Multiple and multigeneric anthelmintic resistance on a sheep farm in Kenya

The anthelmintic efficacy of benzimidazoles (albendazole, fenbendazole and oxfendazole), levamisole, oral ivermectin and closantel was evaluated on a farm in Kenya using faecal egg count reduction test, larval cultures and a controlled slaughter trial. The results of this study indicated simultaneous resistance of Haemonchus contortus against benzimidazoles, levamisole and ivermectin, and of Trichostrongylus colubriformis and Oesophagostomum spp. against levamisole on the same farm. Ivermectin resistance developed to 47% within 15 months of first use. Closantel was effective against the benzimidazoles, levamisole and ivermectin resistant H. contortus.

Comparative in vitro effects of closantel and selected beta-ketoamide anthelmintics on a gastrointestinal nematode and vertebrate liver cells

PNU-87407 and PNU-88509, beta-ketoamide anthelmintics that are structurally related to each other and to the salicylanilide anthelmintic closantel, exhibit different anthelmintic spectra and apparent toxicity in mammals. The basis for this differential pharmacology was examined in experiments that measured motility and adenosine triphosphate (ATP) levels in larval and adult stages of the gastrointestinal nematode, Haemonchus contortus, and in a vertebrate liver cell line and mitochondria. PNU-87407 and PNU-88509 both exhibited functional cross-resistance with closantel in larval migration assays using closantel-resistant and -sensitive isolates of H. contortus. Each compound reduced motility and ATP levels in cultured adult H. contortus in a concentration- and time-dependent manner; however, motility was reduced more rapidly by PNU-88509, and ATP levels were reduced by lower concentrations of closantel than the beta-ketoamides. Tension recordings from segments of adult H. contortus showed that PNU-88509 induces spastic paralysis, while PNU-87407 and closantel induce flaccid paralysis of the somatic musculature. Marked differences in the actions of these compounds were also observed in the mammalian preparations. In Chang liver cells, ATP levels were reduced after 3 h exposures to > or = 0.25 microM PNU-87407, > or = 1 microM closantel or > or = 10 microM PNU-88509. Reductions in ATP caused by PNU-88509 were completely reversible, while the effects of closantel and PNU-87407 were irreversible. PNU-87407, closantel and PNU-88509 uncoupled oxidative phosphorylation in isolated rat liver mitochondria, inhibiting the respiratory control index (with glutamate or succinate as substrate) by 50% at concentrations of 0.14, 0.9 and 7.6 microM, respectively.

Synthesis and endectocidal activity of novel 1-(arylsulfonyl)-1-[(trifluoromethyl)sulfonyl]methane derivatives

We have recently synthesized a series of novel disulfonylmethane compounds that have shown anthelmintic and insecticidal (endectocidal) activity. Several analogues have shown activity against the internal nematode Haemonchus contortus. In sheep studies, these analogues have shown 100% control of this internal parasite at a 10 mg/kg rate. In vitro activity against the biting flies, Stomoxys calcitrans and Haematobia irritans, has been observed at rates as low as 25 and 2.3 ppm, respectively. Only marginal activity against the liver fluke Fasciola hepatica and Trichostrongylus colubriformis was seen. Respiratory control index values on rat liver mitochondria for this series suggested uncoupling of oxidative phosphorylation as a mechanism of action. Compound 1 is considered to be a promising agent for treatment of parasitized sheep.

The nicotinic acetylcholine alpha-subunit gene tar-1 is located on the X chromosome but its coding sequence is not involved in levamisole resistance in an isolate of Trichostrongylus colubriformis

The polymerase chain reaction was used to amplify fragments comprising the known reading frame of the nematode nicotinic acetylcholine alpha-subunit gene tar-1. Sequences were derived from DNA prepared from bulk collections of worms and from individual male and female Trichostrongylus colubriformis. In each case a levamisole-resistant (BCk) and a drug susceptible population were examined. Although several nucleotide transitions were detected no amino acid sequence variations were found between the isolates and between individual worms, indicating that the coding sequence of this gene is not responsible for levamisole-resistance in the isolate tested. However, an intronic allelic T/C variation at position 4955 was observed in both populations. It has been reported that levamisole-resistance in the BCk isolate of T. colubriformis is due to a sex-linked recessive gene or gene complex. A restriction fragment length polymorphism formed by the allelic variation was found and was detectable by digestion with the restriction endonuclease NlaIII. Statistical comparison of allele frequencies from individual male and female worms was consistent with sex-linkage of tar-1 (P < 0.05) but showed no correlation with levamisole resistance status. The polymorphism described will provide a useful X-chromosome marker and represents the first mapped genetic locus in this species.

