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Importance of Worms and Internal Parasites

Gastrointestinal parasites (worms) are a common problem for many sheep and goat owners, particularly those living in humid environments such as the southern and eastern United States. Internal parasites can affect animals in subtle ways resulting in decreased growth, milk production, and lower feed efficiencies. They steal resources from their host sheep or goat meaning your animal needs more feed to achieve the same amount of growth and production than they would without high numbers of worms. Internal parasites can also affect your animals more severely leading to serious disease and death in some animals.

Expectations towards Worms

It is essentially impossible and more importantly unnecessary to eliminate all internal parasites in your sheep or goats. Sheep and goats, also called small ruminants, tolerate some level of internal parasites well with minimal consequences on their health. That said, gastrointestinal parasites pose a greater threat to sheep and particularly goats than to other livestock species. This is because sheep and goats graze closer to the ground than cattle, increasing exposure to parasites. Additionally, goats are less adapted to high parasite loads, are more likely to develop disease from internal parasites, and produce very concentrated feces. These concentrated feces allow worm eggs to reach high levels.

Problems occur when a sheep or goat’s parasite load becomes excessive or their immune system is weakened. The parasite load may become excessive when an animal has too high numbers of parasites in their bodies; this happens at lower parasite numbers if the specific parasite is more likely to cause disease. Sheep and goats have weakened immune systems when they are sick with other diseases, are quite young or old, and during highly stressful events such as lambing. Deworming strategies should seek to protect these higher at-risk groups, controlling parasite levels in all animals to prevent visible effects of parasitism. This is not always possible and some animals may have signs of illness related to parasites.

Common Worms Encountered

The types of worms your small ruminants are likely to encounter depend on where they are located geographically. One of the most commonly troublesome worms for owners throughout the U.S. is the barber pole worm or Haemonchus contortus. This and other worms are briefly summarized in Table 1. Remember that each herd and individual goat/sheep is different and may have unique compositions of worms. Unless there are obvious symptoms, your veterinarian will do some fecal examinations to determine the type of worm(s) affecting your animals.

Another point to keep in mind is that GI parasites tend to cause similar symptoms. Affected animals will have signs of unthriftiness: their growth, weight gain and coat quality will be relatively poor. Other general signs of parasitism include loss of appetite, stomach pain (colic), diarrhea, blood in the feces, and weakness. These nonspecific symptoms may be due to a number of different GI parasites.

Management Practices

The best approach towards controlling worm populations in your small ruminant includes a mixture of good management practices with deworming protocols used sparingly and only when necessary. The appropriate use of dewormers is critically important as there is rampant resistance to dewormers among certain types of worms, making treatment more difficult.

Choosing a Dewormer(s)

There are three main classes of dewormers that can be used in sheep and goats. These are the benzimidazoles, the membrane depolarizers, and the macrocyclic lactones. They are summarized in Table 2. Each dewormer class attacks internal parasites through different modes of action and will have varying effectiveness depending on the type of worm your small ruminants have and any resistance those worms might have. For example, benzimidazoles are not very effective against the barber pole worm, as this worm has developed high rates of resistance against this dewormer class. However, benzimidazoles are more effective against tapeworms, whipworms and threadneck worms. Thus, you can use them in combination with another drug class to treat several types of worms in your sheep and goats at once.

Using a combination of different classes of dewormers at the same time is highly recommended in small ruminants. This strategy provides a broader spectrum of activity and an additive killing benefit when treating resistant worms. When using this strategy in your animals, it is important you give full doses of each medication, all at the correct timing. Some dewormers may be extra-label use for small ruminants, particularly goats, and thus you must work with your veterinarian to use them.

Consult with your veterinarian to determine which dewormer combinations would be best for your animals. Your veterinarian will consider whether or not your animals have worms with resistance to different dewormers. There are specific tests that can be performed to determine if resistance exists in your small ruminants, and if so to which dewormer classes. However, it may not be necessary to test for resistance initially. You and your veterinarian may decide to treat using a routine regimen without testing for resistance to save on expenses or if this is your first time having issues with worms and resistance is not suspected.  

If resistance is a known or suspected problem in your animals, there are several tests available to learn more. There are currently two tests to determine drug resistance: the fecal egg count reduction test (FECRT) and the larval developmental assay (LDA; DrenchRite). With the FECRT, fecal egg counts before and after treatment or the egg counts between treated and non-treated animals are compared. An effective drug will reduce fecal egg counts in your sheep and goats by 95% or more. This test can be labor-intensive, time-consuming and may not give reliable results if you have a smaller herd. The other method to determine dewormer resistance is the DrenchRite test. This involves sending samples of larvae to a laboratory that tests for resistance to all dewormer classes in a single test and identifies the type of parasites in the sample. Results from the DrenchRite tests are reliable but also more expensive.

Different Deworming Strategies

There are a number of deworming strategies, each with their benefits and drawbacks. The three most common and recommended are salvage deworming, tactical deworming, and strategic deworming. Which strategy works best for you depends on your unique situation and animals; it is worthwhile discussing these with your veterinarian.

In many herds a minority of animals (about 20%) host a majority of the parasites (about 80%). Salvage deworming takes advantage of this situation by seeking to control parasite populations through identifying animals with high parasite loads in your herd and only treating those animals. It involves using the FAMACHA program to determine the degree of anemia in your sheep and goats through a scoring system. The advantages of this strategy include a significant reduction in the use of dewormers and resistance. However, this strategy is highly labor intensive as animals need to be scored every two weeks and high scoring animals should be culled or sent to slaughter; a method that can work well in production systems but not if your small ruminants are pets.

Tactical deworming involves treating your animals when parasite egg production is highest, limiting use of dewormer during low parasite concentration. Fecal egg counts are run to determine parasite numbers and animals are dewormed when their parasite eggs per gram (EPG) numbers are over 1,000 in cold seasons and over 2,000 in warm seasons. This strategy is particularly useful when weather conditions are favorable for parasite buildup on pasture.  Favorable weather conditions may vary between different worms but generally occur 10 to 14 days after a rainfall, especially if it is followed by a drought. Moreover, warmer temperatures between 50°-98°F with an optimum of 70°-80°F also favor parasite buildup in the environment.

Strategic deworming targets parasites in your small ruminants instead of in the environment. This involves deworming ewes/does three weeks prior to expected birth of the first lamb/kid and then every 3-4 weeks until the lamb/kid is three weeks old. Advantages of this strategy include addressing the rise of worm egg production in female animals around birthing. However, because it does not treat worms in the environment, dewormer resistance is likely to develop.

Regardless of which deworming strategy you use, smart drenching guidelines should be followed. These are a set of recommendations created to improve the efficacy of dewormers and prevent resistance among worms. Among these is to dose all dewormers except levamisole and moxidectin at twice the cattle dose for the heaviest animal in your group. In addition, all products should be used orally with drenching guns in the throat to ensure the full dose is given. Talk with your veterinarian for a more thorough summary of current recommendations as well as which strategy would work best for your individual farm.

Table 1: Common Worms

Table 2: Classes of Dewormers