A sometimes serious disease spread by fleas is making inroads in Texas, quietly doubling case numbers since 2008, and beginning to encroach on larger metropolitan areas.

While considered a nuisance on cats and dogs, fleas have spread disease around the world, including plague and several types of typhus. In the southwestern US, the cat flea Ctenocephalides felis is a vector for Rickettsia typhi, also known as murine typhus.

Cat fleas become infected with R. typhi by feeding on infected rodents like rats, or opossum, that are reservoir species for the bacteria. Cat fleas then feed on cats, dogs, and sometimes humans, and pass along the infection. Cat fleas are also the vector for Bartonella henselae, commonly called "cat scratch fever."

Though the names sound similar, typhus is not caused by the same bacteria that causes typhoid fever. Cat fleas are the most common species of flea in the US, and feed on cats, dogs, rodents, and other small animals. While humans can contract R. typhi from cat fleas, they are also likely to become infected by coming into contact with the feces of infected fleas, which might be in the fur of companion pets like cats and dogs, or if small wild animals are handled or touched.

Ctenocephalides felis, aka the cat flea. Image by Katja ZSM/Wikimedia Commons

Like mosquitoes and ticks, fleas are arthropods and they require a blood meal during their adult stage. When infected with R. typhi, bacteria can reproduce vigorously enough in the gut of a flea that the flea cannot take in enough blood, and it will bite repeatedly trying to satiate its hunger.

While murine typhus oftentimes causes mild symptoms, an increase in cases means greater likelihood patients will suffer the more serious form of the disease. Because symptoms of R. typhi mimic other illnesses, and doctors may not identify the infection, case counts are probably under-reported. Symptoms of infection with the R. typhi bacteria include:

body aches and muscle pain

chills and fever

loss of appetite and nausea

rash that appears several days after onset

gut pain and cough

While the infection can be treated with antibiotics, severe infections can be fatal.

A recent study in the journal Emerging Infectious Diseases looks at the incidence of R. typhi, and found case numbers are increasing over a larger part of the Lone Star State.

Interested in identifying the spread and incidence of R. typhi, scientists reviewed surveillance data on all probable and confirmed cases of the disease between 2003 and 2013 in Texas, where case reports of the infection are required.

Number of reported cases in Texas, by year, of typhus group rickettsiosis, from 2003–2013. Image via Kristy O. Murray et al. via Emerging Infectious Diseases

In the initial review year, 2003, there were 27 cases reported. By the year 2013, 222 cases were reported, with cases peaking in the summer months, except in southern Texas where another uptick occurs in December and January. When the survey started in 2003, nine counties in Texas reported cases of R. typhi, and by 2013, 41 counties reported confirmed cases.

Illness onset by epidemic week during the study period, by location; <28°N represents south Texas. Image via Kristy O. Murray et al. via Emerging Infectious Diseases

In a media report, Melissa N. Garcia, a pediatric tropical medicine instructor at Baylor College of Medicine in Houston, noted:

However, when we are starting to see more cases in San Antonio, Houston, and Dallas … I think we're starting to see a little bit of a shift.

In this study, the average patient age was 33 years, with the highest number of patients in the five to 19-year-old range. More than half the victims reported noticing a flea bite, or fleas in their home, before onset. Common animals in these homes were dogs, cats, wildlife, and rodents.

Over the range of the study, 1,762 cases were reported. Of these, 1,047 infections, or 60%, were serious enough for the patients to be hospitalized, with four deaths.