A study of some factors influencing the immunization of sheep against Haemonchus contortus using attenuated larvae

The respective roles of age of lamb, size of immunizing dose, previous exposure to infection and anthelmintic therapy on the immune response of Scottish Blackface sheep to gamma-irradiated H. contortus larvae were studied.As previously shown, young lambs immunised at 10 and 14 weeks of age failed to develop a significant resistance to subsequent challenge.In contrast, parasite-naive lambs immunised at 9 and 10 months of age with 10 000 H. contortus larvae irradiated at 60 kiloröntgens developed a highly significant immunity to subsequent challenge. This degree of immunity was not significantly altered by the administration of spaced doses of normal larvae from 10 weeks of age until the point of vaccination.Worm-free lambs vaccinated at 9 and 10 months of age failed to develop any immunity to subsequent challenge when the anthelmintic thiabendazole was administered 3 weeks after each immunizing infection.     

The response of sheep vaccinated with irradiated Trichostrongylus colubriformis larvae to impulse and sequential challenge with normal larvae

Sheep aged 9–10 months were vaccinated with three doses of 20 000 irradiated Trichostrongylus colubriformis larvae at fortnightly intervals. This gave 97–99% protection against subsequent “impulse” challenge with 40 000 normal larvae or “sequential” challenge with 2 000 larvae/week-day for 4 weeks.The sheep were protected against both types of challenge when worms from the immunizing infections were present at the time of challenge, and when these worms were first removed by anthelmintic treatment.     

Dual infection of lambs with Trichostrongylus colubriformis and chlamydiae

Lambs in one group were inoculated with chlamydiae and Trichostrongylus colubriformis larvae simultaneously; lambs in a second group were inoculated with chlamydiae after a T. colubriformis infection was established. Results were compared to those from groups inoculated with T. colubriformis only and chlamydiae only. There were no significant differences in fecundity associated with chlamydial inoculation. Packed cell volumes (PCVs) were depressed in groups inoculated with chlamydiae, and chlamydial antibody formation appeared to be inhibited when chlamydiae was superimposed on a T. colubriformis infection. Relatively few T. colubriformis adults were recovered in lambs inoculated with chlamydiae and T. colubriformis simultaneously.     

Possible relationship of levels of mucosal IgA and serum IgG to immune unresponsiveness of lambs to haemonchus contortus

Abomasal mucus IgA and serum IgG antibodies were studied in adult sheep and young lambs vaccinated with irradiated Haemonchus contortus larvae and subsequently challenged with normal larvae. In the adult sheep protection against challenge was associated with raised levels of these antibodies. Vaccination of the lambs, on the other hand, did not protect against challenge nor did it stimulate either serum IgG or mucus IgA antibodies. The latter remained at levels similar to those of both control lambs given a single challenge infection and worm-free adult sheep.     

Maintenance of acquired immunity to Trichostrongylus colubriformis in Merino sheep that are losing weight

SUMMARY The effect of loss of weight on maintenance of acquired immunity to Trichostrongylus colubriformis in Merino sheep was assessed in 2 experiments. Adult Merino sheep with previously acquired immunity to T colubriformis were housed in individual pens. Sheep were infected 3 times weekly with 2000 T colubriformis larvae. A diet based on chopped oaten and lucerne hay (experiment 1) or a pelleted diet based on lucerne hay and grain products (experiment 2), was offered in amounts to sustain a moderate gain in body weight, or was reduced step-wise to cause a loss in body weight. Worm egg counts were determined weekly. The reduction in feed on offer resulted in a mean weight loss of 4.0 kg over 9 weeks (experiment 1) or 4.8 kg over 7 weeks (experiment 2). Based on worm egg counts, there was no indication of a change in immunity to T colubriformis in either of the experiments. These results suggest that acquired immunity to T colubriformis in adult sheep that are exposed to infection is maintained during periods of moderate loss of weight associated with inadequate nutrition.     

Experimental Ostertagia spp. infection of sheep: Populations and lesions in previously worm-free and previously exposed animals

Sheep raised and maintained under worm-free conditions were subjected to one heavy exposure and several low-level exposures of infective Ostertagia spp. larvae. The infections were terminated by treatment with thiabendazole. These sheep and not previously exposed controls were then challenged with 400 000 or 20 000 Ostertagia spp. larvae and killed after a further 8 days. In the previously exposed sheep, worm numbers, rate of worm development, and pathological effect, were greatly reduced. No significant differences were seen between sheep aged 6 and 18 months at the time of challenge.     

