Peritoneal lavage cells of Indonesian thin-tail sheep mediate antibody-dependent superoxide radical cytotoxicity in vitro against newly excysted juvenile Fasciola gigantica but not juvenile Fasciola hepatica

Indonesian thin-tail (ITT) sheep resist infection by Fasciola gigantica by an immunological mechanism within 2 to 4 weeks of infection yet are susceptible to F. hepatica infection. Studies of ITT sheep show that little liver damage occurs following F. gigantica infection, suggesting that the invading parasites are killed within the peritoneum or shortly after reaching the liver. We investigated whether cells isolated from the peritoneums of ITT sheep could kill newly excysted juvenile F. gigantica in vitro and act as a potential mechanism of resistance against F. gigantica infection. Peritoneal cells from F. gigantica-infected sheep, rich in macrophages and eosinophils, mediated antibody-dependent cytotoxicity against juvenile F. gigantica in vitro. Cytotoxicity was dependent on contact between the parasite and effector cells. Isolated mammary gland eosinophils of F. gigantica-infected sheep, or resident peritoneal monocytes/macrophages from uninfected sheep, also killed the juvenile parasites in vitro. By using inhibitors, we show that the molecular mechanism of killing in these assays was dependent on the production of superoxide radicals by macrophages and eosinophils. In contrast, this cytotoxic mechanism was ineffective against juvenile F. hepatica parasites in vitro. Analysis of superoxide dismutase activity and mRNA levels showed that activity and gene expression were higher in F. hepatica than in F. gigantica, suggesting a possible role for this enzyme in the resistance of F. hepatica to superoxide-mediated killing. We suggest that ovine macrophages and eosinophils, acting in concert with a specific antibody, may be important effector cells involved in the resistance of ITT sheep to F. gigantica.

Acquisition of resistance against Fasciola gigantica by Indonesian thin tail sheep

The plasma levels of GLDH and the dynamics of development of the numbers of surface lesions on livers, and numbers of parasites, within two to three weeks of a challenge infection, indicated that the major component of the high resistance of Indonesian thin tail (ITT) sheep against Fasciola gigantica was acquired, and acted against juvenile parasites. Few parasites reached the livers of ITT (exposed) sheep, but many had been inhibited, probably in the wall of the jejunum. ITT (naive) sheep showed some resistance, compared with control Merino sheep, by three weeks after infection. Manifestation of the resistance of ITT (exposed) sheep was suppressed by administration of the immunosuppressant, dexamethasone. Killing of parasites in ITT sheep appeared to have ceased by 21 to 28 days after infection. The basis of the acquired resistance was deemed to be an exceptional immunological capacity of ITT sheep responding to an antigen, or an immunological suppressant, peculiar to F. gigantica. That molecule, produced by juvenile parasites, warrants further study as a candidate for a vaccine.

Evidence for a major gene determining the resistance of Indonesian Thin Tail sheep against Fasciola gigantica

The genetic basis of the high resistance of Indonesian Thin Tail (ITT) sheep against Fasciola gigantica has been studied by measuring the resistance levels of ITT sheep, St. Croix sheep and their F2 and F3 crossbreds. All of the sheep were bred and reared in the same environment. Approximately half of the F2 and F3 crossbreds were as resistant as the ITT parental breed, one-quarter had the low resistance of the St. Croix parental breed and the rest had an intermediate level of resistance. It is concluded that the high resistance is determined by a major gene with incomplete dominance. The prevalence of the gene in the ITT sheep population is estimated to be about 90%. The resistance may be innate or acquired; but, regardless of the mechanism, it could be used to control fasciolosis caused by F. gigantica by breed substitution or by crossbreeding with other breeds of sheep.

Resistance of Indonesian thin tail sheep against Fasciola gigantica and F. hepatica

High resistance of Indonesian thin tail (ITT) sheep against Fasciola gigantica has been confirmed. Naive ITT sheep had only 17% of the number of mature parasites collected from control St. Croix sheep. In contrast, the level of resistance of ITT sheep against F. hepatica was the same as that of the low resistance Merino breed after both primary and secondary infections. It is suggested that resistance of ITT sheep against F. gigantica was manifested in two phases. The major phase appeared to be specific to the ITT sheep : F. gigantica relationship, acting against immature parasites in both naive and previously exposed hosts and could be innate or acquired. The second phase appeared to be specific to F. gigantica, and was acquired, because it killed young adult parasites only after secondary infection. F. gigantica from ITT sheep were heavier than those from the St. Croix sheep, possible because feeding was impaired by more extensive liver damage in the latter. Approximately 45 F. gigantica killed 25-kg sheep before and during migration from the liver parenchyma into the bile ducts. Death was caused by haemorrhages into the liver parenchyma, bile ducts and peritoneum.

Acquired resistance of merino sheep against Fasciola gigantica

Merino sheep acquired resistance against Fasciola gigantica, which is contrary to previous observations of infections with F. hepatica in that breed. The acquired resistance was manifest against young adult parasites. St. Croix sheep had previously been shown to have more resistance than European sheep against F. hepatica after primary infection; however, in F. gigantica infections in the present study there was no difference between the resistance levels of the breeds. Antigenic analysis of the host:parasite relationships could lead to identification of protective antigens suitable for use as vaccines.