A survey of anthelmintic resistance in nematodes of sheep in The Netherlands

The prevalence of anthelmintic resistance in nematodes of sheep was surveyed in 1994 on 70 farms in the Netherlands. An in vitro egg hatch assay, faecal egg count reduction (FECR) 14 days after treatment, and larval cultures were used as methods of investigation. Oxfendazole was tested on 69, ivermectin on 51, and levamisole on 36 farms. The median effective dose (ED50) of thiabendazole could be determined on 64 farms. On 60 farms (94%) the ED50 value was > or = 0.12 microgram ml-1, which is indicative of the presence of benzimidazole (BZ) resistance. On two farms egg output was too low to do a FECR test. Based on the results of the FECR test, BZ resistance was present on 56 farms (84%), on 2 farms there was a suspicion of resistance and on 9 farms no resistance could be found. No clear indications were found for the presence of resistance against ivermectin or levamisole. BZ resistance was demonstrated in Haemonchus contortus, Cooperia curticei, Ostertagia spp. and/or Trichostrongylus spp. No resistance was observed in species from the genus Nematodirus, Chabertia ovina and/or Oesophagostomum spp.

The results of anthelmintic-abbreviated infections of Trichostrongylus colubriformis and Teladorsagia circumcincta on fecal egg counts in goats on pasture

Twenty, 2-yr-old angora goats kept on nematode larvae-contaminated pasture since birth were divided into 2 equal groups. Goats from group 1 were immunized by drug-abbreviated infection, a procedure that gave high protection against field challenge in 12-moold sheep. Group 1 was orally dosed 3 times with increasing numbers of Trichostrongylus colubriformis and Teladorsagia circumcincta infective larvae. Each time, the infection was abbreviated with Oxfendazole (OXF) 15 days after dosing. Group 2 received only OXF. After the third dose of OXF, the goats were grazed together on the same pasture and fecal egg counts determined. No protection in immunized goats was achieved. In fact, immunized goats produced significantly more nematode eggs than the nonimmunized group.

The death rate of Ostertagia circumcincta and Trichostrongylus colubriformis in lactating ewes: implications for anthelmintic resistance

Lactating adult Romney ewes were infected, 4 weeks post-lambing, with benzimidazole (bz) resistant strains of Ostertagia circumcincta and Trichostrongylus colubriformis. Commencing 4 weeks after the initial infection the ewes were subjected to challenge 3 times weekly with 5000 L3 of bz-susceptible strains of both parasite species. At weekly intervals over the following 6 weeks, groups of ewes were drenched with a bz anthelmintic (oxfendazole) to remove bz-susceptible parasites and slaughtered to determine adult worm burdens of the bz-resistant parasites. The O. circumcincta infection declined exponentially with a mean daily death rate of 10.6% day-1 and no worms were recovered after 4 weeks or more of challenge. The T. colubriformis infection did not decline significantly over the 6 weeks of continuous challenge, indicating that the death rate could not be distinguished from zero. The upper 95% confidence limit for the death rate of T. colubriformis was 4.9%. The implications of these death rates on selection for drug resistance following ewe drenching during the post-partum period are discussed with selection pressure likely to be greater for T. colubriformis than for O. circumcincta.

Efficacy of an albendazole slow-release capsule for the control of susceptible or resistant nematode parasites of dairy goats

The efficacy of albendazole slow release capsules-ASRC- (Proftril-Captec) on gastro-intestinal nematodes of dairy-goats was assessed both for benzimidazole (BZ) susceptible and resistant strains. For BZ susceptible strains, the efficacy of ASRC, assessed by controlled test, ranged from 92% for Trichostrongylus colubriformis to more than 99% for Haemonchus contortus on existing worm burdens. The administration of the ASRC prevented infection with the same strains for 85 to 91 days post treatment. In a dairy goat farm, where both in vitro and in vivo tests indicated a high level of BZ resistance, the efficacy of ASRC on present infections, assessed by Faecal Egg Count Reduction test, ranged from 20 to 60% according to the sampling date (13 to 88 days post treatment). On the same farm the ASRC given at turnout to non infected (primiparous) goats prevented egg shedding up to 72 days (30 for controls). In contrast the ASRC given to infected (multiparous) goats did not prevent egg shedding. The present results show that the ASRC is an efficient device for controlling BZ susceptible nematodes. The ASRC prevents infection with BZ resistant larvae whereas existing infections are not reduced.

The prevalence of anthelmintic resistance in nematode parasites of sheep in southern Latin America: Paraguay

This survey to detect anthelmintic resistance in nematode parasites of sheep was conducted on 11 farms in the Occidental and 26 farms in the Oriental Regions of Paraguay using the faecal egg count reduction test (FECRT). The anthelmintic groups tested were the benzimidazoles, levamisole and avermectins (both oral and injectable). Overall the levels of resistance were 73%, 68%, 73% and 47%, respectively. Levels of resistance were similar for all three important nematode genera, viz. Haemonchus contortus, Ostertagia and Trichostrongylus. This survey clearly indicates that a large, and ever increasing, proportion of sheep farmers are rapidly approaching the time when they will have exhausted all chemotherapeutic options to control parasites. Unless they face having to abandon their sheep farming operations, radical changes will need to be implemented with urgency.