Morphological changes in trichostrongylid eggs after treatment with fenbendazole

The influence of daily doses of 0.05–0.1 mg fenbendazole/kg body weight on egg development was tested in twenty-seven lambs artificially infected with Haemonchus contortus plus Trichostrongylus colubriformis or Ostertagia circumcincta, as well as in two additional sheep naturally infected with Nematodirus spp.Atypical eggs were detected before a reduction in the number of eggs. Atypical eggs had blastomeres of different sizes and shapes; knot-like, crater-like or bubble-like. In isolated cases eggs with atypical first larvae were recognizable. Morphologically changed eggs occurred in the reproductive organs of female O. circumcincta worms.None, or only a very small number of third stage larvae, could be cultured from the faeces of treated sheep.     

Immunization of neonatal lambs against the larvae of Taenia hydatigena, using viable eggs followed by chemotherapy

The subcutaneous injection of viable eggs of Taenia hydatigena into neonatal lambs induced 100% protection against the development of viable T. hydatigena larvae from an oral challenge. However, contrary to some published results, no protection was induced against a simultaneous infection with eggs of T. ovis and Echinococcus granulosus.The short-acting partial resistance to oral infection with T. hydatigena eggs, transferred from infected ewes to their lambs, was not enhanced by hyperimmunization of the ewe. In the lamb, this resistance did not interfere with the development of subcutaneously injected T. hydatigena eggs into immature larvae, or with the consequent induction of resistance to an oral challenge with T. hydatigena eggs. The immunizing lesion regressed rapidly after treatment of lambs with Mebendazole, and after 6 months the lesions were negligible in both treated and untreated lambs.     

Immunizing potential of various stages of Taenia ovis eggs injected subcutaneously into neonatal or sixteen-week-old lambs

When viable eggs of Taenia ovis were given orally to 1-week-old lambs, infection occurred only in those lambs that had been deprived of colostrum. When viable eggs were injected subcutaneously into 1-week-old lambs, no larvae developed at the injection site, and no resistance was stimulated against an oral challenge of T. ovis eggs given 11 weeks later. However, when eggs, oncospheres or developing cysticerci were subcutaneously injected into 16-week-old lambs, all grew at the injection site and stimulated a high degree of immunity to oral infection. Colostrum-derived antibodies against T. ovis apparently suppressed the immunizing potential of T. ovis eggs injected subcutaneously into neonatal lambs.     

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.     

Development of villus atrophy in the small intestine of sheep infected with Trichostrongylus rugatus

Pairs of sheep infected with 120 000 larvae of Trichostrongylus rugatus were killed at intervals from 2 to 56 days after infection (DAI). Worms were located in tunnels in the epithelium of villi or upper intestinal crypts at all stages of development. Villus atrophy developed progressively until 16 DAI, when surface microtopography, characterised by subtotal villus atrophy, stabilised. Most severe lesions were in the first 3 m of small intestine where the density of nematodes was highest. Discontinuities in the epithelium and effusion of inflammatory cells and tissue fluids into the lumen were rare. No effects of infection on body-weight gain, appetite or serum total protein and albumin were evident in the first 20 days of infection. However, it was concluded that T. rugatus fundamentally resembled T. colubriformis and T. vitrinus in the response it elicited in the intestine of sheep, and must be considered potentially pathogenic.     

Traumatic myositis ossificans in a dog

AIM: To determine the distribution of the three common Trichostrongylus spp. infecting sheep and their resistance status on farms throughout New Zealand, using PCR.

METHODS: Cultures were prepared from faecal samples from 70 farms while conducting faecal egg count reduction tests (FECRT) in lambs between 2010 and 2012. Trichostrongylus-type infective stage larvae (L3) were recovered from cultures, derived from untreated control and albendazole-, levamisole- and ivermectin-treated groups of lambs on each of the farms involved, and these were identified to species using PCR analysis of the second internal transcribed spacer region of ribosomal DNA. The species composition of the larvae present in cultures from the untreated control groups was examined across all farms to assess any potential differences in geographical distribution. In addition, the species composition of larvae cultured from the untreated and anthelmintic-treated lamb groups were compared to determine which species exhibited resistance to each of the anthelmintics used in the FECRT.

RESULTS: Of 67 farms with Trichostrongylus spp. present, 42 (63%) cultures from the untreated control groups contained all three Trichostrongylus spp. and no significant geographical patterns in their distribution were detected. Seven samples contained only one species. Irrespective of the anthelmintic efficacy levels, Trichostrongylus colubriformis dominated cultures prepared from lambs following treatment with albendazole (99.1 (95%CI?=?97?100)% of larvae) or levamisole (81.6 (95%CI?=?75.3?87.9)% of larvae), indicating the presence of widespread resistance in this species. In cultures prepared from levamisole-treated lambs, small numbers of T. axei larvae were also frequently present (5.4 (95% CI?=?1.3?12.4)% of larvae). Resistance to ivermectin was not found in any of the three Trichostrongylus spp. after PCR identification. Although larvae were identified, based on length, as being Trichostrongylus spp., for 24 of the 48 samples cultured following treatment with ivermectin, 100% of the larvae present were identified as Teladorsagia circumcincta.

