Paragonimus is agenus of flukes (trematodes) and is the only genus in themonotypic familyParagonimidae. Some tens of species have been described, but they are difficult to distinguish, so it is not clear how many of the named species may besynonyms. The name Paragonimus is derived from the combination of two Greek words, “para” (on the side of) and “gonimos” (gonads or genitalia).[3] Several of the species are known aslung flukes. In humans some of the species occur aszoonoses; the term for the condition isparagonimiasis. The firstintermediate hosts ofParagonimus include at least 54 species offreshwater snails from superfamiliesCerithioidea andRissooidea.[2]
The most prominent species ofParagonimus in human medicine isParagonimus westermani, an infectious lung fluke originating in eastern Asia. Worldwide, about nine species ofParagonimus are known to cause human paragonimiasis in which many of the species reside in East Asia, West Africa, and in North and South America.[4]
Species ofParagonimus vary in size; the adult stage might attain a length of up to 15 millimetres (0.59 in) and a width of up to 8 mm (0.31 in).[5] The adult flatworm has an oval shape body with spines covering its thick tegument. Both theoral sucker andacetabulum are round and muscular. The acetabulum is slightly bigger than the oral sucker – 0.19 mm and 0.12 mm, respectively.[5]Ovaries are located behind the acetabulum and posterior to the ovary are thetestes. Theseminal receptacle, theuterus and its metraterm, the thick-walled terminal part, lie between the acetabulum and the ovary.[5]
The parasite passes through twointermediate hosts, anaquatic snail and acrustacean. It enters itsmammaliandefinitive hosts when they eat infected freshwater crustaceans. Typical hosts include dogs, cats, and humans. Humans usually contract paragonimiasis when they eat undercookedfreshwater crabs (for instance species of the genusNanhaipotamon) orcrayfish, that contain livemetacercariae. In the intestine, the parasite will move into the abdomen and commonly into the lungs. In the lung, the parasites encyst and cross fertilize each other. The cyst eventually ruptures in the lungs and the eggs may be coughed up or swallowed and excreted in the feces. An egg landing in fresh water hatches and releases a ciliated miracidium. A successful miracidium swims about until it finds an intermediate host, usually an aquatic snail. A crustacean in turn becomes infected by eating infected snails. The definitive host completes the cycle if it eats infected crustaceans.[citation needed]
Worldwide roughly 20 million people are infected withParagonimus. Human infections are most common in regions with many human and animal reservoir hosts plus an abundance of intermediate hosts, such as snails, crabs, or crayfish, and where in addition consumption of raw or undercooked seafood is common. Consumption of insufficiently cooked meat from infected land animal hosts, such as wild boar, commonly transmits the infection.[6] The domestic cat is a reservoir for a variety of lung flatworms and cantransmit the infection to humans.
Symptoms of paragonimiasis may includeabdominal pain,diarrhea,fever, andhives. If the infection remains untreated, the symptoms may diminish or disappear after only few months, but sometimes they last for decades.[7] Paragonimiasis is caused by the body's naturalimmune response to the worms and eggs that are present and also migrating from the intestines to the lungs.[citation needed]
As a rule, the parasites begin to cause symptoms about three weeks after ingesting live metacercariae. After about eight weeks, they begin to produce eggs in the lungs. Some patients develop brain damage if parasites establish in the brain and produce eggs. The brain damage commonly causesheadache,vomiting, andseizures.[4] Untreated cerebral paragonimaisis commonly results in death from increased intracranial pressure.
Praziquantel has been used to effectively treat paragonimiasis by separating the tegument. An effectively complete rate of cure may be expected after three days of treatment if there has not been too much permanent damage, such as from intracranial effects.[8] Other medications can also be used such asbithionol,niclofan, andtriclabendazole with high cure rates.[citation needed]
Thorough cooking of an infected crustacean kills all stages of the parasite. Crab meat should not be eaten raw, even if pickled, because the pickling solution often fails to kill all the parasites. Utensils and cutlery boards should be cleaned thoroughly before and after food preparation.[4]
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