Cestodes of the dog and cat

Cestodes (tapeworms) of the Family Taeniidae (Phylum Platyhelminthes, Class Cestoda, Subclass Eucestoda, Orders Cyclophillidea and Pseudophillidea) include various different species that are pathogenic for dogs and cats in their adult stage or both adult and larval stages; some of these (in one or both of the stages) are also pathogenic for humans and, in some cases can cause very severe diseases (Echinococcus spp.). The two Orders (Cyclophillidea and Pseudophillidea) have different biological and epidemiological characteristics as a result of their different life cycles.

CYCLOPHYLLID CESTODES

Cyclophyllid cestodes have an indirect life cycle which usually requires a specific intermediate host.

Infested dogs and cats shed proglottids into the environment which, assumed by the intermediate host, evolve into the cystic infestant larval stage. The infestation is acquired by dogs and cats through the intermediate host carrying the parasite.

According to various epidemiological studies, the prevalence of these parasites ranges from 4% to 60% in dogs and between 1.8% and 52.7% in cats. Data from studies based exclusively on flotation copromicroscopic studies tend to underestimate the prevalence because the proglottids are eliminated locally and may not be present in some samples of faeces from infested subjects.

Dipylidium caninum

This is the most common tapeworm found in dogs and cats which are its definitive hosts (together with other wild carnivores such as the fox) and in which the adult parasite localises in the small bowel. There are reports of sporadic cases of human infestation (particularly in children) following accidental ingestion of fleas.

In fact, in animals the infestation is contracted by ingestion, during licking or grooming, of an infesting stage of D. caninum: the cysticercoid that localises in the muscle of the intermediate host, the flea (Ctenocephalides spp.), or more rarely the louse (Trichodectes canis) which feeds on the proglottids of the parasite itself. It is interesting to note that the localisation of cysticercoids in the muscles of the flea limits the intermediate host’s mobility facilitating its ingestion by the definitive host. The adult parasite is between 15 and 20 cm long and has a scolex with an armed rostrellum (7-8 rose-thorn like rings of hooks), tapered proglottids (shaped like rice grains or cucumber seeds, from which its name “cucumber tapeworm” is derived), two genital pores and a set of male and female genital organs (hermaphrodites) and a uterus divided into interconnecting chambers full of oncospheres (egg packets). The mature proglottids detach and are eliminated on the faeces or migrate actively through the anus of infected subjects, often remaining attached to the fur in the perianal region. Each proglottid contains numerous egg packets (size 200 x 120 microns) within which there are about 20 embyronated eggs with a diameter of about 25-30 microns.

By feeding on the proglottids, the larvae of fleas are infested by the emerging Dipylidium caninum hexacanth larvae which evolve within the arthropods until they reach the infesting stage of cysticercoids. The cysticercoid ingested by the definitive host reaches the adult stage in about 1 month (prepatent period).

All stages of the louse (in which the development to infesting form takes only 30 days, in that it is an obligate parasite passing its whole life on the dog) can ingest the oncospheres, while only the larvae of fleas (which only at this stage have masticatory organs) can be infested. In the flea the maturation of the oncosphere to cysticercoid is synchronous with that of the insect and can take several months (in relation to the duration of the diapause) which could account for cases of delayed re-infestation. The infestation is usually asymptomatic, but sometimes gastrointestinal symptoms (severe diarrhoea) and pruritus in the area around the anus may occur in cases of massive infestation.

The diagnosis is made from direct macroscopic observation of the proglottids on the faeces or adhered to the fur of the affected subjects; much more rarely, it is made from observation of the egg packets during copromicroscopic examination following enrichment techniques such as flotation.

During abdominal ultrasonography, the adult Dipylidium caninum (like other cestodes) can be seen as intraluminal, moderately hyperechoic, linear structures, which do not have the double-walled appearance typical of nematodes (Dirofilaria immitis, ascarids) but which must be differentiated from linear foreign bodies causing gastrointestinal disorders. These latter always have a shadow or “comet tail” (because of their greater acoustic impedance).

Treatment
The treatment is based on administration of praziquantel at a dose of 5 mg/kg orally or parenterally, or in a spot-on formulation for cats (12 mg/kg). Nitroscanate (50 mg/kg) and niclosamide (recommended dose per os: 160 mg/kg in the cat and 150 mg/kg in the dog) are also effective. Mebendazole, administered for several consecutive days at the various doses reported in the literature (e.g. 40 mg/kg per os for 3 consecutive days), is not consistently effective and is inadequate for the treatment of Dipylidium caninum (although it is effective against other cestodes). A plan to control the intermediate hosts (fleas) is essential for the control of the parasite and prevention of re-infestations.

