Controlling canine and feline gastrointestinal helminths
Despite the widespread availability of affordable and highly efficacious anthelmintics, gastrointestinal helminths remain a common finding in dogs and cats.1 Environmental contamination with infectious stages of gastrointestinal helminths is widespread, and the risk of reinfection of pets, particularly those allowed to roam freely outside, is great. Infection with gastrointestinal helminths can adversely affect the health of dogs and cats, and the presence of infectious stages in the environment also poses a zoonotic health risk to people. Ensuring that pets stay free of infection with these parasites requires instituting a parasite control program that focuses on both regular deworming and stringent flea control, together with routine monitoring for worms by performing fecal examinations.
GASTROINTESTINAL HELMINTHS IN DOGS AND CATS
AscaridsAscarid infections are one of the most common nematode infections diagnosed in pet dogs and cats. Surveys have shown that virtually all pups are born infected with Toxocara canis, the common ascarid of dogs.2 A recent survey of samples collected from dogs in shelters across the United States found that more than 30% of dogs less than 6 months old were shedding T. canis eggs.1 Toxocara canis infection is much less common in adult dogs but still occurs.1 Toxocara cati, the common ascarid of cats, is also routinely found; these worms are present in more than 25% of cats in some surveys.3,4 Unlike dogs, in which age-related resistance to intestinal infection is seen, cats remain susceptible to adult T. cati infection throughout their lives.5 Veterinarians, veterinary technicians, and knowledgeable clients commonly refer to ascarids of dogs and cats as roundworms, although this term refers to any nematode.
Fetal pups acquire T. canis in utero when larvae from the dam's somatic tissue stores are activated late in pregnancy to migrate across the placenta. The pups are then born with developing ascarids in the liver and lungs. These larval stages complete a liver-lung migration in the neonates. This migration results in larvae breaking across the alveoli, traveling up the respiratory tree, and then being swallowed to develop to egg-producing adults in the small intestine within three or four weeks of birth. A small number of larvae may also be transmitted to nursing pups through the milk.5 Because somatic infection is common in adult dogs, even in those receiving routine monthly intestinal parasite control, veterinarians should assume that essentially every pup is likely to harbor developing ascarids in the small intestine and, thus, should routinely deworm pups for T. canis. 6
Dogs and cats of any age may acquire infection with Toxocara species by ingesting infective larvae in eggs from a contaminated environment or by ingesting larvae in the tissues of paratenic hosts. In adult dogs, ingesting infectious larvae of T. canis results primarily in the formation of quiescent larval tissue stores rather than patent, egg-producing intestinal infections with adult worms. If a dog infected with tissue stores of quiescent larvae is an intact female, the larvae may later be transmitted to pups in utero.7 However, in adult cats, both somatic tissue stores and patent intestinal infections commonly develop; somatic stores of T. cati larvae in intact female cats may be transmitted to future litters of kittens through a transmammary but not a transplacental route. Cats allowed to roam outside where they are likely to consume infected paratenic hosts are at greatest risk of infection.5
Disease due to T. canis and T. cati is most severe in young animals. Clinical signs may develop in response to migrating larvae or because of large numbers of adult ascarids in the small intestine. Larvae migrating through the lungs induce pulmonary inflammation; affected animals may develop a mild to moderate cough accompanied by a mucopurulent discharge. Large numbers of larvae migrating through the lungs of young pups may result in severe pulmonary edema, hemorrhage, and even death just a few days after birth.7