The effects of ivermectin and moxidectin on egg viability and larval development of ivermectin-resistant Haemonchus contortus

The in vivo effects of ivermectin and moxidectin on egg viability and larval development of ivermectin-resistant Haemonchus contortus were examined over time after anthelmintic treatment of sheep. Twenty merino sheep, (12 months old) were allocated to five treatment groups and infected with ivermectin-resistant H. contortus. Thirty one days later, the sheep were treated with intraruminal ivermectin capsules, oral ivermectin, oral moxidectin or injectable moxidectin at the manufacturer's recommended dosages, or left untreated. At various times up to 112 days after treatment, faecal egg counts (FEC) were determined and development rates of infective larvae (L3) cultured in faeces or on agar were measured. Eggs in faecal cultures from ivermectin capsule treated sheep showed reduced L3 development percentages in comparison to faecal cultures from untreated sheep. Eggs from ivermectin capsule treated sheep, isolated from faeces, and cultured on agar showed similar L3 development to eggs from control sheep. These results demonstrate an inhibitory effect of excreted ivermectin in faeces on larval development of ivermectin-resistant H. contortus. L3 development in faecal culture from animals receiving oral ivermectin were reduced for only 3 days after treatment. Faecal egg counts and development of L3 larvae in both culture systems from moxidectin treated sheep were low, due to the high efficacy of the drug. Egg counts in moxidectin treated sheep were reduced by approximately 90% 24h after treatment, before decreasing to almost 100% at 48h, suggesting that the current quarantine recommendation of holding sheep off pasture for 24h after treatment may still lead to some subsequent pasture contamination with worm eggs.     

Effectiveness of copper oxide wire particles for Haemonchus contortus control in sheep

OBJECTIVE: To assess the efficacy of copper oxide wire particles (COWP) for the control of H contortus infections in grazing sheep. PROCEDURE: In experiment 1, 40 worm-free Merino hoggets (11 to 12 months of age) were divided into four equal groups and allocated to separate 0.8 ha pasture plots. Two groups then received 2.5 g COWP whereas the other two groups were untreated. From 1 week after COWP treatment all lambs received a weekly infection of 2000 H contortus larvae. At week 8, six sheep from the untreated group were then allocated to two groups and treated with either 2.5 or 5.0 g of COWP to establish therapeutic efficacy of treatment. Experiment 2 followed a similar protocol but was conducted with 40 worm-free Merino lambs (3 to 4 months of age) and no assessment of therapeutic efficacy was made. Results: In experiment 1 no significant difference in faecal worm egg counts was observed between treatments and faecal worm egg counts remained less than 3000 epg in all animals. Total worm counts were reduced by 37% by COWP treatment (P = 0.055). Both 2.5 g and 5.0 g doses of COWP at 8 weeks of infection reduced faecal worm egg counts by > 85% with the higher dose giving an earlier response to treatment. In experiment 2, faecal worm egg counts at 4 and 6 weeks were reduced by more than 90% in the COWP treated lambs and worm numbers were 54% lower after 6 weeks when all remaining untreated lambs had to be treated for haemonchosis. Mean faecal worm egg counts in the COWP lambs remained below 3500 epg and clinical disease did not develop in the majority of lambs before the end of the experiment at 10 weeks. CONCLUSION: Treatment with COWPs appears to have the potential to reduce establishment and worm fecundity of Haemonchus spp for an extended period and may offer livestock producers a supplementary means of reducing larval contamination of pasture particularly in areas where anthelmintic resistance is a problem and copper supplementation is likely to be beneficial.     

