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Guidelines for the Control of
Internal
Parasites in Cattle: Flukes
By Dr. Tom Craig and Patricia Holland
The common fluke is widespread in southeastern and northwestern United
States and many places in between. The deer fluke is generally found in
the Great Lakes region, northwest, and Texas Gulf Coast.
Fasciola hepatica (the common fluke) and
Fascioloides magna (the deer fluke) are transmitted where suitable
habitat for the host snail species exists. Flukes arenÕt important
elsewhere unless infected cattle are transported.
Pastures infested with liver flukes have
clay soils and are constantly or periodically flooded, but not inundated
with water. Snails needed to complete the life cycle of flukes live in
shallow water where they feed off algae growing on vegetation. The
damage to cattle and the epidemiology of these two fluke species are
different even though they use the same snails as intermediate hosts.
The life cycle
The common liver fluke has a wide range of hosts, including humans.
Cattle are the important source of infection of common flukes to snails,
but deer flukes are unable to complete their life cycle in cattle
because their eggs are trapped in thick-walled cysts within the
liver.Snails are infected with deer flukes from white-tailed deer or
elk, and only cattle sharing pastures with these species are infected by
these flukes.
Liver flukes are transmitted seasonally.
In southern climates, hot dry conditions cause snails to
aestivate/hibernate over the summer while buried in mud. Snails become
active and reproduce when conditions are favorable. Snails become
infected with fluke larvae that hatched from eggs passed in the hostÕs
feces. Reproduction and development occurs in the snail before they
release hundreds of larvae that encyst on vegetation. These cysts can
survive for some time in the environment if not exposed to excessively
hot or cold temperatures.
These infective cysts on vegetation are
then grazed by cattle. Larval flukes enter the liver and begin
migration. The common fluke eventually enters bile ducts where it
matures and begins producing eggs 12 to 16 weeks after infection.
Disease occurs when flukes reached the
bile ducts and cause calcification of the ducts and scarring of the
liver.
Older cattle that have been repeatedly
exposed to the parasite can temporarily rid themselves of infection of
adult flukes only to be re-infected the following transmission season.
Deer flukes migrate through the liver or
occasionally other tissues. When they encounter other deer flukes, they
then form a cyst containing two or more flukes. They mature and begin
producing eggs. The eggs, however, remain trapped within the cyst.
Production and economic effects of flukes
Effects of fluke infection vary with the number of parasites. If numbers
are sufficient, migration of the common fluke may cause delay in onset
of puberty and subsequent pregnancy in heifers. There is also some
evidence of lowered feed efficiency in cattle with elevated numbers of
common flukes.
Condemnation of livers in infected cattle
is a serious economic consideration. Following treatment, black tracts,
fibrosis, and thick walled cysts in the liver and other organs
associated with deer flukes may be sufficient for condemnation even
though flukes are no longer present.
Treating for flukes
Treating cattle for common flukes benefits individual animals and
reduces infection of snails on the pasture. Reducing numbers of
infective snails helps prevent infection of animals occupying that
pasture.
Based on the time necessary for
development in the environment and within the host, there is only a
narrow time frame where treatment of common flukes will have effect on
control of parasite numbers in the environment.
The anthelmintics available in North
America are effective only after cattle have been infected for several
months. Treating cattle for deer flukes, however, has no effect on
pasture contamination, but may benefit individual cattle. Timing of
treatment in cattle isn't as important because preventing snail
infection isn't an issue.
Ranchers on the Texas Gulf Coast, where
deer flukes are common, routinely administer anthelmintics to treat for
this parasite. They believe treated cattle do better and are culled less
frequently than untreated cattle, despite permanent liver damage. This
perception may be false because anthelmintics effective against deer
flukes are even more effective against common flukes, and improvement
may be due to removal of common flukes. In both cases, the number of
flukes involved determines the level of economic loss associated with
infection.
Clorsulon (Curatrem¨) is effective against
common flukes eight weeks or longer after infection and has some
efficacy against deer flukes.
Ivermectin/clorsulon (Ivomec¨ Plus) is
only effective against adult common flukes 12 to 16 weeks or more after
infection.
Albendazole (Valbazen¨) has efficacy
against adults of both flukes. Cattle should be treated at least two
months after the end of transmission season (usually mid-summer in the
south) to effectively remove adult flukes and reduce numbers of fluke
eggs in the environment.
In the south, treat cattle September through November with products that
control flukes and other intestinal parasites. If weather conditions
were wet and mild the previous winter, treatment for flukes in the
spring may be indicated as well. Vaccinate cattle with an 8-way
clostridial product to protect against Redwater/Bacillary Hemoglobinuria
and Black Disease, which can secondarily result from fluke infections.
Dr. Tom Craig is with the department of
veterinary pathobiology, and Dr. Patricia Holland is with the department
of large animal clinical sciences, Texas A&M University.
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