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.