The prevalence of anthelmintic resistance in nematode parasites of sheep in southern Latin America: Uruguay

This survey was conducted on 252 farms randomly distributed over all the sheep raising areas in Uruguay. The study involved farms with more than 600 sheep, which represented 80% of the total sheep population of the country. Three anthelmintic groups were assessed, namely, benzimidazoles, levamisole and avermectins. Overall, the results showed 80% of sheep flocks had benzimidazole resistance, 71% had resistance to levamisole, and 1.2% of flocks showed resistance to avermectins. Approximately 28% of farms had resistance to one anthelmintic group, 64% to two anthelmintic groups, and 1% resistance to all three groups. Only 7.5% of farms had no detectable levels of anthelmintic resistance. More than 80% of farms had Trichostrongylus populations resistant to both benzimidazoles and levamisole. Resistance was recorded in all three anthelmintic groups for Haemonchus and resistance also occurred to benzimidazoles and levamisole in Ostertagia.

Further novel milbemycin antibiotics from Streptomyces sp. E225. Fermentation, isolation and structure elucidation

Ten novel alpha and beta class milbemycins have been isolated and characterized from the Streptomyces sp. E225, which has previously been shown to produce four related milbemycins. Some of the metabolites contain new structural features including, VM48641 which possesses an alpha-methoxyl substituent at C-27, and VM48642 which contains a furan ring at the terminus of the C-26 side chain. Several of these new compounds were shown to possess potent anthelmintic activity. An analysis of NMR chemical shift trends in this series of metabolites is presented.

Beta-tubulin gene polymorphism and benzimidazole resistance in trichostrongylus colubriformis

The complement of beta-tubulin alleles in Trichostrongylus colubriformis populations was examined and found to undergo changes similar to those previously reported for Haemonchus contortus following selection for benzimidazole (BZ) resistance. Genomic DNA from BZ-resistant and -susceptible strains was probed with a series of overlapping fragments derived from a T. colubriformis beta-tubulin gene. A susceptible population showed a high level of polymorphism (detected as RFLPs with several enzymes and directly by sequence analysis) at a locus, tcb-1, which appears to be the homologue of the gru-1 locus in H. contortus. This polymorphism disappeared following selection for BZ resistance, leaving a single tcb-1 allele in the resistant population. The same single allele was present in 2 additional, unrelated resistant populations. These data support the hypotheses that tcb-1 and gru-1 are major determinants of BZ susceptibility and hence a major target of BZ-resistance selection.

Cryopreservation of the first-stage larvae of trichostrongylid nematode parasites

First stage (L1) larvae of Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta can be cryopreserved in the presence of DMSO using a two-step freezing protocol involving an initial period at -80 degrees C prior to transfer to liquid nitrogen. Thawed L1 larvae continue development in vitro producing third stage (L3) larvae that are infective to sheep when dosed per os. Establishment rates for L3 larvae grown from thawed L1 larvae were 40 and 80% for H. contortus and T. colubriformis, respectively. There was no difference in survival or infectivity between benzimidazole (BZ)-susceptible and BZ-resistant H. contortus isolates and cryopreservation caused no shift in their BZ-resistance status as indicated in an in vitro larval development assay. Cryopreservation also had no effect on the sensitivity of these isolates to the avermectins or levamisole in vitro. High survival rates (60-70%), good levels of establishment and the stability of anthelmintic resistance status of isolates indicate that little if any selection occurs during the cryopreservation process. L1 larvae of all 3 species have been successfully recovered after 16 months storage in liquid nitrogen, cultured to the L3 stage and established in sheep.

The persistence of the efficacy of injectable or oral moxidectin against Teladorsagia, Haemonchus and Trichostrongylus species in experimentally infected sheep

The persistence of the efficacy of moxidectin was evaluated against experimental gastrointestinal nematode infections in 55 lambs randomly allocated to 11 equal groups and infected on day 0. Moxidectin 1 per cent injectable solution was administered at a dose rate of 0.2 mg moxidectin/kg bodyweight to five of the groups on days -42, -35, -28, -21 and -14; five other groups were treated with moxidectin 0.1 per cent oral drench at the same dose rate on days -35, -28, -21, -14 and -7, and the 11th group remained untreated as a control. The lambs were infected experimentally with 8000 Teladorsagia circumcincta, 2000 Haemonchus contortus and 10,000 Trichostrongylus colubriformis infective larvae and killed three weeks later. Both formulations of moxidectin showed excellent activity against T circumcincta and H contortus with almost 100 per cent efficacy against the abomasal parasites for up to 35 days after treatment. The efficacy of moxidectin 1 per cent injectable against T colubriformis was much higher (> 99 per cent) than that of the oral drench and it was highly effective up to 21 days after treatment, and gave a moderate reduction in worm burden for up to 35 days after treatment. No adverse reactions to moxidectin were observed in any of the animals.

Anthelmintic profile of the cyclodepsipeptide PF1022A in in vitro and in vivo models

A novel cyclodepsipeptide of fungal origin, PF1022A, recently was reported to have anthelmintic activity. To supplement published reports and determine potential utility of PF1022A as a ruminant anthelmintic, the compound was examined in in vitro and in vivo models. Assays used measured motility of Haemonchus contortus (intrinsic drug potency), ATP levels (parasite death), and activity against H. contortus, Ostertagia ostertagi, and Trichostrongylus colubriformis in the jird (spectrum, potency, and efficacy by various routes). The potency of PF1022A in reducing motility is greater than commercial anthelmintics. Examination of ATP levels in PF1022A-paralyzed H. contortus indicates that worms are not killed, suggesting the compound acts as a neurotoxin in nematodes. In the jird, PF1022A has activity orally against each of the parasites studied and at doses comparable to all commercial anthelmintics, except the macrocyclic lactones which are more potent. Unfortunately, for some nematode species, parenteral delivery is ineffective at realistic doses.

