Portable near-infrared spectroscopy: A rapid and accurate blood test for diagnosis of Haemonchus contortus infection and for targeted selective treatment of sheep

Haemonchus contortus is the most prevalent and important gastrointestinal nematode (GIN) in small ruminants. Since it reduces the packed cell volume (PCV), causing anemia, early diagnosis can be used for targeted selective treatment (TST) of sheep, reducing antiparasitic drug use and anthelmintic resistance. This study aimed to predict PCV values through near-infrared spectroscopy (NIRS) and to develop a classification and diagnosis model of H. contortus infection using PCV values, eggs per gram of feces (EPG) counts and mean daily weight gain (DWG). A total of 1728 spectra were collected from blood samples of 216 lambs with a portable NIR spectroscope. In parallel, other parameters indicative of infection were measured: PCV by hematocrit, FAffa MAlan CHArt (FAMACHA) scores, EPG and DWG. To evaluate the relationship between NIRS spectra and the evaluated parameters, principal component analysis (PCA) was used for an exploratory analysis, regression by the partial least squares method (PLS) for the prediction of PCV values via NIRS, and PCA linear discriminant analysis (PCA-LDA) as a classification model for diagnosis. The absorption peaks in the NIRS region associated with the excitation of overtones of nitrogen-hydrogen (N-H) functional groups of proteins had a strong impact on the principal components (PCs), indicating that blood proteins, especially hemoglobin, can be estimated by the NIRS technique. The model for predicting PCV by PLS presented a standard error of prediction of 2.53, root-mean-square error of 2.48, and coefficient of determination of 0.84, indicating good correlation between the PCV values predicted by the model and the PCV obtained by hematocrit. The PCA-LDA model presented 93.33% sensitivity and 82.18% accuracy, both higher than those of the FAMACHA method, as was expected for resilient Morada Nova lambs. The multivariate models associated with the NIRS technique reported here can be used in the future as a quick and versatile tool for H. contortus infection diagnosis and TST application in lambs.     

Resistance of exotic and domestic lambs to experimental infection with Haemonchus contortus

Resistance to primary and secondary infections with H. contortus was compared in St. Croix, 3/4 St. Croix, Florida Native, Barbados Blackbelly and domestic crossbred lambs. On the basis of worm counts, fecal egg counts and packed cell volumes, St. Croix and 3/4 St. Croix lambs were the most resistant to secondary infections, Florida Native and Barbados lambs were intermediate in resistance, and the domestic lambs were the least resistant. Breed differences were highly variable during primary infection. Breed differences were most pronounced prior to puberty, with domestic lambs highly susceptible to secondary infections. After puberty, domestic lambs were better able to resist secondary infections. Age apparently had little or no effect on the parasite resistance of exotic breeds. Sex differences were significant only after puberty when ewe lambs were more resistant to secondary infection than ram lambs. In one of three experiments lambs of hemoglobin genotype AB were more resistant to secondary infection than lambs of genotype BB in terms of fecal egg counts, packed cell volumes and the proportion of ‘responder’ and ‘nonresponder’ lambs but not in terms of worm counts, although subtle differences may not have been detected because of small numbers of lambs.     

Responses of hair and wool sheep to a single fixed dose of infective larvae of Haemonchus contortus

Body weights, fecal egg counts (FEC), and packed cell volumes (PCV) of hair sheep and wool sheep crossbred lambs were compared over 8 weeks following administration of a single dose of approximately 10,000 third-stage larvae of Haemonchus contortus. Hair sheep lambs (n=17) were reciprocal crosses between mainland USA populations of Barbados Blackbelly (BB) and Virgin Islands White (VIW) sheep. Wool sheep lambs (n=64) were from a crossbred composite of 50% Dorset, 25% Rambouillet, and 25% Finnish Landrace breeding. Lambs of both breed types continued to grow during the period of infection. Mean weights were higher for wool lambs (39.7±0.8 kg) than for hair lambs (28.2±1.5 kg). FEC increased to week 5 in both breed groups and remained elevated in wool lambs through week 7 but declined sharply in hair lambs after week 5. Mean FEC for weeks 4 through 8 were 4011±361 eggs per gram of feces (epg) in wool lambs, and 1135±196 epg in hair lambs. PCV declined through week 7 in wool lambs but stabilized and then increased after week 4 in hair lambs. Mean PCV in weeks 4 through 8 were 22.4±0.3% in wool lambs and 24.3±0.5% in hair lambs. These results suggest that Caribbean hair breeds may be able to contribute significantly to development of parasite-resistant sheep populations.     

