Hidden health problems caused by fish-borne trematodes

Fish-borne trematode infections tend to go unnoticed and hence be under-recorded, not least because the symptoms are often quite general, and may be mistaken for a range of other infections and conditions. Most who are infected suffer morbidity rather than mortality, often being unable to work. However, given that an estimated 19 million people are infected with fish-borne trematodes in SE Asia and another 20 million in China, this hidden problem clearly affects a great many people. Unfortunately, detecting a low level of trematode infection, which is vital for early and effective treatment, is difficult even for trained technicians. Even more difficult is identification of the species of trematode, information that is important for epidemiological studies. These difficulties have been one of the drivers of the TREMKIT project, and its development of two new tools for detecting and identifying the parasite.

Credit: Prof P. Sithithaworn, Khon Kaen University, Thailand

Fish-borne trematodes have a complicated life cycle with two intermediate hosts. Starting with a human host, the eggs from adult flukes are passed out with the faeces. The eggs must get into water in order to hatch and be able to infect their first host, a freshwater snail. The fluke develops in the snail and then burrows out to seek the second host which is a freshwater fish. Many species of snail and fish may carry the immature flukes, and some of the most common fish hosts are popular food fishes. The immature fluke can wait for a long time inside the fish, waiting for the moment it is eaten by a suitable mammal host, including humans. The immature fluke then makes its way through the intestine wall and begins to migrate to the liver where it tends to take up residence in the gall duct. However, adult flukes may be found in the liver itself.

Credit:P.Sithithaworn

Naturally, the presence of these unwanted guests in the liver has many undesirable effects. Infection with fish-borne trematodes is associated with a most unpleasant cancer of the bile duct which is very difficult to treat, even if it is detected early. Since detection and diagnosis of the cancer tend to be difficult, it is often in an advanced state when it is found and by then, the prognosis is very poor.

The reason that fish-borne trematodes are a problem in some countries comes from the tradition of eating raw or uncooked (typically fermented) fish products. These are very popular in SE Asia and are a traditional food in rural areas. Properly cooked fish poses no risk, but the encysted flukes are quite durable, tending to remain viable in fresh fish and surviving the salty, acid environment of fermented fish for many days. Deep freezing leads to a rapid loss of viability, but is rarely used in rural areas. Quite why infection appears to have increased in recent years is unclear, and several factors may have contributed. Changes in irrigation and particularly the development of large-scale irrigation systems are associated with increasing risk of infection, probably due to the ideal habitat they form for snails. Sewage-fed aquaculture also has had a role as it provides the perfect opportunity for completing the life cycle, especially where latrines are placed over ponds. In a rather odd twist, the increasingly wealthy urban consumers are now more at risk from fish-borne trematodes due to the increasing trend to eat raw fish. Thus, the problem is moving from the rural areas into the cities.

Credit :P.Sithithaworn

The usual method for detection of infection in humans is the examination of faeces for the presence of eggs. However, at low levels of infection, the very small numbers of eggs may easily be missed. For this reason, TREMKIT is developing a test based on monoclonal antibodies. This will be more sensitive than microscopy methods and can be used for identifying infected people who need to be treated. It is intended as a mass-screening method for use by health authorities and will not necessarily differentiate between the three trematode species in the study area (Opisthorchis viverrini, O. felineus and Clonorchis sinensis). Since they all respond to the same antihelminthic drugs, there is currently no need to identify which species is infecting a person. For epidemiological studies, identification to species is essential and for this, TREMKIT is developing a test based on polymerase chain reaction (PCR) technology which relies on detecting trematode DNA. PCR tests are very sensitive and highly specific but relatively expensive, so their use will probably be restricted to research tools for universities and international agencies. It is hoped that a combination of the two tools will greatly improve our knowledge of the trematodes, those at risk and identify methods for reducing levels of infection.

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