Metagonimiasis is a disease caused by an intestinal trematode, most commonly Metagonimus yokagawai, but sometimes by M. takashii or M. miyatai. The metagonimiasis-causing flukes are one of two minute flukes called the heterophyids. Metagonimiasis was described by Katsurasa in 1911–1913 when he first observed eggs of M. yokagawai in feces (date is disputed in various studies). M. takahashii was described later first by Suzuki in 1930 and then M. miyatai was described in 1984 by Saito.
Signs and symptoms The main symptoms are diarrhea and colicky abdominal pain. Because symptoms are often mild, infections can often be easily overlooked but diagnosis is important. Flukes attach to the wall of the small intestine, but are often asymptomatic unless in large numbers. Infection can occur from eating a single infected fish source. Peripheral eosinophilia is associated especially in early phase. When present in large numbers, can cause chronic intermittent diarrhea, nausea, and vague abdominal pains. Clinical complaints can also include lethargy and anorexia. In acute metagonimiasis, clinical manifestations are developed only 5–7 days after infection. Heavy infection has also been associated with epigastric distress, fatigue, and malaise. Occasionally, flukes invade the mucosa and eggs deposited in tissue may gain access to circulation. This can then lead to eggs embolizing in the brain, spinal cord, or heart. Granulomas may form around eggs and can cause seizures, neurologic deficits, or cardiac insufficiency. An interesting case in Japan found diabetes mellitus (DM) to be a sign of chronic infection with intracerebral hemorrhages as the acute sign of aggravation<. Two months after administering praziquantel, the hemorrhages were gone, as was the diabetes. This unique case shows the potential of additional symptoms associated with metagonimiasis that are still unknown.
Cause Metagonimiasis is most commonly caused by one of the two smallest flukes known to infect man, Metagonimus yokogawai, also called the Japanese fluke. More rarely, metagonimiasis can arise from infection with M. takahashii or M. miyatai. Recent studies analyzing the DNA of the three agents causing metagonimiasis found that DNA sequencing supports M. yokagawai and M. takahashii be placed in the same clade, and phylogenic tree analysis supports their genetic similarity. M. miyatai, however, was found to be more genetically distinct, and the authors concluded it should be nominated as a separate species. An additional study examining karyotype data on the three disease-causing agents also supported the nomination of M. miyatai as a separate species. Trematodes are one class of phylum Platyhelminthes from the order Digenea and are generally referred to as flukes. Metagonimiasis is of the family Heterophyidae.
Transmission Transmission requires two intermediate hosts, the first of which is snails, most commonly of species Semisucospira libertina, Semiculcospira coreana, and Thiara granifera. Infection is acquired through the secondary intermediate host, fish, that have not been thoroughly cooked. Metacercariae encyst under the scales or in the flesh of fish from fresh or brackish water. Sweetfish (Pecoglossus altevelis) is one of the most common fish species infected, but others include the golden carp (Carassius auratus), common carp (Cyprinus carpio), Zacco temminckii, Protimus steindachneri, Acheilognathus lancedata, and Pseudorashora parva. Definitive hosts include humans and various fish-eating mammals, primarily dogs, cats, and pigs. Fish-eating birds may also be infected with metagonimiasis.
Reservoirs Reservoirs include fish-eating mammals such as dogs, cats and pigs as well as fish-eating birds. The presence of heterophyid infection in humans is generally caused by a lack of host specificity by the parasites, as seen in the many non-human reservoirs for metagonimiasis. The many reservoirs also have negative implications on the efficacy of prevention and eradication efforts of the disease. Carp have also been identified as reservoirs of the parasite.
Incubation period The incubation period is around 14 days and infestation may persist for more than one year.
Morphology
Eggs The morphology of the eggs is very important for diagnosis, but is difficult as eggs are very small. Eggs have a smooth, hard shell that is transparent and yellow-brown with a more conventional, ovoid egg shape. They are about the same size as those of Heterophyes and Clonorchis, usually measuring 26-28 μm length and 15-17μm width. The egg also has a very slight opercular shoulder, marking the line of cleavage between the shell and operculum, an "escape hatch" for the miracidium. The Clonorchis has more distinctive tapering and a seated operculum that help distinguish it more readily from Metagonimus species.
Adult flukes The body of the adult disease-causing agent of metagonimiasis is often described as leaf-shaped, similar to most trematodes. It is one of the smallest intestinal flukes, and is only slightly larger than Heteropheres. The most prominent feature is that its ventral sucker is deflected to the right of its midline and is closely associated with the opening of the genital pore. The testes are large and diagonal to each other while the smaller ovary is anterior to the testes and the uterus is filled with eggs. The uterus winds forward to the genital pore and is the largest organ in the body. The size of the adult fluke does not exceed 2.5 mm length by .75 mm width.
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