Variation in resistance to haemonchosis: selection of female sheep resistant to Haemonchus contortus.

Seventy female lambs (6-7 months old) which were exposed to natural infections of Haemonchus contortus were designated as responders or non-responders on the basis of 10 weekly cumulative faecal egg counts. Selected responder and non-responder lambs were treated with ivermectin, housed separately and 6 weeks post-housing, seven lambs from each group were given a trickle infection of Haemonchus contortus at 1000 L3 daily for 5 days per week up to 2 weeks and examined weekly for 10 weeks after first infection. Analysis of data revealed significantly lower mean faecal egg counts and non-significantly less weight loss in responder than non-responder lambs. Mean values of haemoglobin, packed cell volume, total serum protein and peripheral eosinophil counts were significantly higher in responders than non-responders. In contrast, serum pepsinogen concentration was significantly less in responders than in non-responders. At 10 weeks post-infection, there were fewer pathological lesions and significantly lower worm burdens in responders than in non-responders. These results demonstrate a distinct resistance in responders to Haemonchus contortus infection.     

Failure to vaccinate lambs against Haemonchus contortus with functional metabolic antigens identified by immunoelectrophoresis.

Metabolic products from Haemonchus contortus larvae cultured in vitro from the infective third to fourth stage were collected and concentrated. Chromatographic and immunoelectrophoretic analyses were made to study the numbers and activity of antigens in the metabolic products derived from the in vitro cultured larvae. Three-month-old lambs were given a series of injections of metabolic antigens with and without adjuvant at dose rates of 0·05, 0·5 and 5·0 mg antigen protein per injection. These animals and saline injected controls were each challenged with 3000 H. contortus infective larvae after the last antigen injection and killed 35 days later. No difference was seen in the faecal worm egg counts or the differential worm counts among the vaccinated and control animals. The antigen preparation of worm metabolic products conferred no resistance to challenge infection with the parasite.     

Egg Laying Capacity of Haplorchis taichui (Digenea: Heterophyidae) in Humans

Quantitative fecal egg counts represented as the number of eggs per gram of feces (EPG) are generally a reliable parameter to estimate the worm burden of intestinal and hepatic parasitoses. Although Haplorchis taichui (Digenea: Heterophyidae) is one of the most common minute human intestinal flukes, little is known about the relationship between EPG and the actual worm burden in patients or the severity of the disease. In the present study, fecal samples were collected from 25 villagers in northern Thailand before and after praziquantel treatment. The EPG values of each participant were determined by the modified cellophane thick smear method, and adult worms were collected from the whole stool after the treatment. Eggs per day per worm (EPDPW) of H. taichui were estimated 82 from egg counts and expelled worms. The EPG was not well correlated with the worm burden, and a reverse correlation was observed between the EPDPW and the worm burden.     

