Parasitic zoonoses (Proceedings)

In recent years, concern about the zoonotic potential of parasites encountered in small animal practice has risen substantially.  Pet owners have greater access to information and misinformation through the internet and veterinarians have become more concerned about issues of liability where zoonotic diseases are concerned.  While we know that zoonotic infections with several helminths of small animals do occur, the actual numbers of cases are difficult to establish because these are not usually notifiable diseases.  Nonetheless, practitioners need to be prepared to discuss these parasites with their clients and should always have supplemental printed or on-line material available for clients.  Generally speaking, medical students receive far less instruction in parasitology than veterinary students and the general perception of physicians appears to be that client education on zoonotic diseases is the responsibility of the veterinarian.

Two of the most common parasites of dogs, Toxocara canis and Ancylostoma spp, the common roundworm and hookworm, respectively, have received considerable notoriety for their ability to cause larva migrans in humans.  Adult Toxocara canis infections are most common in puppies, although adult infections can be encountered in dogs of any age and a recent study found that infection of adult dogs with low numbers of eggs readily produced patent Toxocara infection.  Transplacental transmission of worms from the somatic reservoir of female dogs leads to potentially heavy infections in puppies and ingestion of infective eggs will lead to additional adult worms.  Toxocara female worms are very prolific and many thousands of eggs can be passed daily in the feces of an infected puppy.  Transplacentally acquired infections have a prepatent period of 3 weeks in puppies.  Toxocara eggs require about 3 weeks to develop to the infective stage in the environment so prompt removal of feces is a very effective way of controlling infection in a client's backyard.  Infective eggs in the environment appear to be very persistent and numerous surveys of public parks and other areas show a high rate of contamination with infective eggs. 

Human infection follows ingestions of the infective egg(s).  Most human infections are asymptomatic, although some researchers have suggested that subclinical infection may also have impacts on health and a recent study using samples from a large nutrition survey by CDC found an association between diminished lung function and previous infection with Toxocara (Walsh, 2011).  Overt clinical disease takes the form of visceral or ocular larva migrans.  The CDC has estimated that there are at least 750 cases of ocular larva migrans annually in the U.S. 

Although Toxocara cati infection can also cause larva migrans, it appears to cause disease much less frequently than T. canis.  Another ascarid found in raccoons, Baylisascaris procyonis, can produce severe and often fatal cases of visceral larva migrans.  Any veterinarian or other individuals in contact with raccoons should deworm animals for this ascarid and dispose of raccoon feces promptly.  Raccoons are common in many parts of the U.S. and feces deposited in backyards and public areas should also be removed.  Adult Baylisascaris can be effectively treated with fenbendazole (50 mg/kg PO daily for 3 days) or pyrantel (20 mg/kg PO).  Occasionally, dogs may develop patent B. procyonis infections.

Ancylostoma caninum is the most common canine hookworm in the U.S.  Ancylostoma braziliense is also found in dogs and cats in the Gulf Coast region and Caribbean.  Uncinaria stenocephala in dogs and cats and A. tubaeformae cats have little importance as zoonotic infections in the U.S.  Once hookworm eggs are passed in the feces, larvae are formed and the eggs hatch.  Development to the infective stage can occur in a few days in hot weather so prompt removal and disposal of fecal material is important.  Larvae are able to penetrate the skin of the host and this is a common route of infection for dogs.  Larvae will also penetrate the skin of other animals as well, causing the condition in humans know as cutaneous larva migrans.  Even without treatment hookworm larvae will probably be killed by the human inflammatory response, but lesions can be very irritating and patients usually seek treatment.  Ancylostoma braziliense larvae appear to penetrate human skin more aggressively than those of A. caninum, increasing the risk of CLM in areas where A. braziliense is found. 

