Tuesday, March 26, 2013


Paragonimus life cycle
Paragonimus is a genus of flatworms which causes human paragonimiasis. Contraction of paragonimiasis usually occurs when humans eat undercooked freshwater crustaceans, such as crabs or crayfish. These often contain  metacercariae which are intermediate forms of the flatworm. Once ingested and in the intestine, the parasite will move into the guts and into the lungs. In the lung, the parasites transform into a cyst where they will cross fertilize with one another. The cyst will rupture in the lungs and eggs are either coughed up or swallowed and excreted through feces. Eggs landing in the freshwater will hatch and  infect the first intermediate host which is usually an aquatic snail. A crustacean may also become infected by eating the infected snail. The cycle starts again when the definitive host (e.g. the human) ingests the infected crustacean.

Symptoms of paragonimiasis include abdominal pain, diarrhea, fever, and hives. These symptoms can last for months and sometimes even up to 20 years. Most symptoms are caused by the body’s natural immune response to presence of the worms and eggs and their migration from the intestines to the lungs. On average, it takes about eight weeks for the parasites to start producing eggs in the lungs but only three weeks to develop symptoms,

Paragonimus mexicanus is the causal agent of human paragonimiasis in several countries of the Americas. It is considered to be the only species of the genus present in Mexico, where it is responsible for most human infections.

Mexican researchers have now investigated Paragonimus mexicanus specimens from several locations in Mexico. The provide conclusive morphological, molecular (DNA Barcodes) and geographical evidence that strongly suggest the presence of at least three species in Mexico. One of the three species is likely new to science and another one could be one (Paragonimus ecuadoriensis) that was considered a junior synonym of Paragonimus mexicanus. 

These results have important implications with respect to diagnosis, treatment, and control of human paragonimiasis in Central America. There are more than 50 species described in the genus and nine of them are from the Americas. The authors clearly state that the taxonomic status of some of these species also needs to be reinvestigated to establish the number of causative agents of human paragonimiasis.

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