Stocking rate effects on the worm burdens of Angora goats and Merino sheep

Worm egg counts of yearling Angora or Merino wethers grazing in mixed flocks at either 4/ha or 8/ha were observed over a 5 month period. Based on faecal egg counts infection levels in the sheep increased for the first 3 months and then decreased as the animals developed resistance. Egg counts in the goats did not decrease during the experiment. Post mortem worm counts indicated that there was a significant host species--stocking rate interaction on worm burdens of Haemonchus contortus adults and Ostertagia circumcincta adults and larvae. Goats had more H. contortus and O. circumcincta adults than sheep at the 8/ha stocking rate. At 4/ha goats had fewer O. circumcincta fourth stage larvae than sheep. There was no stocking rate effect on the numbers of Trichostrongylus axei or T. colubriformis. Sheep had more T. axei and fewer T. colubriformis than goats.     

Multiple Anthelmintic Resistance in Haemonchus Contortus on a Sheep Farm in Kenya

Multiple resistance to albendazole, thiophanate, levamisole and orally administered ivermectin was detected in an isolate of Haemonchus contortus in sheep on a farm where benzimidazole resistance had already been identified. Following a faecal egg count reduction test, this was confirmed by both critical and controlled anthelmintic tests. Different groups of sheep infected naturally or given an experimental infection with the benzimidazole-resistant isolate were treated with the recommended doses of various anthelmintics. Compared to the control group, the percentage reductions in the faecal egg counts of sheep treated with albendazole, thiophanate, levamisole and ivermectin varied between 38.2% and 79.1% and the residual worm counts between 27.3% and 57.5%. The results indicate the presence of multiple anthelmintic resistance in this isolate of H. contortus. Sheep treated with closantel showed 100% reductions in faecal egg and worm counts, indicating that this drug was very effective against the population of H. contortus on the farm.     

Doramectin resistance in Haemonchus contortus on an alpaca farm in Belgium

Parasitism by gastrointestinal nematodes is a health concern in New World Camelids (NWC) worldwide, and anthelmintic treatment is often needed for parasite control. Although anthelmintic resistance has been reported in ruminants worldwide, data in NWC are only scarce. In the present study, a case of suspected doramectin resistance in alpacas was examined. A field efficacy study was conducted for the evaluation of two different dosages of doramectin using a faecal egg count reduction test. A group of 8 alpacas was treated with a subcutaneous injection of doramectin at 0.2mg/kg bodyweight. Individual faecal samples were collected before treatment and 7 days after treatment. The faecal egg counts indicated a treatment efficacy of only 68%. To determine whether the treatment failure was caused by true anthelmintic resistance or suboptimal dosage in this animal species, a group of 4 alpacas was subsequently treated at 0.3mg/kg bodyweight. Faecal egg counts 7 days post treatment were reduced by only 41%, indicating that the treatment failure was more likely to be caused by the presence of resistant parasites on this farm. Coprocultures of faecal samples collected after treatment indicated the presence of 98.5% Haemonchus contortus and a small percentage of Cooperia oncophora (<1.5%). A controlled efficacy trial in sheep, for which the optimal dosage of doramectin is known, was conducted to ensure that this truly was a case of resistant parasites. Infective larvae collected from the faeces of these alpacas were used to infect eight nematode-free lambs. These lambs were assigned to one of two groups based on faecal egg counts post infection. One group was treated with doramectin injectable at 0.2mg/kg bodyweight, the other group served as a non treated control group. Pharmacokinetics indicated that the doramectin treatment was adequate, yet an efficacy of only 16% was determined on day 7 after treatment. Identification of the larvae after treatment revealed 100% H. contortus. On day 7 after treatment, H. contortus worm counts were only reduced by 8% in the treated lambs. The results of the present study report for the first time a case of doramectin resistance in alpacas, mainly in H. contortus.     