Vaccination of lambs against Haemonchus contortus infection with a somatic protein (Hc23) from adult helminths

A somatic protein from adult Haemonchus contortus (Hc23), the most abundant component in a low molecular weight fraction with known immunizing effect against experimental haemonchosis, has been purified by immunochromatography. The immunoprophylactic value of Hc23 was tested in groups of 5–6 months old Assaf lambs using Al(OH)₃ or Escherichia coli lipopolysaccharide + inactivated Propionibacterium acnes as adjuvant and the results compared with uninfected control, uninfected and challenged or infected and challenged lambs. Immunization with Hc23 in either adjuvant elicited significant reductions in fecal egg counts after challenge with 15,000 L3s (70.67%–85.64%, respectively) and reduced (67.1% and 86%) abomasal worm counts (45 days post-challenge). Immunized lambs displayed higher peripheral eosinophil counts, were less anaemic and had weight gains than challenged controls. The results suggest that the Hc23 antigen can induce a partially protective response against haemonchosis in lambs.     

Resistance of nonlambing exotic and domestic ewes to naturally acquired gastrointestinal nematodes

No breed differences in Haemonchus contortus burdens were found when nonlacfating exotic (St. Croix and Barbados Blackbelly), domestic (Finn-Dorset × Rambouillet) and $\text{1}\text{2}\text{exotic}\text{-}\text{1}\text{2}$ domestic ewes grazed fall pasture, but all ewes had significantly (p < 0.05) lower worm burdens at necropsy than tracer lambs grazing the same pasture. Florida Native ewes grazing the same pasture showed significantly (p < 0.05) less packed cell volume (PCV) decrease and significantly (p < 0.05) lower fecal egg counts than the other ewe breeds but were not available for necropsy. Tracer lambs had a significantly (p < 0.05) greater PCV decrease and significantly (p < 0.05) higher fecal egg counts than ewes.The results of this study suggest that breed differences in the periparturient rise in fecal egg counts of exotic and domestic ewes observed in an earlier study may have been caused by breed differences in the periparturient relaxation of immunity rather than breed differences in the ability to acquire immunity to worms.     

A preliminary study on the responses to experimental Haemonchus contortus infection in indigenous goat genotypes

A preliminary study on the responses of different goat genotypes to experimental haemonchosis was assessed in 48 castrated 6-months old Boer, Xhosa-Boer, Nguni and Xhosa goats randomly assigned to four balanced groups: non-infected and non-supplemented (NINS); infected, non-supplemented (INS); infected, supplemented (IS) and non-infected, supplemented (NIS). Each goat in the infected groups received a dose of 7200 L3 H. contortus larvae and kept for 90 days. Of the infected groups, the IS Xhosa goats had the lowest faecal larval counts (FLC) and maintained the highest PCV values throughout the study. The INS Xhosa and Nguni goats maintained body weight, but INS Boer goats lost body weight markedly. In all the treatments, the Xhosa goats also maintained high PCV values throughout the study. In conclusion, the Xhosa and Nguni genotypes performed better in terms of tolerance to haemonchosis. With protein supplementation, the Xhosa and Nguni genotypes also appeared to be much more capable of increasing PCV and eosinophil counts as worm counts increased, unlike the Boer and Xhosa-Boer genotypes.     

Experimental concurrent infection of Afar breed goats with Oestrus ovis (L1) and Haemonchus contortus (L3): interaction between parasite populations, changes in parasitological and basic haematological parameters

The study was conducted to examine the occurrence and interaction between Oestrus ovis and Haemonchus contortus in experimentally infected Ethiopian Afar breed of goats. Twenty goats were divided into four groups (O, OH, H, and C) of five animals each. Each animal of groups O and OH received weekly infections for 5 weeks with 66 first instar larvae (L₁) of O. ovis. Then animals of groups OH and H were infected with a single dose of 5000 third stage larvae (L₃) of H. contortus. Goats of group C were kept free of any infection as non-infected control. Faecal egg count (FEC), blood cell count, total serum protein level and body weight were recorded weekly throughout the study period. At necropsy worm burden, female worm length, fecundity and larval burden of O. ovis in the nasal-sinus cavities of infected animals were assessed. The results showed that the presence of H. contortus in the abomasum of goats of group OH had no influence on the development of O. ovis. On the contrary, a significant reduction (P < 0.05) in FEC, worm burden, fecundity and female worm length was revealed in group OH animals compared to the mono-infected animals (group H). This was associated with eosinophilia and reduced packed cell volume.     

