Internal parasite (worm) control
In most sheep production areas, internal or gastro-intestinal
parasites (i.e. 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
are slow to acquire immunity. It takes 10 to 12 months for most
lambs to develop immunity to parasites. Sheep also suffer a loss
of immunity at the time of lambing, which does not restore itself
until approximately four weeks after lambing.
Heavy stocking rates and insufficient pasture rest periods
further 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 arid regions, because parasites require
moisture for their development.
In the past, sheep producers relied heavily on anti-parasitic
drugs, called "anthelmintics" 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 ivermectin and the benzimidazoles.
In the U.S., few anthelmintics are FDA-approved for use in sheep
and lambs, and no new drugs are likely to be developed. As a
result, producers must develop more integrated programs for
controlling parasites, which do not rely exclusively on drug
Gastro-Intestinal Worms (roundworms, nematodes, stomach
In warm, moist climates, the parasite that causes the most problems
to sheep and lambs 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 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). The other worm
species are more likely to cause diarrhea. The barber pole worm
is difficult to control because it has a short, direct life
cycle and is a prolific egg producer.
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.
The stomach worms usually of secondary importance are Trichostrongylus
spp. and Ostertagia spp. 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. Ostertagia
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,
Ostertagia circumcincta is the dominant nematode of sheep
Nematodirus is not usually a primary pathogen in 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. Experts disagree about the importance
of effects of the parasite. Although dramatically large numbers
of tapeworms may occupy the small intestine, damage to sheep
is generally much less than that done by the gastrointestinal
nematodes such as Haemonchus and Ostertagia.
In extreme cases, tapeworms may cause intestinal blockages.
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)
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 rain 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 be diagnosed in the live animal.
Treatment usually involves high, repetitive doses of anthelmintics,
along with steroids and other supportive therapies. 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 has been advocated.
Coccidia (Eimeria spp.)
Coccidia are single-cell protozoa that damage the lining of
the small intestine. They are host-specific, meaning the species
that affects cattle, swine, and poultry does not affect sheep
and vice versa. Coccidiosis is very common in sheep, especially
young, growing lambs. Older sheep serves as sources of infection
for young sheep.
Lambs in lambing pens, intensive grazing areas, and feedlots
are at greatest risk. Transmission of coccidiosis to lambs favors
warm, wet environmental conditions. Stress often induces outbreaks
of coccidiosis. Coccidiosis often follows weaning or shipping
Clinical signs include diarrhea (sometimes containing blood
or mucous), dehydration, fever, weight loss, loss of appetite,
anemia, and death. The coccidia organism does not respond to
the standard deworming products. Medications used to treat clinical
coccidiosis differ from medications use to prevent it.
Outbreaks of coccidiosis are usually treated with sulfa drugs
and amprolium (Corid). These drugs must be prescribed by a veterinarian.
Feed additives for the prevention of coccidiosis in lambs are
currently in use by the sheep industry. They include two FDA-approved
products, lasalocid (Bovatec®) and decoquinate (Deccox®),
and one non-approved product, monensin (Rumensin®). Rumensin®
is very toxic to horses. Bovatec® and Deccox® should
not be fed to horses or other equines.
Monensin requires a veterinary prescription. Preventive medications
such as monensin, lasalocid, and decoquinate, are collectively
referred to as coccidiostats, meaning that they slow down the
shedding of coccidia into the environment. They are only effective
in preventing disease if they are added to the feed before lambs
become exposed. On the other hand, treatment medications such
as sulfa compounds and amprolium are coccidiacidal, meaning
that they actually kill the coccidia organisms in the intestine
of the treated animal.
Integrated Parasite Management (IPM)
Internal parasite control starts with good management and common
sense. Sheep should not be fed on the ground. Feeders which
cannot easily be contaminated with feces should be utilized
for grain, hay, and minerals. Water should be 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 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 the 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 than have been recently established
or renovated by tillage. While burning a pasture will remove
worm larvae, there are no pasture treatments that will effectively
eliminate or reduce worm larvae.
