Evidence for multiple anthelmintic resistance in two nematode parasite genera on a Saanen goat dairy

Multiple resistance was demonstrated within the experimental milking goat herd at the Ruakura Agricultural Centre. The development of this resistance was probably exacerbated by the atypical animal management and parasite control programmes under which the dairy was managed. Resistance by Teladorsagia (Ostertagia) spp. to members of the three broad-spectrum families of anthelmintic is reported for the first time. Worm counts in a controlled slaughter trial demonstrated multiple anthelmintic resistance in a mixed population of Teladorsagia (T. circumcincta and T. trifurcata) and Trichostrongylus spp. The efficacies of oxfendazole, morantel citrate and ivermectin in reducing the Teladorsagia population were 43.8%, 75.6% and 93.1%, respectively. Ivermectin removed more than 99% of the small intestinal Trichostrongylus spp. but oxfendazole and morantel were less effective, removing 37.9% and 87.7%, respectively. Only treatment with ivermectin totally eliminated faecal nematode egg counts 8 days after treatment. The lack of agreement between faecal nematode egg depressions and worm counts in lambs given ivermectin suggests that the drug may have affected the fecundity of the surviving nematode population or the assay lacked the sensitivity required. These data suggest that the sole use of faecal egg depression would be inappropriate for accurate assessment of anthelmintic effectiveness.     

Safety and efficacy against fourth-stage gastrointestinal nematode larvae, of monepantel in 6-week old lambs

A controlled, blinded study was undertaken in 6-week old, pre-weaned lambs to demonstrate the safety and efficacy against fourth-stage gastrointestinal nematode larvae, of monepantel administered per os at 2.5mg/kg body weight. Worm burdens of 10 monepantel-treated lambs were compared to those from 10 untreated control lambs. Geometric mean derived efficacies of 100, 100, 96.4 and 99.9% were demonstrated against Haemonchus contortus, Teladorsagia spp., Cooperia curticei and Trichostrongylus colubriformis, respectively. These results, considered in the light of an earlier series of studies demonstrating the efficacy of monepantel in older animals, and an absence of any adverse events, provides strong support for the use of monepantel as a safe and effective anthelmintic in lambs from six weeks of age.     

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

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

Parasites of domestic and wild animals in South Africa. XLII. Helminths of sheep on four farms in the Eastern Cape Province

At monthly intervals for periods ranging from 9 to 25 consecutive months between three and four Merino yearling-sheep and two Merino lambs on three farms in Eastern Cape Thornveld, and two Dorper yearling-sheep on a farm in Valley Bushveld were slaughtered and examined for helminths. The Merino sheep were infected with 15 nematode species, of which Haemonchus contortus, Nematodirus spathiger and Trichostrongylus spp. were the most numerous, and with four cestode species. The Dorper sheep were infected with 17 nematode species, of which Trichostrongylus rugafus and N. spathiger were the most numerous, and with two cestode species. Haemonchus contortus was most numerous in older sheep during the summer months and exhibited no tendency to over-winter as arrested fourth stage larvae. The largest numbers of N. spathiger were present in sheep on the most coastally located of the farms during autumn and winter; on a farm further inland during spring; and on an inland farm in Valley Bushveld during spring and summer. As Merino lambs progressed from birth to 9 months of age a larger proportion of their dwindling burdens of N. spathiger was present as fourth stage larvae. Dorper sheep in Valley Bushveld harboured most Teladorsagia circumcincta in May and from October to January. Trichostrongylus rugatus was most numerous in Merino sheep on one of the Thornveld farms during September, whereas all Trichostrongylus spp., including T. rugatus, were most numerous in Dorper sheep on the Valley Bushveld farm during January.     

Chick embryo tracheal organ: a new and effective in vitro culture model for Cryptosporidium baileyi