Is the establishment rate and fecundity of Haemonchus contortus related to body or abomasal measurements in sheep?

The relationship among parasitological parameters, abomasal size and body size measurements was investigated in lambs following an experimental infection with Haemonchus contortus. In total, 100 lambs from five different genotypes (German Merino (GM), Texel × GM, Suffolk × GM, German Blackhead Mutton × GM and Ile de France × GM) were experimentally infected with 5000 infective third stage larvae of H. contortus at the time of weaning at 12 weeks of age. Four and six weeks after infection, individual faecal samples were collected for estimation of faecal egg counts (FECs). Furthermore, wither height, shoulder width, heart girth, loin girth and body length were taken at 18 weeks of life. Lambs were slaughtered and necropsied 7 weeks post-infection, and worm counts, abomasal volume and surface area were determined. Positive correlations were found between different body size parameters, body weight and abomasal sizes. FEC and worm counts were not significantly correlated either with body size parameters or with abomasal size. The mean worm burden was higher in GM than in crossbred lambs. There was no significant difference in abomasal size between GM and crossbred lambs. The results suggest that the variations between animals in worm burden following an experimental infection with H. contortus (worm resistance) are not influenced by body size parameters or abomasal sizes. Therefore, other factors, including genetic-based differences in resistance, must cause these findings between and within breeds.     

The response of lambs to vaccination at weaning with irradiated Trichostrongylus colubriformis larvae: segregation into 'responders' and 'non-responders'

Dineen J. K., Gregg P. and Lascelles A. K. 1978. The response of lambs to vaccination at weaning with irradiated Trichostrongylus colubriformis larvae: segregation into ‘responders’ and ‘non-responders’. International Journal for Parasitology8: 59–63. Groups of colostrum fed and colostrum deprived lambs were vaccinated with irradiated Trichostrongylus colubriformis larvae at weaning (3 months) and challenged with normal infective larvae. As there was no significant difference between the worm counts of these groups after challenge, it was concluded that the failure of lambs, generally, to respond to vaccination as vigorously as mature sheep, was not due to ‘feed-back inhibition’ of the immune response by the action of maternal antibody.However the results showed that the lambs segregated into ‘responders’ and ‘non-responders’. The mean worm count of responders was 1560 whereas for non-responders this was 24,340. A responder was defined as an animal whose worm count was less than the lower limit of the 99 % confidence interval of unvaccinated controls. These results suggest that genetically-determined factors play an important role in the responsiveness of lambs to vaccination.Whereas counts of globule leucocytes in duodenal tissues were negatively correlated with worm counts (i.e. positively correlated with resistance), counts of both eosinophils and neutrophils were positively correlated with worm counts. These findings suggest that either globule leucocytes are involved in the resistance mechanism, or they are by-products of cellular events involved in resistance. On the other hand it is unlikely that either eosinophils or neutrophils are involved in the mechanism of resistance.     

Haemonchus contortus: A multiple-resistant Brazilian isolate and the costs for its characterization and maintenance for research use

