What are my chances of catching an interesting parasite?

For most of the time that I have worked at CABI I have dealt, among other things, with articles on parasitology, and reading about some of the things that can be caught from undercooked meat or fish has made me tend to avoid it. However, on a recent holiday on the Trans-Siberian railway I stopped in Irkutsk near Lake Baikal, decided to try eating one of the local fish species, the Baikal Omul, Coregonus migratorius, and discovered that it was served raw. Reckoning that the risk was small and it would cause too much trouble to send it back, I ate it anyway, and found the taste not unpleasant, although I preferred the smoked form that I tried a couple of days later.

On my return, colleagues gleefully added to what I already knew about the tapeworm Diphyllobothrium latum, which can apparently reach 10 metres in length, and I decided to investigate what the risks were. Although C. migratorius is not widely distributed, there were a number of records in CAB Abstracts mentioning it (or synonymous names); I found some (for example Pronin, N. M. et al.: Meditsinskaya Parazitologiya i Parazitarnye Bolezni (1988) no. 4, pp. 64-67) indicating that a high proportion of fish were infected with larvae of species such as Diphyllobothrium dendriticum, but that these were mainly found in the viscera and not in the muscle which is what you eat. Having said that, fish roe also featured on the menu, and that can be infected.

Broadening the search to look for all mentions of Diphyllobothrium in people in Russia found many more records; most indicated prevalences of a few per cent in local populations, suggesting that the likelihood of becoming infected in a brief visit is small, although a few particular areas such as some lakeside villages have much higher prevalences. The main focus of D. latum is in north-western Russia but other species are found in Siberia. There were few records mentioning other tapeworms being transmitted by fish in Russia; broadening the search further, I found that the bile duct fluke Opisthorchis felineus is quite common, but that its main intermediate hosts are cyprinid fish (carp and their relatives) rather than salmonids like C. migratorius.

Looking up the symptoms of Diphyllobothrium infection in a textbook, I found that in the great majority of cases there are none (a few people suffer from pernicious anaemia, particularly if their intake of vitamin B12 is inadequate), and that as Diphyllobothrium usually sheds microscopic eggs rather than complete segments, infected people are often unaware of being infected. The same book indicated that larvae can be killed by freezing fish before eating it, but I don’t know how often this is done.

It seems therefore that the risk of my catching anything is small, and the chance of knowing about it is even smaller; in most respects this is good news, but it might deprive me of an interesting subject for a future blog entry.

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