CONCLUSIONS: As in previous surveys, all three Trichostrongylus spp. were common throughout New Zealand and no geographical patterns were detected in the current study. On all farms where resistance to albendazole and/or levamisole was indicated (i.e. efficacy <95%), the species identified as being resistant was T. colubriformis. Even where efficacies were >95%, T. colubriformis still tended to dominate in post-treatment cultures. While this could reflect a lower susceptibility of T. colubriformis to these anthelmintics, it seems more likely to indicate the presence of resistant genotypes in these populations. Similarly, T. axei also tended to be present after treatment with levamisole, which likely reflects a known lower susceptibility of this species to these anthelmintics.     

Partial protection of lambs against Haemonchus contortus by vaccination with a fractionated preparation of the parasite

Florida Native lambs, less than 6 months of age, were successfully vaccinated against Haemonchus contortus with a high mol. wt fraction (greater than 30,000 daltons) derived from a somatic extract of H. contortus larvae (SEL) and excretions and secretions (ES) of larvae isolated during in vitro development from the infective 3rd to 4th stage. A 59% reduction in adult worm numbers was obtained in vaccinates compared to naive lambs following challenge. The protection in vaccinated lambs was similar to that seen in lambs exposed to a primary infection of H. contortus larvae which had been cleared with anthelmintic prior to the challenge infection. The unfractionated SEL/ES preparation and a low mol. wt fraction gave no significant protection against challenge infection.     

Effects of non-genetic factors on responses to gastro-intestinal nematode infections in Ethiopian sheep

The effects of non-genetic factors on responses to gastrointestinal nematode infections were examined in artificially infected indigenous Horro and Menz lambs in the central highlands of Ethiopia. The experiment involved lambs of two sexes, infection and monitoring regimens (3 levels), dam parity (4 levels) and birth types (2 levels). A total of 152 weaner lambs (103 Menz and 49 Horro) were used. Endoparasite challenge involved three infection periods (primary, secondary and tertiary) each separated by an anthelmintic treatment. Data were collected on faecal egg counts (FEC), packed cell volume (PCV), worm count, and weight gain. Female lambs were found to harbor lower numbers of T. colubriformis counts and had lower (p < 0.01) FEC in tertiary infection compared to males. Younger animals were more susceptible to the nematode infections than older animals as measured by all the parameters studied (p < 0.01). Twins had higher FEC in both secondary and tertiary infections compared to singles. Lambs born to dams of first parity had higher PCV and lower FEC values during the tertiary infection compared to other parities. These results would be used as basic information in designing a more rational control strategy and should also help to advise on better management care for less resistant sheep.     

Selective and on-demand drenching of lambs: Impact on parasite populations and performance of lambs

AIM: To determine whether drenching regimes for lambs by which a proportion (10%) of the heaviest animals was selectively left untreated, or animals are only drenched ‘on demand’ when faecal nematode egg counts (FEC) exceeded a threshold level, would result in measurable increases in parasite larval challenge in the autumn and/or decreases in the performance of lambs.

METHODS: A replicated study compared three drenching strategies in which mobs of lambs (n=360 in total) received either: a five-drench preventive programme, administered to all animals (Treatment 1); a five-drench preventive programme, but the 10% heaviest animals left untreated each time (Treatment 2); or drench treatments administered only when FEC exceeded 500 eggs per gram of faeces (epg) (Treatment 3). After the five-drench programme, animals in Treatments 1 and 2 were treated according to FEC as for Treatment 3. A triple-combination drench containing ivermectin, oxfendazole and levamisole, administered orally, was used for all treatments. There were nine farmlets, allowing three replicates of each treatment, in a completely randomised design. Parasite infestations on pasture were measured in autumn by pasture plucks, and worm burdens were monitored in tracer lambs, while the performance of lambs was assessed by liveweight gains, fleece weights, and body condition and dag scores.

RESULTS: Increased numbers of Haemonchus contortus and Trichostrongylus colubriformis larvae on pasture were found in the autumn on farmlets treating selectively or on-demand (Treatments 2 and 3). No differences were detected in other parasite species. Mean liveweight gains did not differ between treatments but some differences were detected between drenched and undrenched lambs in Treatment 2. Mean body condition and mean dag scores of lambs in Treatment 3 tended to be lower and higher, respectively, than those of lambs in Treatment 1; Treatment 2 was generally intermediate.

CONCLUSIONS: Drenching strategies for lambs designed to slow the development of anthelmintic resistance, by increasing the pool of susceptible worms available to dilute resistant survivors after treatment, resulted in increased numbers of H. contortus and T. colubriformis but not other species of parasite on pasture. The increased parasite challenge to lambs in the autumn was associated with small production losses, which may be acceptable to farmers wishing to implement such strategies. It is clear that further work is required on the interaction between management practices and the population dynamics of parasites, especially with regard to creating pools of susceptible genotypes to slow the development of drench resistance.     

