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PubMed1989

A preliminary evaluation of factors affecting an experimental system for vaccination-and-challenge with Haemonchus contortus in sheep.

A preliminary evaluation of factors affecting an experimental system for vaccination-and-challenge with Haemonchus contortus in sheep. International Journal for Parasitology 19: 169-175. Studies were made with Haemonchus contortus in sheep to ascertain the influence of a range of factors in the domain of the host, the parasite or the vaccine on the formulation of protocols for vaccination-and-challenge to be used in identifying protective immunogens. The results corroborate earlier findings that protective immunity can follow vaccination with homogenates of parasites and show that initial processing of parasites for a vaccine leaves protective immunogen in a functional state. Sonicates of adult worms produced protective immunity and were identified as raw stock in which to prospect for candidate immunogens. By contrast, sonicates of infective larvae and exsheathing fluid invoked no significant protection and were not accredited for the same purpose. In an experiment unaccompanied by protective immunity, ewes contained lower worm burdens than castrate males indicating that vaccination experiments should be made with hosts of one sex only. Again in an experiment unaccompanied by protective immunity, Freund's complete adjuvant increased susceptibility to infection compared with Freund's incomplete adjuvant or no adjuvant implying a profound and persistent interference from killed mycobacteria on resistance against H. contortus.

Animals↗
PubMed1989

No association between the ovine leucocyte antigen (OLA) system in the Australian merino and susceptibility to Haemonchus contortus infection.

A genetic analysis has been made of the Ovine Leucocyte Antigenic (OLA) system in Australian merinos. The animals consisted of sires, dams and their progeny. The typing data were consistent with previous findings of a high degree of polymorphism. At least two closely linked loci with several alleles at each are necessary to explain the data. No evidence was found for an association between OLA types and three measures of susceptibility to infection by the blood-sucking parasite Haemonchus contortus. Attention is drawn to the utility of half-sib data for analysis of the genetic control of resistance to parasites in sheep and other animals with a similar breeding structure.

Animals↗
PubMed1989

Cross-immunity between Haemonchus contortus and Trichostrongylus colubriformis in sheep.

Cross-protective immunity between the nematode parasites, Haemonchus contortus and Trichostrongylus colubriformis, was examined in sheep vaccinated with irradiated larvae of either species. Secondary immunological responsiveness stimulated in this manner protected only against challenge infection with the species used for vaccination. Significant cross-protective immunity was not observed. Titres of serum antibody to an extract of adult but not infective larval T. colubriformis reflected the specificity for protective immunity. Immediate hypersensitivity skin reactions to nematode extracts did not reflect the antigen-specificity for protective immunity.

Animals↗
PubMed1989

Experimental Haemonchus contortus infections in guinea pigs.

Approximately 40% of exsheathed Haemonchus contortus larvae administered to guinea pigs established in the stomach and developed into fourth stage larvae. Most worms were then lost between 5 and 7 days after infection and the guinea pigs were resistant to a second infection. Haemorrhage, oedema and infiltration with inflammatory cells, especially eosinophils, developed in the stomach wall of infected guinea pigs and reactive hyperplastic changes occurred in the gastric lymph node. H. contortus infection of guinea pigs has some potential as a model for study of the pathology, immunology and chemotherapy of gastric nematodiasis.

Animals↗
PubMed1990

Response to helminth infection of sheep selected for resistance to Haemonchus contortus.

Lines of Merino sheep selected for increased (IRH) and decreased (DRH) resistance to Haemonchus contortus were compared with an unselected (CH) line, after approximately four generations of selection. Measurements were recorded on 69 IRH, 47 DRH and 84 CH animals. Following artificial challenge with H. contortus, the IRH line had significantly (P less than 0.001) lower faecal egg counts than the CH and DRH lines (2730, 12,720 and 17,400 epg, respectively). Significant differences (P less than 0.05) were found between all lines in the minimum packed cell volumes during artificial infection (25.7, 22.0 and 20.3%) and in faecal egg counts after natural infection (140, 3590 and 8750 epg). Differences were also recorded (P less than 0.05) following artificial challenge with Trichostrongylus colubriformis (490, 840 and 1340 epg). On a percentage basis, faecal egg counts in the IRH line deviated less from the CH line following artificial infection with T. colubriformis (42%) than with H. contortus (79%). The reverse was true for the DRH line (60 and 37%, respectively). Differences in egg output of this magnitude should have a marked effect on requirements for anthelmintic treatment, rate of development of drug resistance and level of pasture contamination when the lines are grazed separately.

