Researchers have identified how a specific strain of salmonella is getting into Bermuda’s drinking water system, prompting environmental health officials to reiterate advice on how to avoid getting ill.

The research team, led by doctoral student Shervon De Leon, took faecal samples from 273 creatures on the Island and discovered that feral chickens and pigeons were the main carriers of salmonella mississippi.

They concluded that although chickens cannot access rooftops, they can pass on the disease-causing bacterium to other animals, such as pigeons and lizards, at shared feeding grounds, which can lead to the contamination of water tanks and the spread of salmonella mississippi to humans.

Elaine Watkinson, a senior public health analyst, told The Royal Gazette that householders should treat untreated or “raw” water like raw meat, adding: “You have to do something to your food, such as cooking raw meat or washing vegetables and fruit. Don’t assume the water is safe.”

Susan Hill Davidson, acting chief environmental health officer, added: “We have more detail. It’s an opportunity for us to again get the message across and maybe some people like to listen to the science and that might be the thing that might spur them [to treat their water].”

David Kendell, director of the Department of Health, said adult Bermudians who had lived here all their lives might have developed some immunity to bacteria in tank water but others were at risk, especially bottle-fed infants whose formula was made using untreated water, and those with compromised immune systems.

“People need to really look at it in terms of protecting the health of their children,” he said.

The joint study into the source of salmonella mississippi — which is the predominant strain of salmonella in Bermuda and on the Australian island of Tasmania, but in very few other countries — was prompted by an earlier Caribbean-led burden of illness report.

That report found from the testing of human stool samples that salmonella poisoning accounted for almost half of the gastroenteritis cases in Bermuda, with some 70 per cent of those salmonella cases involving salmonella mississippi.

Gastroenteritis is a public health concern on the Island, with an annual incidence of one episode per person per year.

Mr De Leon, a student at the University of the West Indies, said his team suspected birds would play a major part in the transmission of salmonella mississippi and the study bore that out.

“We looked at feral chickens, birds, frogs, rats, and we found a high prevalence in feral chickens and in pigeons and in some of the other birds,” he said.

“[In the case of chickens], they pass it on to other birds that share feeding grounds because they all poop where they eat. The other birds that can fly will get on to rooftops and then rainfall washes that poop into your water tank and then people drink that water. If it’s not treated, you have people getting sick from that water.”

Of the 63 feral chickens that were tested, salmonella was detected in 35 and samples from 14 tested positive for salmonella mississippi.

The researchers also tested tap and tank water samples from 102 randomly chosen households, most of which were untreated.

The samples were not tested for specific pathogens but for general “coliform bacteria” that would indicate the possible presence of harmful, disease-causing organisms in the water. Nearly all tap and tank samples — 88 and 89 per cent, respectively — contained coliforms.

A questionnaire on use and treatment of rooftop-collected water from those households discovered that 65.7 per cent did nothing to treat their water.

Almost 100 per cent used the water for cooking, 89 per cent used it for drinking and 100 per cent used it for cleaning dishes.

The researchers, who presented their findings at the third international One Health Congress in Amsterdam last year, concluded: “Most Bermudian residential water is contaminated by bacterial faecal indicator species.

“Depending on the indicator, between 67 per cent and 90 per cent of tap samples surpassed government safe drinking water standards and few households treat their water. Drinking water is likely a source of gastroenteritis.”

The team said its findings suggested “no improvement” to household water supplies since the last study ten years ago.

Ms Hill Davidson said householders could take simple steps to protect themselves and their families, including chlorinating tank water regularly, boiling water for a minimum of three to five minutes, or installing a UV treatment system. Brita and similar filters do not remove bacteria.

She said the aim was not to scare anyone as drinking water was a healthy lifestyle choice, but to make people aware that untreated tank water could cause illness.

“Our budget constraints mean that we can’t do as much as we might like from the public education point of view,” she said. “[But] repeating the message is something we have done annually and certainly over and over again.”