Internal parasite (worm) control
In most sheep production areas, internal parasites or gastro-intestinal
nematodes ( worms) are usually the primary disease affecting
sheep and lambs. Sheep are more susceptible to internal parasites
than most other types of farm livestock for several reasons. Their
small fecal pellets disintegrate very easily thus releasing the
worm larvae onto pastures.
They graze close to the soil surface and to their feces. They
acquire immunity more slowly. It takes 10 to 12 months for many
lambs to develop immunity to parasites. Sheep also suffer a temporary loss
of immunity around the time of lambing, called the periparturient egg rise.
Heavy stocking rates and insufficient pasture rest periods
contribute to the incidence of parasitic disease in
sheep and lambs. Internal parasites tend to be much less of
a problem under range-type conditions where sheep do not graze
the same pasture twice in the same grazing season. They are
also less of a problem in dry climates (or during dry periods) because parasites require
moisture for their development.
In the past, sheep producers relied heavily on anti-parasitic
drugs called "anthelmintics" (dewormers) to control internal
parasites in their flocks. But the long-time use and in some
cases misuse of these drugs has resulted in parasites that have
become increasingly resistant to anthelmintics. Drug resistance
has been documented in all three drug families and is most commonly
reported with the benzimidazoles and avermectins. Some farms are experiencing complete anthelmintic failure.
In the U.S., few anthelmintics are FDA-approved for use in sheep
and lambs, and no new drugs are likely to be approved. No combination products are available. As a
result, producers must develop more integrated programs for
controlling parasites, which do not rely exclusively on drug
therapy. They must also learn to use the existing drugs more judiciously.
Gastro-Intestinal Worms (roundworms, nematodes, stomach
In warm, moist climates, the parasite that causes the most problems
is usually Haemonchus Contortus, better
known as the "barber pole" or wire worm. The barber
pole worm is a blood-sucking parasite that pierces the lining
of the abomasum (the sheep's fourth or "true" stomach),
causing blood plasma and protein loss to the sheep.
Females are identified as barber pole worms because their white
ovaries are wound around their red blood-filled intestine. Male
worms are red. The barber pole worm is the largest and most
deadly stomach worm. The worms are visible during necropsy.
The symptom most commonly associated with barber pole worm infection
is anemia, characterized by pale mucous membranes, especially
in the lower eye lid; and "bottle jaw," an accumulation
(or swelling) of fluid under the jaw. Infections with barber
pole worm rarely result in diarrhea (scours). Other worm
species are more likely to cause diarrhea.
The barber pole worm
can be difficult to control because it has a short, direct life
cycle and is a prolific egg layer.
A female barber pole worm can produce 5,000 to 10,000 eggs per
day. The barber pole worm is also capable of going into a "hypobiotic"
or arrested state when environmental conditions are not conducive
to its development and resuming its life cycle once environmental
conditions improve. Some worm larvae are able to survive on
pastures over the winter, depending upon climate.
Worms usually of secondary importance include Trichostrongylus spp. and Teladorsagia (Ostertagia). Their importance is usually
as an additive effect in mixed infections with Haemonchus. However,
in warmer sub-tropical areas, Trichostrongylus spp. are
important pathogens in grazing ruminants. Teladorsagia appears to be much less important in the United States than
in cooler parts of the world such as Northern Europe and the
In the southern United States, Ostertagia circumcincta is of no real significance in small ruminants because the hot
and often dry summers are hostile to the survival of its pre-parasitic
stages In the western U.S. particularly the cooler, wetter,
coastal areas of Washington, Oregon and Northern California, Teladorsagia is the dominant nematode of sheep
Nematodirus is not usually a significant problem in small ruminants
in North America. However, Nematodirus battus does cause
significant disease in lambs in Britain because of its unusual
hatching requirements. Cooperia infections are usually secondary
contributors to parasitic disease.
Tapeworms (Moniezia spp.)
Because tapeworm segments can be seen in sheep feces, they often
cause alarm to producers. While experts disagree about the importance
of effects of tapeworms, there is little evidence to suggest that treatment is beneficial to the host. Although dramatically large numbers
of tapeworms may occupy the small intestine, damage to sheep
is generally much less than that done by the gastrointestinal
In extreme cases, tapeworms may cause intestinal blockages.
They may affect gut motiblity, which could predispose lambs to enterotoxemia. There is some evidence that lamb growth rates may be affected
when large numbers of tapeworms are present. Tapeworms have
an indirect life cycle. They require pasture mites to complete
their life cycle. Only certain anthelmintics (benzimidazoles, praziquantel)
are effective against tapeworms.
