7: Treatment of Veterinary Parasites

Treatment of Veterinary Parasites


A wide variety of safe, effective parasiticides can eliminate infections and infestations or prevent development or exacerbation of parasitic disease. Despite the availability of these powerful treatments, parasite control continues to be an unmet need for many domestic animals due to lack of use or incomplete knowledge on efficacy. Choosing the parasiticide treatment most appropriate for a particular clinical situation requires careful assessment of risk factors as well as veterinary expertise regarding individual patient, herd, or flock characteristics and needs. Rarely is one treatment or a single protocol the best option for all patients, even when general concerns about the type of parasitic disease of greatest risk are similar. In this chapter we summarize information about anthelmintics, ectoparasiticides, and antiprotozoals currently available to veterinarians. Only products approved by FDA, EPA, or similar regulatory agencies and that have been confirmed to be safe and efficacious will be discussed here; availability of some approved products or formulations may be limited. Products covered in this chapter are largely restricted to those available in the United States and Canada. Veterinarians should be familiar with and consult current regulatory‐approved label information prior to prescribing or recommending any specific product.


Effective control of helminths involves more than just selection of a safe, effective anthelmintic. Effective parasite management programs must incorporate likelihood of environmental contamination and thus risk of re‐infection, seasonality of transmission, age and immune status of animals, and zoonotic risk of allowing infections to persist. Selection of the best anthelmintic for a given clinical situation is often based on factors other than efficacy, including safety, application route, persistence, and knowledge about resistance. Traditionally, anthelmintics were administered in liquid oral formulations either directly or via nasogastric tube. However, many anthelmintics are now given with the feed, injected, or as a transdermal product that is administered topically and absorbed systemically (Table 7.1).

Table 7.1. Common administration routes for anthelmintics

Application route Examples Species available for
Tablets/chewable Milbemycin oxime +/‐ praziquantel Cats, dogs

Ivermectin +/‐ pyrantel +/‐ praziquantel Cats, dogs

Pyrantel/praziquantel +/‐ febantel Cats, dogs
Paste Fenbendazole, ivermectin, moxidectin, oxibendazole, pyrantel pamoate Horses

Fenbendazole aCattle
Liquid Albendazole, closantel, fenbendazole, ivermectin, levamisole, moxidectin, oxfendazole aCattle, asheep, agoats
Water additive Fenbendazole aPigs
Feed additive Fenbendazole, morantel tartrate aCattle

Dichlorvos, fenbendazole, ivermectin, levamisole, piperazine, pyrantel tartrate aPigs
Block / mineral Fenbendazole aCattle
Pellets Fenbendazole, pyrantel tartrate Horses
Injectable Melarsomine dihydrochloride Dogs

Moxidectin Dogs

Doramectin, ivermectin aCattle, apigs

Levamisole, moxidectin aCattle

Eprinomectin extended release parasiticide aCattle

Praziquantel Cats, dogs
Transdermal Moxidectin, selamectin Cats, dogs

Emodepside/praziquantel Cats

Eprinomectin/praziquantel Cats

Doramectin, eprinomectin, ivermectin, moxidectin aCattle

a Products listed not approved for use in all food animals; always consult label prior to administering.

Anthelmintics also differ in the degree to which they persist in the animal after administration, with some products being cleared within 24 hours and others lasting and remaining efficacious as long as 6 months. Persistency is often desired to reduce the number of times animals must be treated but can affect withdrawal times (Table 7.2) and influence selection for resistance. With use of any antiparasitic drug treatment, including anthelmintics, selection for resistance will occur. Anthelmintic resistance has reduced the utility of these drugs in certain clinical situations. Accordingly, recognizing resistance when it occurs and developing appropriate strategies to protect animal health in the face of diminished anthelmintic efficacy are important clinical skills. Resistance to parasiticides is discussed further below.

Specific Anthelmintics

Macrocyclic lactones, also referred to as avermectin/milbemycin compounds, constitute some of the safest, most effective, and most widely used anthelmintics ever developed. First marketed in the early 1980s, the class now includes abamectin, doramectin, eprinomectin, ivermectin, milbemycin oxime, moxidectin, and selamectin. These compounds cause flaccid paralysis by impeding glutamate‐gated chloride channels; removal of gastrointestinal parasites follows via peristalsis and stages in tissues are cleared by the immune system. Although the specific efficacy of individual compounds differs and formulations influence spectrum of activity, in general, members of this class are effective against a diverse array of nematodes and arthropods. All available formulations are safe when administered according to label directions. However, extralabel use of high doses can result in toxicity, particularly in collies and other dogs that carry the MDR1 mutation and in certain exotic animals, including chelonians and some other reptiles.

Table 7.2. Examples of withdrawal times for selected anthelmintics

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Sep 19, 2022 | Posted by in GENERAL | Comments Off on 7: Treatment of Veterinary Parasites

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Anthelmintic Species treated aWithdrawal times
Albendazole Beef cattle 27 d oral liquid

Goats, sheep 7 d oral liquid
Dichlorvos Pigs 0 d feed additive
Doramectin Pigs 24 d injectable

Beef cattle 35 d injectable; 45 d pour‐on

Dairy cattle Injectable only in heifers <20 mo. of age
Eprinomectin Beef cattle 0 d pour‐on; 48 d injectable

Dairy cattle 0 d pour‐on; injectable only in heifers <20 mo. of age
Fenbendazole Pigs 0 d feed additive

Beef cattle 8 d oral liquid, paste; 11 d molasses block; 13 d flaked meal, free‐choice mineral, pellets; 16 d protein block

Dairy cattle 0 d oral liquid, flaked meal, free‐choice mineral, paste, pellets

Goats 6 d oral liquid
Ivermectin Pigs 5 d feed additive; 18 d injectable

Beef cattle 35 d injectable; 48 d pour‐on

Dairy cattle Injectable only in heifers <20 mo. of age

Sheep 11 d oral liquid
Levamisole Pigs 3 d water or feed additive

Beef cattle 2 d bolus, oral liquid, feed additive; 7 d injectable; 9 d pour‐on

Sheep 3 d oral liquid
Morantel tartrate Beef cattle 14 d feed additive

Dairy cattle 0 d feed additive
Moxidectin Beef cattle 0 d pour‐on; 21 d injectable

Dairy cattle 0 d pour‐on; injectable only in heifers <20 mo. of age

Sheep 7 d oral liquid
Oxfendazole Beef cattle 7 d oral liquid
Piperazine Pigs 21 d water or feed additive