Lung flukes are parasitic worms that primarily infect the lungs of mammals, including humans. These organisms belong to the genus Paragonimus, with Paragonimus westermani being the most common species affecting humans. Understanding their physical characteristics, both as adults and eggs, aids in identification and diagnosis.
Appearance of Adult Lung Flukes
Adult lung flukes, particularly Paragonimus westermani, have distinct physical characteristics. They are reddish-brown and ovoid or coffee bean-shaped. These adult worms measure approximately 7.5 to 12 millimeters (mm) in length and 4 to 6 mm in width, with a thickness ranging from 3.5 to 5 mm.
Their outer covering, or tegument, is covered with small, scale-like spines, giving it a textured appearance. They also have two muscular suckers: an oral sucker at the anterior end and a ventral sucker positioned slightly anterior to the middle of the body. Both suckers are similar in size, allowing the parasite to attach firmly to host tissues.
Internally, adult flukes are hermaphroditic, meaning each worm possesses both male and female reproductive organs. They have a lobed ovary situated near the center of the body, and two lobed testes located in the posterior region. These internal structures, along with extensive vitelline glands that produce yolk for the eggs, fill much of the worm’s body, contributing to its thick and robust form.
Microscopic Appearance of Lung Fluke Eggs
Lung fluke eggs are important for diagnosis and are observed microscopically. They are yellow-brown, often appearing golden. They have an ovoid or elongated shape, often asymmetrical with one end slightly flattened.
Their size ranges from 80 to 120 micrometers (µm) in length and 45 to 70 µm in width. A distinguishing feature is their thick shell. At the broader, flattened end, a distinct lid-like structure called an operculum is visible. The opposite, abopercular end is thickened.
When passed in the sputum or feces of an infected host, these eggs are unembryonated, meaning they do not yet contain a developing larva. This morphology, including the operculum and thick shell, helps distinguish Paragonimus eggs from those of other parasites.
Geographic Distribution and Common Hosts
Lung flukes of the genus Paragonimus have a widespread geographic distribution, affecting various regions across the globe. These parasites are particularly prevalent in parts of East and Southeast Asia, including countries such as China, Japan, Korea, the Philippines, and Vietnam. Endemic areas also extend to parts of Africa and the Americas, encompassing both North and South continents.
The life cycle of lung flukes involves multiple hosts. Definitive hosts, where adult worms reside and reproduce, include a variety of mammals. Humans are a common definitive host, but the parasites also infect other carnivores such as dogs, cats, pigs, tigers, foxes, weasels, and minks. These animals can serve as reservoirs for the parasite, contributing to its persistence in certain environments.
The parasite’s life cycle involves two intermediate hosts. The first intermediate hosts are freshwater snails, with various species acting as suitable hosts, such as Semisulcospira species. Larval stages develop within these snails before emerging to infect the second intermediate hosts. The second intermediate hosts are freshwater crustaceans, primarily crabs and crayfish. Humans and other definitive hosts acquire the infection by consuming these crustaceans raw or undercooked.