The aim of this work was to determine the resistance level of Haemonchus contortus isolated from the Santa Inês flock of the Embrapa (Brazilian government's Agricultural Research Company), Southeast Livestock Unit (CPPSE), as well as to determine costs of characterizing and maintaining this isolate in host donors. Forty-two male Santa Inês lambs were experimentally infected with 4000 H. contortus infective larvae of the field isolate of CPPSE, called Embrapa2010, and divided into six treatment groups, which received triclorfon, albendazol plus cobalt sulfate, ivermectin, moxidectin, closantel and levamisole phosphate, as well as a negative control group (water). Egg per gram (EPG) counts were performed at 0, 3, 7, 10 and 14 days post treatment when the animals were slaughtered for parasite count. The data were analyzed using the RESO statistical program, considering anthelmintic resistance under 95% of efficacy. EPG and worm count presented a linear and significant relation with 94% determination coefficient. The susceptibility results obtained by RESO through both criteria (EPG and worm count) were equal, except for closantel, showing that the isolate Embrapa2010 is resistant to benzimidazoles, macrocyclic lactones and imidazothiazoles. The need of a control group did not appear to be essential since the result for susceptibility in the analyses with or without this group was the same. Suppression in egg production after treatment did not occur in the ivermectin and moxidectin groups. In the control group, the establishment percentage was just 12.5 because of the low number of third-stage larvae, resistance (innate and infection immunity) of the animals studied plus good nutrition. Drug classes presented similar efficacy between adults and immature stages. The costs for isolate characterization were calculated for 42 animals during 60 days. The total cost based on local market rates was approximately US$ 8000. The precise identification of Brazilian isolates and their establishment in host donors would be useful for laboratorial anthelmintic resistance diagnoses through in vitro tests, which has an annual cost of approximately US$ 2500 for maintenance in host donors.     

The periparturient rise in fecal egg counts in three strains of Florida native ewes and its value in predicting resistance of lambs to Haemonchus contortus

The periparturient rise in fecal egg counts in three strains of Florida Native ewes and its value in predicting resistance of lambs to Haemonchus contortus. International Journal for Parasitology16: 185–189. Significant (P < 0.05) differences in the magnitude of the periparturient rise in fecal egg counts (PPR) occurred in ewes of three strains of Florida Native sheep. Non-lambing ewes (n = 12) and lactating University strain ewes (n = 31), a strain selected over a 26-year period for parasite resistance by survival at pasture without anthelmintic treatment, had lower fecal egg counts than ewes of either Maxcy strain (n= 17), a commercial strain treated regularly with anthelmintics, or Backlinie strain (n = 16), a strain of Florida Native sheep that had some crossbreeding to Hampshire and Suffolk in their genotype and were regularly treated with anthelmintics. Whereas both Maxcy and Backlinie ewes showed a pronounced PPR, none occurred in non-lambing ewes (n=12) and the slight rise in fecal egg counts observed in University ewes was not significantly different from that of the non-lambing ewes. Backlinie ewes having twin lambs (n = 5) had a higher PPR than Backlinie ewes with single lambs (n = 11) or Maxcy ewes with single lambs (n = 15), but all three groups had higher PPRs than University ewes with single lambs (n = 27). No differences occurred in fecal egg counts of non-lambing ewes regardless of strain. Six weeks after weaning fecal egg counts of all ewes were uniformly low regardless of strain or prior lactation status.Ram lambs born to these ewes showed no significant strain differences in Haemonchus contortus burdens after experimental infection, deworming with levamisole and reinfection, although University lambs (n= 13) had lowest mean worm burdens at necropsy (2437) followed by Maxcy lambs (n= 12, mean =3106) and Backlinie lambs (n= 13, mean =3670). There was no significant correlation between worm burdens in these lambs and the magnitude of the PPR in their dams. Eight to 16 weeks after turn out to a common parasite-infested pasture ewe lambs (n = 13) born to University ewes had lower fecal egg counts than similar ewe lambs (n=8) born to Backlinie ewes. Prior to this time differences were not significant and egg counts were generally low. The magnitude of the PPR in their dams was not significantly correlated with fecal egg counts or PCVs of individual ewe lambs.     