The influence of a heavy infection with sensitive and resistant strains of Ostertagia circumcincta and with Trichostrongylus colubriformis on the pharmacokinetics of febantel in lambs

Plasma concentrations of febantel and its major metabolites, fenbendazole, oxfendazole and fenbendazole sulphone, were determined after oral administration of 7.5 mg/kg febantel in lambs before and 28 days after infection with 100,000 L3 larvae of a benzimidazole (BZ)-sensitive or BZ-resistant strain of Ostertagia circumcincta or with 75,000 L3 larvae of a BZ-sensitive Trichostrongylus colubriformis strain. The febantel concentrations were always low, and in only a few samples were higher than the limit of detection. A mean decrease in the area under the curve (AUC) for the three metabolites of 10.2%, 16.4% and 4.9% in lambs infected, respectively, with BZ-sensitive O. circumcincta, BZ-resistant O. circumcincta and T. colubriformis was observed. The Cmax for all the metabolites was higher in the BZ-sensitive O. circumcincta group than in the naive sheep, while the Tmax occurred earlier. The Cmax and the Tmax values for all the metabolites were lower in the BZ-resistant O. circumcincta group than in their own naive controls. In the T. colubriformis group the Cmax values of the metabolites were lower and the Tmax occurred much later.

Physiology of two strains of Trichostrongylus colubriformis resistant and susceptible to thiabendazole and mucosal response of experimentally infected rabbits

Two strains of Trichostrongylus colubriformis of ovine origin, one resistant and one susceptible to thiabendazole, were compared during experimental infection in rabbits. Groups of rabbits were infected with 5000 infective larvae (L3) of either the resistant or the susceptible strain. On days 21 and 42 post-infection (p.i.), 5 rabbits of each group were killed and the small intestine divided into three sections for worm counts and histological or biochemical analysis. Faecal egg counts were performed twice a week from day 15 to day 42 p.i. The physiology of the worms was characterised by in vitro acetylcholinesterase secretion of adult worms. The host inflammatory response was determined by peroxidase activity in mucosal homogenates and by histological counts of mast cells and eosinophils. Infectivity was not significantly different between the two strains. However, egg production was delayed and was significantly lower for the resistant strain. Conversely, in vitro secretion of resistant worms was significantly higher than the susceptible ones. The mucosal homogenetates of rabbits infected with the resistant strain had significantly higher levels of peroxidase activity in the duodenum on days 21 and 42 p.i. Mast cell counts were also significantly higher in the duodenum on day 42 p.i., in rabbits infected with the resistant strain. No difference was observed in the eosinophil counts. These results suggest that genetic variation in the nematode, such as anthelminthic resistance, is associated with variations in worm biology and physiology as well as differences in the inflammatory response of the host.

The effect of reduced feed intake on the efficacy of oxfendazole against benzimidazole resistant Haemonchus contortus and Trichostrongylus colubriformis in sheep

Within 12 h of reducing the daily intake of a 50:50 lucerne and wheaten hay ration from 800 to 400 g, there was an increase in digesta marker concentration indicating a reduction in the flow rate of digesta fluid and particulates through the abomasum of sheep. The efficacy of the recommended dose of oxfendazole (OFZ) was then tested against benzimidazole resistant Trichostrongylus colubriformis and Haemonchus contortus in sheep whose feed intake was halved 36 h before and after drug treatment. In animals fed the reduced ration there was a 60% reduction of T. colubriformis and 94% reduction of H. contortus compared with a 19% and 60% reduction respectively on the higher feed intake. It is therefore suggested that greater anthelmintic efficacy can be achieved in sheep by temporary feed reduction.

Survey for drug-resistant trichostrongyle nematodes in ten commercial goat farms in west Malaysia

The prevalence of drug-resistant parasites in goats in West Malaysia has rarely been reported. Ten commercial goat farms were surveyed for resistance to anthelminthics by measuring the reduction in faecal egg counts (FECs) after treatment with levamisole, pyrantel pamoate and fenbendazole. Resistance to fenbendazole was seen in most farms; there was no evidence of resistance to levamisole but some resistance to pyrantel pamoate was detected on one farm. The significance of the findings are discussed.

Prevalence of fenbendazole resistance in ovine nematodes in north west India

A survey was undertaken between March and September 1992 to assess the prevalence of fenbendazole resistance in sheep nematodes in 32 traditionally managed rural flocks and 22 intensively managed flocks on 3 farms. Fenbendazole, a commonly used anthelmintic was used at the recommended dose of 5 mg/kg body weight. Efficacy was determined on the basis of percentage reduction in strongyle faecal egg counts (FECR%) and larval cultures before and between 7 and 10 days after treatment. None of the traditionally managed rural flocks showed resistance and all had greater than 99% reduction in faecal egg counts after treatment. Of the 22 intensively managed flocks, however, 15 had slight fenbendazole resistance (between 60 and 90% reduction in egg counts) and 4 had severe resistance (less than 60% reduction).

