Immune responses in Indonesian thin tail and Merino sheep during a primary infection with Fasciola gigantica: lack of a specific IgG2 antibody response is associated with increased resistance to infection in Indonesian sheep.

After a primary infection with Fasciola gigantica, the immune responses in a resistant (Indonesian thin tail) and a susceptible (Merino) breed of sheep were analysed. The number of adult flukes recovered from the livers of the Indonesian thin tail sheep were significantly lower than those found in the Merino animals. On days 8, 14 and 25 p.i., Indonesian thin tail sheep exhibited a significantly higher eosinophilia than Merino sheep, whereas neutrophilia was significantly elevated in the Indonesian thin tail sheep on days 36 and 48 p.i. Serum from both sheep breeds demonstrated IgM, IgG1 and IgE responses to F. gigantica. In contrast, the Indonesian thin tail sheep produced significantly lower levels of IgG2 antibodies relative to the high level detected in Merino sheep. The IgE response was biphasic in both sheep breeds with the first response detected by day 14 and the second response developing from days 30 to 60 p.i. Western blotting showed that a similar profile of adult fluke antigens was recognised by IgG1 and IgE antibodies in both the Indonesian thin tail and Merino sheep. The IgE response was directed to a major antigen at about 92 kDa. We postulate that IgG2 could act as a blocking antibody for protective effector responses against F. gigantica in sheep and that the Indonesian thin tail sheep, by downregulating IgG2 responses, have an enhanced capacity for killing F. gigantica in vivo.

Immune responses of cattle to experimental anti-Fasciola hepatica vaccines.

Fasciola hepatica infection of cattle and sheep is an important cause of clinical disease and production losses, and is controlled at present by a combination of chemotherapy and management measures. However, the prospects for the control of F. hepatica infection by vaccination are good, and we have previously shown substantial protection of cattle against experimental challenge infection following immunisation with a combination of the purified fluke-derived enzymes cathepsin L1 (CATL 1), cathepsin L2 (CATL 2) and fluke-derived Hb fraction (FHB). This and other recent studies have also demonstrated fundamental differences between protective and non-protective immune responses to liver fluke infection. In this present study we have further analysed the response of animals to liver fluke challenge following experimental vaccination. Calves were vaccinated with either CATL 2 plus FHB, or CATL 1 plus CATL 2. Partial protection against challenge infection was achieved in both vaccinated groups, with the greatest level of protection (55 per cent reduction in fluke burdens) recorded in the group vaccinated with CATL 1 plus CATL 2. This latter group also showed the greater level of lymphocyte proliferation and the greater production of gamma-INF in response to stimulation with fluke antigen in vitro following challenge. These results are significant in our attempts to characterise the elements within the immune response to vaccination which are protective.

Lymphocyte and cytokine responses of young cattle during primary infection with Fasciola hepatica.

An experiment was designed to characterise the early immune responses of cattle given a trickle infection of the trematode Fasciola hepatica. Lymphocyte responses and IFNgamma production following stimulation with Fasciola antigens were measured. Animals were sequentially slaughtered at weeks 5, 7, 9 and 11 post-infection. Results demonstrated responses to fluke antigens and cytokine production (IFNgamma) from weeks 2-5 post-infection. These findings may suggest an early stimulation of IFNgamma production by peripheral blood lymphocytes following infection with F hepatica in cattle.

Correlation of specific antibody titre and avidity with protection in cattle immunized against Fasciola hepatica.

Cattle produce specific serum antibody mainly of the IgG1 isotype in response to infection with the liver fluke, Fasciola hepatica. In these animals a positive correlation between fluke-specific serum IgG1 levels and fluke-burden in non-immunized infected animals was observed. In contrast, immunization of cattle with a combination of the fluke-derived antigens cathepsin L2 (CL2) and fluke haemoglobin (FHb) in Freund's complete/incomplete adjuvant (FCA/FLA) induced a specific antibody response involving IgG2, as well as IgG1. These immunized animals also exhibited very high (72%) levels of protection against a subsequent challenge infection. When the vaccine was administered in FIA alone the specific antibody response, while still involving IgG1 and IgG2, was of lower magnitude (10-fold and 100-fold, respectively) and no significant reduction in fluke burden was observed following challenge. Nevertheless, in these animals, a strong IgG2 response was associated with low fluke burdens. These results provide further evidence of the non-protective nature of specific immune responses in cattle following F. hepatica infection, and demonstrate that vaccination can induce a qualitatively different, and protective, response.

Immune responses of chronically infected adult cattle to Fasciola hepatica.

Eight adult cows, with an existing chronic Fasciola hepatica infection, were experimentally infected with 1300 metacercariae of F. hepatica, given as trickle infections, over two separate 10-day periods. Two fluke-naive heifers were similarly treated. Analysis of parasite-specific immunoglobulin isotypes IgM, IgG1, IgG2 and IgA showed IgG1 to be the dominant isotype in both chronically infected and previously naive animals. Lymphocyte proliferation assays demonstrated (a) an association between lymphocyte response and mature fluke burden in the chronically infected cattle and (b) no association between lymphocyte response and mature or immature fluke burden in naive heifers. There was no production of gamma-interferon (IFN gamma) by lymphocytes responding to adult fluke antigen. At post-mortem examination the burden of immature flukes in chronically infected and previously naive heifers was similar. This suggests that chronically infected animals may be tolerant to a moderate superinfection and that the prevailing immune mechanism in operation may be a non-protective response generated by the Th2 lymphocyte subset.

A bolus for the administration to cattle of metacercariae of the liver fluke Fasciola hepatica.

A gelatin-agar bolus, designed and developed for the administration of metacercariae of the liver fluke Fasciola hepatica, was evaluated in adult Holstein Friesian cattle. The metacercariae, enclosed within a gelatin capsule, were placed inside the bolus and delivered to each animal using an oesophageal balling gun. At slaughter, 13 weeks after challenge, an average of 25% of the challenge dose was recovered from each liver. This percentage recovery is similar to that obtained with other known methods. The new bolus, however, offers improved handling qualities. In addition, the bolus also has potential for improving a number of other techniques including those for the administration of other parasites, compounds or chemotherapeutic agents.

A novel method for the administration of metacercariae of Fasciola hepatica to adult cattle.

A gelatin bolus containing the infective stage of Fasciola hepatica was prepared and administered to adult cattle. The bolus was formed to fit into the nozzle of a standard size bolus-type dosing gun. Administration proved easy and effective causing minimal stress to the animal, yet ensuring that experimental infection took place.