The economic impact of anthelmintic resistance was investigated in lambs by comparing productivity parameters in groups of animals treated either with a highly effective anthelmintic, or an anthelmintic to which three species of resistant worms were known to be present. Ten farmlets, each stocked with 30 lambs, were rotationally grazed for 5 months, with monthly treatments of either albendazole, to which resistance existed, or a new combination product containing derquantel and abamectin (DQL-ABA) to which there was no resistance. Stock on five farmlets were treated with each anthelmintic and productivity measures, including liveweights, body condition and faecal soiling were assessed throughout. In addition, fleece weights and information on carcass weight and quality was collected at the end of the trial. Anthelmintic efficacy was measured at the last two treatment dates by faecal egg count reduction test with larval cultures. Albendazole demonstrated efficacies of 48.4% and 40.9% for Trichostrongylus spp. and Teladorsagia circumcincta respectively. By contrast, the DQL-ABA treatments were >99% effective against all genera. The difference in live-weight gain was 9 kg in favour of the DQL-ABA treatments. This translated into a 4.7 kg increase in carcass weight with a 10.4% increase in carcass value. Significant differences in body condition scores, faecal breech soiling and fleece weights were also recorded, all in favour of the DQL-ABA treatments. The time required for 50% of the animals to reach a target live-weight of 38 kg was significantly shorter (by 17 days) in those animals treated with DQL-ABA. The results show that the production cost of subclinical parasitism as a result of using an anthelmintic product which is less than fully effective due to resistance can greatly exceed the cost of routine testing of anthelmintic efficacy and the adoption of new anthelmintic classes. There is a strong case for many farmers to re-evaluate their position on some of these issues in order to optimise financial performance.     

Evolution of nematode community in grazing sheep selected for resistance and susceptibility to Teladorsagia circumcincta and Trichostrongylus colubriformis: a 4-year experiment

Field trials were undertaken to compare nematode population dynamics in two experimental flocks of rams selected for high and for low faecal egg counts (FECs) following two doses with 20000 infective larvae, 12000 Teladorsagia circumcincta (Tcirc) and 8000 Trichostrongylus colubriformis (Tcol) separated by a treatment. The selection was done by dosing 200 ram lambs (INRA 401 breed) and FECs proceeded with egg identification on days 28 and 35 post-each-infection. The 30 lambs with the lowest FECs constituted the group of permanent resistant rams (R), the 30 with the highest values the group of susceptible ones (S). Each group grazed separated pastures from April to November, this during 4 consecutive years. The contamination (number of eggs deposited) was estimated every 3 weeks on each pasture. The larval populations was measured by successive groups of tracer lambs. The infection of the permanent rams was done by slaughtering eight R and S rams on mid and end of each grazing season. Each autumn, new R and S ram lambs were selected to complete the R and S groups before turning out in spring. On third year, a third species (Haemonchus contortus, Hcon) was added to compare its behaviour with the two others.The regulation of parasite populations were studied by comparing what happens in R rams and pasture to S ones. The selected R ram lambs had a FEC of Tcirc 50% lower than S ones. This induced a similar lowered pasture contamination, a 25% decrease of infective larval population in years 2 and 3, and the worm burdens in R rams were the half of that in S rams after 2 years. The FEC of Tcol was 75% lower at the selection and induced similar difference in pasture contamination, but 65 and 96% reduction in L3 population, respectively, after 1 and 2 years. The worm burden was reduced by 99%. Adding Hcon species during the third year, in year 4, the L3 population was half in R pasture compared with S one, and R rams had 60% less worms of this species than S ones.This selection without any drench prevent outbreaks observed in young tracer lambs on S pasture. The comparison of selected resistant rams to selected susceptible ones and not to unselected controls gave more rapid information about the tendency of the regulation of the communities of nematode parasites.     

Studies on Haemonchus contortus. IX. The effect of Trichostrongylus axei in Merinos on natural pasture

Four groups of 6-7-month-old Merino lambs were each dosed with 40 000 infective larvae of Trichostrongylus axei on 2 November 1976 and subsequently exposed to challenge with Haemonchus contortus on natural grazing at the University of Pretoria's Experimental Farm in the eastern suburbs of Pretoria. One of these groups and one group of controls were killed every 8 weeks from the end of December 1976-June 1977. Predosing with T. axei was greater than 50% effective against 5th stage and adult H. contortus in greater than 50% of sheep for 164 days (Class C), improving to greater than 60% in greater than 60% of sheep (Class B) 220 days after dosing T. axei. The numbers of retarded 4th stage larvae (L4) of H. contortus in the undosed controls as well as in the sheep predosed with T. axei rose from a low level in summer (December) to a peak in late Autumn (June).     