Pilot project to investigate over-wintering of free-living gastrointestinal nematode larvae of sheep in Ontario,Canada

This study investigated the overwintering survival and infectivity of free-living gastrointestinal nematode (GIN) stages on pasture. The presence of GIN larvae was assessed on 3 sheep farms in Ontario with a reported history of clinical haemonchosis, by collecting monthly pasture samples over the winter months of 2009/2010. The infectivity of GIN larvae on spring pastures was evaluated using 16 tracer lambs. Air and soil temperature and moisture were recorded hourly. Free-living stages of Trichostrongylus spp. and Nematodirus spp. were isolated from herbage samples. Gastrointestinal nematodes were recovered from all tracer lambs on all farms; Teladorsagia sp. was the predominant species. Very low levels of Haemonchus contortus were recovered from 1 animal on 1 farm. The results suggest that Haemonchus larvae do not survive well on pasture, while Teladorsagia sp., Trichostrongylus spp. and Nematodirus spp. are able to overwinter on pasture in Ontario and are still infective for sheep in the spring.     

Predicting resistance of sheep to Haemonchus contortus infections

In vitro cultures of lymphocytes from worm-free sheep responded to larval and adult H. contortus antigens by undergoing blast transformation. The level of response varied considerably between animals but was heritable and positively correlated with resistance to subsequent primary, secondary and trickle infection by the parasite, but not with age of the sheep older than 5 months. Lymphocyte responses to phytohaemagglutinin and H. contortus antigens were not present at birth in lambs but developed during the first 5 months in the absence of infection. The proportion of the infecting dose of larvae which was inhibited at the early fourth stage correlated with age of the sheep and with the increase in lymphocyte response to H. contortus antigens during primary infections with H. contortus. Sheep which were most susceptible to infection had the lowest lymphocyte responses to H. contortus antigens but had the highest rate of weight gain during infection. It is suggested that man has selected the most productive animals at the expense of resistance to H. contortus. The definition of genetically controlled markers on lymphocytes is seen as a means by which disease resistant strains of animals may be selected.     

Growth response in lambs injected with a long acting microencapsulated Vitamin B 12

Nematode parasites have been a major factor limiting sheep production in New Zealand for more than 100 years. Twenty-nine species of nematodes were unintentionally introduced with sheep into New Zealand, but it is principally species of Haemonchus, Ostertagia, Trichostrongylus, Nematodirus and Cooperia that are associated with production losses and clinical disease.

The seasonal dynamics of nematode infection are the consequence of complex inter-relationships between the sheep, their husbandry and the prevailing climate. The patterns of pasture contamination by nematode eggs and then larvae, and the subsequent levels of infection in ewes and lambs are broadly similar throughout New Zealand. Numbers of infective larvae on pasture build up over summer to a peak in autumn/early winter with, in some years, a spring peak derived from the parturient rise in faecal nematode egg counts (FEC), expressed in eggs per gram of faeces (epg), in lactating ewes. The immune capability of lambs is initially low but increases with the magnitude and duration of exposure to infection. Once significant immunity has developed (usually by 10–12 months of age), sheep are capable of markedly restricting parasite infection, except during times of disease, malnutrition or stress.

For the effective control of nematode parasites, farmers have come to rely almost exclusively on broad-spectrum anthelmintics. However, issues relating to resistance, residues and eco-toxicity increasingly threaten the sustainability of chemotherapy. In order to maintain present levels of parasite control and productivity in the long term, farmers need to integrate management practices aimed at minimising animal exposure to parasites with reduced reliance on anthelmintics.     

The epidemiology and pathogenic significance of haemonchosis in a merino flock in East Africa

The epidemiology of H. contortus was studied for 21 months in 80 set-stocked Merino ewes and their grazing in an endemic area in Kenya. Observations included faecal egg counts (weekly), worm burdens (monthly), haematological indices and clinical signs. The levels of infective larvae on the pasture were estimated using tracer sheep.The classical acute form of haemonchosis was seen with equal intensity in both ewes and lambs during periods of high rainfall and self-cure was confirmed as a flock phenomenon which occured simultaneously on one occasion in sheep of all ages.Not previously recognised were the uniformity of the faecal egg counts in both ewes and lambs over long periods when the worm burdens fluctuated greatly, the marked decrease in worm burdens without the occurence of classical self-cure during periods when the intake of infective larvae was low, and the inverse relationship between the individual worm-weight and the total worm population. It was also concluded that one of the most significant features of haemonchosis is not the acute syndrome so characteristically associated with the disease, but the fact that a moderate infection of a few hundred worms persisting over a period of several months will produce chronic anaemia and, ultimately, severe loss of bodily condition and deathes in ewes and lambs grazing on poor quality pasture.