Larvae will develop in sandy loamy soil and close contact with the soil, either through working or leisure activities like sunbathing, can lead to infection.  Ancylostoma larvae are much less resistant to environmental conditions than Toxocara eggs.  Freezing conditions will kill most hookworm larvae so parasite contamination will largely die back during winter months in areas with prolonged cold periods.  There are currently no products available that can be used to treat the environment for hookworm without having detrimental effects on vegetation and/or other invertebrates.  Rarely, humans will develop intestinal infections of Ancylostoma caninum that are associated with eosinophilic enteritis.  The worms in these infections do not produce eggs and are difficult to diagnose.  Ancylostoma caninum will also form a somatic reservoir of larvae in dogs and transmammary infection of puppies is common.  These transmammary infections have a prepatent period of about 11-14 days.

Ancylostoma and Toxocara are 2 of the big 3 common helminths in dogs in the U.S. (the third being the whipworm, Trichuris vulpis). Because these parasites are so common the infection risk for dogs is high. Groups like the Companion Animal Parasite Council (www.capcvet.org) have recommended that dogs and cats be placed on monthly heartworm preventatives that are also effective in treating roundworms and hookworms.

Other common helminths with limited zoonotic importance are Dipylidium caninum, the common dog and cat tapeworm and Dirofilaria immits, the canine heartworm.  Dipylidium is transmitted to dogs and cats through ingestion of the flea intermediate host containing the larval tapeworm.  Occasionally children become infected through ingestion of infected fleas or possibly by “kissing” an animal that may contain portions of an infected flea around the mouth.  Symptoms caused by the tapeworm do not extend beyond mild intestinal signs, but parents are understandably distressed by the presence of tapeworm segments.  This is one of the few instances when a physician would be correct in telling a concerned parent that the child acquired the infection, albeit indirectly, from the dog or cat.  Treatment of dogs and cats with praziquantel when infection is observed, and effective flea control will prevent exposure in the household.

Infections with canine heartworm occur rarely in humans and are usually of little clinical significance.  Radiographic lesions caused by Dirofilaria in the human lung can be confused with neoplasia.

Strongyloides stercoralis is a nematode parasite capable of infecting both dogs and humans. Probably most infections in dogs 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.   If infection is diagnosed in a pet, owners, particularly those with young children must be advised that infection is zoonotic and that larvae can penetrate the skin.   Infection in immunosuppressed human patients may be a severe and even fatal disease. The most effective treatment for the infection in dogs is ivermectin (0.2-0.8 mg/kg, Mansfield and Schad, 1992).

The protozoan parasite of greatest zoonotic concern in small animal practice is Giardia.  The current understanding of host specificity of this parasite is based on genotypic analysis of human and animal isolates from various regions of the world.  Giardia isolates are placed in one of several “assemblages.”  Assemblages C and D seem to be specific to dogs and Assemblage F appears to occur only in cats, but assemblages A and B infect both humans and a variety of other animals.  The proportions of the assemblages in isolates from companion animals seems to vary considerably with geographic region and a recent paper found a high prevalence of infection with zoonotic strains in asymptomatic dogs from the western US (Covacin et al, 2011).  However, our understanding of the natural history of Giardia is still quite limited and the actual risk presented by contact with these infected animals is unknown.  Because of our limited understanding, it is reasonable to treat asymptomatic animals if Giardia is detected.

References

Covacin C, Aucoin DP, Elliot A, Thompson RCA. 2011.  Genotypic characterization of Giardia from domestic dogs in the USA.  Vet. Parasitology. 177:28-32.

Gauthier JL, Richardson DJ.  2002.  Knowledge and attitudes about zoonotic helminths:  a survey of Connecticut pediatricians and veterinarians.  Comp Con Ed Prac Vet.  24 (5A) 4-9.

Gavin PJ, Kazacos KR, Shulman ST.  2005.  Baylisascariasis.  Clin Micro Rev.  18:703-718.

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.

Walsh MG. 2011.  Toxocara infection and diminished lung function in a nationally representative sample from the United States population.  Int J. Parasitol. 41:243-247.