Lungworms (Dictyocaulus filaria, Muellerius capillaris)
Wet, low-lying pastures and cool, damp weather favor the development
of lungworm disease in sheep. Lungworm eggs are passed in the
feces. After the eggs hatch and are ingested by the sheep, they
travel through the sheep's tissues to the lungs (trachea and
bronchi). Only in severe infestations do lungworms produce clinical
disease, causing fever, coughing, nasal discharge, rapid breathing,
and poor performance. Secondary infection by bacteria may cause
Liver Flukes (Fasciola hepatica)
Liver flukes can cause death in sheep and lambs or liver damage
in sub-acute cases. In the U.S., they are primarily a concern
in California, the Gulf States and Pacific Northwest. Liver
flukes require snails as an intermediate host. Two drugs are
available in the United States for the treatment of liver flukes:
Clorsulon and Albendazole.
Meningeal Worm (Paralaphostrongylus tenius)
The meningeal (deer or brain) worm is an internal parasite of
white tailed deer. The life cycle of the meningeal worm requires
terrestrial snails or slugs as intermediate hosts. Sheep are
unnatural, dead-end hosts for the parasite. When sheep ingest
snails containing infective larvae, the parasite moves into
the brain and/or spinal cord causing often fatal neurological
The neurological signs observed in infected sheep depend upon
the number of larvae present in the nervous tissue and the portion
of the brain or spinal cord that has been affected. A mild infection
may produce a slight limp or weakness in one or more legs, while
a more severe infection may cause an animal to be partially
or completely paralyzed.
Meningeal worm infection cannot easily be diagnosed in the live animal.
Treatment usually involves high, repetitive doses of anthelmintics,
along with steroids and other supportive therapies. Research by Cornell University has demonstrated good efficacy with high doses of fenbendazole, along with anti-inflammatory drugs. Preventative
measures include fencing off areas which receive high deer utilization
and removing sheep from pastures before weather turns cool and
Fencing sheep away from likely snail and slug habitats (e.g.
ponds, swamps, wetlands, low lying and poorly drained fields,
and woodlands) may also help to prevent the problem. In high
risk areas, monthly deworming (with ivermectin) has been advocated.
Coccidia (Eimeria spp.) is discussed in another chapter.
Integrated Parasite Management (IPM)
Internal parasite control starts with good management and common
sense. Sheep should not be fed on the ground (unless the ground is frozen). Feeders which
cannot easily be contaminated with feces should be favored for grain, hay, and minerals. Water should be kept clean and free
from fecal matter. Pastures and pens should not be overstocked.
When new sheep are acquired, they should be isolated from the
rest of the flock for at least 30 days and aggressively dewormed to prevent
the introduction of drug-resistant worms.
Use of Clean or Safe Pastures
Clean or safe pastures are pastures which are not contaminated
with worm larvae that affect sheep. Examples of clean pastures
include pastures that have not been grazed by sheep (or goats)
for the past 6 to 12 months; pastures which have been grazed
by horses or cattle; pasture fields in which a hay or silage
crop has been removed; pasture fields which have been rotated
with field crops; and pastures that have been recently established
or renovated by tillage.
Pasture Rest and Rotation
It is a common misconception that rotational grazing
controls internal parasites in sheep. Intensive rotational grazing
may actually contribute to parasitic problems. This is because
rotating large groups of ewes and lambs through small paddocks
concentrates livestock and infective parasite larvae onto the
same small area.
Researchers in the Netherlands found that it takes three months
of rest for an infected pasture to return to a low level of
infectivity. Researchers at Langston University (Oklahoma) determined
that a 65-day rest period was sufficient (for goats). Rotational
grazing is an effective management tool for managing parasites,
but only if pasture rest periods are long enough (i.e. 60 days
or more). On the other hand, better nutrition provided by rotational
grazing may offset the effects of higher parasite loads on the
Approximately 80 percent of the worm larvae can be found in
the first two inches of grass. Therefore, sheep grazing taller
forages should have fewer parasite problems. Sheep should not
be allowed to graze forages shorter than 3 inches in height.
Sheep that browse also have fewer parasite problems. Another
grazing strategy is to wait until the dew has lifted from the
grass or grass has dried after a rain. Dry conditions force
parasites to stay at the base of the plants where they are less
likely to be consumed by the livestock.