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.

Acetylcholinesterase in infective-stage larvae of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis resistant and susceptible to benzimidazole anthelmintics

The amounts of acetylcholinesterase (AChE) in homogenates of infective-stage larvae of strains, from different countries, of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis resistant or susceptible to benzimidazole (BZ) anthelmintics, were compared using a rapid colorimetric assay. The levels of AChE were much greater in those strains which were BZ-resistant than in BZ-susceptible populations. The larvae of susceptible and resistant H. contortus contained less AChE than did the larvae of the other two species. One-way analysis of variance showed that the observed differences between susceptible and resistant strains were significant in each of the species studied. It is suggested that the extra enzyme may act as an anthelmintic-detoxifying enzyme, may inactivate the active site of the anthelmintic or may prevent binding of the anthelmintic to its receptor.

Mutual resistance to avermectins and milbemycins: oral activity of ivermectin and moxidectin against ivermectin-resistant and susceptible nematodes

To determine whether there is mutual resistance to avermectin and milbemycin anthelmintics, ivermectin and moxidectin sheep drenches were tested against ivermectin-resistant and susceptible isolates of Ostertagia circumcincta and Trichostrongylus colubriformis in sheep. None of the isolates had been exposed to moxidectin previously. The dosage of ivermectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubriformis were 23 times and six times larger, respectively, than the dosages required to remove the same percentage of susceptible isolates. The dosages of moxidectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubriformis were 31 times and nine times larger, respectively, than the dosages required to remove the same percentage of susceptible isolates. It is concluded that the worms resistant to ivermectin were also resistant to moxidectin.

Further novel metabolites of the paraherquamide family

Four novel metabolites of a Penicillium strain, IMI 332995, which has previously been reported to produce paraherquamide and a number of related metabolites, are herein described. VM55596 is the first N-oxide to be found in this family of compounds. Unusual oxidative substitution is also seen in VM55597. VM55599 appears to be the first documented example of the hexacyclic indole species that have long been postulated as biosynthetic precursors of metabolites of the brevianamide, paraherquamide and marcfortine families.

Comparative kinetic disposition of oxfendazole in sheep and goats before and during infection with Haemonchus contortus and Trichostrongylus colubriformis

The kinetic disposition of [14C]-oxfendazole (OFZ) and its metabolites, fenbendazole (FBZ) and fenbendazole sulphone (FBZ.SO2), in plasma and abomasal fluid were determined in Merino sheep and Angora goats before and during infection with Trichostrongylus colubriformis and Haemonchus contortus. The systemic availability (area under the plasma curve, AUC) of OFZ was significantly lower in goats (13.5 micrograms.h/ml) than in sheep (22.2 micrograms.h/ml) and was reduced with infection in goats (5.6 micrograms.h/ml) and sheep (15.1 micrograms.h/ml). The elimination of plasma [14C] was faster in goats than in sheep. The responses observed for [14C] were a reflection of the behaviour of OFZ. The concentration of OFZ and metabolites in abomasal fluid were similar in both species in the absence or presence of infection. However, as the mean flow rate of abomasal fluid was slower in goats (240 ml/h) than in sheep (488 ml/h), only 7% of the dose passed the pylorus in abomasal fluid of goats compared with 14% in sheep. The presence of gastrointestinal nematodes generally increased abomasal fluid flow rate but neither species nor infection had any effect on the rate or extent of [14C] excretion in urine or faeces. It is suggested that goats possess a faster hepatic metabolism than sheep resulting in more rapid elimination of OFZ.

In vitro activity of paraherquamide against the free-living stages of Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta

Paraherquamide, an oxindole alkaloid recently reported to have potent nematocidal activity, was shown to have a marked inhibitory effect on the motility of the free-living larval stages of H. contortus, T. colubriformis and O. circumcincta. The effect of paraherquamide on larval motility could be distinguished from that caused by levamisole and the avermectins. After 72 h exposure, the concentration of paraherquamide required to inhibit the motility of 50% of L3 larvae present was 0.033, 0.058 and 2.7 micrograms ml-1 for O. circumcincta, T. colubriformis and H. contortus, respectively. Ivermectin (IVM)-resistant isolates of H. contortus were significantly more sensitive to the paralytic effects of paraherquamide than IVM-susceptible isolates of this species. Paraherquamide had no effect on the time for development from the egg to the L3 larval stage of H. contortus, T. colubriformis and O. circumcincta.