The effect of sire selection on the response of lambs to vaccination with irradiated Trichostrongylus colubriformis larvae

Dineen, J. K. and Windon, R. G. (1980). The effect of sire selection on the response of lambs to vaccination with irradiated Trichostrongylus colubriformis larvae. International Journal for Parasitology10: 189–196. Rams selected for responsiveness and unresponsiveness to vaccination with irradiated T. colubriformis larvae at an early age were mated to unselected random bred ewes. Progeny were vaccinated with 20,000 irradiated larvae at 8 and 12 weeks of age, given anthelmintic treatment at 16 weeks and challenged with 20,000 normai larvae at 17 weeks. The results, based on wether worm counts and ewe faecal egg counts, showed significant differences between responder and non-responder progeny. There was a significant correlation between worm counts and faecal egg counts of half-sibs from the same sire group. The occurrence of globule leucocytes was inversely related to worm burdens of wether progeny, however, no clear relationship was found with eosinophils. In vitro lymphocyte stimulation using T. colubriformis L₃ antigen, concanavalin A and lipopolysaccharide showed that statistically defined responder progeny, pooled from both responder and non-responder sire groups, gave higher responses than non-responder lambs after vaccination. The results confirm that genetically-determined factors are involved in the response of lambs to vaccination at an early age, and indicate that rapid genetic progress may be achieved in the type of mating usually carried out under fleld conditions.     

Regulation of Haemonchus contortus populations in sheep exposed to continuous infection

Groups of sheep were infected three times weekly for 15 weeks with infective larvae of Haemonchus contortus at four rates ranging from 600 to 4800 larvae per week. At 3-weekly intervals, sheep from each group were killed for total worm counts after receiving a dose of radiolabelled larvae which enabled a direct measurement of establishment of incoming larvae during the continuing infection. Peak H. contortus burdens were reached between 6 and 9 weeks of infection, and were related to infection rate. In the groups receiving 2400 and 4800 larvae per week, worm numbers then declined rapidly and by the end of the experiment were lower than those in the groups given 600 or 1200 larvae per week. Establishment of incoming larvae was not influenced by infection rate, and declined from 45% in the first 4 weeks of infection to insignificant levels during the final 6 weeks. The proportion of incoming larvae arrested in their development increased as the infection progressed. It was concluded that H. contortus numbers were regulated by development of resistance to infection and by a loss of established worms which was related to the current rate of larval intake, and to the host's previous experience of infection.     

Changes in blood leukocytes,bone marrow and lymphoid organs in sheep infected with Haemonchus contortus

Haemonchus contortus infection, which causes a blood loss anaemia in sheep, depleted leukocytes and produced leukopenia and a mild lymphopenia. Thymus atrophy, a decreased size of spleen and enlargement of adrenal glands occurred concomitantly in infected sheep which suggested that they were caused by the stress of infection. The overwhelming change in bone marrow of infected sheep was a 4-fold increase in erythroid series cells.Primary immunization with rat erythrocytes produced similar haemagglutinating antibody responses in infected sheep and in sheep allowed to recover from infection after treatment with thiabendazole. This suggests that extant infection with H. contortus was not associated with a secondary immunodeficiency. Blastogenic responses of blood lymphocytes from infected sheep to larval antigen correlated with faecal egg counts but not with haematocrit or leukocyte values. These results suggest that the unresponsiveness of lymphocytes to worm antigen which occur during infection with H. contortus is more closely related to contact with the parasite than to the pathological effects of infection.     

The response of young Romney lambs to immunization with Trichostrongylus colubriformis larvae

1988. The response of young Romney lambs to immunization with Trichostrongylus colubriformis larvae. International Journal for Parasitology 18: 1035–1038. Groups of weaned Romney ewe lambs were immunized with two doses of 28,000, 35,000 or 42,000 (2000 kg⁻¹) infective larvae of Trichostrongylus colubriformis at 8, 12 or 16 weeks of age, respectively. Each group and helminthfree control lambs of similar age were challenged with T. colubriformis at the same dose rate as used for immunization. Faecal egg counts (FEC) and haematological observations were made during the experiment, and at slaughter, 42 days after challenge, worm burdens were determined and small intestinal histology was examined.

Lambs in each immunized group were identified as ‘responders’ or ‘non-responders’ on the basis of both FEC and worm burdens. A significant (P<0.001) decrease in the worm burdens recovered as a proportion of the challenge infections in both unimmunized and immunized lambs with increasing age was observed.