Sheep (and goats) are generally not affected by the same internal
parasites as cattle and horses. Consequently, pastures grazed
by cattle and horses are safe(r) for sheep (and goats) and vice versa.
Sheep can be co-grazed with cattle and/or horses. A leader-follower
system can be utilized or pastures can be alternated between
sheep and cattle and/or horses.
There are numerous other benefits to multi-species grazing.
Each species has different grazing behavior that complements
one another. For example, sheep prefer to eat weeds and short,
tender grasses and clover, while cattle prefer to eat taller
grasses. Cattle may offer some protection from predators.
Some pasture plants have deworming properties, such as those
containing condensed tannins. Research has shown that sheep
grazing tannin-rich forages have lower fecal egg counts than
animals grazing traditional grass pastures. The tannins may
also decrease the hatch rate of worm eggs and larval development
Forage species which contain high levels of condensed tannins
include sericea lespedeza, birdsfoot trefoil, and chicory. Sericea
lespedeza is a warm, season legume. Birdsfoot trefoil is a long-lived
perennial legume. Chicory is a low-growing, leafy perennial.
Generally speaking, trees and shrubs contain higher levels of
tannins than pasture grasses, and tropical legumes contain more
condensed tannins than temperate legumes.
BioWorma® is the tradename for a product which contains spores of a fungus called Duddingtonia flagrans. When fed to grazing animals, the fungus spores trap and kill worm larvae in the manure, thereby preventing contamination of pastures. The fungus does not have any effect in the animal. Parasitized animals still require deworming. It works on the free-living stage of roundworms. It needs to be fed daily to have any effect.
BioWorma® is being marketed by an Australian company. It was approved for sale in the US in 2018 and became available for purchase in 2019. There are two products. BioWorma® contains 34% fungal spores and can be purchased by veterinarians and feed mills. Livamo® with BioWorma® contains 2.4% fungal spores and is available for purchase by producers. Premier 1 Supplies, of Iowa, was the first US distributor of Livamol® with BioWorma®. It began selling BioWorma® by itself in 2020.
Earthworms have been shown to ingest worm eggs and larvae, either
killing them or carrying them below the soil surface. Certain
types of fungi will trap and kill parasitic larvae. Dung beetles
ingest and disperse manure, thus keeping eggs and larvae from
developing. Anything that is done to maintain soil health and
promote these types of organisms will aid in parasite control.
Scientists are examining the possibility of feeding nematophagous
fungi to livestock to kill larvae in manure piles.
Supplemental feeding can aid in the control of parasites. Sheep and lambs on a higher plane of nutrition
mount a better immune response to internal parasites than animals
whose nutritional status is compromised. Animals on low protein
diets are more susceptible to infection because they produce
less IgA (immunoglobulin). Higher levels of protein have been
shown to improve the pregnant ewe's immune response to parasites
after lambing. Lambs receiving protein supplementation usually have
reduced fecal egg counts.
Keeping sheep and/or lambs in confinement or dry lot (i.e. "zero grazing")
is a proven means of reducing parasitism and preventing reinfection.
Under a zero grazing situation, sheep and/or lambs do not have
access to vegetation for grazing. They are housed in a bedded
barn, dirt lot, or facility with slatted floors. Feed should
be fed off the ground in feeders. Watering containers should
be kept free from fecal matter. Slatted floors offer the best
protection against internal parasites because sheep generally
do not come into contact with their feces.
Genetics is probably the best long term strategy for controlling internal
parasites in sheep. Some sheep breeds are naturally more resistant and
resilient to internal parasites. They include the "native" breeds of the Southeast (Florida, Louisiana, and Gulf Coast Native and the Florida Cracker) and hair sheep breeds with tropical origins: St. Croix, Barbados
Blackbelly (and its derivatives), and hair sheep composites, such as the Katahdin. Grazing resistant
breeds of sheep with susceptible breeds, may act to sweep
pastures and reduce contamination to susceptible animals.
Regardless of the breed raised, producers can breed sheep
which are more resistant to parasites by culling ewes that are
persistently affected by parasites and favoring parasite resistant
ewes and rams in their selection programs. Both fecal egg counts
and FAMACHA© scores can be used to identify sheep with
resistant and susceptible genetics.