Pasture Rest and Rotation
It is a common misconception that rotational grazing helps to
control 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 will have fewer parasite problems. Sheep should not
be allowed to graze forages shorter than 2 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 conversely.
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 anthelmintic 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./////
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 should not be overlooked as a means to
control 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 have
reduced fecal egg counts.
Keeping sheep and/or lambs in confinement (i.e. "zero grazing")
is a means of reducing parasitism and preventing reinfection.
Under a zero grazing situation, sheep and/or lambs do not have
access to any vegetation for grazing. They are housed in a bedded
barn, dirt lot, or facility with slotted floors. Feed should
be fed off the ground in feeders. Watering containers should
be kept free from fecal matter. Slotted 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 weapon against internal
parasites in sheep. Some sheep breeds are more resistant and
resilient to internal parasites. They include the Florida (or
Gulf Coast) Native and the hair sheep breeds: St. Croix, Barbados
Blackbelly (and its derivatives), and 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 also 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. Scientists
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 properly to 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 approximated accurately in order to calculate the
proper dosage of medicine. Underdosing results in the survival
of worms which are resistant to the anthelmintic used.
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.
Using the same anthelmintic or drugs from the same chemical
family will increase the rate at which worms become resistant
to the drugs. To lessen the effect, anthelmintics (or drug groups)
should be rotated on an annual basis. On the other hand, anthelmintics
should not be rotated after each use; otherwise, worms will
develop resistance to each drug simultaneously.
If you do not deworm your animals much (e.g. < 3 times per
year), you might not have much of a drug resistant problem.
But if you introduce new animals to your flock, you could introduce
drug-resistant worms. This is especially true if you purchase
animals from a farm that deworms frequently.
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.
Anthelmintics (dewormers) available in the U.S.
|Prohibit®, Levaso®l, Tramisol®
Rumatel®, Nematel®, Strongid®
|Only Valbazen® drench, Ivomec® drench, Levamisol® drench and oblets, and Cydectin® drench are FDA-approved for use in sheep in the United States. Levamisol is currently off the market.
Currently, there is a lot of interest in using "natural"
products as an alternative to chemical control of parasites.
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.
Copper oxide particles (administered as a bolus) have been
shown to reduce barber pole worm infections in sheep. They have
been used with mixed results in goats. Copper oxide is available
for cattle as a supplement to alleviate copper deficiency and
has been used in sheep for the same purpose. Scientists are
currently evaluating different dosage rates to avoid copper
toxicity in sheep.
Worms in "refugia" are those which have not been exposed
to drug treatment. They include free-living stages on pasture
and worms in untreated animals. Refugia are being viewed as
an important tool to slow down anthelmintic resistance. 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.
Fecal Egg Analysis
Fecal egg analysis is an important part of a internal parasite
control program. Primarily, a fecal analysis tells you how contaminated
your pastures are. Fecal analysis 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
analysis is determining drug resistance.
The test to determine drug resistance is called the fecal egg
count reduction test (FECRT). To conduct a FECRT, animal are
weighed and dewormed with the anthelmintic you wish to test.
Fecal samples are collected twice: first at the time of deworming
and second, 7 to 10 days later. Six or more (ideally 10 ) animals
should be tested for each anthelmintic. Fecal samples should
also be collected and analyzed for a similar group of untreated
For an anthelmintic to be considered effective it should reduce
fecal egg counts by 90 percent (ideally 95%). There is severe
drug resistance if treatment fails to reduce egg counts by more
than 60 percent.
To do your own fecal analysis, 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 popsicle stick.
If you want to count eggs, you want to get a McMaster Egg Counting
slide available HERE. The McMaster slide has chambers that making
egg counting easier. The Paracount-EPG Fecal Analysis
Kit with McMaster-Type Counting Slides is available for $40
from the Chalex Corperation.
Identifying Worm Eggs. "Strongyle-type" eggs (Haemonchus,
Ostertagia 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. F
luke 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 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 should
be treated 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.