Web edition: May 25, 2005
Mention Salmonella and most people think of chickens and eggs. Indeed, some 118,000 cases of food poisoning each year in the United States trace to consumption of undercooked eggs contaminated with these bacteria. Scores more people contract the foodborne disease from handling or eating undercooked chicken.
However, barnyard fowl are far from the only animals that transmit Salmonella. Indeed, a new report documents more than a dozen Salmonella poisonings throughout the eastern United States last year due to germs shed by "pocket pets" such as mice and hamsters. This is the first confirmed outbreak linked to pet rodents, according to an investigation related by health officials in the May 6 Morbidity and Mortality Weekly Report.
The multistate investigation that uncovered the new Salmonella source was triggered last year by the suspicions of a Minnesota veterinarian.
Under contract, the vet checked a large pet distributor's animals weekly for signs of disease. While investigating one newly arrived shipment of 780 hamsters, she made some unsettling observations, notes Stephen Swanson, an epidemic-intelligence officer with the Centers for Disease Control and Prevention (CDC). "The animals were less active than you would have anticipated," he says. Moreover, many hamsters were sweaty, had crusty eyelids, were hunched over, and suffered from diarrhea.
Because mass-reared and transported pocket pets have high death rates, he says, "few veterinarians would have bothered to send some sick ones off for testing." This one did.
As she dispatched a pair of sick animals to the University of Minnesota Diagnostic Laboratory, she directed the pet distributor to quarantine the rest of the animals. For reasons that remain unclear, Swanson says, the distributor nevertheless started shipping out the hamsters to pet stores throughout the upper Midwest. When test results came back showing the sick rodents were suffering from Salmonella infections, more than 200 animals had been distributed to 15 pet stores. The ones left behind were dying fast. Sixty percent succumbed in the next few days. So the vet sent seven more animals out for testing as the pet distributor began destroying the remainder of its stock of hamsters to try to contain the contagion.
Especially troubling, the diagnostic lab found Salmonella in the animals' internal organs, such as the lungs, liver, and spleen. Ordinarily, Swanson says, the germs show up only in an animal's gut and feces. To find the body riddled with Salmonella "indicated the hamsters had overwhelming systemic infection, which is not normal," Swanson told Science News Online.
At this point, the vet called the Minnesota Department of Health to report the incident, explains Swanson, who's currently assigned to work in that state office. His team then got samples of the germ for analysis.
There are some 2,000 different types of Salmonella, and this was a type known as Typhimurium. The researchers took a DNA fingerprint of the bacterium, then compared it with other DNAs in a CDC database. The effort revealed that the Salmonella was "a very rare strain," Swanson says. Because the infected pets could have spread the bacteria to their handlers and owners, his group decided to probe the CDC database of people infected with this rare germ since Jan. 1 of last year.
To their surprise, the investigators turned up 28 cases in people from 19 states. Twenty-two of the patients were later interviewed, and most of them had been exposed to pet-store rodents within 8 days of developing Salmonella poisoning. Some of the animals that these people had handled were mice or rats purchased to feed pet snakes. At least two individuals picked up their disease from other people who had handled sick rodents. Although no one died, six people were hospitalized because of these pet-spread infections.
This suggested a significant outbreak in the eastern United States, Swanson explains, because with this type of infection, "we know that there's somewhere around 38 cases that go undetected for every one that's identified."
Don't kiss that mouse
As Swanson's group began questioning each family struck by the rare Salmonella, it encountered a 5-year-old Minnesota boy who had developed a 2-week, gut-wrenching siege of diarrhea after playing with his new but lethargic mouse. It, too, had diarrheabefore it died.
The boy's mother put two and two together: The boywho sometimes kissed the mouse on its mouthhad caught its illness. But the family's doctors, the pet-store owner who sold the mouse, and a veterinarian all discounted the mother's idea.
So did Minnesota Department of Health officials, until they realized that the DNA fingerprint of the germ that had made this child sick matched that of bacteria isolated from the lot of Minnesota hamsters that had the rare Salmonella. Swanson's team was able to make the match only because the boy's mother had put the dead mouse in the freezeron the off chance somebody would decide to follow up on her suspicions that the animal might be linked to her child's illness.
Swanson's team reports that these rodent-triggered Salmonella poisonings were difficult to treat because the specific strain of bacteria involved proved immune to the standard antibiotics ampicillin, chloramphenicol, streptomycin, sulfizoxazole, and tetracycline. CDC investigators, who traced back many sick animals to pet breeders and distributors, discovered that at least four of the facilities routinely used these antibiotics in attempts to prevent diarrheal disease in their animals.
The researchers suspect that this prophylactic use of antibiotics allowed some Salmonella bacteria to develop resistance to the drugs. These germs could then move about the animal colonies with impunity, eventually infecting pet owners.
With this new report, rodents join the list of pets with a demonstrated ability to host and spread Salmonella.
Two years ago, CDC scientists reported that people in six states had suffered Salmonella poisoning from contact with pet reptiles. One case involved a 3-month-old infant whose father was a biology teacher. In his classroom, the dad had a boa constrictor that he frequently draped over his shoulders. Though he washed his hands after handling the animal, the man didn't change his shirt before playing with his child. Stool samples from the child and the snake showed that the two harbored the same Salmonella.
Many pet birds can host Salmonella germs, as can cats, dogs, amphibians, pet hedgehogs, and horses. Once people come into contact with contaminated pets, they can develop the diarrhea that marks the disease. Because the germs are shed in the feces, if sick children or the adults who care for them aren't careful about thoroughly washing their hands after each bathroom visit, residual germs on a person's hand can be transferred to objects, such as kitchen countertops, around the home.
Swanson, a pediatrician specializing in infectious-disease control, now advises that careful hand washing should follow any contact with pets and their cages, even if an animal appears to be healthy. Reptiles are known to carry and shed Salmonella for years without developing disease, says Swanson. It's possible, he says, that the same may be true of some rodents and other pets.
He notes that the infamous Typhoid Mary Mallon of the early 1900s was infected with the form of Salmonella that causes typhoid fever. "She shed that germ in her stool for decades," Swanson notes, "but was never sick from it." Unfortunately, the same was not true for many people who encountered her.
Because pets may also serve as carriers, he advises "never letting them run around in areas where you prepare fooda kitchen countertop or sink." Doing so, he says, risks turning a veterinary problem into a case of food poisoning.
National Center for Infectious Diseases
Centers for Disease Control and Prevention
1600 Clifton Road
Atlanta, GA 30333
Centers for Disease Control and Prevention
Minnesota Department of Health
PO Box 9441
Minneapolis, MN 55440
U.S. Food and Drug Administration
5600 Fishers Lane
Rockville, MD 20857-0001
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