Echinococcuss pp.

As far as concerns public health, the most important cestodes are definitely those of the genus Echinococcus (from the Greek: Echínos = spiny; cóccos = berry) given the frequency and severity of the anthropozoonosis. This genus includes two species: Echinococcus granulosus and Echinococcus multilocularis which cause, respectively, cystic echinococcosis (hydatidosis) and alveolar echinococcosis.

Cystic echinococcosis is one of the main parasite infections of livestock and is considered an extremely important indirectly transmitted parasitic anthropozoonosis.

Echinococcus granulosus

The adult parasite is from 3 to 9 mm long and is composed of a scolex at its head equipped with four suckers and a central rostrum, at the tip of the scolex, to which 28 to 50 hooks and a body (strobila) normally consisting of three or four proglottids are attached. The proglottids closest to the neck are sexually immature; these are followed by mature proglottids (containing reproductive organs) and then pregnant ones (in which eggs are present). Each proglottid is a hermaphrodite, being equipped with both male and female genitalia and has a single genital pore. The eggs are difficult to distinguish from those of other species of taeniae and measure 32 to 36 microns.

The diagnosis is also difficult because the proglottids are small (2-3 mm) and expelled intermittently. The faecal examination must, therefore, be associated with more specific techniques such as a search for coproantigens (by an enzyme-linked immunosorbent assay) or biomolecular studies (polymerase chain reaction) followed by sequencing to determine the genotype.

The oncospheres survive in the external environment, remaining viable and infestant, for about 2 years.

Dogs and other definitive hosts infested by Echinococcus granulosus are almost always asymptomatic, even in the presence of thousands of cestodes.

In intermediate hosts (e.g. sheep, goats, cattle, buffaloes, swine), hydatid disease may remain subclinical for a long time, particularly if the sites involved are the liver, lungs or kidneys, while clinical manifestations can be seen in the case of cysts in the bones or central nervous system.

In humans (who are infested by eating fruit or vegetables contaminated by faeces or accidental ingestion of oncospheres after contact with soil), the infestation is always symptomatic and the severity of the disease is related to the number and sites of the hydatids and the possibility of their rupture (spontaneous or during surgical removal) with consequent severe anaphylactic reactions.

Treatment and prophylaxis

The prophylaxis of this extremely important anthropozoonosis is based predominantly on health education and hygiene rules aimed at preventing dogs and canids from eating the viscera of intermediate hosts, particularly in abattoirs (and above all in the context of domestic slaughtering), ensuring that all parasitized viscera, the only source of infestation for the definitive host, are confiscated and destroyed. For this reason veterinary police forbid dogs from entering slaughterhouses.

It is sound practice not to give raw viscera to dogs even if the provenance of the offal is known. Furthermore, in endemic areas, dogs should be treated periodically with specific antihelminthics. The treatment should be given after the dog has been confined to a kennel so that all the faeces produced in the 48 hours after the treatment can be destroyed, thus preventing the eggs from contaminating the environment (the antihelminthics used to do not kill the eggs).

As far as concerns humans, it is good practice not to eat unwashed or poorly washed raw vegetables and to wash the hands after having touched soil or other substances potentially contaminated by eggs of Echinococcus granulosus excreted with the faeces of dogs.

Isoquinolines (praziquantel and epsiprantel) are the drugs of first choice for the treatment of dogs. Other compounds such as nitroscanate and various benzimidazoles are partially effective against Echinococcus granulosus, but none is as efficient as the isoquinolines.

Mesocestoides spp.

The adult stage of cestodes belonging to this genus (e.g. Mesocestoides lineatus, Mesocestoides corti) infests dogs, cats and wild carnivores in Europe, North America and Asia. The life-cycle of Mesocestoides is not yet completely clear. Unlike the other Cyclophillidea it requires two intermediate hosts and a definitive host. The first intermediate host is presumed to be the oribatid mite which directly ingests the eggs expelled with the faeces of the definitive host. The second infestant stage (tetrathyridium) can be present in numerous hosts, such as small reptiles, amphibians, birds and rodents. The tetrathyridia invade the peritoneal cavity or coelum, multiplying by budding or binary fission, and can be present in both solid and cystic (acephalic) forms. Ingested by the definitive host, the tetrathyridia mature in the small bowel of this latter until the adult stage.

