As human cases of flea-borne murine typhus continue to occur in South Texas, researchers are working to better understand the role cats and their fleas may play in the disease's transmission cycle.
In a recent study , researchers at the Texas A&M College of Veterinary Medicine and Biomedical Sciences (VMBS) detected DNA from Rickettsia typhi — the bacterium that causes flea-borne murine typhus — in cat fleas (Ctenocephalides felis) collected from domestic cats in the Rio Grande Valley, a region that consistently reports some of the highest numbers of human cases of murine typhus in Texas.
The findings provide new insight into the pathogens carried by cat fleas and demonstrate how closely animal, human and environmental health are linked.
Tracking an emerging threat
Flea-borne murine typhus is a bacterial disease that can infect humans when flea feces containing R. typhi enter the body through a bite site or mucous membrane and can typically be treated with antibiotics. Its incidence has steadily increased in Texas over the past decade, particularly in South Texas — with more than 6,700 cases being reported in the region between 2008 and 2023, according to the Texas Department of State Health Services.
"The last time flea-borne typhus was rampant, rats and rat fleas were the main reservoir and vector," said Dr. Sujata Balasubramanian , an associate research scientist in the VMBS and first author of the study. "Now, there is a layer of cat fleas contributing to this uptick."
Despite the Rio Grande Valley consistently reporting some of the state's highest numbers of flea-borne typhus cases, relatively little is known about the prevalence of R. typhi among cats and cat fleas in the region.
"People have cats as pets and often befriend and feed stray cats," Balasubramanian said. "Despite the awareness that cat fleas can carry and transmit R. typhi, we do not have a good understanding of R. typhi carriage in cats or cat fleas, not even in the areas of higher incidence of flea-borne typhus."
Fleas reveal multiple zoonotic pathogens
When the research team tested fleas collected from domestic cats for several disease-causing pathogens, it detected R. typhi in fleas removed from six of the 167 cats sampled, providing some of the first data on the pathogen's presence in cat flea populations in the region.
Researchers also identified Bartonella henselae and Bartonella clarridgeiae in the fleas and cats. Both bacteria are associated with cat scratch disease, a zoonotic infection that can be transmitted from cats to humans.
Although R. typhi was detected in fleas from a relatively small number of cats, researchers say the findings help fill an important knowledge gap about the pathogens carried by cat fleas in an area where human cases of murine typhus continue to occur.
While the study did not directly examine transmission between cats, fleas and people, the findings provide a clearer picture of the pathogens circulating within flea populations in a region where human cases continue to occur.
The study also highlights the importance of a One Health approach, which recognizes the connections among animal health, human health and the environment.
"Like all vector-borne diseases, there are many pieces of the puzzle needed for the successful transmission of murine typhus," said Dr. Sarah Hamer , a professor in VMBS' Department of Veterinary Integrative Biosciences . "We must think about the ecology of the disease, which includes the interactions of the right species of flea with cats or opossums — both of which can serve as hosts for the fleas and bacteria. Humans serve as an accidental host when hungry fleas find us instead of an animal."
Several factors may be contributing to the continued presence of flea-borne murine typhus in South Texas.
"There are only a few places in the U.S. where murine typhus is recognized to be transmitted with some regularity and, unfortunately, that includes South Texas," Hamer said. "The increased abundance of pet cats and stray or feral cats, socioeconomic conditions that prevent access to affordable flea protection on cats, and warming temperatures likely all promote flea infestation of cats and transmission of the bacteria that causes typhus."
Because the disease depends on interactions among fleas, animals, people and the environment, researchers say understanding those relationships is essential for identifying risks and developing effective prevention strategies.
Protecting pets and people
Although cats typically do not become seriously ill from R. typhi, controlling flea infestations remains an important step in protecting both animal and human health.
"If we can help to control flea infestations on cats, we also reduce the chance that fleas will become infected with the bacteria and reduce the chance that infected hungry fleas will hop onto a human instead of a cat," Hamer said. "What we do to help animal health can also protect human health."
Rather than indicating that cats themselves post a threat to people, the findings highlight the importance of controlling flea populations.
"Fleas and other ectoparasites can spread disease agents that not only make our pets sick, but they can make us sick as well," she said. "Keeping cats indoors where they can't interact with wildlife and working with your veterinarian to provide flea and tick control for cats can help minimize chances of disease transmission."
Continuing the search for answers
Researchers say the study is only the beginning of efforts to better understand how flea-borne murine typhus circulates among animals, fleas and people in South Texas.
"We know rats, rat fleas, cats, cat fleas and opossums can carry R. typhi," Balasubramanian said. "More sampling and more studies are imperative."
Future work would focus on identifying factors that influence its circulation and determining how risks vary among pet, stray and feral animal populations.
"Because of all the changes in wildlife populations, climatic conditions and human land use, continued surveillance and research is the only way we can gather data to understand these changing risks for human and animal health," Hamer said.
By Camryn Haines, Texas A&M University College of Veterinary Medicine and Biomedical Sciences
