''Paragonimus kellicotti'', the North American lung fluke, is a species of parasitic

trematode Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive host ...

in the genus ''

Paragonimus ''Paragonimus'' is a genus of flukes (trematodes) and is the only genus in the monotypic family Paragonimidae. 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 b ...

''. This species of ''Paragonimus'' has an intricate lifecycle, and although its name may suggest that it is only a health concern in North America, it is also prominent in Southeast Asia and China.

Morphology

''Paragonimus kellicotti'' parasites have dorsoventrally flattened, brown, oval-shaped bodies. They are soft-bodied parasites with oral and ventral suckers that are relatively similar in size. They use their ventral suckers to attach to a host. Their tegument is covered with spines, which is a characteristic that sets them apart among other ''Paragonimus'' species. Lung flukes are hermaphroditic, having both the female and male organs. Thus, they are able to self-fertilize.

Habitat

Early larval stages are found in snails and later move on to crustaceans, primarily crawfish and crabs. ''P. kellicotti'' parasites may live in fresh or salt water. Thus, they may be found within hosts of swamp, coastal, or forest areas.

Development

Lung fluke eggs are passed through feces or soil. Eggs then hatch into miricidia within three weeks. A miricidium infects the primary host, snails. Within snails, the miricidium develops into a cercaria. The cercariae eventually leave the snail and infect the second intermediate host,

crustaceans Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

. The lung fluke encysts within the tissue of crustaceans until the crustacean is consumed by humans or other animals. Once the fluke has been ingested,

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

s within the digestive tract of the consumer break down the parasitic cysts. The immature parasite continues to mature within the lungs of its new host, feeding on its intestine, and lay eggs. Mature lung flukes may breed year-round. ''P. kellicotti'' may live up to 20 years within a human host.

Infection

Infections of ''P. kellicotti'' affect 21 million people worldwide. However, humans rarely become infected in North America. The frequent victims of this infection in North America are small animals that feed on

crayfish Crayfish are freshwater crustaceans belonging to the clade Astacidea, which also contains lobsters. In some locations, they are also known as crawfish, craydids, crawdaddies, crawdads, freshwater lobsters, mountain lobsters, rock lobsters, mu ...

, such as

otter Otters are carnivorous mammals in the subfamily Lutrinae. The 13 extant otter species are all semiaquatic, aquatic, or marine, with diets based on fish and invertebrates. Lutrinae is a branch of the Mustelidae family, which also includes wea ...

skunk Skunks are mammals in the family Mephitidae. They are known for their ability to spray a liquid with a strong, unpleasant scent from their anal glands. Different species of skunk vary in appearance from black-and-white to brown, cream or ginge ...

s, and

mink Mink are dark-colored, semiaquatic, carnivorous mammals of the genera ''Neogale'' and '' Mustela'' and part of the family Mustelidae, which also includes weasels, otters, and ferrets. There are two extant species referred to as "mink": the A ...

. Occasionally, dogs and cats become infected, as well. If these animals are left without treatment, they eventually die. ''P. kellicotti'' is a food-borne trematode infection. Humans become infected with the parasite by eating raw or undercooked crayfish meat that contains the parasite larvae. These infections are common in

sub-Saharan Africa Sub-Saharan Africa is, geographically, the area and regions of the continent of Africa that lies south of the Sahara. These include West Africa, East Africa, Central Africa, and Southern Africa. Geopolitically, in addition to the List of sov ...

. After the parasite has been ingested, it will travel from the

intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...

s until it reaches the

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

Signs of infection

Several diagnostic signs indicate a lung fluke infection. These include a cough,

fever Fever, also referred to as pyrexia, is defined as having a body temperature, temperature above the human body temperature, normal range due to an increase in the body's temperature Human body temperature#Fever, set point. There is not a single ...

, and

weight loss Weight loss, in the context of medicine, health, or physical fitness, refers to a reduction of the total body mass, by a mean loss of fluid, body fat (adipose tissue), or lean mass (namely bone mineral deposits, muscle, tendon, and other conn ...

. Pleural effusion, a condition in which a surplus of fluid accumulates around the lungs, is another sign of an infection. ''P. kellicotti'' infection may sometimes be

misdiagnosed A medical error is a preventable adverse effect of care ("iatrogenesis"), whether or not it is evident or harmful to the patient. This might include an inaccurate or incomplete diagnosis or treatment of a disease, injury, syndrome, behavior, i ...

tuberculosis Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, in ...

. However, a lung fluke infection is differentiated from tuberculosis by increased eosinophils in the blood and pleural fluid.

Treatment

A ''P. kellicottii'' treatment in humans consists oral medication of praziquantel over a minimum of three days to achieve a 100% cure rate when the parasite is located in the lungs. Other ectopic infections may require surgical removal. Surgery does not guarantee all larvae will be removed.

References

* * * {{Taxonbar, from=Q3363448 Plagiorchiida Animals described in 1908