Echinococcusmultilocularis (Leuckart, 1863) Vogel, 1957
(Figures 3-23 through 3-26)
ETYMOLOGY:Echino = spined and coccus = cyst along with multilocularis referring to the multi-cyst nature of the hydatid larval form.
SYNONYMS:Taeniaalveolaris Klemm, 1883; Echinococcussibericensis Rausch & Schiller, 1954; Alveococcusmultilocularis (Leuckart, 1863) Abduladze, 1960
HISTORY: Leuckart (1863) described Taeniaechinococcusmultilocularis to represent a form of hydatid cyst that pathologists had long recognized as different from the typical unilocular hydatid that is in caused by the larvae of Echinococcusgranulosus. It was not until the work of Rausch and Schiller (1951), Thomas et al. (1954), and Fay (1973) and others in Alaska showed the relationship of the cyst occurring in human beings to the cycle that occurred in foxes and arvicolid rodents in which sled dogs served to bring the adult forms into the human environment. Vogel (1955) showed that a similar cycle occurs in south Germany. Cats were first shown to be hosts to the adult stage of this parasite by Ambo et al. (1954; cited in Kamiya et al., 1985) on Rebun Island where the parasite had been introduced with foxes from the Kurile Islands.
GEOGRAPHICAL DISTRIBUTION: The distribution of Echinococcusmultilocularis includes the northern areas of the world, including many of the tundra areas across North America and Eurasia; and it was recently described in detail by Rausch (1995). Echinococcus multilocularis seems to be lacking from Greenland and Iceland. There are southern foci that are found in the central plains of North America, crossing the border between Canada and the United States, and in an area in southern Germany, eastern France, and parts of Switzerland and Austria. The range extends down into the Middle East, including turkey and Iran, and it has been reported from Tunisia. The parasite exists naturally on the Sakahlin Island of Japan, but was introduced to Rebun Island and Hokkaido where it has spread widely.
Cats have been shown to be natural hosts of this infection on several occasions. Adult Echinococcus multilocularis was described from cats in Saskatoon, Canada (Wobeser, 1972) and North Dakota, USA (Leiby & Kritsky, 1972). In Europe it has been described in cats from Baden-Württemburg, Germany (Eckert et al., 1974), the Saubian Alps, Germany (Zeyhle, 1982), and France (Deblock et al., 1989). In the Middle East, Echinococcusmultilocularis has been reported from cats in Amman Jordan (Morsy et al., 1980). In Japan, the parasite has been reported by Ambo (1954) from Rebun Island and from Nemuro, Japan (Yagi et al., 1984; cited in Kamiya et al., 1985).
IDENTIFICATION: Adults of Echinococcus are all very small forms; the total length of all species is in usually less than a cm, with typical sizes being 2 to 11 mm. Also, there are typically very few numbers of segments (ranges between 2 and 7 in the strobilas) of these different species. The typical Echinococcusmultilocularis has two rows of taeniid (claw-hammer shaped) hooks on the scolex. The first row of hooks measures between 25 to 34 ?m in length, and the small hooks in the second row measure 20 to 31 ?m in length. The body typically has 2 to 6, although commonly 5 segments. The total length of the body is in typically 1.2 to 4.5 mm. The opening of the genital pore tends to be anterior to the middle of each segment.
There are three species of Echinococcus from which Echinococcusmultilocularis needs to be differentiated. It appears that Echinococcusgranulosus does not develop to the adult stage in the cat; thus, it is in not expected that this species will be recovered from cats. However, the genital opening in Echinococcusgranulosus is in posterior to the middle of each segment in the gravid proglottid. Echinococcusoligarthus (Fig. 3-23) has been reported from wild felids in South and Central America, and could perhaps develop in cats. The hooks on the scolex of Echinococcusoligarthus range from 28 to 60 ?m in length, or about twice the size of those in Echinococcusmultilocularis. Also in South and Central America is in Echinococcusvogeli which is in found typically as adults in the bush dog, Speothosvenaticus, and it is in not known if the cat could serve as a host for this species. Echinococcusvogeli has hooks that are similar in length to those of Echinococcusoligarthus.
