Alveolar Hydatid Disease in a Chipmunk
Echinococcus multilocularis is a small (3-5 mm) tapeworm found in the intestinal tract of infected canines, including foxes, coyotes and domestic dogs. As is
typical of many species of tapeworms, the life cycle includes a mature stage in the definitive carnivore host and an immature stage in an intermediate host, which is usually a prey species. Small rodents are the most common hosts for the immature stage, which takes the form of multiple, slowly growing cystic structures, each of which contain large numbers of developing tapeworms. The cysts grow slowly, and resemble a tumour, for which they may be mistaken. The cycle is completed when the carnivore final host kills and eats the infected intermediate host.
This parasite is a public health concern, due to its ability to infect people. Humans who inadvertently ingest tapeworm eggs from carnivore feces may become infected with the intermediate stage of the tapeworm, producing the same slowly-growing, multicystic masses that come to occupy a large part of the abdominal cavity, in the condition known as alveolar hydatid disease.
Parasite only recently discovered in Southern Ontario
There are parts of the world in which the tapeworm is well established. In Central Europe, the definitive host is the red fox, a species that commonly occurs in and around urban areas. The range of the parasite in Europe is expanding, with recent invasion of parts of Scandinavia. In North America, the tapeworm has long been known to occur in the arctic, and in the northern plains including the southern parts of the three Canadian prairie provinces, with eastward extension into the Midwestern states as far as Michigan. Foxes and coyotes are the common hosts. The tapeworm was not known to occur in southern Ontario or other parts of eastern North America until quite recently.
Over the past 4 years, a small number of dogs (5) have been diagnosed with infection with E. multilocularis. Somewhat unusually, they have all been found with the intermediate stage of the tapeworm, although dogs are also capable of being definitive hosts. These dogs came to veterinary care because of the presence of large, space-occupying masses in their abdomens, due to the growth of alveolar hydatid cysts. The dogs presumably were infected through the consumption of tapeworm eggs in the feces of an infected canine, such as another domestic dog or perhaps an infected coyote or fox. The concern is that the natural cycle of this parasite has become established in Ontario wildlife, with infected wild canines and intermediate rodent hosts.
Monitoring of tapeworms in wild carnivores has begun
Working in collaboration with CWHC in Ontario, Drs. Andrew Peregrine and Claire Jardine and graduate student Jonathan Kotwa in the Department of Pathobiology at the Ontario Veterinary College have begun an investigation of the occurrence of the tapeworm in wild carnivores,. Hunter and trapper-harvested fox and coyote carcasses have been collected and sampled for the presence of adult tapeworms. Preliminary results from the first year of monitoring should soon be available.
Looking for the intermediate stage of the parasite in infected wild rodents is likely a less productive method of finding the parasite in the environment. Many more rodents would have to be sampled in order to find an infected animal. The carnivores effectively concentrate the sample by eating numerous rodents, making themselves a better source of information.
Chipmunk with alveolar hydatid disease discovered by chance
By a stroke of good luck and the existence of the CWHC passive surveillance program for disease in wildlife, a case of alveolar hydatid disease in a wild rodent has been discovered. A veterinary technician from a southern Ontario veterinary clinic was walking her dog in a nearby conservation area where the dog caught a chipmunk. The chipmunk’s large abdomen suggested that it was pregnant, but upon further examination, this was found to be not the case, rather the abdomen was filled with hydatid cysts. The CWHC was contacted and we collected the carcass of the chipmunk for post-mortem examination. The abdomen was filled with multicystic masses that distorted abdominal shape and pushed normal viscera out of the way. The diagnosis of alveolar hydatid disease due to infection with E. multilocularis was confirmed using histology, PCR, and genetic sequencing.
This is the first evidence of a locally-established sylvatic cycle of the tapeworm. This provides an opportunity for more focused geographic surveillance to try and find evidence of the occurrence of either the adult tapeworm in carnivores or the intermediate stage in rodents.
Veterinary and human public health measures will be required in order to prevent exposure of people to this parasite. Humans are exposed through accidental contact with the feces of infected carnivores, which makes domestic dogs a particularly likely source. Regular deworming with a tapeworm drug for dogs, particularly those that roam outdoors and hunt, is a critical part of control of this parasite. From the human side, awareness of the potential risks and the application of basic hygiene measures are all that is required.
Submitted by Doug Campbell, CWHC Ontario/Nunavut