Trapping efficacy of Duddingtonia flagrans against Haemonchus contortus at temperatures existing at lambing in Australia

The aim of this study was to determine the trapping efficacy of Duddingtonia flagrans against Haemonchus contortus at the temperature ranges experienced around lambing in the major sheep producing regions of Australia. Faeces were collected from Merino wethers, maintained in an animal house and which had received either D. flagrans chlamydospores for a 6-day period (DF) or not (NIL). Faeces were incubated at one of four daily temperature regimens which were composed of hourly steps to provide 6-19 degrees C, 9-25 degrees C, 14-34 degrees C and 14-39 degrees C to mimic normal diurnal air temperature variation. Enumeration of the number of preinfective and infective larvae that had migrated from or remained in faecal pellets was used to calculate percentage recovery and trapping efficacy of D. flagrans. Recovery of H. contortus larvae of both stages was significantly lower in DF faeces but the magnitude of the effect was considerably greater for infective larvae. Mean recovery of infective larvae from NIL and DF faeces was 10.6 and 0.4%, respectively, indicating a mean trapping efficacy of 96.4%. The lowest trapping efficacy (80.7%) was observed at 6-19 degrees C but total recovery of infective larvae, from DF faeces, was greatest at the two highest temperature regimens, although still less than 0.9%. The results of this study indicate that typical Australian lambing temperatures should not be a barrier to the use of D. flagrans as an effective biocontrol of H. contortus in Australia.     

Haemonchus contortus and Trichostrongylus colubriformis in pen-trials with Javanese thin tail sheep and Kacang cross Etawah goats

Weight gain costs due to infection were higher in sheep than goats, 28 and 17.5%, respectively, for Trichostrongylus colubriformis and 48.7 and 32.2%, respectively, for Haemonchus contortus. The extent of bodyweight cost attributed to anorexia in sheep infected with H. contortus was higher (13.5 g/day) than in sheep infected with T. colubriformis (2.3 g/day). On the other hand, bodyweight cost due to the other pathogenic effects in sheep infected with T. colubriformis were higher (35.6 g/day) compared to sheep infected with H. contortus (10.9 g/day). A strong relationship between faecal egg count and worm count (r=0.79, P=0.006) was shown only in sheep infected with T. colubriformis. About half of the infected sheep and goats had low or zero faecal egg counts throughout the study. In about 40% the egg count rose initially but became low by weeks 10-16, whereas in about 10% counts increased progressively throughout the period of observation and these animals also had the highest numbers of worms at slaughter. Packed cell volume was reduced in sheep and goats infected with H. contortus but serum protein and haemoglobin levels were unaffected. Sheep infected with T. colubriformis had a higher level of eosinophilia after 8 weeks (18.4%) than sheep infected with H. contortus (11.4%), whereas this pattern was reversed in goats and levels were also lower (4.1 and 8.9%, respectively). There was no apparent relationship between eosinophilia and resistance to infection with H. contortus or T. colubriformis.     

The response of Awassi and Merino sheep to primary infection with Haemonchus contortus

The possible existence of breed differences in the response of sheep to primary infection with Haemonchus contortus was examined by comparing worm establishment and pathogenic effects of the parasite in Awassi and Merino sheep of haemoglobin type B infected with 500 third stage H. contortus larvae per kg body weight. The results showed that the Merino sheep had lower faecal egg counts and worm burdens and suffered less severe clinical disturbances than sheep of the Awassi breed. This suggests that genetic resistance operates primarily at the level of worm establishment.     

Seasonal variations in the numbers of trichostrongylid nematode eggs and their larvae in the faeces of farmed goats in Malaysia.

Faecal worm egg counts of goats from two farms in Penang Island, West Malaysia, were monitored over a period of 14 months. The faecal egg count pattern followed that of total rainfall. The humid tropical environment was favourable for the development of various species of trichostrongylid nematodes, namely Haemonchus contortus, Trichostrongylus spp., Oesophagostomum spp. and Cooperia spp. Generally, H. contortus was observed to be the predominant species, more so in the monsoon months of the year.     

The effect of single and divided dose administration on the efficacy of fenbendazole against adult stages of benzimidazole resistant sheep trichostrongylids.

