Comparative pathogenesis and immunochemical analysis of Fasciola gigantica infections in cattle and sheep

Accurate economic quantification of the effects of Fasciola gigantica infections in cattle and sheep is essential to justify allocation of resources for control programmes. The present studies considered the comparative pathogenesis and economic loss caused by F. gigantica infection in young cattle and sheep of different breeds. Young cattle (Friesian and Boran) and sheep (Dorper and Red Maasai) were infected with single doses of F. gigantica metacercariae to produce chronic infections in cattle and acute, subacute and chronic infections in sheep. The animals were monitored parasitologically, biochemically and serologically. Immunochemical analysis of F. gigantica excretions/secretions using Western blotting and 35Smethionine biosynthetic radio-labelling were carried out with a view to identifying polypeptides of potential use in early prepatent diagnosis and protection and also to determine if there were any apparent species or breed differences in humoral antibody response to the parasite which may correlate with the differences in susceptibility/resistance to infection noted in the two species.

When examined by routine meat inspection procedures, all the livers of infected cattle and sheep were condemned as unfit for human consumption. Based on the prepatent periods, weight loss and degree of liver damage, production losses' were found to be less severe in the Friesians than the Borans. Friesians. infected with 0.95 flukes/kg live weight lost an equivalent of 63g/fluke/year while the Borans, which had 1 fluke/kg live weight lost 157g/fluke/year. The Borans had higher xi haematological values (RBC counts, PCVs and haemoglobin concentrations) suggesting that they were less liable to develop anaemia as a result of infection than the Friesians.

Interbreed differences were also seen in live weights and carcass weights of sheep. Mortalities were encountered at all levels of infection in sheep but were heaviest in acute infections where animals died at 12 weeks p.i.. Subacute infections caused heavier live weight losses than chronic infection (4.2 and 4.5 g/fluke/week in the infected Dorpers and Red Maasai respectively and there was a lower recovery from the latter, while chronic infections resulted in a loss of 2.4 and 3.1 g/fluke/week respectively). The subacute infections (5.4 and 7 flukes/kg live weight in Dorpers and Red Maasai respectively) also caused greater dressed weight losses of 2.3 and 2.2g/fluke/week while chronic infections (3.8 and 4.4 flukes/kg live weight in the Dorpers and Red Maasai respectively) resulted in a loss of 0.14 and 0.12 g/fluke/week. One Red Maasai with a very high peripheral eosinophilia lost infection and was considered to have been resistant to fasciolosis.

Elevated glutamate dehydrogenase (GLDH) activity, eosinophilia and an antibody response were associated with prepatent infection in all hosts while increased yglutamyl transferase (GGT) activity, anaemia, weight loss and hypoproteinaemia occurred after entry of flukes into the bile ducts. Humoral antibody responses, as detected by the enzyme linked immunosorbent assay (ELISA), did not appear to be related to the intensity of infection.

The changing protein structure of the maturing parasite was demonstrated by the differences observed in the excretions/secretions of Do/i, D)4 and adult flukes, which excreted 9, 11 and 13 polypeptides respectively some of which were excreted from Do/i to adult and some were age specific. Western blotting analysis revealed that cattle have a different recognition pattern for D14 and adult F. gigantica ES antigens compared to sheep. An invariable shift in antigen recognition from high (cl90kDa) to low molecular weight (c22kDa) antigens around the time of entry of flukes into the bile ducts was observed in cattle but not sheep. Sera of Red Maasai sheep with acute infections recognized two Do/i antigens in the 46-69KDa region. A Di4 46KDa antigen was recognized by both the Dorpers and Red Maasai with acute infections while the latter also recognized a c22kDa antigen (from week 4 post infection (p.i.)) which was also weakly recognized by sheep with chronic infections after week 9-10 p.i. A cl91KDa doublet present in adult ES was recognized by sheep with acute, subacute and chronic infections before patency. No interbreed differences in antigen recognition patterns were apparent either in sheep or cattle. The high molecular weight antigens recognized by both sheep and cattle before patency (c 190kDa) may be of value in protection against the immature F. gigantica in both species.

From the results of these studies it is suggested that the ability to recognize the low molecular weight antigens(s) by cattle but not by sheep may be related to their resistance to infection.

Biosynthetic radio-labelling of F. gigantica antigens revealed that D0/i antigens of high molecular weight were of low immunogenicity while those excreted by D14 and adults were strongly immunogenic. A 38-40 kDa protein present in D14 and adults was strongly recognized by infected cattle and sheep sera. Additionally, cattle and sheep sera strongly recognized a 26kDa antigen present in biosynthetic radio-labelled adult ES. The 40 and 26kDa polypeptides in adult ES and the 38kDa antigen in D14 ES, which were strongly recognized by both sheep and cattle sera in the prepatent phase may have potential as protective antigen and may be involved in stimulating a protective concomit ant immunity.