For those in the northern hemisphere, summer holidays are upon us and with them comes the pleasure of taking part in fresh water sports. But beware, minuscule parasites may be lurking in seemingly tranquil waters ready to burrow into exposed skin and cause an extremely itchy rash to develop.

Swimmer’s itch was first described in 1928 in Michigan, US and incidences have since been recorded worldwide. In addition to its medical label of cercarial dermatitis, it is also known as: duck rash, clam digger’s itch or lake itch in the US, duck lice in Canada, pou du canard in Switzerland and rice paddy itch in rice growing areas. Incidences of infection are now increasing worldwide and swimmers itch is now regarded as an emerging infectious disease.

The parasites that cause swimmers itch are the larval stages, or cercariae, of flatworms of the schistosome genus Trichobilharzia. They emerge from aquatic snails during the summer months and, using their forked tails, swim in search of their next host, usually a water bird, and burrow into exposed skin. If the cercariae invade the correct host they mature into adult worms and produce eggs that escape from the host in the faeces.

Once in fresh water these eggs hatch, releasing another type of larvae that are attracted to specific aquatic snails where they multiply many times to produce thousands more cercariae. Once infected, snails shed cercariae for the rest of their lives. Click here to see this amazing shedding on YouTube.

Although humans are not their correct hosts and they cannot develop further in them, these cercariae will also burrow into human skin if they encounter it. This causes an allergic reaction to occur. Almost immediately after leaving the contaminated water, an intense itching or burning sensation develops, followed a day or so later by the appearance of red pimples that turn into blisters. The reaction becomes more rapid and more severe if several exposures to the parasite occur. After a week or so the itching fades away, usually leaving no lasting damage. Relief can be provided using cold compresses or applying an anti-itch cream, but scratching should be avoided for fear of causing secondary contamination with bacteria.

The parasites can appear in lakes frequented by migratory water fowl or with resident ducks, geese or swans. Although a particular species of cercariae can develop to an adult in a variety of birds, they usually have a very specific species of snail as an intermediate host, possibly because they have evolved to evade the immune response of that particular snail.

There are over 40 species in the genus Trichobilharzia but, until recently, no species causing swimmers itch in the UK had been accurately identified. Scott Lawton and his team sampled a lake in Hampshire that is used for recreational fishing and gathered Radix auricularia, the snail host. Cercariae emerging from 3 of the snails were collected, examined by light and scanning electron microscopy and their DNA was extracted. Ribosomal RNA was subjected to sequencing and phylogenetic reconstructions and these molecular techniques showed that they were Trichobilharzia franki. This UK T. franki population were found to be closely associated with populations in France. The authors conclude that movement of migratory waterfowl infected with T. franki between the UK and the rest of the continent facilitates gene flow between parasite populations across Europe. Compared to the rest of Europe, there are few recorded cases of cercarial dermatitis in the UK. This may be due to under-reporting as symptoms are generally mild and could be confused with insect bites. However, some consider that increased use of fresh water bodies for recreational purposes and the influence of climate change that could favour increases in populations of the snail hosts and cause changes to wildfowl migratory routes may now lead to a re-emergence of this disease.

In addition to the epidemiology of species of Trichobilharzia, some fascinating studies of the biology of these cercariae have been made. Work by Wilfried Haas in the 1980s demonstrated the pattern of behaviour that the cercarial larvae of Trichobilharzia ocellata undergo to find and penetrate the skin of their primary hosts. After leaving their snail hosts they swim upwards and rest at the surface of the water. Disturbance of the water or a passing shadow that might indicate the presence of a duck causes them to swim downwards towards the ducks feet. Detection of warmth and the presence of lipids, cholesterol and ceramides present in the skin stimulate attachment and they usually penetrate the skin near a wrinkle of beside a hair follicle. Special histolytic enzymes secreted from the head gland assist in skin penetration.

Finally, how can swimmers, fishermen and sailors protect themselves from swimmer’s itch? People swimming or wading into potentially contaminated water are recommended to rub exposed skin vigorously with a towel immediately they exit the water. As snails are usually found near the margins of lakes, particularly in weedy areas, these areas should be avoided and authorities should be encourages to put up warning notices and discourage the feeding of ducks and geese near areas used for swimming, fishing or water sports.