salmonella i karet

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Nemo Beware: Fish Tank Can Be a Haven for Salmonella

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By DEBORAH FRANKLIN

Published: April 18, 2006

Tropical fish seem the tidiest of pets: they never lick your face, leap from the cat box to the kitchen counter or take pleasure in rolling around in some wad of mysterious rot.

But recent findings from Australia confirm that fish owners should nonetheless take care when cleaning an aquarium or otherwise interacting with finned friends or the water they swim in.

Researchers reported in the March issue of Emerging Infectious Diseases on cases of bacterial infections serious enough to send some children to the hospital with high fever and bloody diarrhea. The infections stemmed from a multidrug-resistant strain of Salmonella paratyphi B that was traced via DNA analysis to fish tanks in the patients' homes.

"The fact that 12 to 14 percent of Australian households have ornamental fish and as many as 12 million American and 1 million Canadian families own domestic aquariums, together with the young age of most affected patients," make the risk of contamination from the tanks a matter of public health, the scientists concluded in their report.

The risk of catching salmonella bacteria from pet reptiles, chicks, ducklings and other animals has been widely recognized for decades.

In the 1970's, the Food and Drug Administration banned the sale and distribution of nearly all turtles with shells less than four inches long (a size children can easily hold and put in their mouths) because as many as 250,000 children were thought to have contracted salmonella infections from them.

More recently, links to pet fish have been suspected in isolated cases in North America and Europe as well as in Australia over the last 10 years, according to Dr. Diane Lightfoot, a microbiologist and salmonella specialist at the University of Melbourne, who contributed to the Australian study.

But the number of salmonella outbreaks linked to pet fish and aquariums is relatively small and widely scattered. That makes them harder to trace than the vastly more numerous infections from raw or undercooked poultry, other meats, raw eggs or other contaminated food.

Most salmonella infections entail a few days of cramping, vomiting and diarrhea that may not even be reported to a doctor, let alone a public health department. Only rarely do the illnesses require treatment with antibiotics.

Still, young children, pregnant women, the elderly and others with weakened immune systems are at greater risk of developing much more serious or even fatal salmonella infections.

In a phone interview, Dr. Lightfoot emphasized that she and her colleagues were not opposed to having fish as pets. In fact, she said, she keeps a few ornamental carp in her own backyard pond.

"The world would be a terrible place without fish tanks," she said. "We're just calling on people to use common sense. Wash your hands frequently with soap and water. And when Mum's cleaning the tank, a child shouldn't play with the pebbles or sticks or splash in the water. It's easy to get infected."

Dr. Colette Gaulin, who has been tracking aquarium-linked salmonella for the health ministry in Quebec, said health workers there developed posters and brochures with "tips on handling your aquarium" for distribution to consumers at local pet shops.

The list includes admonitions to replace one-third of the tank's water twice each month and to strictly follow the manufacturer's instructions regarding filtration.

"Do not wash aquarium accessories in the kitchen sink, bathroom sink or bathtub," another item on the list advises.

"If you have no other option," it continues, "then thoroughly clean and disinfect all the surfaces used with a bleach solution of four tablespoons per liter of lukewarm water. Rinse these surfaces well before reusing."

Though the particular drug-resistant strain identified in Australia has yet to be reported from American aquariums, Dr. Fred Angulo of the Centers for Disease Control and Prevention said that taking the sorts of precautions that Dr. Lightfoot and Dr. Gaulin recommended made good sense.

All three researchers agreed that beyond any immediate public health risk, a potentially bigger problem is the makeup of the strain of Salmonella paratyphi B making the rounds in Australian fish tanks.

The strain was loaded with five different genes that enable any bacteria carrying them to be impervious to at least as many different antibiotics.

There are still effective drugs that, if needed, will readily kill the strain, and it shows no signs of spreading to the human food supply or through it.

But where is such multipronged drug resistance likely to have developed? Almost certainly not in the aquariums of individual pet owners, or even in the larger network of aquariums in local pet shops or in veterinary clinics, Dr. Lightfoot said.

She said she believed it came from Southeast Asia, which exports most of the world's ornamental fish. The use of antibiotics in the region's aquaculture tends to be heavy and not tightly regulated.

Since the 1990's, the researchers noted, multidrug-resistant versions of the same salmonella strain have been increasingly isolated from infected people in different parts of the world.

"We can't tell from our work if this multidrug-resistant strain is coming in through the fish, the snails or even the seaweed," Dr. Lightfoot said, "but we can be fairly certain that it's not in the fish food, and also fairly certain that we are importing it."

Michael Feldgarden, who helps track resistant bacteria, said he agreed that the blanketing doses of antibiotics necessary to create that deep pattern of resistance were probably given much earlier in the distribution chain of the fish — probably on fish farms in Southeast Asia.

He is part of a Boston-based surveillance network, the nonprofit Alliance for the Prudent Use of Antibiotics, that, among other activities, works to identify the reservoirs of drug resistance among nonpathogenic bacteria.

One long range concern, Dr. Feldgarden said, is that now that the five resistance genes seem to be established in that strain of salmonella, they may fairly easily jump as a unit to another strain — or even to a completely different, nastier organism living in the same soup.

And those new changes may prove even more dangerous to people, he said.

"I'd be interested to culture some of those fish and see what else is infecting them," Dr. Feldgarden said.

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