Differential stability of the benzimidazole (BZ)-tubulin complex in BZ-resistant and BZ-susceptible isolates of Haemonchus contortus and Trichostrongylus colubriformis

The binding of [3H]mebendazole ([3H]MBZ) to tubulin from BZ-susceptible (BZ-S) and BZ-resistant (BZ-R) isolates of Haemonchus contortus and Trichostrongylus colubriformis was investigated using charcoal extraction and gel filtration techniques. The amount of [3H]MBZ bound at infinite free ligand concentration (Bmax) was significantly reduced for the BZ-R isolate compared with the BZ-S isolate in both species when assayed by charcoal extraction. However, Bmax was increased to comparable levels for both BZ-S and BZ-R isolates of each species when assayed by the less stringent gel filtration technique. These results indicate that the BZ-tubulin interaction in trichostrongylid nematodes is comprised of a minimum of two components. As similar levels of total [3H]MBZ binding were observed for both BZ-S and BZ-R isolates of each species when assayed by gel filtration, it is suggested that the reduction in the pseudo-irreversible BZ binding component in BZ-R isolates results in an increase in the level of reversible BZ binding and therefore provides a survival advantage to BZ-R nematodes.

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.

Effects of dietary molybdenum on nematode and host during Trichostrongylus vitrinus infection in lambs

The addition of molybdenum (0.05 mmol kg-1 dry matter) to the diet of lambs exposed for four weeks to a trickle (2500 third stage larvae per day) infection with Trichostrongylus vitrinus reduced the number and length of adult worms retrieved from the small intestine 11 days later: both effects were particularly marked in female worms from female lambs (P less than 0.01). Worms from lambs given molybdenum contained less proteinase enzyme activity and secreted less proteinases in culture irrespective of the sex of the host. Pathogenicity was not attenuated by molybdenum. Damage to the intestinal mucosa was severe in both dietary groups but infected females given molybdenum developed lower plasma albumin concentrations and lighter dressed carcases than those not given molybdenum. Neither the effects on the parasite nor those on the host could be attributed simply to molybdenum-induced copper depletion, using conventional measures of copper status. Molybdenum may be toxic to T vitrinus but may also facilitate or enhance the inflammatory process limiting larval establishment or increasing parasite rejection.

Temperature dependent binding of mebendazole to tubulin in benzimidazole-susceptible and -resistant strains of Trichostrongylus colubriformis and Caenorhabditis elegans

The binding of [3H]mebendazole ([3H]MBZ) to tubulin in benzimidazole-susceptible (BZ-S) and benzimidazole-resistant (BZ-R) strains of Trichostrongylus colubriformis and Caenorhabditis elegans was examined in order to investigate the biochemical changes to tubulin that result in BZ resistance in parasitic and free-living nematodes. In both species the extent of [3H]MBZ binding to tubulin was significantly reduced in the BZ-R strain compared with the BZ-S strain. The decrease in [3H]MBZ binding in the BZ-R strain of each species was the result of a significant reduction in the amount of charcoal stable [3H]MBZ-tubulin complexes and was not related to a change in the association constant of the [3H]MBZ-tubulin interaction. [3H]MBZ binding to tubulin was temperature dependent, reaching maximum levels at 37 degrees C in BZ-S T. colubriformis and 10 degrees C in BZ-R T. colubriformis. Both the BZ-S and BZ-R strains of C. elegans displayed maximum [3H]MBZ binding at 4 degrees C. Resistance ratios derived from the amount of [3H]MBZ binding in the BZ-S and BZ-R strains and in vitro development assays demonstrated that the temperature dependence and extent of drug binding was indicative of BZ resistance status and was species specific in the BZ-S isolates. These results indicate that biochemical differences exist in the binding of benzimidazole carbamates to tubulin in nematode species, and suggest that the susceptibility of the parasitic nematodes to the benzimidazole anthelmintics is the result of a unique high affinity and/or high capacity interaction of benzimidazole carbamates with tubulin.

Effect of a controlled-release albendazole capsule on parasitism and productivity of sheep

The efficacy of intraruminal albendazole (ABZ) capsules (Profitril-Captec) and the effect of treatment on productivity were studied in 300 ewes infected with gastrointestinal nematodes and the trematode Dicrocoelium dendriticum. Coprological tests revealed that treated animals remained negative for 10 weeks after the administration of capsules. Contamination of pasture with nematode larvae was significantly reduced during the whole experiment. Necropsy of 14 animals (seven treated and seven untreated) showed 96.9-99.2% efficacy against the nematodes Nematodirus spp., Oesophagostomum spp., Cooperia spp., Trichostrongylus spp. and Trichuris ovis, while efficacy was 88.5% against D. dendriticum. During the 6 month pasture season (May-October 1989), treated ewes produced on average 2.56 kg cheese and 0.6 kg wool per ewe more than untreated controls. Our study confirms the reliability of the ABZ slow-release capsules over 90 days and the positive effect of treatment on nematode contamination of pasture and ewe productivity.

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.

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.

Field evaluation of a mixture of albendazole sulphoxide and levamisole against Ostertagia and Trichostrongylus spp in sheep

The efficacy of a mixture of albendazole sulphoxide and levamisole, 3.6 and 8.25 mg/kg respectively, at single and double dose rates, was compared with the recommended dose rate of each anthelmintic alone. The comparison was conducted on groups of 6 to 14-week-old lambs on 22 farms, 16 of which had evidence of multiple resistance to benzimidazole and levamisole. A single dose of the mixture reduced mean egg counts by 95% on half the farms with multiple resistance and on all the remaining farms. Consequently, the mixture should be included in an assessment of effective anthelmintics on farms to determine its value for nematode control. A double dose rate of mixture was effective on all but 4 farms. Reductions caused by the mixture were due to the additive actions of the drugs on 18 of 22 farms; synergistic action was noted on only 3 farms. It was concluded that the mixture of albendazole sulphoxide and levamisole offered many farmers an effective anthelmintic for use in preventive control programs. Recommendations for such programs include annual rotation of effective anthelmintics as a means of delaying selection for drug resistance.

