Cite as: Jacela JY, DeRouchey JM, Tokach MD, et al. Feed additives for swine: Fact sheets – carcass modifiers, carbohydrate-degrading enzymes and proteases, and anthelmintics. J Swine Health Prod. 2009;17(6):325–332.
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Keywords: swine, carcass modifiers, carbohydrate-degrading, enzymes, proteases, anthelmintics
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Parasite control, in addition to control of viruses and bacteria, must be part of a comprehensive herd-health program in every swine production system. Gastrointestinal worm infections may result in significant economic losses. Signs of infection are general and not readily apparent, since worm infections rarely cause elevated mortality levels. Some worms commonly found in pigs are roundworms (Ascaris suum), nodular worms (Oesophagostomum species), intestinal threadworms (Strongyloides ransomi), whipworms (Trichuris suis), kidney worms (Stephanurus dentatus), and lungworms (Metastrongylus species). Anthelmintics or “dewormers” are chemical substances that can be added to pig diets to control parasitic worms.

What are the consequences of worm infection?

Worms are parasites that deprive the pig of nutrients, negatively affecting pig growth and feed efficiency. Heavy infestation in some cases can lead to condemnation and loss of carcass value. An example is liver condemnation due to larval migration of A suum. During their development, the larval forms of this worm pass through the liver and create white scars known as “milk spots.”

What products are available for use as anthelmintics in swine feed?

Dichlorvos. Dichlorvos is indicated to remove and control mature and immature forms of the most common pig worms. However, it is relatively ineffective in controlling early larval forms of roundworms. Two consecutive days of feeding is recommended when dichlorvos is added to pig diets. No withdrawal time is required when this product is used at the approved dose.

Fenbendazole. Fenbendazole has a relatively broad spectrum of activity. It is effective against mature and immature forms of common worms that infect pigs. However, fenbendazole has a higher activity when given at low doses for several days (9 mg per kg body weight with the dose divided over 3 to 12 days) than when single-dosed. No withdrawal time is required when this product is used at the recommended dose.

Ivermectin. Ivermectin is highly effective against immature and adult forms of most gastrointestinal roundworms, as well as against pig external parasites such as lice and mange mites. Ivermectin is available in an injectable preparation as well as in the premix form. The premix product is labeled to be fed for 7 consecutive days. A withdrawal time of 5 days is required when this product is administered in feed.

Levamisole. Levamisole is effective against mature roundworms, but only moderately effective against nodular worms. This anthelmintic has a negative effect on diet palatability. Thus, it is more commonly administered through drinking water to insure intake. When levamisole is administered in pig diets, withdrawal of regular feed overnight is recommended prior to feeding the medicated diet the following morning. Treated pigs should be fed the regular diet once the medicated diet is completely consumed. A withdrawal time of 3 days is required.

Piperazine. Piperazine has a relatively narrow spectrum of activity. It has good efficacy against roundworms and moderate efficacy against nodular worms, but is ineffective against other types of pig worms. This drug is more commonly available commercially as a water-soluble product, but it is also approved by the Food and Drug Administration for use as a feed additive. The main advantage of piperazine is that it is relatively inexpensive and is administered as a 1-day single treatment. However, a withdrawal period of 21 days is required.

Pyrantel tartrate. Pyrantel tartrate is fed for 3 consecutive days to remove large roundworms or continuously to prevent migration and establishment of roundworms and nodular worms. This drug is photodegradable and, hence, must be used immediately upon opening the package. It also should not be mixed in diets containing bentonite. A withdrawal time of 24 hours is required.

Additional detailed information on dewormers approved for swine can be found in the Feed Additive Compendium¹ or on the Food and Drug Administration (FDA) Web site.²

When is it necessary to treat pigs with anthelmintics to control worms?

Worm infections occur more frequently in pigs raised in outdoor lots than in conventional confinement facilities. Therefore, production design is one consideration in terms of determining how frequently pigs should be fed anthelmintics. Breeding stock should be given anthelmintics after arrival at the farm and before introduction to the herd. Sows are a common source of worm eggs for piglets and should be dewormed several days before farrowing and before moving to the farrowing room. Scrubbing the sow to remove the worm eggs attached to her body before transfer to the farrowing barn also can reduce exposure of baby pigs.

Knowledge of the specific parasites present in the herd and their life cycle is helpful in establishing an effective control program. Prepatent period (Table 1) refers to the period between the time when the infection occurs and when the adult worms begin shedding eggs. Some worms produce eggs several days after infection, while others take months to begin producing eggs. Most anthelmintics are not able to destroy the egg and larval forms that develop into adults after several days. The interval for repeating deworming can be determined on the basis of the prepatent periods. Deworming must be repeated before the minimum prepatent period to kill the adult forms and prevent them from laying eggs.

Choosing the appropriate anthelmintic

Anthelmintics have different modes of action and vary in their effectiveness against different species of pig worms. Therefore, choosing the proper anthelmintic to be used in the feed will depend on the specific worm problem. The relative effectiveness and spectrum of activity of common anthelmintics are listed in Table 2. Brand names of products available in the United States are enumerated in Table 3. It should be noted that anthelmintics, like antibiotics, may require specific withdrawal periods (Table 4).

Summary

Worm control is an important component of every herd-health program. Many anthelmintics are effective against different types of worms. Therefore, selection of an appropriate anthelmintic will depend on the type of worm to be controlled. Also, use of anthelmintics must not be relied on as the sole approach in controlling worms, but must be combined with good sanitation and production practices to be successful.

References

1. Feedstuffs. Feed Additive Compendium. Minnetonka, Minnesota: Miller Publishing Co; 2008.

2. Animal Drugs @ FDA. US Food and Drug Administration Center for Veterinary Medicine Web site. Available at: http://www.accessdata.fda.gov/scripts/animaldrugsatfda/. Accessed 1 July 2009.

3. Myers GH. Strategies to control internal parasites in cattle and swine. J Anim Sci. 1988;66:1555–1564.

4. Myer RO, Brendemuhl JH. Controlling Internal Parasites In Swine. Gainesville: University of Florida Institute of Food and Agricultural Sciences; 2009; Publication AS50. Available at http://edis.ifas.ufl.edu/pdffiles/AN/AN03900.pdf. Accessed 30 June 2009.