Dogs and cats (and, sporadically, humans) are definitive hosts of the parasite, but can sometimes act as second intermediate hosts because of the accidental ingestion of oribatid mites in the ground (e.g. by truffle hounds) and develop so-called peritoneal cestodiasis due to localisation of the larvae on the peritoneal surface or, much more rarely, also on the pleural surface. In some cases, however, the peritoneal cavity can be invaded by perforation of the gut by tetrathyridia which have developed from the adult. This is a particularly serious clinical condition and is probably underdiagnosed. While intestinal infestation by the adult parasite rarely causes gastrointestinal symptoms (mild diarrhoea), most dogs with peritoneal cestodiasis have anorexia, altered appetite, abdominal distension and lethargy.

Tetrathyridia in the dog are usually the acephalic (cystic) form, while only solid tetrathyridia are present in the cat.

The diagnosis is made mainly on the basis of findings of an ultrasound examination, which readily shows the cystic forms in the dog; the tetrathyridia are often (but not always) associated with a peritoneal effusion. In the cat the finding is usually incidental, during abdominal laparotomy for surgical interventions, but also the solid forms of tetrathyridia can be visualised by ultrasonography in this species. Cats do not usually develop a peritoneal effusion

The fluid collection in the dog has the characteristics of an exudate. Macroscopically, the fluid is rarely limpid but rather it is usually brownish because of the tissue debris suspended in it. Cytological analysis of the abdominal fluid reveals numerous calcareous bodies (rounded or oval, pale yellow structures) typical of infestation by Mesocestoides spp.

Polymerase chain reaction analyses of the effusion (and/or of the fluid present in the cysts, aspirated by ultrasound-guided needle biopsy) enables the definitive diagnosis to be made.

Treatment
The treatment is extremely difficult and the mortality rate of dogs affected by peritoneal cestodiasis is very high. Only fenbendazole, administered orally at a dose of 100 mg/kg twice daily for a prolonged period (up to 9 months) and with constant monitoring, has been demonstrated to be able to provide a parasitological cure. In the case in which the abdominal fluid is septic (or there are clinical signs of sepsis), concomitant antibiotic treatment is indicated.

The prevention of peritoneal cestodiasis in cats is based entirely on reducing or eliminating predatory behaviour; dogs should be dissuaded from digging holes, or at any rate from ingesting earth or soil.

PSEUDOPHYLLID CESTODES

Pseudophyllid cestodes have an indirect life cycle that requires two intermediate hosts.

The adults live in the small bowel of dogs and cats, releasing operculated eggs into the environment which transform into the ciliated larval stage (coracidium) able to infest the first intermediate host, a copepod (small crustacean).

The coracidia present in the copepod, ingested by a secondary intermediate host (rodents, small reptiles, amphibians), transform into infesting larvae (plerocercoids) able to evolve into the adult stage. Dogs and cats are infested by eating the second intermediate host (subjects with predatory behaviour are at higher risk of infestation).

Spirometra spp.

This is definitely the most commonly found cestode of the Order Pseudophyllidea found in dogs and, in particular, in cats in Italy. The adult stage of the parasite (25-75 cm long) localises in the small bowel of dogs and cats following ingestion of the second intermediate host (rodents, small reptiles, amphibians, birds) which contain the infestant second larval stage (plerocercoid). The prepatent period is about 10-20 days (following which eggs begin to be excreted with the faeces).

The infestation can be asymptomatic, but not uncommonly the parasitized subject has diarrhoea, vomiting, loss of weight and a general decline in condition.

The diagnosis is made from the copromicroscopic finding in the faeces of the characteristic operculated eggs (morphologically very similar to the eggs of Diphyllobotrium latum or of Trematodes) sized 57-72 x 30-37 µm or from the finding of fragments of the adult parasite in vomit or in the faeces. The adults can be identified by the presence of a distinctive medial genital pore.

As for D. caninum, the adult parasites can be seen by abdominal ultrasound examination.

Treatment and prophylaxis

Prophylaxis in cats and dogs is based on limiting predatory behaviour. The treatment is oral administration of praziquantel as a single high dose (30 mg/kg) in the cat and at a dose of 7.5 mg/kg for two consecutive days in the dog.

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