The efficacy of mixtures of albendazole sulphoxide and levamisole against sheep nematodes resistant to benzimidazole and levamisole

Faecal egg count reduction tests and an anthelmintic efficiency assay were used to assess the efficacy of combinations of albendazole sulphoxide and levamisole against populations of Ostertagia and Trichostrongylus sp. which contained different proportions of worms resistant to both benzimidazole and levamisole anthelmintics. Compared to the effects of either drug alone, significantly greater efficacy was obtained using combinations which included dose rates similar to those recommended for the separate components. At these dose rates, the mixtures reduced mean faecal egg counts by 95% or more, and caused a reduction of 68% in adult Ostertagia sp. and more than 95% for 4th stage Ostertagia and T colubriformis. The increased efficacy of the mixtures could be accounted for by actions of the drugs acting independently.

Efficacy of ivermectin administered via sustained-release bolus against gastrointestinal nematodes in cattle

Twelve calves (mean weight, 175.5 kg) were used to confirm efficacy of ivermectin delivered from a prototype sustained-release bolus against naturally acquired gastrointestinal nematodes including early fourth-stage (inhibited) larvae of Ostertagia ostertagi. The calves were allocated by restricted randomization on weight to 1 of 2 groups: controls, to which a placebo bolus was given orally, and treated calves, to which a sustained-release bolus designed to deliver 8 mg of ivermectin/day at a steady rate was given orally. After treatment, the 2 groups were housed in separate pens with concrete flooring. Twenty-eight days after treatment, all calves were euthanatized and necropsied. The ivermectin-treated calves had no larval or adult Ostertagia spp and significantly (P less than 0.01) fewer adult Trichostrongylus axei and adult Cooperia (C oncophora, C punctata and C surnabada) than control calves. Efficacy of ivermectin was greater than 99% for Cooperia spp, and 100% for other parasites. Drug-related adverse reactions were not observed.

Efficacy of some commercial formulations of anthelmintics on Haemonchus contortus and Trichostrongylus columbianum

The anthelmintic action of five commercially formulated anthelmintics: Assiazine, Banminth, Nemafax, piperazine and levamisole has been evaluated by observing the effect on egg hatch, motility and mortality of the larvae of Trichostrongylus columbianum and Haemonchus contortus. Assiazine and levamisole inhibited egg-hatch of the helminth ova dose-dependently. Nemafax, Banminth, and piperazine did not inhibit egg hatch. Levamisole and Banminth affected the motility of the helminth larvae with subsequent death after 24h. The other anthelmintics used did not adversely affect the helminth larvae as compared with the untreated control group.

Population dynamics of Trichostrongylus colubriformis in sheep: computer model to simulate grazing systems and the evolution of anthelmintic resistance

A computer model was developed to simulate Trichostrongylus colubriformis populations, their level of resistance to the common anthelmintics, host mortalities and acquired immunity. Predictions were based on sheep management practices such as lambing, weaning, sheep/paddock rotation, anthelmintic treatment, the use of controlled release devices (capsules) for anthelmintic delivery and daily meteorological records to determine the development and survival of infective larvae (L3) on pasture. Evolution of drug resistance was determined by a simple genetic system which allowed for up to three genes, each with two alleles, to give a maximum of 27 genotypes associated with one drug or three genotypes for each of three drugs. The model was validated against egg counts, L3 counts on pasture and host mortalities observed in a grazing trial, however, aspects of the model such as the development of drug resistance and use of the model in a variety of climatic zones have yet to be tested against field observations. The model was used to examine the impact of grazing management and capsule use on anthelmintic resistance and sheep production over 20 years using historical weather data. Predictions indicated that grazing management can play a dominant role in parasite control and that capsule use will reduce sheep mortalities and production losses, and in some circumstances will not cause a substantial increase in anthelmintic resistance for up to 5 years.

Levamisole resistance in Trichostrongylus colubriformis: a sex-linked recessive character

Reciprocal crosses between susceptible and levamisole resistant strains of Trichostrongylus colubriformis produced F1 offspring consistent with resistance being inherited as a sex-linked recessive character. The resistance status of the offspring of the backcrosses of the F1 to both parental strains supported this hypothesis. The results are consistent with resistance being controlled by a single gene, or a tightly linked group of genes, but indicate that other autosomal loci have minor effects. The results contrast with the reported observations that resistance to the benzimidazole anthelmintics is polygenic and autosomal. The results are discussed relative to a general evolutionary model for anthelmintic resistance which predicts that selection from the upper extreme of an anthelmintic tolerance distribution results in polygenicity.

