Los Angeles is in the midst of a typhus outbreak, but researchers say the usual suspects—cats and rats—aren’t to blame.
Instead, they believe the opossum, the marsupial becoming a growing nuisance in the city, is the culprit. Scientists have known for three decades that opossums house the fleas that carry the bacteria for murine typhus, Rickettsia typhi. But what they haven’t figured out is exactly how opossums have become hosts.
“Opossums are an unusual carrier of parasites,” David H. Walker, the executive director at the Center for Biodefense and Emerging Infectious Diseases at the University of Texas in Galveston, told The Daily Beast.
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“They might be in some ways be a better host than other animals, a good habitat. But I’m speculating; to understand that, we need to know more.”
The typhus hitting Los Angeles has been described as an “epidemic,” but it’s not. Epidemic typhus—often found in dense, poorer countries with underdeveloped sanitation—is spread by lice carrying the bacteria Rickettsia prowazekii.
The typhus affecting the L.A. area is an endemic typhus; instead of an outbreak, it’s concentrated in certain regions. In this case, murine typhus caused by Rickettsia typhi is found on rat or cat fleas; in the United States, it’s often found in Los Angeles and southern Texas.
Typically, fleas nest in these animals, passing the infection on to their hosts who don’t show signs of the infection; the animals act as vessels to people who might touch the animals, get flea poop on their hands, and smear it on an open pore or orifice.
Walker said he isn’t shocked about the cases in Los Angeles, which has a trifecta of perfect conditions for a typhus outbreak: warm weather, lots of people, and a vibrant population of street animals that includes rats, cats, and opossums.
“Fleas play a role in infecting opossums,” he said, adding that there is not enough research to really understand that role.
Lucas Blanton, a research physician at the University of Texas Medical Branch, has investigated the medical mystery of murine typhus’ transmission.
“The classic cycle is the urban cycle, which involves rats and rat fleas and is well established,” he told The Daily Beast. “It’s been demonstrated that they become infected and don’t get sick. They have the bacteria in their systems for a long period of time and they can efficiently transmit the bacteria.
“But the same situation in an alternate cycle—a suburban cycle—of murine typhus isn’t well established,” Blanton continued. “To my knowledge, no one has ever taken a group of opossums, infected them in a lab, set fleas on them, and seen if they acquire infection like rats.”
Blanton and Walker said that most of the medical community’s knowledge of murine typhus is circumstantial. They know that opossums have antibodies against Rickettsia typhi. They know that opossums like to hang out with feral cats (Blanton cited the practice of feeding stray cats cheap cat food as a potential way opossums picked up the bacteria from felines). They know the disease is spread through flea feces, which people pick up and rub into their eyes or scratch themselves with and get infected. But there are many unanswered questions about the connection between the disease and the sharp-toothed scavengers.
Los Angeles has a history of grappling with murine typhus—as a press release from the L.A. County of Public Health pointed out.
“In L.A., murine [typhus] used to be in the city center,” Blanton said. “It was more urban, more related to rats. Then cases began popping up sporadically in the 1960s, and people noticed they were in the hills and suburbs. They didn’t find rats or rats exposed to the bacteria—but they did find opossums that had been exposed.”
Blanton said that without DNA analysis, researchers would crush fleas and inject them into guinea pigs to see if the type of flea, Ctenocephalides felis, was consistent with carriers of murine typhus. In 1992, researchers published the most thorough report yet on opossums and rat fleas in the Journal of Clinical Microbiology—but couldn’t establish how opossums got the Rickettsia typhi.
“The opossums I’ve studied, they’re just infested,” Blanton said. “Fleas have no issues being on an opossum. But we don’t really know: Is the opossum getting infected for a long period of time and then infecting fleas? Or is it a domestic animal that passes it to an opossum? Or do the fleas jump ship to a new host?”
In Galveston, Texas, Blanton said, 0.3 percent of feral cats carried Rickettsia typhi; that number was 7 percent with opossums. “I’d say that passes the [statistical] significance test,” Blanton said.
Right now, the theories about how opossums get cat fleas range. There’s the shared cat food theory. Blanton also some theorize the bacteria is aerosolized and carried in the air, much like pollen. Walker said a recent outbreak in Austin, Texas, involved houses built on uplifted piers, beneath which opossums like to live.
Luckily, getting murine typhus is far from deadly. “You don’t need a flea collar, and it’s not like rats spreading the plague,” Walker said. “It’s an under-recognized disease and it’s misdiagnosed a lot. But the fatality rate is one percent at the most.”
Will global warming contribute to a spread of typhus-carrying opossums? Blanton said it’s still too early to say.
“In Texas, it [murine typhus] does look like it’s marching northward,” he said. “But will the suburban environment alter density [of opossums, who are further spread out in the north than the south]? Are they just concentrating more towards garbage cans and in places where stray cats and dogs feed? We don’t know.”
