Last updated on April 17, 2014 at 8:50 EDT

Paragonimus westermani

Paragonimus westermani is a species of fluke, or flatworm that is classified within the Platyhelminthes phylum. This species is abundant in South America and Asia and affects the lungs of humans and other hosts. It was first discovered in 1878 in Europe after two Bengal tigers died. In 1879, Ringer found this species in the lungs of a human. Manson and Erwin von Baelz identified the sputum and eggs separately in 1880, after which Manson asserted that a snail was most likely the worm’s intermediate host. Alongside his Japanese colleagues, Manson described the life cycle of the species between 1916 and 1922.

Paragonimus westermani can reach an average length between .2 and .4 inches. Its body, color, shape, and size resemble a coffee bean and its skin is covered with small spines, which can help distinguish it from other species within its genus. This species has two suckers that are similar in size on the front and back of its body. Like all species within its class, this worm is hermaphroditic and has a pair of testes and an ovary.

The lifecycle of Paragonimus westermani begins when a unembryonated egg passes with the sputum from the lungs of a human or reservoir host. These hosts can be a number of carnivorous species including felids, mustelids, canids, and some rodents. After two weeks of development within the eggs, miracidia hatch and move through the water until they find a suitable snail host. Once the miracidia penetrate through the snail, they form “mother” sporocysts that create “daughter” rediae. This process allows many cercariae larvae to develop within one snail. Once the larvae emerge from the snail, they will swim through fresh water to locate a crab. The cercariae will encyst within the crab and develop into metacercaria. Once a carnivore or human consumes infected crabmeat, the digestive fluids will break down the cysts, releasing metacerciaria larvae, which will then penetrate through the intestinal lining and travel to the lungs. This species is unique in that it requires two intermediate hosts, a snail and a crab, to develop into infective larvae.

Carnivorous animals contract Paragonimus westermani worms after eating raw infected meat, while humans can contract the worm by eating raw and undercooked crabmeat. The rate of infection is higher in areas where raw crabmeat is a dietary staple, and the worm is able to spread rapidly due to its wide range of available hosts. However, human infection can occur by less direct means, when utensils or other food has been exposed to infected raw meat. Infection can also occur if a human consumes an infected crab-eating animal, like a boar, that has not been properly cooked.

Symptoms of a Paragonimus westermani infection begin when the worms enter the intestine and migrate to the lungs, causing fever, abdominal pain, diarrhea, coughing, pulmonary abnormalities, and other lung related issues. After longer periods of infection, hosts may experience coughing, discolored sputum, and other symptoms. Once the worm has reached the lungs and laid its eggs, the eggs may encyst and become attached to the tissue. If the tissue of the lung heals over the cyst, the eggs and cyst become pseudotubercles. Because of this, and the symptoms that are related to the lungs, it can be difficult to distinguish an infection from tuberculosis. Patients displaying signs of tuberculosis in areas where Paragonimus westermani is endemic should also be tested for an infection of the parasitic worm. If a worm or cyst dislodges from the lung and travels to the spine, the host may experience paralysis, and if it travels to the heart, the host may die.

In order to diagnose a Paragonimus westermani infection, also known as paragonimiasis, eggs must be present in a sputum or stool sample, but this method is only successful after two or three months of an infection. Other methods of diagnosis include a comparison with other types of parasitic worm and antibody detection tests in lighter infections. X-ray tests can be conducted to locate encysted eggs within the lungs, but this method can be misleading in appearance, causing a possible misdiagnosis of pneumonia, spirochaetosis, or tuberculosis. A lung biopsy may be used to determine whether a nodule is related to a worm infection.

The most common treatment for paragonimiasis is a drug called praziquantel, which can eliminate Paragonimus westermani from a host within days of periodic treatment courses. Bithionol can also be used to treat this type of infection, but is often associated with negative side effects like rashes. Methods to prevent paragonimiasis should include public education about sanitary food preparation and cooking methods, as well as elimination of its first primary hosts within water and general hygiene standards. Research about the habits of the species and how the infection spreads is also important. One study, conducted in the Namback District of Laos between the years of 2003 and 2005, showed that out of 308 adult villagers and 633 students, 156 villagers and 92 students tested positive for an infection. This study also focused on the transference means of the worm within the area and concluded that raw crab was the main method of transference.

Image Caption: An adult specimen of Paragonimus westermani (Digenea: Plagiorchiida) stained with carmine. Credit: Division of Parasitic Diseases and Malaria/Wikipedia

Paragonimus westermani