April 2007 | Back to Table of Contents

Pulse

The Case of the Ruined Rodents

A sleuthing vet and doctor find the solution to a mystery in the hamster cage.

Veterinarian Amanda Covington was making her weekly rounds at a Minneapolis pet distributor one day in August of 2004 when a cage full of hamsters caught her eye. She was under contract with the distributor to examine animals before they were shipped to pet stores to ensure they were healthy. Occasionally, she’d see signs of “wet tail,” a lay term for diarrhea that might be caused by anything from the stress of shipping to infection with some sort of bug, commonly Lawsonia intracellularis.

Hamsters with wet tail sometimes can look fine. But on that morning, Covington noticed that the hamsters were hunched up and sweaty, although none of them had diarrhea. A few days later, Covington got a call. The hamsters were dying. All had diarrhea and some had bloody noses and crusty eyes. “I’d been working there almost three years, and I’d never seen anything like this,” she says.

Covington instructed the distributor to immediately send several of the dead hamsters to the Veterinary Diagnostic Laboratory at the University of Minnesota for necropsy—the animal equivalent of an autopsy. At that point, the distributor stopped shipping the animals to pet stores.

When the necropsy results came back later in the week, Covington was shocked. The animals were infected with Salmonella.

The Minnesota Department of Health was contacted, and the remaining hamsters in the batch were euthanized. But some animals in the diseased group, Covington discovered, already had been shipped to pet stores. She was worried they would infect the people who bought them and called the Minnesota Department of Health.

Mouse Detectives
Infectious diseases expert Stephen Swanson, M.D., was working as an epidemic intelligence service officer at the Minnesota Department of Health (MDH) when he heard about a large die-off of hamsters that had occurred at a Minneapolis pocket pet distributor.

Normally, Swanson says, public health officials wouldn’t be contacted about an animal die-off. But this case involved 780 hamsters, 243 of which already had been shipped to 15 retailers in four Upper Midwestern states—and it involved Salmonella. The pet stores were beginning to report that the animals they had received were dying.

The MDH obtained the bacteria isolates from the dead hamsters and performed further testing. Serotyping and pulse-field gel electrophoresis subtyping (a sort of DNA fingerprinting) confirmed that all bacterial isolates were the same strain of Salmonella enterica serotype Typhimurium. A couple of cases of disease in humans that were caused by this same strain of Salmonella Typhimurium had recently been reported to the Centers for Disease Control and Prevention (CDC). So the health department decided to look into the case and put Swanson in charge.

Swanson, who speaks cautiously, as if he’s thinking two sentences ahead and carefully choosing his words, says Salmonella Typhimurium is a known cause of enteric infection in humans that’s most often transmitted by ingestion of contaminated food. Prior to this investigation, there were no reported outbreaks of human salmonellosis associated with exposure to rodents with the exception of one dating back to the 1960s. That one only suggested that two people might have gotten sick from guinea pigs.

Swanson and his team began their investigation by looking at the two cases of Salmonella Typhimurium in humans that matched the strain responsible for the hamster die-off and had been reported to the CDC. One was a 4-year-old South Carolina boy who had been hospitalized for five days with fever, watery diarrhea, and abdominal cramping. But this boy had become ill two months before the hamster die-off. The second, a 5-year-old in Minnesota, had developed abdominal cramps, vomiting, bloody diarrhea, and a high fever—also several weeks prior to the outbreak of disease in the Minneapolis hamsters. But both the South Carolina and Minnesota families, the MDH learned, had purchased rodents in the days before the children got sick.

Still, there was no proof that rodents had caused the disease. That is, until Swanson discovered that the Minnesota boy’s mother had, on a hunch, decided to freeze the pet mouse that had died in case it could be examined later. The MDH team performed a necropsy and isolated the same strain of Salmonella Typhimurium that the boy had.

During the next five months, the MDH team looked nationwide for people who had become infected with the strain of Salmonella Typhimurium responsible for the outbreak. They identified 15 confirmed human cases and, as a result, documented the first known outbreak of human salmonellosis associated with direct contact with commercially distributed rodents.

In addition, the MDH team determined that the strain involved in the outbreak was multidrug-resistant and found evidence of closely related Salmonella Typhimurium strains in rodent cages and transport containers at a Georgia-based pet distributor.

Pocket Pet Protection
The significance of the investigation is twofold, according to Swanson, lead author of an article about it that was published in a January issue of the New England Journal of Medicine.

First, he says, the outbreak was almost certainly larger than the number of human cases they identified. “We have reason to believe, based on prior epidemiologic work, that somewhere between 30 and 40 times that number of human cases probably occurred. We think that only one out of every 38 individuals who gets sick from non-typhoidal Salmonella is ultimately identified by a public health agency like the CDC or MDH.”

Second, he emphasizes that this is yet another example of zoonotic disease from exposure to pocket pets—monkey pox, tularemia infection, and lymphocytic choriomeningitis virus infection have also been identified in rodents.

As a pediatrician, Swanson is especially concerned about young children, whom he says should thoroughly wash their hands with soap and water after handling rodents and their cages. People who play or work with rodents should be aware that rodents can shed Salmonella and that rodent feces are potentially infectious. And he notes that immunocompromised people and pregnant women may wish to avoid contact with the animals altogether. The New England Journal article describes the case of a 23-year-old pregnant woman who got sick from this infection, was hospitalized, and went into preterm labor. Her baby was also infected and hospitalized for nearly two months.

Swanson also notes that the fact that the Salmonella strain proved to be multidrug-resistant may be influenced by the widespread use of nontherapeutic antibiotics within the pocket pet industry. Four of the five breeders and distributors investigated used antimicrobials to prevent diarrhea.

Pet Smarts
Surprisingly, neither Swanson nor Covington are warning people not to have pocket pets.

“In the end, we’re not saying that these are not appropriate pets and that humans should avoid them,” Swanson says. Instead, he wants the public to know that they should wash their hands after handling pocket pets, supervise young children who handle them, and avoid putting animals in areas where food is being prepared. He wants physicians to ask patients with salmonellosis about their exposure to animals other than reptiles. And he wants industry to reduce the use of nontherapeutic antimicrobials and to routinely sanitize cages and transport containers.

The take-home message from Covington is that any pet can carry disease. Before going to vet school, she got sick from dogs in a pet store where she worked. She insists that her own children wash their hands after handling any animal, including the family’s cow, horse, parakeet, dog, and four cats. “My dad was a physician, so that’s how I was raised. Every time you touch an animal, you wash your hands. You’re always careful.”

Covington is actually supportive of the animal distributor she was working for. “They were trying to do the right thing,” she says. “This company just got a bad batch of hamsters, and this was the result.”—Carmen Peota