Worms are the most common endoparasites of companion animals and infestations rank among the most critical of parasitic infestations in cats, dogs, and horses. There are two main classes of worms that affect companion animals: tapeworms and roundworms.

Tapeworms, or cestodes (classification: phylum Plathelminthes, class Cestoda), whose individual segments hang together like a chain, can grow to considerable lengths in the intestine of the final host. If the segments are excreted, they are sometimes visible in feces or on the coat near the anus. They reproduce in various intermediate hosts, such as fleas, which act as effective transmitters of tapeworm infestation.

Roundworms, or nematodes (classification: phylum Nemathelminthes, class Nematoda), such as the common roundworm and hookworm, are also troublesome. Infestation with roundworms takes place through the ingestion of infectious eggs (common roundworm), infectious larvae (hookworms), or active penetration of the larvae through the epidermis (hookworms). However, the mother's milk can also carry roundworms when dormant larvae in the mother become active and reach the mammary glands.

Harmful effects of worm infestation

Worms may be there, but you can't always see them. The indications of disease depend on the defenses of the affected animal and the extent of the infestation. They range from general ill-health with a dull coat and occasional vomiting to a weakened immune system through massive loss of blood with resulting anemia and even intestinal obstruction and death.

Worm prevention and control

In contrast to farm livestock, control measures for companion animals are often inadequate, but in light of the consequences of worm infestation, it's easy to understand why veterinarians recommend that cats, dogs, and horses be wormed regularly (at least four times a year: for details see animal specific chapters). Broad-spectrum anthelmintics effective against all major roundworms and tapeworms are available and are administered once per worming. In the case of a high risk of tapeworms, extra treatment especially for tapeworms may occasionally be necessary as these parasites in particular grow much more quickly. Young, pregnant, and lactating animals should be wormed according to a special schedule.

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