Direct anthelmintic and immunostimulatory effects of oral dosing semi-purified phytohaemagglutinin lectin in sheep infected with Teladorsagia circumcincta and Trichostrongylus colubriformis

Lectins are plant secondary compounds that can have anthelmintic properties in vitro. In particular, the phytohaemagglutinin lectin extracted from Phaseolus vulgaris has been shown to inhibit the feeding of Trichostrongylus colubriformis and Teladorsagia circumcincta L1 larvae. However, little is known about the potential anthelmintic properties of this lectin in vivo and its suitability to control gastrointestinal parasite infections in lambs. In a 2 × 2 study, lambs were either orally dosed, or not, with 2.3mg semi-purified PHA lectin per kg live weight (LW) per day, whilst concurrently infected, or not, with 1000 T. circumcincta and 1000 T. colubriformis L3 infective larvae per day for 42 days. There were no adverse clinical effects observed with this dose of PHA lectin. Although worm burdens were similar, animals dosed with PHA lectin had reduced concentration of nematode eggs in the faeces compared with their non-lectin dosed counterparts (P=0.026), suggesting that there may be a direct effect of PHA lectin on parasite fecundity. Irrespective of infection, PHA lectin had immune-stimulatory properties with increased eosinophillia in both abomasal and small intestine tissue sections taken at slaughter on day 42 (P<0.02 for both) and a tendency for decreased ability of Teladorsagia larvae to penetrate abomasal tissue explants (P=0.06). Compared with infection alone, concurrent PHA lectin dosing and infection further increased the number of eosinophils (P<0.01), PAS-positive (mucin-producing cells) (P=0.03) and tended to increase the number of T helper cells (P=0.06). No interactions were observed for cell populations in small intestine tissue sections. These results suggest PHA lectin could have two possible modes of action against T. circumcincta and T. colubriformis, a direct anthelmintic effect on nematode fecundity and an indirect effect through enhancing local immune responses in the host.     

Dose determination studies for monepantel, an amino-acetonitrile derivative, against fourth stage gastro-intestinal nematode larvae infecting sheep

Monepantel is the first compound from the recently discovered amino-acetonitrile derivative (AAD) class of anthelmintics to be developed for use in sheep. Three dose determination studies were conducted in Australia and Switzerland to identify the minimum therapeutic dose of monepantel when formulated for the oral treatment of sheep to control fourth stage (L4) gastro-intestinal nematode larvae. In each study, sheep infected with the target nematodes (selected from Haemonchus contortus, Teladorsagia (Ostertagia) circumcincta, Teladorsagia trifurcata, Trichostrongylus axei, Trichostrongylus colubriformis, Trichostrongylus vitrinus, Cooperia curticei, Cooperia oncophora, Nematodirus battus, Nematodirus filicollis, Nematodirus spathiger, Chabertia ovina and Oesophagostomum venulosum) were treated with either 1.25, 2.5 or 5.0mg monepantel/kg liveweight. Following euthanasia and worm counting, efficacy was calculated against worm counts from untreated control groups. Monepantel proved highly effective at 2.5 and 5.0mg/kg, but was only moderately effective against some nematode species (L4 stage) at 1.25mg/kg. The results also confirmed that monepantel will effectively control L4 stages of nematodes resistant to at least some of the currently available broad-spectrum anthelmintic classes (macrocyclic lactone resistant strains were not included in the studies). It was concluded that 2.5mg/kg would be a suitable minimum dose rate for a commercial product. No adverse events related to treatment with monepantel were detected.     

Resistance of Teladorsagia circumcincta in goats to ivermectin in the Netherlands

Resistance of Teladorsagia circumcincta in goats to ivermectin In 2005, suspected ivermectin resistance of gastrointestinal nematodes was investigated in 17 goats on a farm in the Netherlands, using a faecal egg count reduction test combined with faecal larval cultures. Eight goats were treated orally with 0.2 mg/ kg ivermectin on 1 August 2005 and the other nine goats served as untreated controls. Faeces were collected from all goats on 1 August and on 12 August. Faecal egg counts were reduced by 80.6% in goats receiving ivermectin. On the basis of faecal larval cultures and the egg counts, ivermectin was found to be 100% effective against Haemonchus contortus, 15.0% effective against Teladorsagia circumcincta, and 93.3% effective against Trichostrongylus spp. The effect of levamisole and oxfendazole on gastrointestinal nematodes was also evaluated. Nine goats were treated subcutaneously with 5 mg/kg levamisole and seven goats orally with 4.5 mg/kg oxfendazole on 12 September On the basis of faecal egg counts performed on 12 and 22 September, levamisole reduced the total egg count by 95.4% and oxfendazole reduced the egg count by 85.8%. Differentiation of pooled faecal larval cultures revealed that levamisole was 99.9% effective against H. contortus, 81.9% effective against T. circumcincta, and 99.5% against Trichostrongylus spp. Oxfendazole was 75.5% effective against H. contortus, 33.2% effective against T. circumcincta, and 100% effective against Trichostrongylus spp. On 28 October the nine goats that were still present on the farm were orally treated with 0.4 mg/kg ivermectin. Faecal egg counts and differentiation of pooled larval cultures of faecal samples collected on 28 October and 9 November, respectively, revealed that total egg counts were reduced by 93.1%. Ivermectin was 49.1% effective against T. circumcincta but 100% effective against H. contortus and Trichostrongylus spp. Thus H. contortus appears to be resistant to oxfendazole and T. circumcinta appears to be resistant to oxfendazole and ivermectin. This is the first documented case of ivermectin resistance of a gastrointestinal nematode of small ruminants in the Netherlands.     

