Tapeworms in all the wrong places: some unusual presentations of common parasites (Proceedings)

Raw food diets – when will we learn that truly raw foods are the pathway for disaster? In addition to bacterial problems, most tapeworm infections in dogs and cats are a result of eating raw food (prey) or ingesting untreated water. Pets should be fed cooked or prepared food and provided with fresh, potable water.

All tapeworms of dogs and cats have an indirect life cycle. The definitive host is the one in which the tapeworm matures, reproduces and generates eggs. The intermediate host is the host 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.

Taenia

There are over 60 species of Taenia worldwide, many of which use canid and felid definitive hosts. Within the US, more species tend to occur in domestic dogs than domestic cats (Table 1).

Table 1. Some species of Taenia in domestic dogs and cats.

The 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 as it sits on the fecal. 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 5-12 weeks, depending on species. How long untreated tapeworms will survive is not known for sure but, T. taeniaeformis has been reported to remain patent in cats for as long as 34 months.

Intestinal infections tend to not be pathogenic. Infections are generally diagnosed based on finding proglottids in the feces although eggs can be found on fecal flotation if the proper specific gravity is used. Praziquantel, epsiprantel, and fenbendazole are approved for the treatment of Taenia infections in dogs and cats.

Occasionally, dogs and cats develop cysticercosis as a result of infection with T. crassiceps. The metacestode of this species of tapeworm is unique in that it is prolferative cysticercus that develops asexually through budding. Consequently, ingestion of 1 or a few organisms can result in massive infections. Intraperitoneal, intrapleural, intracardiac, intracranial and subcutaneous cystcercosis have been documented, most with fatal results. Why these animals develop cysticercosis is uncertain; however, an impaired immune system is thought to play a key role. 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 autoinfection, are considered the most likely source.

Cysticercosis associated with T. crassiceps occurs in people as well. Although the source of infection (wild vs domestic canid) is usually not known, at least one case was linked to the family dog.

Dogs can also develop cysticercosis as a result of infection with T. solium. Although uncommon any more in the United States, this parasite is responsible for cerebral cysticercosis in humans in many areas of the world. In these same areas, dogs also can 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 euthansia. While canine neurocysticercosis due to T. solium has not been weel recognized in the US, it does exist in Mexico where many rescue groups travel to bring dogs back to the US.

Spirometra

As with other tapeworms, the life cycle of Spirometra species is indirect. However, 2 intermediate hosts are needed rather than just 1. Mature tapeworms live in the small intestine of the definitive host; eggs are released from the proglottid and pass in the feces to the environment. If deposited or washed into water, the egg hatches and the ciliated form is eaten by a freshwater copepod crustacean. The tapeworm develops into the procercoid, which is the infective form for the next intermediate host. The second intermediate host range is quite diverse and includes most vertebrates except fish. Once ingested by the vertebrate, the tapeworm develops a little more into another form (plerocercoid – also called a sparganum), which is now infective to the definitive host. Conversely, if the second intermediate host is eaten by another non-fish intermediate host, the sparganum remains viable and migrates back into tissues. Within the definitive host, the sparganum remains in the small intestine where it attaches to the mucosa and matures. The prepatent period is as little as 10 days.

Intestinal infections tend to cause little problems. Intermittent diarrhea or vomiting may occur. Owners may not associate these problems with tapeworm infections unless long chains of senile proglottids are found. Praziquantel is the treatment of choice for this parasite.

Dogs can also act as second intermediate hosts if they drink water with infected copepods. In these cases, the spargana develop within the tissues of the dog. Sparganosis can be proliferative or non-proliferative. Most infections are nonproliferative such that ingestion of a single procercoid results in the development of a single plerocercoid. In proliferative sparganosis, however, ingestion of a single procercoid can result in numerous spargana as a result of the asexual replication of the larvae. The larvae grow and repeat the process, which can ultimately lead to death of the host.

There is no approved treatment for sparganosis in our domestic animals. Various anthelmintics have been tried, most with poor results. The lack of treatment options for proliferative sparganosis generally warrents a poor prognosis for survival.

Mesocestoides

Although Mesocestoides first were described in 1863, we still do not know the complete life history of this parasite. Species of Mesocestoides occur worldwide. Mature tapeworms live in the small intestine of a variety of birds and mammals, including domestic dogs and cats. It appears that some type of coprophagous invertebrate is the most likely first intermediate host. The typical metacestode stage, the tetrathyridium, occurs within the second intermdiate host, which includes amphibians, reptiles, birds and mammals. These tetrathyridia tend to localize within the peritoneal cavity, lungs, and liver. When eaten by the definitive host, they mature in 16-21 days. Certain strains of the parasite can asexually reproduce within the intestinal tract of the definitive host as well as within the tissues of the intermediate host. One study showed ingestion of 500 tetrahtyridia could result in as many as 17,000 mature tapeworms in the dog.

Intestinal infections in the definitive host are not usually associated with clinical signs. However, dogs (and cats) can also serve as second intermediate hosts and infection can lead to a condition called Canine Peritoneal Larval Cestodiasis (CPLC). Clinical signs of CPLC range from none to abdominal enlargement, ascites, anorexia, vomiting and peritonitis. Clinical signs are noted to be present for as long as a year before medical intervention was sought.

Benzimidazoles have been used in treating the metacestode infections, but, progress is slow and recurrence is common; thus, life long treatment may be warrentd. Fenbendazole and praziquantel have also been used to treat mature, intestinal infections with good results.