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Managing Internal Parasitism in Sheep and Goats - AS-573-W
Managing Internal Parasitism
in Sheep and Goats
Kate Hepworth, Animal Sciences Student; Mike Neary, Extension Sheep Specialist;
Terry Hutchens, Extension Associate for Goat Production, University of Kentucky
Purdue
extension
AS-573-W
Animal Sciences
Purdue University Cooperative Extension Service, West Lafayette, IN 47907
Internal parasites are a significant threat facing
todays small ruminant producer. Problems associated
with parasites, particularly those of the gastrointestinal
tract of sheep and goats can cause irreversible damage
or even death to the animal, reduced performance and
economic loss for the producer. Animals that are
overburdened with parasites can be hindered in their
reproductive performance, experience reduced growth
rates, and become less productive overall, whether
their purpose be meat, fiber, or milk. Prevention and
control of the parasites that infect sheep and goats are
becoming increasingly difficult due to generations of
overuse and improper use of the available anthelmintic
dewormers, which results in increasing resistance by
parasites to common anthelmintics.
This paper will be used to give producers a general
understanding of the parasites that affect their animals,
how they live, and methods that can be used to lower
their costs and losses due to parasite infestations.
Sheep and goats are affected for the most part by the
same parasites. By far the most deadly internal parasite
to small ruminants in the Midwestern United States is
the gastrointestinal roundworm Haemonchus
contortus, also known as the barber pole worm and a
variety of other names. Haemonchus contortus is a
blood sucking parasite that can cause severe anemia,
protein loss and death in goats and sheep, and thus is
the most important to control. Some other parasites
that affect sheep and goats are those of the
Trichostrongylus family, particularly Teladorsagia
circumcincta and Trichostrongylus axei, and the
protozoa coccidia. Haemonchus contortus is the most
dangerous parasite to sheep and goats in many parts of
the United States, although other parasites may be
important in different regions outside the Midwest.
However, by developing a parasite control plan aimed
at Haemonchus, the majority of other dangerous
parasites will be controlled as well.
Lifecycle
Sheep and goats are generally affected by the same
parasites, although certain parasites may affect one
species more severely than the other, and treatment
methods may vary between sheep and goats. In order
to know how best to prevent future problems with
parasites and control current infestations, it is
necessary to understand the general life cycle of the
parasites most common to sheep and goats.
Figure 1: The lifecycle of a gastrointestinal parasite. This image is
courtesy of Virginia Tech Cooperative Extension. Purdue Extension Knowledge to Go

Haemonchus has a lifecycle that takes approximately
21 days to complete (see Figure 1). The cycle begins
when the larvae in the infective L3 stage of development
are ingested from the grass and travel to the abomasum,
or true stomach, of the host. Once in the abomasum the
larvae will follow one of two paths. They may proceed
with further larval stages and the eventual development
into adults, or they will go into hypobiosis. This is an
inhibited, or arrested development state, that occurs
when conditions are not conducive for the entire life
cycle to be complete.
When the L3 stage larvae enter the abomasum,
provided that environmental conditions are favorable,
they will molt into the L4 stage of larval development
and will then molt once more into adults. Factors that
induce the molting of the L4 larvae into adults include:
greening of grass, a rise in environmental temperature,
rain following a drought period, increased estrogen
levels in the host, and possible even a photoperiod
stimuli. Once the molt into an adult form is complete
adults then begin to lay eggs in the abomasum.
In the case of Haemonchus and Ostertagia, the L4
larvae can go into arrested development, or hypobiosis.
Hypobiosis is a period of dormancy that occurs when
the environment is not conducive to the lifecycle of
these parasites. Larvae in different regions may go into
hypobiosis in different times during the year,
depending on the environment. For example, in the
Midwestern states, Haemonchus larvae will likely
become dormant during harsh winters. In southern
parts of the country, hypobiosis may occur when the
weather is too hot or dry for larvae to survive.
During hypobiosis the L4 larvae hibernate in glands
in the abomasum without developing further or
causing problems for the host. They remain metabolically
inactive until they receive signals that indicate it is
time for them to resume development and then begin
to lay eggs. The signals that spur the L4 larvae to come
out of hypobiosis are the same signals mentioned
above that indicate to them to develop in the first
place. Once larvae leave hypobiosis, they resume
the normal lifecycle and begin to lay eggs.
Haemonchus adults require about 14 days to begin
laying eggs (see Figure 2) in the stomach after
reaching adulthood. The Haemonchus adult female
can lay up to 5,000 eggs per day, yet another reason
why Haemonchus is so difficult to control and so
dangerous to sheep and goats. Females that have gone
through hypobiosis over the winter generally resume
development two to four weeks prior to lambing or
kidding and begin to produce eggs. This phenomenon
is called the periparturient rise in fecal egg counts
(will be discussed later). The eggs laid in the
abomasum are expelled from the body via the feces.
Eggs in the feces generally remain inactive for a few
days in the environment, until the environment and
temperature become favorable for the development of
these eggs into larvae. The larvae hatch from the egg
and then emerge from the pellets and move through
larval stages L1, L2 and L3 of development. Once the
larvae reach the L3 stage, the infective stage, they
emerge from the fecal pellet and climb up onto blades
of grass (see Figure 3) where they wait to be ingested
by a grazing animal, thus completing the lifecycle. In
Figure : This image shows magnified eggs of Haemonchus Contortus.
Figure 3: This image depicts larvae inside a droplet of water on a blade
of grass. Purdue Extension Knowledge to Go
3
order for the larvae to emerge from the fecal pellet in
which it was expelled from the body, the environment
has to be in a condition that keeps the pellet moist and
pliable. Optimal conditions for the L3 larvae to emerge
from the pellet are warm, wet conditions, like those
that usually occur in the early spring.
An important characteristic of these larvae that must
be taken into account when designing a parasite
control plan is that the L3 larvae can survive on
pasture for extended periods of time, making pasture
management a key component in the prevention and
control of parasite infection. The L3 larvae can survive
on pasture for up to 90 days in the summer, and up to
180 days in the fall or winter. The Haemonchus larvae
thrive in temperatures ranging from 70° to 80°
Fahrenheit where there is an average of approximately
2 inches of rainfall per month. Extremely hot or dry
environments will cause egg laying to stop, but the
larvae can survive temperatures below 32° F.
Ostertagia larvae prefer cooler temperatures, with
their optimum temperature for development being
around 40° Fahrenheit. These larvae are very resistant
to adverse winter conditions, and can over-winter on
pasture.
Animal Susceptibility
Individual goats and sheep vary in their degree of
susceptibility to Haemonchus and other parasites.
Some animals, by means of their genetics, are much
more resistant or resilient to parasitic infections, and
can survive parasite levels without showing any
symptoms while another animal may be killed by that
level of infestation. This genetic resistance can be used
when selecting breeding stock, since a herd that
exhibits more innate resistance to parasites will cost
less to maintain and will ultimately be more profitable
to the producer.
Animal age and stage of development also have a
significant impact on the susceptibility to parasites. As
animals age, they are exposed to more and more
parasites and develop some immunity to infection.
This is more likely with sheep than with goats.
Resista