great lakes dairy sheep symposium - the Department of Animal ...

STRATEGIC CONTROL OF GASTRO-INTESTINAL PARASITES IN SHEEP
Gastro-Intestinal Parasitism in Sheep
D.H.Bliss, Ph.D.
Veterinary Parasitologist
MidAmerica Ag Research
Verona, Wisconsin,USA
Internal parasites often cause severe problems in sheep. If left untreated, sheep will suffer
from levels of parasitism which not only interferes with efficient production but can also lead to
death. Compared with cattle, clinical parasitic disease is a much greater problem with sheep
although production losses are equally high in both species. From a production stand point,
internal parasites, even in low numbers, cause considerable economic loss from reduced milk
production, reduced breeding efficiency, reduced weight gains, reduced feed efficiency, reduced
carcass quality, reduced hair or wool quality, and reduced immune status of the infected animals
to fight off other disease conditions such as coccidiosis.
Sheep develop high levels of parasitic worm burdens and suffer from heavy parasitism for a
number of reasons:
● First of all, sheep graze closer to the ground than cattle where parasitic larvae are often
concentrated. Parasitic larvae hatch from eggs passed in the fecal material; these larvae
undergo several molts until they reach an infective stage. This infective stage is mobile
and moves with moisture trails onto nearby vegetation to be eaten by its intended host.
Parasitic larvae can build up to very high numbers on summer pasture as the animals
constantly re-infect themselves.
● Secondly, fecal material excreted by sheep is very concentrated and, therefore, worm egg
counts in sheep are often very high. Fecal worm egg counts from sheep can be 5 to 10
times greater than normal counts found in cattle. A small amount of sheep feces can
produce a high level of parasite contamination. Preventing worm egg shedding during the
first part of the grazing season in sheep is critical to prevent environmental contamination
later on.
● Thirdly, parasite control programs for sheep over the years have been more therapeutic,
aimed at controlling high worm burdens after they have occurred rather than trying to
prevent parasite contamination of the animal’s environment. Treating sheep after heavy
parasite loads are encountered has little impact on reducing future contamination.
Furthermore, once high worm burdens are encountered, complete control is hard to
achieve and, often, heavily infected sheep continue to shed worm eggs even after
treatment. A commonly recommended practice over the years has been to deworm sheep
every eight weeks while on pasture. The time it takes for adult worms to develop after the
animals ingest infective larvae is approximately three weeks or less, depending upon the
particular species of parasite ingested. The recommended practice of treating every eight
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