Anthelmintic activity of the macrocyclic lactone F28249-alpha in sheep

The macrolytic lactone F28249-alpha was titrated in experimentally infected sheep and found to be highly effective against most of the common gastrointestinal nematodes as a single oral dose, given at a rate of 0.025, 0.05, or 0.1 mg/kg. Specifically, maximal activity was evident at even the lowest dosage against adult Haemonchus contortus, Ostertagia circumcinta, Trichostrongylus axei, and T colubriformis and L4 O circumcinta. Activity against Oesophagostomum columbianum was also high at all dosages, with a calculated ED95 of 0.029 mg/kg. Cooperia curticei was eliminated at 0.1 mg/kg, but control was erratic at the lower dosages. The greatest weakness of this compound was its activity against C oncophora. The activity against this parasite was weak (less than or equal to 85%) at all dosages, and the dosage-response curve was flat, suggesting dosages substantially higher than those given would be necessary for high-order control of this species.

A field strain of Trichostrongylus colubriformis resistant to levamisole and morantel in South Africa

A strain of Trichostrongylus colubriformis from Nottingham Road, in Natal, was found to be solidly resistant to levamisole and morantel at the recommended dose levels. Untreated control sheep in fact harboured fewer worms at slaughter than either of the 2 treated groups. In contrast, the benzimidazoles and ivermectin were more than 99.9% effective against this worm strain. The possible implications of escalating resistance to anthelmintics in the gastrointestinal nematodes of sheep are discussed.

Laboratory selection of a benzimidazole-resistant isolate of Trichostrongylus colubriformis for ivermectin resistance

In vivo ivermectin resistance was selected in an isolate of Trichostrongylus colubriformis (TcR) already known to be benzimidazole resistant. This was accomplished in sheep by using levels of ivermectin calculated to reduce the fecal egg output from each generation of T. colubriformis by congruent to 95%. The first indication of ivermectin resistance was observed with the F10. A dosage-titration trial comparing the parent TcR with the ivermectin-selected F21 demonstrated that the latter was congruent to 20 times more resistant to oral ivermectin therapy in experimentally infected sheep than was the parent isolate. Treatment of the F16 generation with 50 mg/kg of thiabendazole resulted in only 54% egg reduction and confirmed that benzimidazole resistance was stable.

A larval paralysis assay for the detection of thiabendazole resistance in trichostrongyles

Infective-stage larvae of trichostrongyle nematodes, either resistant or susceptible to thiabendazole (TBZ), were incubated in eserine, an acetylcholinesterase inhibitor. Paralysis occurred in larvae treated with eserine but significant differences were observed in the percentage of larva immobilized between TBZ-resistant and TBZ-susceptible strains of the nematodes. These differences are probably related to the presence of higher levels of acetylcholinesterase in the TBZ-resistant strains than in the susceptible strains. This could be used as a rapid and inexpensive method of detecting resistance to TBZ in trichostrongyles.

Effects of ivermectin on control of gastrointestinal nematodes and weight gain in weaner-yearling beef cattle

Four groups of 16 crossbred beef calves were used in evaluating different anthelmintic treatment schedules: group 1 was given ivermectin (IVM) at weaning only (October 31) and grazed on initially safe pasture; group 2 was given IVM at weaning, on January 28, and on April 22, and grazed on contaminated pasture; and group 3 was given IVM at weaning and on April 22, and grazed on contaminated pasture; and group-4 was group treated with fenbendazole (FBZ) at weaning only, with provision for individual salvage treatment, and grazed on contaminated pasture. The investigation was from Oct 31, 1984, to Oct 9, 1985. Initially high fecal egg counts at weaning were more effectively reduced by IVM than by FBZ, and the effect of safe pasture was evident in minimal worm burdens in tracer calves grazed with group-1 cattle during November and least amount of weight loss in group-1 yearlings during winter. Fecal egg counts, pasture larval counts, and plasma pepsinogen concentrations remained low in group 2 after the January treatment. Fecal egg counts of the other groups increased substantially during late winter and spring, but pasture larval counts increased only on group-1 and group-4 pastures. During spring, highest worm burdens were found in group-1 and group-4 tracer calves (grazed in April) and in group-3 and group-4 yearlings (slaughtered in early April). Six cattle of group 4 were salvage treated with FBZ in February and April. Greatest gains were observed from March through June, with group-2 and group-1 cattle gaining the most.(ABSTRACT TRUNCATED AT 250 WORDS)

Detecting benzimidazole resistance with faecal egg count reduction tests and in vitro assays

Composite strains of Trichostrongylus colubriformis and Ostertagia spp consisting of 0, 1, 10, 25, 50, 75, 90, and 100% of known resistant strains were prepared and tested for benzimidazole resistance using faecal egg count reduction tests, in vitro egg hatch assays and tubulin binding assays. All tests detected resistance where the proportion of the resistant strain in the composite was 50% or more, whereas none of the tests unequivocally detected resistance below 25%. Egg count reduction tests were no less sensitive than the in vitro tests in detecting low levels of resistance but the egg hatch and tubulin binding assays provided a better quantitative estimate of moderate to high levels of resistance. Faecal egg count reduction therefore, provides a suitable means of detecting resistance in the field but tests, more sensitive to low levels of resistance are required. Results indicate that the use of post-treatment counts alone provides an adequate indication of anthelmintic efficiency.