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Two Suffolk x Katahdin lambs

Lambs grazing

Baa Baa Black Sheep 

Periparturient egg rise
Periparturient egg rise

May grazing
Close grazers

 Sheep feces
Sheep feces

Barber pole worm
Barber pole worm
Image source: Hair sheep Workshop

Parasitized lamb
Parasitized lamb


 Lamb with bottle jaw
Bottle jaw

 Tapeworm segments
Tapeworm segments

Sick lamb

 White tail deer
Deer transmit meningeal worm

At the mineral feeder
Coccidiostat in mineral

 Harvesting excess pasture growth
Safe pasture

 Rotational grazing
Rotational grazing

 Co-grazing in Vermont
Mixed species grazing

Gulf Coast Native sheep
Image source:

St. Croix ewes
St. Croix ewes
Image source:

Katahdin ewe and lamb

Fecal sampling
Fecal sampling  

Filling a McMaster slide
McMaster slide

Strongyle eggs
Strongyle eggs
Image source: Hair sheep Workshop

FAMACHA(c) card

 FAMACHAŠ scoring a goat


 Deworming lambs
Administer all anthelmintics orally

 Oral dosing syringes
Syringes for deworming

Copper oxide wire particles
Copper oxide wire partices  

Sericea lespedeza
Sericea lespedeza

 Close-up of chicory

supplementla feeding
Supplemental feeding

Zero grazing
Zero grazing controls parasites


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.

The Parasites

Gastro-Intestinal Worms (roundworms, nematodes, stomach worms)
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 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, Teladorsagia is the dominant nematode of sheep (and goats).

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 Haemonchus.

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 death.

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 disease.

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 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 (with ivermectin) has been advocated.

Coccidia (Eimeria spp.) is discussed in another chapter.

Integrated Parasite Management (IPM)

Good Management
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 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 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.

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 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.

Alternative Forages
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 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.

Nematode-trapping fungus
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.

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.

Nutritional Management
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.

Zero Grazing
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.

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 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.

Combination treatments
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.
Drug class
Ingredient Trade names
Benzimidazole Fenbendazole

Panacur®, SafeGuard®

Nicotinic agonist
Prohibit®, Levasol®, Tramisol®, LevaMed®
Rumatel®, Nematel®, Strongid®
Macrocylic lactone


Ivomec®, Primectin®

Cydectin®, Quest®
Only Valbazen® drench, Ivomec® drench, Prohibit® and LevaMed® drench, and Cydectin® drench are FDA-approved for use in sheep in the United States.

Anthelmintic resistance

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.

Alternative Dewormers

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 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. 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.


Eye color
Packed cell
volume (PCV)
> 28
< 12

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.

The FAMACHA© system results in fewer animals being treated, which slows down drug resistance. It identifies wormy animals that require treatment. Persistently wormy animals, especially breeding rams, should be culled. The process of inspecting the eyes is quick and can be incorporated with other management practices. Proper handling facilities will facilitate the use of the FAMACHA system.

The FAMACHA© system is only effective for the barber pole worm and other blood-feeding parasites. It should not be used in isolation. 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 genetic selection. FAMACHA© should only be used by properly trained individuals. To get a FAMACHA© card, producers must take an approved training. It is now possible to receive online training.

Five Point Check©

The same South African researchers who developed the FAMACHA© system have developed the Five Point Check© for targeted selective treatment of internal parasites in small ruminants. The Five Point Check© expands the utility of the FAMACHA© system by incorporating other check points to encompass the symptoms and deworming need for other internal parasites of economic significance. It is also useful for making deworming decisions for animals with FAMACHA© scores of 3.

What to check
Which parasites
Eye Paling of ocular membranes
FAMACHA© score
Barber pole worm
Liver fluke
Back Body condition score All

Dag score
Fecal soiling
evidence of scouring

Brown stomach worm
Hair worm
Nodule worm
Jaw Sub-mandibular edema
"bottle jaw"
Barber pole worm
Liver fluke
Nose Nasal discharge Nasal bots

A happy sheep is a productive sheep

The Happy Factor™ is another decision support model that has been used to determine which animals would benefit from anthelmintic treatment. It uses performance-based criteria. Animals which do not reach predicted target weights are dewormed. The Happy Factor™ is especially adaptable to automated weighing and drafting systems.

Internal parasite (worm) control


Late updated 19-Apr-2021 by Susan Schoenian.
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