Paragonimus skrjabini

Paragonimus skrjabini is classified as a species in the genus Paragonimus, which consists of many species of lung flukes that result in the food-borne parasitic disease paragonimiasis. Scientists have identified P. skrjabini, along with several other species including P. westermani and P. miyazakii, to be key pathogens in causing paragonimiasis in humans, primarily in Asian regions of the world. P. skrjabini is especially prevalent in 26 provinces in China with cases appearing more recently in India and Vietnam as well. From a morphological and genetic standpoint, P. skrjabini is most closely related to the species P. miyazakii, so much so that two sub-species have been classified separately within the P. skrjabini complex: P. skrjabini skrjabini and P. skrjabini miyazakii. Doanh PN (2007) establishes the importance of learning more about P. skrjabini, asserting that 'among Paragonimus species, P. westermani followed by P. skrjabini complex are the major pathogens for human paragonimiasis in Asia.' Max Braun in 1899 first defined the genus Paragonimus, which initially only included the species P. westermani. In Vietnam, since paragonimiasis was first reported there in 1906, it was presumed for 89 years that only one species of Paragonimus lung fluke, P. westermani, caused paragonmiasis in humans. However, scientists have been conducting many studies of crabs and humans infected with paragonimiasis, leading to the discovery of several previously unknown species. The few cases reported in North America of paragonimiasis are most likely the result of diseased individuals traveling to the area from a different country or people consuming infected, imported food. 'P. skrjabini' is endemic in China and was first described in the Guangdong Province in 1959, originating from a viverrid's lungs. Today, according to the World Health Organization, estimates put the number of persons afflicted with paragonimiasis at 20.7 million and the number at risk of contracting the disease at 293 million. For P. skrjabini, it stands as a public health threat in certain areas of the world, such as the Three Gorges Reservoir where it is the primary paragonimiasis causing parasite. Changes in the environment such as pollution and the persistence of individuals' consumption habits of raw crab puts paragonimiasis epidemics at high risk. 'Skrjabini' is named in honor of the Russian Helminthologist, Konstantin Skrjabin (1878–1972). The life cycle of P. skrjabini involves three hosts. The first intermediate host is a mollusk (typically a snail), the second intermediate host is a crustacean (typically a crab), and the definitive host is a mammal such as a dog, cat, or a human. Among others, freshwater crab species of the genus Nanhaipotamon are known to be second intermediate hosts for P. skrjabini. The mammal is the definitive host because it is the site where sexual reproduction occurs and adult P. skrjabini flukes develop. Infection begins when humans consume raw or uncooked crustaceans such as crabs that contain metacercariae of P. skrjabini. P. skrjabini metacercarie are typically located in the muscles of the crabs's bodies (Zhang et al. 2012). Next, in the animal or human's small intestine, the metacercarie excyst (emerge from a cyst) and travel to the abdomen before ultimately moving into the lungs. There, adult worms begin to grow and develop. P. skrjabini in humans, however, are known to often fail to make it to the lungs and thus don't reach the stage of adult development. Rather, immature P. skrjabini parasites stay undeveloped and enter the human host's brain, muscles, and various other subcutaneous tissues, leading to extrapulmonary neurologic and abdominal paragonimiasis. P. skrjabini trematodes in the mammal produce and fertilize eggs that then exit the host, typically through feces. In water, the eggs hatch and release miracidium that in turn infect a snail. A sporocyst that contains germinal cells forms in the snail's body cavity, and, following asexual reproduction, produces rediae. Rediae produce cercariae (the larval form of the parasite). The cercariae migrate from the snail to a crab, entering either through direct penetration or by the consumption of the snail by the crab. Often, multiple species of Paragonimus can be found coexisting in one crustacean, suggesting that metacercariae of different species do not compete with each other within the host. The life cycle of P. skrjabini starts over again as mammals or humans eat the crab.