In New Zealand, it is possible to select rams that shed 60 to
70 percent fewer parasite eggs than historical averages. In the US, the National Sheep Improvement program (via Australia's LambPlan) provides EBVs for parasite resistance (fecal egg counts) in Katahdin sheep. Other breeds are now beginning to submit fecal egg count data. EBVs can be calculated for other breeds once fecal egg count data is submitted to NSIP.
are currently looking for genetic markers for worm resistance
so that a DNA test could be used to show producers which of
their animals are resistant to internal parasites.
Proper Anthelmintic Use
Anthelmintics are still an important part of parasite control.
However, they must be used judiciously ensure effectiveness
of treatment and slow down the rate by which worms develop drug
resistance. To start with, the weights of sheep and lambs must
be known or accurately approximated in order to calculate the
proper dosage of medicine. Underdosing results in the survival
of worms which are resistant to the anthelmintic used. Underdosing is one of the major causes of increasing anthelmintic resistance.
Flocks should be divided into groups for deworming or drenching
equipment should be calibrated for the heaviest animals in the
group. Oral drenching is the recommended method of treatment
for sheep. Oral medications should be delivered over the tongue.
If the medicine is deposited into the mouth to stimulate the
closure of the esophageal groove and bypass the rumen.
If an anthelmintic is more slowly absorbed in the gut, drug
levels are prolonged and the treatment may be more effective.
Thus, fasting sheep for up to 24 hours may improve efficacy
of dewormers, especially when using benzimidazoles and ivermectin.
However, water should not be restricted.
To prevent the introduction of drug-resistant worms, you should
deworm newly purchased animals with drugs from at least two
of the three anthelmintic families. Moxidectin should be favored
over ivermectin due to its superior potency. Levamisole should
probably be the other choice, since widespread resistance is
believed to exist in the benzimidazole group of dewormers. After
deworming, the animal(s) should be released onto a wormy pasture
to help dilute any "super-resistant" worms that may
remain in his system.
It is now recommended that clinically-parasitized animals (as evidenced by FAMACHA© score, Five Point Check© or other criteria) be given combination treatments. A combination treatment is when you give different drugs to kills the same worms. Treatment is maximized because of the additive effect of the different dewormers. The recommendation is to give the most potent drug from each drug class, usually albendazole (Valbazen®) + moxidectin (Cydectin®) + levamisol (Prohibit® or LevaMed®). Ideally, each drug is dosed using a separate syringe. The drugs should not be mixed because they are not chemically compatible. The drugs are administered sequentially at the labeled dose. The withdrawal period is the withdrawal period of the drug with the longest withdrawal.
Anthelmintics (dewormers) available in the U.S.
|Prohibit®, Levasol®, Tramisol®, LevaMed®
Rumatel®, Nematel®, Strongid®
|Only Valbazen® drench, Ivomec® drench, Prohibit® and LevaMed® drench, and Cydectin® drench are FDA-approved for use in sheep in the United States.
It is recommended that producers test for anthelmintic resitance every 2 to 3 years. There are two ways to test for anthelmintic resistance. The primary method is the fecal egg count reduction test (FECRT). Before and after fecal egg counts are compared. An effective treatment should reduce fecal egg counts by 95 percent or more. A low level of resistance is present if egg counts are reduced by 90 to 95 percent. If treatment fails to reduce egg counts by 60 percent or more, drug resistance is considered high.
The DrenchRite® assay is another method for determining anthelmintic resistance. The DrenchRite® test determines anthelmintic resistance to all drug classes from a single pooled sample. The University of Georgia College of Veterinary Medicine has the only lab that performs this test in North America.
Currently, there is a lot of interest in using "natural"
substances as an alternative to synthetic dewormers.
Such products include herbal dewormers and diatomaceous earth.
Unfortunately, there is no research to indicate that any of
these products have a substantial effect on internal parasites
in sheep, only testimonials. However, this is an area of increasing
research interest and hopefully recommendations will be forthcoming
in the years ahead.
The exception is copper oxide wire particles (administered as a bolus), which have been
shown to reduce barber pole worm infections in sheep (and goats). Copper oxide wire particles are available in bolus form as copper supplements for cattle and goats. The boluses can be repackaged into smaller doses for sheep, usually 0.5 to 2 g. Dosage is based on age, not weight. Lambs should be given a 0.5 to 1 g bolus. Mature sheep can be given a 1 to 2 g dose. Boluses sold for goats should not be used because they contain too much copper for anthelmintic treatment.