Sheep infected with benzimidazole resistant strains of Haemonchus contortus and Trichostrongylus colubriformis were used to compare the anthelmintic efficacy of fenbendazole given as a single dose or administered in a divided dose regime over five days. Statistical analysis showed no significant difference between the two methods of administration for H contortus. On the other hand, divided dose fenbendazole was significantly less effective than single doses against adult T colubriformis at dose rates of 5 and 7.5 mg/kg. In the case of H contortus a highly significant correlation coefficient between post treatment egg counts and worm counts (r = 0.789) was obtained. This suggests that reduction in faecal egg output following drug treatment would provide a useful field indication of anthelmintic performance of fenbendazole (and possibly related compounds) against benzimidazole resistant strains of this parasite.     

The Effect of Anthelmintic Capsules on the Egg Output and Larval Viability of Drug-resistant Parasites

Challenge with an equal mix of drug-resistant and drug-susceptible larvae of Teladorsagia circumcincta resulted in infections in groups of lambs (n = 6) either untreated or given controlled-release capsules, containing either albendazole or ivermectin. Lambs treated with albendazole capsules contained similar numbers of adult worms at necropsy to the other groups but had no detectable faecal egg count. Animals treated with ivermectin capsules had similar worm burdens and faecal egg counts to the control group but the worms had significantly higher numbers of eggs in utero. These results provide evidence for suppression of egg production by both anthelmintic treatments. The observation that albendazole caused a significant reduction in the developmental success of parasite eggs also has implications for the use of faecal egg count as an indicator for pasture contamination with resistant parasites. In two further groups of lambs, either untreated or given albendazole capsules, treatment caused a significant reduction in egg count and adult worm burden of Trichostrongylus colubriformis. No significant effects were observed on in utero egg counts or egg viability and the apparent effect on the number of eggs produced in faeces per adult female was not significant (p = 0.077). There was, therefore, no evidence that albendazole controlled-release capsules caused suppression of egg output in this species.     

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.     

Strongyle infections of small ruminants in Nigeria

A survey of strongyle infections was conducted in sheep and goats reared in a traditional e extensive husbandry system in two ecological zones if Nigeria. One zone had a seasonal pattern of infection. The majority of animals had faecal worm parasite egg counts of below 500 eggs per gram. Kids, and lambs younger than 3 months did not carry strongyle worm burdens, and the highest infection rate was found in the 7–12 month age group. A high proportion of small ruminants shed strongyle eggs during the postparturient period.The helminth species found by the use of larval culture techniques on the faeces were: Haemonchus contortus, Trichostrongylus colubriformis and Oesophagostonum columbianum. Adults of the same species were found in the few animals necropsied. The significance of the findings is discussed.     

Precision, repeatability and representative ability of faecal egg counts in Heterakis gallinarum infected chickens

This study investigated whether a precise and repeatable quantification of Heterakis gallinarum egg excretion, which considerably reflects the actual worm burdens, can be achieved based on collection of the daily total amount of faeces from chickens. Three-week-old birds (N=64) were infected with 200 embryonated eggs of H. gallinarum, and placed into individual cages 3 wk after infection for 5 wk to collect daily faeces (N=2240). The total daily faeces was mixed and a randomly taken sample per bird was analyzed to estimate the numbers of eggs per gram of faeces (EPG) and total number of eggs excreted within 24h (EPD). A total of 235 daily faecal collections were randomly selected and further examined to determine between and within sample variations of EPG counts as a measure of precision. For this, two random faecal samples were taken from the daily produced faeces by a bird, and the EPG was determined for each of the samples (EPG1 and EPG2). The second faecal sample was analyzed once more to determine a parallel EPG2 count (EPG2a) of the suspended sample. Precision of an EPG count was defined as its relative closeness to the average of two EPG counts using a relative asymmetry index (Index(EPG)). At an age of 11 wk, i.e. 8 wk p.i. the birds were slaughtered and their worm burdens were determined. There were no significant differences between EPG1 and EPG2 (P=0.764) nor between EPG2 and EPG2a (P=0.700), suggesting that the differences between or within the samples were not different from zero. Correlations between EPG counts, as between and within sample coherences, were r=0.85 and r=0.86, respectively. Precision of EPG counts, as measured by Index(EPG), was not influenced by consistency (P=0.870) and total amount of faeces (P=0.088). However, concentration of eggs in faeces (mean EPG) had a significant effect on the precision of the EPG counts (P<0.001). Similar results were also observed for the within sample precision (Index(EPG2)). A segmented regression analysis indicated an abrupt change in the precision of EPG counts as the response to changing egg concentration in the examined faecal samples. The precision of analyses remarkably heightened up to a breakpoint with an EPG count of ≤ 617. A similar breakpoint was also determined for within sample precision (EPG2 ≤ 621). Moderate repeatabilities (R=0.49) for EPG and EPD were estimated in the first week of egg excretion, whereas the estimates were higher (R=0.67-0.84) in the following weeks. Correlations between number of female worms with daily measured EPG and EPD increased to an almost constant level (r ≥ 0.70; P<0.05) in a few days after the nematode excreted eggs and predominantly remained so for the rest of the sampling period. It is concluded that mixing daily total faeces provides samples with random homogenous distribution of H. gallinarum eggs. Precision of the EPG counts increases as the egg concentration in faecal sample increases. Egg excretion of H. gallinarum, quantified either as EPG or EPD, is highly repeatable and closely correlated with the actual worm burden of birds starting as early as in 5 th wk of infection.     

