What the cat brought in--familiar and unfamiliar nematodes in dogs (Proceedings)

One of the most familiar genera of nematodes in small animals, Toxocara, has been the focus of several recent studies and the results of these studies alter some of our basic understanding of these parasites.  Firstly, it has been widely understood that adult Toxocaracanis infections are much more likely to occur in puppies in the first few months of life, with susceptibility to patent infection declining with age so that few adult infections are seen in adult dogs. 

Epidemiological data clearly shows that eggs are much more prevalent in the feces of puppies than adult dogs.  In terms of public health, emphasis has been placed on controlling the infection in puppies.   However a recent study showed that patent infections were easily induced in older dogs as well when low infective doses were used. These lower doses are probably more similar to those encountered by most animals than high egg numbers used in older studies.  These studies reinforce the recommendation that adult, egg-laying Toxocara infection can be found in dogs of any age. Another study produced results that contradict our general understanding of Toxocara cati in kittens. 

Toxocara cati is the most common helminth of cats and it is generally believed that transmission can occur through milk, by ingestion of infective eggs and by ingestion of transport hosts.  It has been widely believed that cats maintain a somatic reservoir of infection like that of T. canis and that in cats, like dogs, activation of the reservoir occurs during late preganancy with movement of larvae to the mammary glands and transmission in milk.  But Coati et al (2004) have found that transmammary infection to kittens only occurred if the dam was infected during late pregnancy.  They also found no evidence of a somatic worm burden in adult cats. 

Further studies need to be done to understand the importance of persistent infection in cats.  Practitioners sometimes encounter cases of repeated T. cati in entirely indoor cats but these study results indicate that resumption of development of somatic tissue larvae is not involved in these cases.

Occasionally, unfamiliar nematodes are found in fecal flotation exam preparations or dogs and cats may present with clinical signs caused by unusual parasites.  Here are two of these organisms that are encountered infrequently but may cause disease and require treatment and recommendations for prevention of reinfection to be provided to owners.

The first of these parasites in this list is not encountered very often, but is probably a more common infection in dogs than we expect.  Strongyloides stercoralis is a small intestinal nematode.  All parasitic adults are females, approximately 6 mm in length, and could easily be missed during necropsy.  Eggs produced by the females hatch rapidly and usually exit the animal as first stage larvae in the feces.

In the environment, larvae may develop to the infective larval stage or undergo a free-living cycle of male and female reproduction, which then produces infective larvae.  The infective larvae can penetrate the skin or be ingested.  Larvae migrate through the tissues to reach the small intestine.  The main route of transmission of Strongylides spp in livestock hosts appears to be transmammary from a larval tissue reservoir, but there is no experimental evidence that transmammay infection plays an important role in dogs. 

Probably most infections are subclinical, but in some animals bronchopneumonia and diarrhea may develop particularly in puppies, which are more often clinically affected than adults.  Immunosuppressed animals are also at risk of developing severe disease.  Strongyloides infection is sporadic, but seems to be more common in kennels.  Diagnosis is made by finding first stage larvae in fresh fecal samples. Samples must be absolutely fresh because free living nematode larvae could invade a sample and be confused with Strongyloides.  In addition, hatched hookworm eggs could also be mistaken for Strongylides larvae.  Larval production does not appear to be consistent or prolific and infections may not be detected on fecal exams. 

Several products have been used in treatment, but the most effective appears to be ivermectin (0.2-0.8 mg/kg, Mansfield and Schad, 1992).  This parasite would be of limited interest except that Strongyloides is considered a zoonosis.  Humans are infected with the same species as dogs and has proven to be a very serious infection in immunocompromised humans because of the potential for autoinfection and massive migration of larvae through tissues.  The actual risk of transmission of canine Strongyloides to humans is not clear, but the seriousness of the infection in immunocompromised humans should be considered.  Even when family members are immunocompent, parents are understandably concerned about risks to young children from skin-penetrating larvae.

Another nematode that occurs more often than fecal exam results indicate is the spirurid nematode, Physaloptera.  This is a gastric nematode found commonly in raccoons, opposums and skunks.  In a survey of coyotes in Virginia, we found Physaloptera  more often than common helminths like hookworm and roundworm.  Physaloptera is transmitted by an intermediate host (insects) so dogs and cats that consume insects may become infected.  Subclinical cases of infection are unlikely to be detected because the eggs of spirurid nematodes are quite dense and do not float well in most flotation solutions.  Sheather's sugar solution has the highest specific gravity of the common flotation solutions and may float eggs, but a negative fecal exam does not rule out the possibility of Physaloptera infection. 

Physaloptera eggs are about 50 microns in length, have a thick shell and contain a coiled larva when passed in the feces.  Chronic vomiting is the primary clinical sign caused by infection with this parasite and some infections are diagnosed by gastroscopy.  Occasionally adult worms are recovered in vomitus and may be misidentified as ascarids because Physaloptera may be up to 6 cm in length and is relatively thick-bodied.  Unlike ascarids, however, Physaloptera tends to assume a C-shape when it is removed from the animal.  Dogs have been successfully treated with fenbendazole at 50 mg/kg for 3 days and cats with 0.2 mg/kg ivermectin, but pyrantel pamoate also appears to be effective.

References

Bowman DD. 2009.  Parasitology for Veterinarians 9th Ed.  Saunders Elsevier, St. Louis, MO.

Coati  N, Schnieder T, Epe C. 2004. Vertical transmission of Toxocara cati Schrank 1788 (Anasakidae) in the cat, Parasitol. Res. 92:142–146.

Fahrion AS, Staebler S, Deplazes P. 2008. Patent Toxocara canis infections in previously exposed and in helminth-free dogs after infection with low numbers of embryonated eggs, Vet. Parasitol. 152:108–115.

Lee A, Schantz PM, Kazacos KR, Montgomery SP, Bowman DW.   2010.  Epidemiologic and zoonotic aspects of ascarid infection in dogs.  Trends in Parasitology. 26:  155-161.

Mansfield LS, Schad GA.  1992.  Ivermectin treatment of naturally acquired and experimentally induced Strongyloides stercoralis infections in dogs.  J Amer Vet Med Assoc. 201:726-730.