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Cestode Zoonoses - SPARGANOSIS
The second larval stage (plerocercoid or sparganum) of the pseudophyllidean cestode of the genus Spirometra (Diphyllobothrium, Lueheela). Several species of medical interest have been described: Spirometra mansoni, S. mansonoides, S. erinacei-europaei, S. theileri, and S. proliferum. These are the most commonly accepted species at the present time, but it should be noted that they are difficult to differentiate and that the taxonomy remains in doubt. The definitive hosts are mainly domestic and wild canids and felids. The development cycle requires two intermediate hosts. The first is a copepod (planktonic crustacean) of the genus Cyclops. which ingests coracidia (free, ciliated embryos) that develop from Spirometra eggs when they reach the water with the feces of dogs or cats (definitive hosts). In the tissues of the copepod, the coracidium turns into the first larva, or procercoid. When a second intermediate host ingests an infected copepod, the procercoid develops into a second larval form, the plerocercoid or sparganum. The plerocercoid larva can be harbored by many vertebrates, including amphibians, reptiles, birds, small mammals (rodents and insectivores), man, nonhuman primates, and swine. Fish do not become infected. Some researchers believe that the second intermediate host is usually an amphibian, but can vary according to region. Numerous species of vertebrates become infected with plerocercoids by feeding on amphibians, but they may also develop plerocercoids after ingesting water containing copepods infected by procercoids (first larva). Several animal species that are not definitive hosts function as paratenic or transport hosts, since the larvae they acquire by feeding on animals infected with plerocercoids encyst again after passing through the intestinal wall and migrating to other tissues. This transfer process is undoubtedly important in the life cycle; but the fact that many species of secondary hosts can be infected directly by ingestion of copepods containing procercoids is probably no less important. When the sparganum reaches the intestine of the definitive host, it attaches to the mucosa; in 10 to 30 days, it matures into an adult cestode, completing the cycle. The adult S. mansonoides reaches about 25 cm in length in the intestine of the definitive hosts (cat, dog). The sparganum found in the tissues of the secondary intermediate hosts and paratenic hosts, including man, varies from 4 to 10 cm in length.
GEOGRAPHIC DISTRIBUTION AND OCCURRENCE:
Sparganosis is found throughout the world, but human infection is not common. Just over 450 cases are known, mostly from Japan, China, Korea, and Southeast Asia. In the United States, about 60 cases have been described# in Latin America and the Caribbean, the disease has been recorded in Uruguay, Ecuador, Colombia, Venezuela, Guyana, Belize, and Puerto Rico; about 30 cases have been diagnosed in Africa. Infections in the Far East are attributed to plerocercoid larvae of Spirometra mansoni: in the United States, to S. mansonoides; in Europe, to S. erinacei-europaei; and in Africa, to S. theileri. However, as has already been mentioned, species identification can be difficult and therefore uncertain.
THE DISEASE IN MAN:
The incubation period, determined in a study of 10 patients who ate raw frog meat, lasts from 20 days to 14 months. The most common localizations of the sparganum are subcutaneous connective tissue and superficial muscles. The lesion is nodular, develops slowly, and can be found on any part of the body. The main symptom is pruritus, sometimes accompanied by urticaria. The lesion is painful when there is inflammation. The patient may feel discomfort when the larva migrates from one location to another. The subcutaneous lesion resembles a lipoma, fibroma, or sebaceous cyst. Ocular sparganosis occurs mainly in Vietnam, Thailand, and parts of China. Its main symptoms consist of a painful edema of the eyelids, with lacrimation and pruritus. A nodule measuring I to 3 cm forms after 3 to 5 months, usually on the upper eyelid. Migration of the sparganum to internal organs can give rise to the visceral form of the disease. The preferred localizations are the intestinal wall, perirenal fat, and mesentery; vital organs are rarely affected. When the plerocercoid invades the lymphatic system, it produces a clinical picture similar to that of Eosinophils are abundant in the was near the parasite; examination of blood samples reveals mild leukocytosis and increased eosinophilia. An infrequent but serious form is proliferative sparganosis caused by Spirometra proliferum. The sparganum of S. proliferum is pleomorphic, with irregular branches and proliferative buds that detach from the larva and migrate to different tissues in the host, where they repeat the process and invade other organs. The life cycle of S. proliferum is not known.
THE DISEASE IN ANIMALS:
The adult cestode, which lodges in the intestine of the definitive host, generally does not affect the health of the animal. In cats, however, it may produce weight loss, irritability, and emaciation, together with an abnormal or exaggerated appetite. Infection by the larvae or spargana can be clinically apparent when their number is large and especially when they invade vital organs. In the intermediate host, the disease is almost always asymptomatic if the number of parasites is relatively small.
SOURCE OF INFECTION AND MODE OF TRANSMISSION:
Sparganosis is maintained in nature primarily by contamination of natural or artificial bodies of water (lagoons, marshes, lakes, and others) with feces from felids and canids infected with Spirometra spp. Contamination of water with eggs of Spirometra spp. leads to the infection of copepods and, consequently, of the second intermediate hosts that ingest these crustaceans. An important means of infection is the transfer of the second larva (sparganum, plerocercoid) from one secondary host to another, which increases the number of animal species and individuals infected. The common route of infection is ingestion; various mammal and bird species become infected by feeding on parasitized frogs or snakes. The high rate of infection in wild pigs in Australia may be due to this mechanism, although it may also stem from ingestion of copepods with drinking-water from lagoons. In any case, contamination of the water by wild canids (definitive hosts) that share the habitat assures that the cycle is perpetuated. The infection rate in man is low, compared to the rate in other animals. Man acquires sparganosis mainly by ingesting larvae contained in raw or undercooked meat of animals infected with spargana, such as amphibians, reptiles, birds, and wild mammals. Another mode of infection, also by larval transfer, is by contact. In Vietnam and Thailand, frogs are popularly believe to have an antiphlogistic effect, and their muscles are applied as poultices. This custom is responsible for ocular sparganosis. It is also probable that man can acquire sparganosis via drinking-water by ingesting copepods infected with procercoids (first larvae). Man is an accidental host and does not usually play any role in the life cycle of the parasite. However, under ecologic conditions in some regions of central Africa, it is suspected that man acts as an intermediate host. In this region, hyenas are the definitive hosts of Spirometra, and man is apparently the only host infected with spargana. In these circumstances, the infection cycle is maintained as a result of tribal custom of letting hyenas devour human corpses.
Specific diagnosis can be made only by removing the nodular lesion and confirming the presence of the plerocercoid. Attempts have been made to identify the species of Spirometra larvae by infecting dogs and cats via the digestive route. For reasons already mentioned, differentiation of species has proven difficult. Diagnosis in definitive hosts infected with adult cestodes can be made by coprologic examination or autopsy.
Human sparganosis can be prevented by avoiding ingestion of contaminated water that has not been treated, and by making sure that meat that might contain spargana is sufficiently cooked. In the Far East, public health education should emphasize the danger of using the tissue of frogs or other cold-blooded animals for medicinal purposes.