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.
نویسنده: دکتر مهدی مسیبی(دکتری تخصصی انگل شناسی پزشکی)
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