Control of helminthosis in lambs by strategic treatment with closantel and broad-spectrum anthelmintics

Treatment of ewes with broad-spectrum anthelmintic in August (pre-lambing) and early November, and of lambs in early November and early February, was effective in controlling infections with Trichostrongylus spp in lambs reared on contaminated pastures under set-stocked conditions. It was ineffective in controlling infections with Haemonchus contortus; 82% of lambs had to be withdrawn from the experiment because of severe haemonchosis. Treatment with closantel (7.5 mg/kg) at the same times was very effective against H. contortus but ineffective against Trichostrongylus spp; 25% of lambs had to be withdrawn because of severe trichostrongylosis. The same schedule using broad spectrum anthelmintic and closantel administered concurrently was effective against both parasites; no lambs had to be withdrawn and the bodyweight gain of lambs was higher than in lambs treated with broad-spectrum anthelmintic or closantel alone. The results provide a basis on which to develop a preventive anthelmintic treatment program to control haemonchosis and trichostrongylosis in sheep which will allow the current high frequency of treatment with broad-spectrum anthelmintics to be reduced. Such a program may retard selection for anthelmintic resistance in Trichostrongylus spp.     

Selection of ivermectin-resistant Trichostrongylus colubriformis in lambs

A mixed population of Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis was passed for four generations through lambs. In successive passages and on Day 6 (D6), lambs were dosed subcutaneously with ivermectin at 100, 200, 250 and 225 micrograms kg-1 body weight, respectively. No larvae of H. contortus were recovered after the first passage and following the third drug exposure only T. colubriformis was present. Larvae of the selected strain of T. colubriformis developed to third stage, sheathed larvae in vitro in concentrations of ivermectin that inhibited development of larvae from the original strain of T. colubriformis. A definitive control test was then run with 45 lambs, to compare this selected strain to the original strain. The treatments were 150 micrograms kg-1 subcutaneous (SC) D6, 200 micrograms kg-1 SC D6, 200 micrograms kg-1 intraruminal (IR) D6 and 200 micrograms kg-1 SC D21; all treatments were compared to non-treated controls, selected or original strain as applicable. The D6 IR and D6 SC 150 micrograms kg-1 treatments were greater than 99% effective in both strains, although the D6 SC 200 micrograms kg-1 treatment was 85 and 48% effective in the original and selected strain, respectively. The D21 200 micrograms kg-1 was only 58% effective against the selected strain. There was significant (P less than 0.001) differences between the original and selected strains of T. colubriformis for both fecal egg output and worm counts. The exposure of four generations of immature T. colubriformis to ivermectin selected for partial resistance.     

Towards finding effective indicators (diarrhoea and anaemia scores and weight gains) for the implementation of targeted selective treatment against the gastro-intestinal nematodes in lambs in a steppic environment

The effective application of targeted selective treatment (TST) against gastro-intestinal nematodes depends on the accurate identification of those animals in need of anthelmintic treatment. Finding cost and labour effective measures to identify such animals is paramount. This study tested the efficacy of three indicators on lambs in a farm in Eastern Algeria: anaemia score (FAMACHA(?)), diarrhoea score (DISCO) and weight gain. These were contrasted against traditional parasitological infection measures (nematode eggs per gram of faeces: EPG). Thirty lambs were used in the study; every second month they were sampled (FAMACHA(?), faecal samples for EPG and DISCO, weight gains) and two of the original 30 lambs were necropsied for adult worm counts. The main gastro-intestinal nematodes were Teladorsagia circumcincta, Marshallagia marshalli, Nematodirus helvetianus, Trichostrongylus vitrinus, and Haemonchus contortus. DISCO proved to be the most effective indicator correctly identifying 80% of the sheep in need of treatment. This was followed by FAMACHA(?) with a 50% accuracy level and finally weight gains, which were not found to be a useful indicator at all.     

