Scientists in the US and Ireland are concerned by indications that liver fluke, which normally affects sheep, may be infecting humans are a higher rate than previously estimated.
The latest issue of the Lancet today gives details of two cases involving US tourists to Connemara, who got the disease after they ate wild watercress.
These are the first confirmed cases of the extremely rare condition in Ireland since the 1960s. The liver fluke is a tiny flattened worm which usually infects sheep in wet areas but can cause severe illness, known as fascioliasis, in humans by infesting bile ducts within the liver.
The Lancet outlines the remarkable circumstances in which the two tourists - a prominent New York cardiologist and his wife - became infected by Fasciola hepatica. They ate the watercress in a sandwich after finding it growing beside a river. it is frequently used in salads.
On returning to New York in late 1998, he (and later his wife) became severely ill. Despite an intensive investigation involving a number of specialists, their condition remained undiagnosed for several months and deteriorated.
Then his colleague, consultant gastroenterologist Dr Jon La Pook, an associate professor at Columbia University, was asked for his opinion. Early last year he began an exhaustive search to find the cause. It culminated in their total recovery after going down a few blind alleys and encountering a "false positive".
Dr LaPook said he did not want to start a scare but his research letter, "Sheep, Watercress and the Internet", showed cases were arising and the disease "was not on our radar". Equally, it was "something travellers should be aware of".
He told The Irish Times it amounted to an "unbelievable story". Moreover, it was a cautionary tale about the Internet and computer technology.
First, it delayed diagnosis as a search generated 750 articles to be gone through. Then it helped to track down a French scientist who helped to test for the worm, and then to find the appropriate medication from Switzerland.
Above all it showed up the need for an "international case report database" to alert people on usual and unusual cases, he said.
According to liver fluke expert Prof John Dalton of Dublin City University, who assisted Dr LaPook, interest in the US will be further heightened by another research paper citing another case in Florida, to be published by the New England Journal of Medicine.
These papers raise the issue of many cases going undiagnosed and possible implications for tourism, he said.
A Bord Failte spokesman said the cases were obviously a once-off among six million visitors a year. But it was unfortunate two people had become ill while on holiday here. Adult flukes normally infest sheep and produce eggs that are passed in their faeces. This is a problem for livestock in counties with high rainfall. The eggs are eaten by snails from which immature forms of the fluke emerge. These become enclosed in a sac on aquatic vegetation, particularly wild watercress. Watercress is grown in glasshouses for sale in food outlets.
The human version has two stages. In the first, young flukes migrate through the liver causing tenderness, organ enlargement, fever, night sweats and sometimes a rash. This may lead to the second; inflammation of the bile duct or its obstruction with associated jaundice.
Prof Dalton and other researchers have been focusing most on South America, where humans and livestock are known to share water sources. Through the molecular parasitology laboratory in DCU, a rapid diagnostic test for liver fluke has been developed. They are also developing a vaccine for animals.
Prof Dalton said human cases could involve severe abdominal pain, but in the developing world the infection in people with compromised health could be a lot worse.
