

A strain of Haemonchus contortus showing moderate levels of resistance to thiabendazole (TBZ) (egg hatch assay LC50 2.1 microM) was subjected, in alternating sequence, to 5 in vitro and 4 in vivo selections with TBZ. This resulted in a significant increase in resistance (egg hatch assay LC50 2.8 microM). The isolates obtained after each combined in vitro and in vivo selection were compared in terms of egg hatch assay, establishment, net egg output, per capita egg output and degree of pathology caused in infected hosts. The increase in resistance was accompanied by a significant increase in larval establishment in sheep (22.8-39.2%) and was associated with more severe pathology, higher egg count and worm recovery 3 months post-infection. The resistant strains were also compared with a TBZ-susceptible strain (egg hatch assay LC50 1.0 microM). The TBZ-susceptible strain had the highest net egg output, it caused the most severe pathology and it resulted in higher worm recovery than any of the TBZ-resistant strains, although the most resistant strain was very similar to the TBZ-susceptible strain. Neither the per capita egg output nor biological features of the free-living stages were significantly affected by the degree of resistance. The implications of these observations for anthelmintic use and for management of nematode anthelmintic resistance in the field are discussed.

A novel anthelmintic model utilizing jirds, Meriones unguiculatus, infected with Haemonchus contortus

Currently, no in vivo laboratory model is available for evaluating anthelmintics against the important ruminant helminth Haemonchus contortus. This report outlines a novel anthelmintic assay utilizing immunosuppressed (0.02% hydrocortisone in feed) jirds, Meriones unguiculatus, infected with H. contortus. Immunosuppressed jirds were inoculated with approximately 1,000 exsheathed infective larvae of H. contortus, treated per os on day 10 postinoculation (PI), and necropsied on day 13 PI. Each stomach was removed, opened longitudinally, incubated in distilled water at 37 C for 5 hr, fixed in formaldehyde solution, and stored for subsequent examination. Stomach contents were examined using a stereomicroscope (15-45x). A variety of standard anthelmintics has been evaluated in the model; modern broad-spectrum ruminant anthelmintics (benzimidazoles, febantel, ivermectin, levamisole hydrochloride, and milbemycin D) are active uniformly and in most cases at doses (mg/kg) comparable to those required for efficacy against H. contortus in ruminants. This model provides an important new tool to assess preliminarily the activity of experimental drugs against H. contortus in vivo prior to studies in ruminants and also may provide a useful tool for studying host-parasite interactions for H. contortus.