Managing anthelmintic resistance: untreated adult ewes as a source of unselected parasites, and their role in reducing parasite populations

AIMS: To test the hypotheses that when untreated adult ewes are rotationally grazed (follow behind) on pastures after lambs receiving routine anthelmintic treatments, the ewes can function as a source of unselected parasites in refugia, capable of slowing the development of anthelmintic resistance, and suppress the build-up of parasites resulting from the development of anthelmintic resistance. METHODS: Firstly, the potential of untreated adult ewes to slow the development of anthelmintic resistance, and to suppress parasite populations under differing levels of anthelmintic efficacy, was investigated using a simulation model. Secondly, a field trial with three replicates of each treatment compared two grazing systems (lambs only vs lambs followed by ewes) and two types of anthelmintic, viz albendazole (ALB), to which resistance was present (faecal nematode egg count reduction (FECR)=57-59%) and ivermectin plus levamisole (IL), to which resistance was absent (FECR=97-99%), in a factorial treatment structure. Parasite populations were monitored using faecal nematode egg counts (FEC), faecal larval cultures, pasture larval sampling, and slaughter of tracer lambs. Animal performance was measured using liveweight, dag score, body condition score, and fleece weights. RESULTS: Model simulations indicated that parasites cycling in the untreated ewes could slow the development of resistance being selected for by the anthelmintic treatments given to lambs and this could occur without a nett increase in larval numbers on pasture. Further, as worm control in the lambs declined with increasing levels of anthelmintic resistance the ewes increasingly functioned as nett removers of parasite larvae, effectively reducing parasite population size. In the field trial, untreated adult ewes contributed to pasture infestations of most parasite species, but not Nematodirus spp. Parasite species on pasture and infecting lambs changed when ewes were present, but larval populations on pasture in the autumn were no greater than when lambs grazed alone. In the presence of anthelmintic resistance, parasite populations were reduced when ewes grazed in rotation with lambs, implicating the ewes as nett removers of parasite challenge. CONCLUSIONS: Untreated adult ewes were a source of unselected genotypes, capable of slowing the development of anthelmintic resistance in most, but not all, parasite species. Further, the potential of adult ewes to remove from pasture more parasite larvae than they contribute through faecal contamination indicates a potentially useful role in suppressing parasite populations, particularly when worm control in lambs is less effective as a result of anthelmintic resistance.     

Brave or gullible: testing the concept that leaving susceptible parasites in refugia will slow the development of anthelmintic resistance

AIM: To test the theory that creating a reservoir of unselected worms by leaving a proportion of lambs in a flock untreated with anthelmintic, i.e. in refugia, will slow the development of anthelmintic resistance in nematode parasite populations. METHODS: Newly weaned Romney lambs (n=180) were infected with two nematode parasite species, Teladorsagia (= Ostertagia) circumcincta and Trichostrongylus colubriformis. For each species, the challenge doses contained a mixture of infective larvae from benzimidazole-resistant and -susceptible isolates calculated to yield, from the combined population, a 95% reduction in faecal nematode egg counts (FEC) following treatment with albendazole. Once the infections were patent, the lambs were divided into nine groups of 20 animals, and each group was allocated to one of three treatments. In Treatments 1, 2 and 3, 100%, 90% and 80% of animals were treated with an anthelmintic, respectively. For treatments 2 and 3, the heaviest animals remained untreated. Following treatment, each group was moved to its own previously prepared low-contamination pasture. Lambs grazed this pasture for 7 weeks before again being treated and moved to new low-contamination pastures (Shift 1 and Shift 2). The parasite populations on pasture resulting from the different treatments were subsequently sampled using tracer lambs, and worm eggs derived from these were used in both egg-hatch assays (EHA) and larval development assays (LDA), to measure albendazole-resistance status. RESULTS: Treating all animals each time the groups were moved to new low-contamination pastures resulted in higher levels of albendazole resistance (p<0.05), measured using EHA and LDA, in subsequent parasite generations than when either 10 or 20% of animals were left untreated. However, higher FEC in the tracer lambs grazed on pastures in Treatments 2 and 3, compared with Treatment 1, indicated an increased level of pasture contamination as a result of leaving some animals untreated. CONCLUSIONS: The results demonstrate that creating a reservoir of unselected parasites slows the development of anthelmintic resistance, and emphasises the risk of treating all animals prior to a shift on to low-contamination pasture. However, higher levels of pasture contamination, resulting from untreated animals, indicate the difficulty in managing both worm control and resistance.     