Animals↗
PubMed1990

Infection of immunosuppressed mice with the abomasal nematode parasite of ruminants, Haemonchus contortus.

Infective larvae of Haemonchus contortus established in mice either immunosuppressed with the corticosteroid, dexamethasone, or the cytotoxic drug, cyclophosphamide, or treated with the histamine H2 inhibitor, cimetidine. Infections persisted for as long as the immunosuppressive treatment (7 days) and growth of larvae was similar to that seen in sheep. Virtually no larvae survived in untreated mice. Accordingly, it would appear that adaptive immunity is an important barrier against primary infection by H. contortus in mice and is a determinant of host-range for this parasite. Antibody raised in either sheep or mice against soluble extracts of adult H. contortus precipitated with different but overlapping sets of worm antigens. This suggests that the unique antigens recognized by the mouse compared with the sheep are crucial for the rapid protective responses which prevent primary infection.

Abomasum↗
PubMed1991

Immunity to Haemonchus contortus and the cellular response to helminth antigens in the mammary gland of non-lactating sheep.

Cellular exudates induced by infusion with helminth antigens were examined in non-lactating mammary glands of ewes immune to infection with the abomasal nematode, Haemonchus contortus. Secondary immunological responsiveness was expressed in two ways. Firstly, antigens from adult H. contortus elicited larger eosinophil-rich cellular exudates in immune compared to non-immune ewes. In this situation, secondary responsiveness in the mammary gland must have been generated through abomasal infection with the parasite. Secondly, repeated infusion with the antigens from adult H. contortus increased the size of cellular exudates in both immune and non-immune ewes. Eosinophils predominated but numbers of macrophages and lymphocytes were also increased. In this second situation, secondary responsiveness must have been either supplemented in immune ewes or derived completely in non-immune ewes by contact with helminth antigens through the mammary gland. The helminth antigens which induce eosinophil exudates in the mammary gland may not be potently protective against H. contortus. Furthermore, eosinophil exudation may not be an in vivo correlate of immunity which is directly useful for discriminating protective antigens and applicable to vaccine development. Infusion with antigens from adult forms of either H. contortus or Trichostrongylus colubriformis elicited cellular exudates equally well in immune ewes primed by infusion with H. contortus adult antigens 7 days beforehand. In addition, antigens from infective larvae of H. contortus elicited cellular exudates more potently than antigens from adult worms. However, vaccination with irradiated larvae has shown that species-specific protective immunity for H. contortus is stronger than cross-protective immunity conferred by T. colubriformis.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

A study of the mortality and fecundity of Haemonchus contortus in sheep following experimental infections.

The regulation of the fecundity and mortality of H. contortus in sheep was examined using a series of mathematical models. Six-month-old Dorset crossbred lambs were infected once only with various doses of infective H. contortus larvae (500-20,000 larvae). Parasite mortality was found to be an increasing linear function of the magnitude of the initial infection over the range of doses examined. Parasite fecundity was found to remain constant over the intensity and duration of the infection. The average fecundity for H. contortus at the time of slaughter was found to be 7037 eggs per female worm per day. There was no evidence of time-dependent changes in fecundity or density-dependent regulation of fecundity.

Animals↗
PubMed1992

Selection of Merino sheep for increased and decreased resistance to Haemonchus contortus: peri-parturient effects on faecal egg counts.

Peri-parturient worm egg counts were compared in 395 Merino ewes bred for either increased (IRH) or decreased (DRH) resistance to Haemonchus contortus, or from an unselected control flock (CH). Following a 10 month period with no anthelmintic treatment, a rise in egg counts began about 4 weeks before parturition and continued into lactation. At all stages, egg counts were significantly lower in IRH ewes, even those failing to conceive. Before lambing, IRH ewes averaged 263 epg, compared with 1113 epg in CH and 1618 epg in DRH ewes. After 2 weeks of lambing, lactating ewes averaged 1050, 2173 and 3708 epg, respectively, in the three lines. At the end of lambing, egg counts had increased to 1645, 3959 and 4124 epg, respectively. Counts appeared to have peaked in IRH ewes but not in DRH ewes. Ewes with twins had higher counts than those with singles, and ewes suckling lambs had higher counts than those losing lambs. No significant variation was associated with age of ewe (2-7 years). Although there were several nematode species present, the results suggest that, in comparison with DRH ewes, IRH ewes were particularly effective in reducing pasture contamination with H. contortus, both before and during the peri-parturient period.

Animals↗