Medicated feed additives are medicines that are fed to theLayer house. Picture from animals, alone or mixed with their feed or water. This is the most common method in pig and poultry operations, and is often used for fattening cattle that are not on pasture.

Most antiparasitic feed additives target parasitic worms (roundworms, tapeworms) or fly species that develop in livestock manure (e.g. houseflies, horn flies). They are often marketed as premixes that are directly offered for animal consumption.

For dogs and cats there are also parasiticides, mostly wormers, which are added to the food or drinking water. They are mostly formulated as liquid drenches or as tablets, pills, etc. that can be crushed.

Active ingredients and formulations of feed additives

The active ingredients most used in anthelmintic feed additives are:

Mixtures of various active ingredients are also used.

Whereas feed additives are a usually a reliable delivery form for pig and poultry, when used on cattle it is not always easy to ensure that each individual animal gets the correct dose according to its weight. It is therefore very important that all cattle get access to the feeding or drinking troughs and that enough place is allocated to each animal. It is also necessary to identify animals that are reluctant to eat from the feeding troughs in order to treat them individually.

For poultry there are also feed additives that contain fly larvicides against houseflies. After feeding, such larvicides go unchanged into the manure where they kill the fly maggots. This is why this approach to fly management is also called feed thru control. It makes a lot of sense because it ensures that all the manure is treated, also the small amounts that are difficult to eliminate through normal hygienic procedures. Feed thru larvicides contain mainly insect development inhibitors such as cyromazine.

There are so far no such feed thru larvicides for pig, cattle or sheep.

Mineral blocks are sometimes also used to deliver anthelmintics (e.g. benzimidazoles) or larvicides (e.g. methoprene) to livestock, mainly to cattle. Such larvicides for cattle are used against horn fly larvae that develop in the cow pats. However, it is almost impossible to ensure that each animal gets the correct dose corresponding to its weight, and the risk of underdosing or overdosing is considerable. Such medicated blocks should be used only on animals that are really accustomed to use them, and the un-medicated blocks have to be removed when the medicated ones are offered. 

Safety of feed additives

Anthelmintic feed additives are usually save for livestock. The active ingredients used are well tolerated and the safety margins of the products used are usually rather high, i.e., the risk of unexpected adverse reactions in case of slight overdosing is quite low.

Parasite resistance to feed additives

Resistance of several gastrointestinal roundworms to benzimidazoles, levamisole and macrocyclic lactones is already a serious problem worldwide, especially in sheep and to a lesser extent in cattle. Therefore there is a real risk that products with such active ingredients do not achieve the expected efficacy. There are so far no reports on resistance to such active ingredients in pig, poultry, dogs or cats.

There are reports on tolerance of houseflies to cyromazine after feed thru administration. Tolerance means rather a reduced susceptibility than a clear resistance. But it is a warning and such tolerance may explain some product failures.