The comparative efficacy of abamectin, monepantel and an abamectin/derquantel combination against fourth-stage larvae of a macrocyclic lactone-resistant Teladorsagia spp. isolate infecting sheep

Anthelmintic resistance by gastrointestinal nematodes of sheep continues to be an issue of global interest. While the recent introduction in some countries of one or two new anthelmintic classes (amino-acetonitrile derivatives [AAD] and spiroindoles [SI]) has been welcomed, it is important that there is no relaxation in parasite control and the management of drug resistance. Monepantel (an AAD) was the first new anthelmintic to be approved for use (New Zealand, 2009) and was followed a year later in the same country by a combination of derquantel (a SI) and abamectin. The present study determined the efficacy of the new anthelmintic products and abamectin against fourth-stage larvae of macrocyclic lactone-resistant Teladorsagia spp. in lambs. Efficacies were calculated by comparing post-mortem nematode burdens of treated animals with those of untreated control sheep, and were 98.5, 86.3 and 34.0% for monepantel, abamectin/derquantel and abamectin, respectively. The nematode burdens of monepantel- and abamectin/derquantel-treated sheep were significantly lower than those sheep treated with abamectin and the untreated controls. Similarly, the burden of the monepantel group was significantly lower than that of the abamectin/derquantel group. These findings provide an opportunity to reinforce the recommendation that farmers and animal health advisors need to know the resistance status of nematode populations on subject farms to ensure effective control programs are designed and implemented. Such control programs should include an appropriate choice of anthelmintic(s), monitoring parasite burdens for correct timing of treatments, and pasture management to reduce larval challenge balanced with the maintenance of drug-susceptible populations in refugia.     

In vitro activity of Peltophorum africanum Sond. (Fabaceae) extracts on the egg hatching and larval development of the parasitic nematode Trichostrongylus colubriformis

Trichostrongylus colubriformis is an important cause of parasitic gastroenteritis in ruminants, where it causes protracted diarrhoea, rapid loss of weight, loss of production and death. The in vitro efficacy of extracts of Peltophorum africanum was determined against this parasitic nematode. Eggs and larvae of T. colubriformis were incubated at 23 degrees C in the extracts of the leaf, bark and root of P. africanum at concentrations of 0.008-25 mg ml-1 for 2 and 5 days, respectively. Thiabendazole and water were used as positive and negative controls, respectively. Inhibition of egg hatching and larval development increased significantly (P<0.05) with increasing concentrations of the extracts. Concentrations of 0.2-1.0 mg ml-1 of the extracts of leaf, stem bark, and root bark of P. africanum completely inhibited the hatching of eggs and development of larvae. No eggs and larvae of T. colubriformis could be observed in wells incubated with all the three extracts at concentrations of 5 and 25 mg ml-1. The in vitro model results support the traditional use of P. africanum against nematode parasites. Further research is required to isolate and structurally identify the active anthelmintic compounds, and to improve methods of plant extraction of the effective anthelmintic components that will be readily adaptable for use by rural communities against helminthosis.     

A microlarval development assay for the detection of anthelmintic resistance in sheep nematodes.

A microlarval development test for the detection of anthelmintic resistance in nematodes is described. Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis eggs were cultured to third stage larvae in the presence of Earle's balanced salt solution, yeast extract and bacteria in a total volume of 150 microliters. Good dose-response data were obtained with thiabendazole, levamisole, pyrantel tartrate and ivermectin allowing the determination of the 50 per cent lethal concentration and of resistance factors when resistant strains were available. The test was found to be accurate, sensitive, easy to carry out and applicable to the routine detection of resistance.     

Overberg Research Projects. IV. The development of resistance to Nematodirus in lambs and spontaneous cure of Teladorsagia in weaners

Soon after 12-week old suckling lambs reached a peak infestation of Nematodirus resistance developed, worm burdens falling by 43.1-97.5% in 18 week and 11-month old sheep respectively. Both Teladorsagia (syn. Ostertagia) and Trichostrongylus reached a peak geometric mean (G) of 18,099 and 3,278 worms on 02 December 1987 in weaners 5-6 months old. Within 6 weeks-6 months of sheep grazing on safe pastures, spontaneous cure of Teladorsagia occurred, worm burdens falling by 77.3-98.7% but not of Trichostrongylus which was only reduced by 34-40%.     

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.