It is recommended that sheep producers learn the copper status of their farm and sheep before using copper oxide wire particles as a primary method of parasite control. Copper sulfate is a oldtime dewormer that was responsible for many sheep deaths. As compared to copper oxide, it is more readily absorbed. It is generally not recommended as a dewormer (for sheep). Nor is it recommended that copper be added to mineral or salt mixtures for sheep.
Worms in "refugia" are those worms which have not been exposed
to drug treatment. They include free-living stages on pasture
and worms in untreated animals. Refugia are essential to maintaining the effectiveness of anthelmintics and slowing the development of resistant worm populations. To increase
refugia, it is suggested that a portion of the flock not be
Fecal egg counts and FAMACHA© scores can be used to identify
which animals do not require deworming. Another strategy for
increasing refugia is to return treated animals to a wormy pasture.
The reason for this recommendation is because if treated animals
are moved to a "clean" pasture, the only worms that
will be on that pasture will be resistant to anthelmintic treatment. For similar reasons, it is recommended that sheep not be held in dry lot after treatment.
Fecal Egg Counting
Fecal egg counting can be important part of an internal parasite
control program. Primarily, a fecal egg count will tell you how contaminated
your pastures are. Fecal egg counts can also be used to make selection
and culling decisions by identifying animals with both high
and low egg counts. Probably the most valuable use of fecal
egg counts is determining drug resistance.
To do your own fecal egg counts, you need a microscope, flotation
solution, mixing vials, strainer, stirring rod, slides, and
cover slips. You do not need an elaborate microscope. 100X power
is sufficient. You can purchase flotation solution from veterinary
supply companies or make your own by mixing a saturated salt
or sugar solution. Your mixing vials can be jars, pill bottles,
film canisters, test tubes, or something similar. You can use
a tea strainer or cheese cloth to strain the feces. The stirring
rod can be a pencil or craft stick.
A McMaster egg counting slide has chambers that making
egg counting easier. There are several source of McMaster slides.
Identifying Worm Eggs. "Strongyle-type" eggs (Haemonchus,
Teladorsagia, and Trichostrongylus) are elliptical or oval, with
smooth, thin shells. Nematodirus eggs are the largest strongyle-type
eggs, but eggs of the species in the group cannot usually be
identified precisely. Worming recommendations can be based on
the quantity of strongyle eggs.
Since fecal counts only estimate the parasite load, there is
no clear cut level at which worming is indicated. As a general
guide, a level of about 500 eggs per gram of feces would indicate
that worming is needed for sheep. A more effective way of deciding
when to treat would be to monitor fecals every 4-8 weeks and
deworm when there is a dramatic rise in egg counts.
Tapeworm eggs are square or triangular. Tapeworm (Moniezia sp.)
eggs may be seen in fecal examination but they are in no way
indicative of the level of infection. Since lungworm eggs hatch
before being passed in the feces the eggs generally are not
seen by the flotation method. Nematode larvae, when present
in the feces, are indicative of lungworm.
Fluke eggs are oval and have a smooth shell with a cap or operculum
at one end. Liver flukes are prolific egg producers, but egg
counts are not necessarily a good indication of infection levels.
Coccidia eggs (oocysts) are very small, about a tenth the size of a Strongyle
egg. Coccidia oocysts are passed in the feces of most livestock.
Oocysts are only a moderate indicator of level of infection.
The FAMACHA© system was developed in South Africa due to
the emergence of drug-resistant worms. The system utilizes an
eye anemia guide to evaluate the eyelid color of a sheep (or
goat) to determine the severity of parasite infection (as evidenced
by anemia) and the need for deworming.
A bright red color indicates that the animal has few or no worms
or that the sheep has the capacity to tolerate its worms. An
almost white eyelid color a warning sign of very bad anemia;
the worms present in the sheep's gut are in such numbers they
are draining the animal of blood. If left untreated, such an
animal will soon die.
The FAMACHA© chart contains five eye scores (1-5), which
have been correlated with packed cell volumes (percentage of
blood made up of red blood cells, also called haematocrit).
Animals in categories 1 or 2 (red or red-pink) do not require
treatment whereas animals in categories 4 and 5 (pink-white
and white) do. Animals in category 3 may or may not require
treatment depending upon other factors.
Mature sheep in category 3 (pink color) probably do not requiring
treament, whereas lambs or other susceptible animals might require (or benefit from) treatment if they are in category 3. The frequency of examination
depends upon the season and weather pattern, with more frequent
examination usually necessary in July, August, and September,
the peak worm season.