by
Susan Schoenian
Date of last revision:
08-Jun-2006 19:05
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The
primary causes of internal parasitism in sheep and lambs are overstocking
of pastures and insufficient pasture rest periods.
Photo: British Virgin Islands |
In most sheep-raising areas, internal 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. 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 therapy.
Click HERE to see a table listing the Internal Parasites of Sheep.
Gastro-Intestinal
Worms (roundworms, nematodes, stomach worms)
In warm, moist climates, the parasite that causes the most problems to sheep
and lambs is usually Haemonchus Contortis, 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 British Isles. 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 (and goats). 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.
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Click HERE to view the Stomach Worm Life Cycle.
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.
Click HERE to view the Tapeworm Life Cycle.
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 death.
Click HERE to view the Lungworm Life Cycle.
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.
Click HERE
to view the Liver Fluke Life Cycle.
Click
HERE
to see a map showing the Approximate Geographic Distribution of Sheep Liver
Flukes in the U.S.
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 disease. 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 wet. 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.
Click HERE to read an article on the Meningeal Worm (sheepandgoat.com).
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Lambs
in lambing pens, intensive grazing areas, and feed lots are at greatest
risk for coccidiosis.
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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 stresses. 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®). 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.
Rumensin® is very toxic to horses. Bovatec® and Deccox® should not be fed to horses or other equines.
Click HERE
to view the Coccidia Life Cycle.
Click HERE
to view a table of the Drugs Used to Treat and Prevent Coccidiosis (sheepandgoat.com).
Good
Management and Common Sense
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 pasture.
Grazing
Strategies
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.
Multi-species
Grazing
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.
Alternative
Forages
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 in feces.
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.
Click HERE
to read Tannins
for Suppression of Internal Parasites by Langston University.
Healthy
Soil
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.
Click HERE to read Dung Beetle Benefits in the Pasture Ecosystem from ATTRA.
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Hair
sheep breeds are more resistant and resilient to internal parasites.
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Parasite
resistance - the ability to limit
fecal egg counts (FEC) when challenged with infective larvae and thus
limiting or preventing infection. Resistance is moderately heritable and
results in lower pasture contamination. Parasite resilience - the ability to maintain a reasonable productivity when subjected to a parasite challenge. It is less heritable and more difficult to measure. With barber pole worm infections, resilience is measured by "packed cell volume (PCV), a measure of red blood cells. |
Nutritional
Management
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.
Zero
Grazing
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
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 less 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.
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Click HERE to view a table of Anthelmintics (sheepandgoat.com).
Alternative
Dewormers
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.
Click HERE
to go to a page on Alternative Dewormers (University of Aberdeen).
Refugia
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 dewormed.
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 animals. 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.
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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 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.
FAMACHA©
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.
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The FAMACHA© system results in fewer animals being treated, which slows down drug resistance. It identifies wormy animals that require treatment. Persistently wormy animals should be marked for culling. The process of inspecting the eyes is quick and can be incorporated with other management practices.
The FAMACHA© system is only effective for the barber pole worm. It should not be used in a vacuum, but rather it should be incorporated into an integrated worm control program that includes other management practices, such as pasture rest, good nutrition, multi-species grazing, alternative forages, zero grazing, and strategic deworming. FAMACHA© should only be used by properly trained individuals.
Click HERE for a list of FAMACHA© training opportunities in the U.S.
Click HERE
to read Smart Drenching and FAMACHA© by the University of Georgia.
To learn more about internal parasites, visit the Southern Consortium for Small Ruminant Parasite Control.