The epidemiology of nematode and fluke infections in cattle in the Red River Delta in Vietnam

Over a period of 13 months, faecal samples were collected monthly from approximately 45 cattle over 3 months of age. Additionally, 74 calves of 1-2 months were sampled to determine the presence of Toxocara vitulorum eggs. Individual egg counts and infective strongyle larvae from pooled faecal samples were examined. Post-mortem worm counts were carried out on six groups of tracer calves (n=12) that had been kept for 4 weeks on pasture in and around the village studied. The following helminths were identified: T. vitulorum, Cooperia punctata, C. pectinata, C. oncophora, Oesophagostomum radiatum, Trichostrongylus axei, T. colubriformis, Haemonchus spp., Fasciola spp. and Paramphistomum spp. In 8% of the samples collected from young calves, individual egg counts for T. vitulorum were found indicative for pathogenic worm burdens. Strongyle egg counts and worm counts indicated that transmission is low without a distinct seasonality. In animals of 3-9 months old, a strongyle egg count peak can be demonstrated which at a higher age steadily and significantly decreased. In faecal cultures Cooperia spp. were most prominent in all age groups throughout the year with the exception of the period September-November when Haemonchus spp. and Oesophagostomum spp. were most prevalent. Fasciola spp. eggs were found in 22% of the collected faecal samples and the egg counts were low indicating that the intensity of Fasciola spp. infection is mild. Based on the present data, regular anthelmintic treatments seem not to be justified, except for a single treatment at the age of 2 weeks against toxocariosis.     

Fibrinogen-degrading proteins from Haemonchus contortus used to vaccinate sheep.

Sixteen nonsibling sheep, approximately 12 months old, that were raised in a helminth-free environment, were used for 2 protection studies 6 months apart. Sheep were vaccinated weekly for 5 weeks by IM injection of fibrinogen-degrading proteins derived from the intestinal tract of adult Haemonchus contortus. Ten days after the last vaccination, sheep were given 2,500 infective H contortus larvae by intraruminal injection. Vaccinated sheep produced specific antibodies, and were protected from the worm challenge. Significant differences in mean fecal worm egg counts for 56 days after worm challenge, in mean numbers of H contortus worms, and female fecundity ratios at necropsy were detected in vaccinated sheep, compared with those in control sheep. These data suggest that the fibrinogen-degrading proteins have a protective role in vaccination of sheep against H contortus.     