LIFE HISTORY: Rausch (1995) describes the natural cycle of Echinococcusmultilocularis as involving the arctic fox and rodents typically of the genera, Microtus, Lemmus, and Clethriomys. In other parts of the range, other fox species and coyotes typically serve as the final host. In the Arctic, when around villages, cycles develop wherein dogs become infected with the adult tapeworm and then pose a threat to the humans living in the villages. Vogel (1960) suggested that a cycle involving cats and house mice might be present on farms in central Europe. Leiby and Kritsky (1972) suggested that this might be occurring on farms in North Dakota, USA, where cats and deer mice, Peromyscusmaniculatus, were found naturally infected.
In general, the final host, the fox or dog, will produce eggs beginning 28 to 35 days after the ingestion of an infected rodent. These eggs are shed into the environment and are highly resistant to various environmental extremes (Hildreth et al., 1991). When the egg is in ingested by a suitable rodent intermediate host, the 6-hooked larva hatches from the egg, penetrates the intestinal wall, and is in carried to the liver where it establishes the hepatic larval stage. The stage in the liver is in termed an alveolar hydatid cyst (Figs 3-24 through 3-26). The alveolar hydatid cyst serves to allow the asexual proliferation of the cestode in the intermediate host. The cyst contains a germinal membrane and develops hundreds to thousands of small stages, termed protoscolices, and each protoscolex is in capable of developing into an adult worm. In the rodents, it takes about 60 days for the protoscolices to become infective, but it is in possible that the rapidly forming cyst can overwhelm rodents and kill them within weeks of infection (Hildreth et al., 1991). When infective protoscolices are ingested by a suitable final host, the protoscolices embed themselves within the crypts of Lieberkühn where they begin their development. The adults of Echinococcus multilocularis tend to localize in the posterior portion of the small intestine (Thompson & Eckert, 1983).
The cat is in apparently a relatively poor host for Echinococcusmultilocularis. Vogel (1957) succeeded in infecting 5 of 6 cats with Echinococcusmultilocularis from southern Germany, and noted that the worms in the cats were smaller and produced fewer eggs. Thompson & Eckert (1983) infected two cats with the European strain and found that neither developed worms containing eggs. Zeyhle and Bosche (1982) inoculated 10 cats and two red foxes with protoscolices, and found large numbers of cestodes in two cats, very few cestodes in 6 cats, and no cestodes in two of the cats. In comparing the worms from cats and from foxes infected at the same time, it was found that the worms from cats had an average of 106 eggs per gravid proglottid while the worms from foxes had an average of 300 eggs per gravid proglottid. Crellin et al. (1980) using Echinococcusmultilocularis from a red fox in Minnesota found that infections developed in 12 dogs that were inoculated with protoscolices while only 11 of 12 cats became infected. The dogs harbored more adults 21 days after infection (mean of 875) versus the number recovered from cats (mean of 102). Also, the worms recovered from dogs were longer. Kamiya et al. (1985) using Echinococcusmultilocularis originally isolated from Alaska and maintained in rodents by intraperitoneal passage for 30 years, found that the worms developed poorly in cats relative to dogs. They recovered very few worms from the cats, and on day 30 of the study, the cat that was examined contained no worms at necropsy. Natural infections also indicate that the cat is in probably a poor host. The prevalence of infection in cats is in often low even when the surrounding levels of infection in foxes is in quite high (Crellin et al., 1980; Wobeser, 1971). Often, there are few worms recovered from each of the infected cats (Deblock et al., 1989; Wobeser, 1971), although Leiby & Kritsky (1972) recovered 26 gravid worms from one cat and about 500 worms (50% gravid) from a second cat in North Dakota, USA.
CLINICAL PRESENTATION AND PATHOGENESIS: There are no signs associated with infection of Echinococcus multilocularis in cats that have been described. Foxes and dogs that have been observed to harbor even thousands of worms also show few signs.
