All tapeworms of dogs and cats have an indirect life cycle. The definitive host is the host in which the tapeworm matures, reproduces, and generates eggs. The intermediate host is the one in which the metacestode (immature) form of the parasite develops. The definitive host becomes infected by eating the intermediate host, while the intermediate host becomes infected by eating eggs in an environment contaminated by the definitive host. In general, dogs and cats serve as definitive hosts, although metacestodes can sometimes develop within them.
Lora R. Ballweber, DVM, MS
TAENIA SPECIES: CYSTICERCOSIS
More than 60 species of Taenia species exist worldwide, many of which have canid and felid definitive hosts. For most Taenia species in the United States, the domestic definitive hosts are dogs and cats (Table 1).
Taenia species life cycles are fairly straightforward. Gravid proglottids break free of the tapeworm and are shed in the feces. Eggs are released from the segment either as it travels through the digestive tract or after it is eliminated in the feces. These eggs are then ingested by the intermediate host, the embryo hatches and migrates to its developmental site, and the metacestode develops. When the intermediate host is ingested by the definitive host, the metacestode is digested free, the scolex embeds itself in the mucosa of the small intestine, and the neck begins to grow, forming proglottids. The prepatent period is usually reported as five to 12 weeks, depending on species. How long untreated tapeworms will survive is not known for sure, but Taenia taeniaeformis has been reported to remain patent in cats for as long as 34 months.
Intestinal infections tend to be nonpathogenic. Infections are generally diagnosed based on finding proglottids in the feces, although eggs can be found on fecal flotation if a solution with the proper specific gravity is used. Praziquantel, epsiprantel, and fenbendazole are approved for the treatment of Taenia species infections in dogs and cats.
Occasionally, however, dogs and cats develop cysticercosis as a result of infection with Taenia crassiceps.1,2 The metacestode of this species of tapeworm is unique in that it is a proliferative cysticercus that develops asexually through budding (Figure 1). Consequently, ingestion of one organism or a few organisms can result in massive infections. In dogs, intraperitoneal, intrapleural, intracardiac, intracranial (also in cats), and subcutaneous (also in cats) cystcercosis have been documented, most with fatal results.1-3 Why these animals develop cysticercosis is uncertain; however, an impaired immune system is thought to play a key role.2-4 The route of infection is also speculative. Ingestion of eggs from the environment, autoinfection via eggs from gravid tapeworms within the small intestine, and ingestion of cysticerci in intermediate hosts have all been proposed. Eggs, either ingested from the environment or through autoinfection, are considered the most likely source.
Figure 1. Numerous Taenia crassiceps cyscticerci from the subcutaneous tissue of a dog.
Cysticercosis associated with T. crassiceps occurs in people as well. Although the source of infection (wild vs. domestic canid) is usually unknown, at least one case was linked to the family dog.5-8
Dogs can also develop cysticercosis as a result of infection with Taenia solium. Although now uncommon in the United States, this parasite is responsible for cerebral cysticercosis in people in many areas of the world. In these same areas, dogs can also be infected by ingesting eggs. If the cysticercus localizes to the brain, the dog can become aggressive. In developing countries where rabies is endemic, aggressive behavior is often sufficient evidence for a diagnosis of rabies, resulting in euthanasia. Therefore, dogs with cysticercosis are sometimes mistakenly thought to have rabies and are euthanized. While canine neurocysticercosis due to T. solium has not been well-recognized in the United States, it does exist in Mexico, where many rescue groups travel to bring dogs back to the United States. Current treatment recommendations include albendazole and prednisone.