Pathogenicity of O. circumcincta, O. ostertagi and H. contortus in weanling stag fawns (Cervus elaphus)

Groups of four stag fawns, weighing 51 +/- 0.8 kg, were-infected with 30,000 larvae of one of three nematode parasites: O.circumcincta, O.ostertagia, or H.contortus. A further four remained as control. Individual feed intake, body-weight, faecal egg count and plasma pepsinogen concentration were measured during seven weeks after infection. Only infection with H.contortus caused significant elevation of pepsinogen concentration to 380ml/ L and consistent depression of feed intake (15%). Worm egg counts were below 300 e.p.g. in deer infected with all worm species. It is concluded that while resistance to development of parasites common to other species appears to be high further investigation of the susceptibility of deer to H. contortus infection would be justifiable.     

STUDIES ON NEMATODE INFECTIONS OF BEEF CATTLE IN NEW SOUTH WALES

The occurrence and seasonal trends of nematode parasite infections in beef cattle on the Tablelands and North Coast regions of New South Wales are described, based on worm counts from 627 spring-born steers slaughtered at 2-monthly intervals from 6 to 24 months of age. The predominant parasites were Ostertagia ostertagi, Trichostrongylus axei and Cooperia spp. Large burdens of adult Ostertagia were seen in 12-month-old animals in late winter and early spring, and later in 18- to 20-month-old cattle in the following late summer and autumn. These infections often resulted in outbreaks of clinical parasitism. Massive numbers of inhibited early fourth stage larvae (EL4) also accumulated in the yearlings during their first spring, reached peaks in mid-summer and then declined. The possibility of their resumption of development to mature worms in the late summer and autumn period is discussed. Adult T. axei showed a similar seasonal trend to Ostertagia and may have increased the severity of outbreaks of clinical disease. Cooperia populations of EL4 and adult worms were highest in weaners during winter, but lower numbers thereafter indicated a strong resistance to re-infection. In addition, Haemonchus placei occurred frequently on the North Coast both as EL4 and adults in cattle of all ages up to 20 months. Of the other cattle nematodes, Oesophagostomum radiatum and Trichuris spp occurred in low numbers, mainly in weaners. Bunstomum phlebotomum and H. contortus occurred sporadically on the North Coast and Tablelands respectively. Intestinal Trichostronglyus spp, Oesophagostomum venulosum and Dictyocaulus viviparus were seen occasionally and Nematodirus spp were not seen. There was a poor relationship between worm counts and faecal egg counts. Cooperia spp dominated the egg counts, while those for other genera were generally low and did not reflect the relative abundance or seasonal changes in worm numbers.     

The epidemiology of abomasal nematodes of sheep in Sweden, with particular reference to over-winter survival strategies

In May 2002, studies on the seasonal patterns of nematode infection of sheep were undertaken on four commercial sheep farms in southern Sweden, which had previously reported problems with nematode parasitism, especially due to Haemonchus contortus. One farm was used for intensive investigation. This entailed the establishment of two replicate groups of sheep, each consisting of 20 ewes and their lambs, on adjacent pasture paddocks. The seasonal patterns of nematode infection were followed by regular (approximately monthly) sampling of both ewes and lambs for nematode faecal egg counts and larval differentiation, and the sequential use of replicate groups of tracer lambs. H. contortus and Teladorsagia circumcincta were the most abundant nematode species, with the former most prevalent in the post-parturient faecal egg counts of ewes. Tracer worm counts showed almost 100% arrested development in the early fourth larval stage for H. contortus as early as mid-summer and the numbers of parasites progressively increased during the season. T. circumcincta also showed high levels of arrested development, but not as early, or as absolute, as for H. contortus. Tracers allocated to the paddocks at the time of turn-out following winter in May 2003, showed virtually a total absence of H. contortus in contrast to exceedingly high infections with T. circumcincta. Results of the three additional monitoring flocks supported these findings. It can be concluded that under Swedish sheep farming conditions, H. contortus has evolved to survive the long, cold winters entirely within the host as the arrested larval stage, relying on the lambing ewe to complete its life cycle. The peri-parturient relaxation of resistance in the ewes triggers the resumption of development to the adult egg-laying parasites to result in pasture contamination and the completion of just one parasite generation/year. In contrast, T. circumcincta can survive well over-winter, both on pasture and within the host.