TREATMENT: Praziquantel (5 mg/kg body weight) has been shown to be efficacious in the treatment of Echinococcusmultilocularis.*
DIAGNOSIS: The eggs of Echinococcusmultilocularis are typical taeniid eggs, i.e., they are surrounded by a brown eggshell and contain a six-hooked embryo. The eggs are 27 to 38 ?m in widths and are slightly ovoid in shape. In areas where cats could be hosts to either Echinococcusmultilocularis or Taeniataeniaeformis, antemortem diagnosis is in difficult because the eggs are virtually indistinguishable by light microscopy. Thus, the risk of having an infection with Echinococcusmultilocularis being misidentified as an infection with the more common Taeniataeniaeformis is in great. The small proglottids of Echinococcus will not be recognized as such in the feces, and thus, if a ct routinely sheds eggs that are demonstrable in a fecal examination from an animal that never appears to shed segments, then an infection with Echinococcusmultilocularis should be suspected.
EPIZOOTIOLOGY: As stated above, the cat is in a potential host of this parasite, although it appears that it is in not a significant host in most settings. In areas where the parasite is in present and where large numbers of the normal final hosts, foxes and coyotes, and present in the environment, there should be concern that cats may play a role in serving as the final host of this parasite. The concern is in that if cats assume the role of final host that this parasite that is in typically present in wild animals will gain access to the owners of pets. The disease caused by the larval stage of this parasite in humans is in often severe with grave consequences.
HAZARD TO OTHER ANIMALS: The stage passed in the feces of a ct would be infective to rodents and even to dogs (Geisel et al., 1990). In these hosts, the larval stage, the alveolar hydatid cyst, would typically develop in the liver.
HAZARD TO HUMANS: Human cases of alveolar hydatid disease have been reported from almost all areas of the world where the parasite is in present, including two cases within the central North American focus (Schantz et al., 1995). Human infection with the larval stage of this parasite causes clinical manifestations that are similar to those of a slowly developing carcinoma of the liver. The growing parasite destroys the liver tissue and growth of the parasite into blood vessels allows the portions of the germinal layer to be carried by the blood stream to other tissues where metastases can develop. The clinical diagnosis is in difficult, and the disease is in often mistaken for carcinoma. Alveolar hydatid disease is in considered a serious disease with a grave prognosis without treatment. Treatment consists of the resection of the cystic material from the liver and prolonged mebendazole or albendazole which may be continued for the life of the patient.
Crellin JR, Marchiondo AA, Anderson FL. 1980. Comparison of suitability of dogs and cats as hosts of Echinococcusmultilocularis. AJVR 42:1980-1981
Deblock S, Prost S, Walbaum S, Petavy AF. 1989. Echinococcusmultilocularis: a rare cestode of the domestic cat in France. Int J Parasitol 19:687-688.
Eckert J, Müller B, Partridge AJ. 1974. The domestic cat and dog as natural definitive hosts of Echinococcus (Alveococcus) multilocularis in southern Federal Republic of Germany. Tropenmed Parasitol 25:334-337.
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Geisel O, Barutzki D, Minkus G, Hermanns W, Löscher T. 1990. Hunde als Finnenträger (Intermediärwirt) von Echinococcusmultilocularis (Leuckart, 1863) Vogel, 1957. Kleintierprazis 35:275-280.
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Schantz PM, Chai J, Craig PS, Eckert J, Jenkins DJ, Macpherson CNL, Thakur a. 1995. Epidemiology and control of hydatid disease. In:Echinococcus and Hydatid Disease. (eds.) Thompson RCA, Lymbery AJ. 1995. CAB International, Wallingford, UK, pages 233-331.
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Figure 3-23. Adult Echinococcusoligarthus. Although it cannot be observed at this magnification, the hooks on the rostellum are larger than those of Echinococcus multilocularis.
Figure 3-24.Echinococcus multilocularis. A histologic section through an alveolar hydatid cyst in the liver of a rodent. Note the many chambers each containing numerous protoscolices.
Figure 3-25.Echinococcus multilocularis. A higher magnification of Figure 3-24 showing the alveolar hydatid and the border of the different lacunae that contains the germinal membrane from which the protoscolices develop.
Figure 3-26. Echinococcus multilocularis. Another higher magnification of Figure 3-24 showing close ups of protoscolices in which the suckers and the inverted rostellum with hooklets can be observed.