Globule leukocyte numbers increased with the age of lambs. In addition, within each age group globule leukocyte numbers reflected individual responsiveness to immunization, significantly (P<0.01) greater numbers being present in ‘responders’ than ‘non-responders’ or unimmunized lambs. No difference in haematological responses were found in relation to the lambs' responsiveness to immunization.     

The response of lambs to vaccination and challenge with Trichostrongylus colubriformis: Effect of plane of nutrition on,and the inter-relationship between,immunological responsiveness and resistance

Wagland B. M., Steel J. W., Windon R. G. and Dineen J. K. 1984. The response of lambs to vaccination and challenge with Trichostrongylus colubriformis: effect of plane of nutrition on, and the inter-relationship between, immunological responsiveness and resistance. International Journal for Parasitology14: 39–44. Merino lambs weaned at 8 weeks of age were fed either ground and pelleted (high plane, HP) or chopped (low plane, LP) lucerne hay ad libitum to achieve an approximate 2-fold difference in liveweight gain. When aged 17 and 21 weeks, 15 of the 20 lambs in each diet group were vaccinated with 80,000 irradiated T. colubriformis larvae. At 25 weeks, vaccinated and unvaccinated lambs were treated with anthelmintic and one week later challenged with 30,000 normal larvae. Four weeks after challenge the animals were killed for worm counts. After vaccination HP lambs had higher titres of antibodies to the parasite and after challenge had lower worm egg outputs, and lower worm burdens than LP lambs. Immunological responsiveness (serum titre of complement-fixing antibodies against worm antigen) and manifestations of resistance (eggs produced per female worm per day and percent protection calculated from worm counts) were significantly correlated within dietary groups. Percent protection and egg production per female worm were highly correlated (r = ?0.81) in individual animals pooled over dietary groups, suggesting that both manifestations of resistance respond to essentially the same immunological mechanism. Failure to obtain significant correlation between weight gain pre-vaccination and immunological and resistance parameters indicated that animal production and resistance to infection are not genetically linked. Negative correlation of weight gain during the vaccination period with serum antibody titre at challenge suggests that the developing immune response competes with weight gain for limited physiological resources of the animal.     

Control of nematode parasites by grazing management—II. Decontamination of sheep and cattle pastures by varying periods of grazing with the alternate host

Commencing in December 1970, paddocks of a uniform series of sheep pastures were grazed for 6, 12 or 24 weeks by either yearling steers is or yearling ewes. Cattle pastures were treated similarly. All ewes and steers were pre-dosed with anthelmintic.At the conclusion of alternate grazing the effectiveness of the grazing treatments was evaluated by grazing each paddock for 1 month with either ten worm-free lambs (sheep pastures) or three wormfree calves (cattle pastures). The test animals were then slaughtered for total worm counts. The grazing of sheep pastures with cattle for 6, 12 or 24 weeks from December onwards resulted in reductions in numbers of Haemonchus contortus and Trichostrongylus colubriformis in test lambs. In comparison with continuous grazing by sheep, Nematodirus spp was only reduced after 24 weeks grazing by cattle. Cattle pastures grazed by sheep for 6 weeks showed no reduction in numbers of Ostertagia ostertagi or Cooperia oncophora in test calves. However after 12 weeks with sheep, numbers of O. ostertagi though not C. oncophora were reduced and after 24 weeks of alternate grazing both these species were reduced.Calves following the 6 week sheep treatments acquired both Haemonchus contortus and Trichostrongylus colubriformis and the calves from the 12 week sheep treatment paddock also carried H. contortus. For sheep the only evidence of cross-transmission was the occurrence of small numbers of Cooperia oncophora in test lambs from the 24 week cattle grazing treatment.The results provide evidence that sequential stocking with cattle and sheep in conjunction with anthelmintic treatment is an effective management strategy for preparing parasitologically safer pastures, but further information is required to determine the optimum timing of sequential stocking in farming situations.