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 therapy.
The Parasites
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 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 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.
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 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 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.
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®). 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)
Good Management 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./////
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 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.
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.
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.
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.
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 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.
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.
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.
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.
