On October 4, the Los Angeles County Department of Public Health (LAC DPH) announced it was investigating an outbreak involving 9 cases of flea-borne typhus in downtown Los Angeles. On October 12, LAC DPH identified an additional cluster involving 4 cases of flea-borne typhus in the city of Compton. Of note, 6 of the 9 cases confirmed in downtown Los Angeles occurred in persons experiencing homelessness.

Flea-borne typhus is endemic in Los Angeles County, which means that cases occur annually. However, according to LAC DPH, “in recent years the average number of reported cases has doubled to nearly 60 cases per year.” The county has experienced 63 cases of typhus so far in 2018. The Los Angeles Times reports that there have been 20 cases of typhus this year in Pasadena alone, which usually experiences only five cases annually.

The current outbreak has prompted local officials, including Mayor of Los Angeles Eric Garcetti, to form a task force aimed at containing the spread of infection. Below, we explore possible reasons why typhus, a group of diseases which is typically associated with overcrowding and poor hygiene, appears to be making such a comeback in Los Angeles County.

Background: What is typhus?

Typhus (not to be confused with typhoid fever) is a group of diseases caused by several different kinds of bacteria that are transmitted to humans by pests such as fleas, lice, and chiggers. There are three main types of typhus: epidemic, flea-borne (also known as murine or endemic typhus), and scrub typhus.

The outbreak in Los Angeles County is caused by flea-borne typhus, and it is spread to people via contact with fleas infected with Rickettsia typhi bacteria. It is also known as murine typhus because rats are the primary animal host for infected fleas, although cats and opossums may also harbor fleas capable of transmitting the infection, leading to zoonotic infection of humans. The CDC notes that flea-borne typhus is endemic in many tropical and sub-tropical climates, and most cases in the US are reported in southern California, south Texas, and Hawaii.

Human infection occurs when humans are bitten by infected fleas or when feces of infected fleas is rubbed into human cuts or scrapes in the skin. Person-to-person transmission has not been documented. Symptoms of flea-borne typhus include fever and chills, headache, muscle and body aches, nausea, and vomiting. A retrospective study of flea-borne typhus in Texas from 1985 to 2015 revealed a case fatality of less than 1%; however, some cases can be severe or life-threatening, leading to hospitalization. The LAC DPH reports that 3 of the 4 cases in Compton required hospitalization. A blood test is used to diagnose flea-borne typhus, and the disease can be effectively treated with the antibiotic doxycycline. There is no vaccine for murine typhus.

In contrast with flea-borne typhus, epidemic typhus, which killed millions of people during the Middle Ages, is transmitted via body lice infected with the bacteria Rickettsia prowazekii. Because of the severity of disease and its potential for use as a biological weapon, R. prowazekii is regulated by the US Federal Select Agent Program. Fortunately, due to advances in hygiene, sanitation, and living conditions which have led to a reduction in lice infestations, epidemic typhus is now considered a rare disease.

Why is flea-borne typhus on the rise?

The incidence of flea-borne typhus in Los Angeles County appears to be rising. In its 2016 annual morbidity report, LAC DPH states that the county “continues to document high numbers of typhus compared to the previous decade, in which the count did not exceed 20 cases per year.” According to California Department of Public Health data from 2001 to 2018, this upward trend is also notable in Orange County, which is adjacent to and southeast of Los Angeles County. While there is no single reason for the uptick in typhus cases, several factors may be to contributing to this increase.

First, case counts for flea-borne typhus may previously have been underreported due to limited surveillance and low clinical suspicion. According to experts with experience controlling typhus in Los Angeles County, “there are probably cases of murine typhus that are undiagnosed because of mild symptoms and lack of physician awareness.” Typhus is not a notifiable disease in California. In recent years, growing awareness of the disease via public health education efforts and media reports may serve to heighten clinical suspicion, leading to increased diagnostic testing for and reporting of typhus.

Second, Los Angeles County has been experiencing a major homelessness and housing crisis in recent years, which may be linked to the increase in flea-borne typhus. As mentioned above, 6 of the 9 typhus patients in downtown Los Angeles reported “experiencing homelessness” (although the Los Angeles County health alert does not specify how recently), and all patients have a history of living or working downtown. Homeless populations often live in close proximity with others and have limited access to facilities and services that support good personal hygiene. In addition, as noted in the Los Angeles Times article above, trash has accumulated in and alongside streets, sidewalks, and alleyways where homeless individuals congregate. LAC DPH warns that “places where there is an accumulation of trash that attract wild animals like feral cats, rats, and opossums that may carry an infected flea can increase the risk of exposure.” Similar factors have contributed to hepatitis A outbreaks among similar populations in Los Angeles County and other urban areas. The state of California declared a state of emergency last year in response to the hepatitis A outbreak.

Third, while typhus has historically been associated with densely-packed living conditions, this does not necessarily explain the typhus clusters in Compton and Pasadena, which are more suburban in nature. In fact, for decades typhus cases have been reported “in upper-middle-class and middle-class persons who live in the suburban foothills in central Los Angeles County.” These cases may be due in part to “the natural relocation of host animals (opossums and feral cats) to regions not previously enzootic for typhus.” An “urban cycle” of typhus transmission involving rat fleas is well established, but there may also be a “suburban cycle” involving cats and opossums found in residential communities, which typically accounts for most reported cases in Los Angeles County. Opossums captured in Los Angeles County in the 1990s were found to harbor large numbers of fleas infected with R. typhi.

Finally, it is possible that rising global temperatures (eg, via climate change) are contributing to an increase in the habitable range of infected species. While this has not yet been demonstrated for murine typhus, studies have shown possible links between rising temperatures and an increase in scrub typhus cases. Most cases of flea-borne typhus occur in the summer and fall in southern California, and spring and summer in south Texas, when temperatures are hottest.

Conclusion

The recent outbreak of flea-borne typhus in Los Angeles County, which has so far experienced 63 cases in 2018, is part of an upward trend of annual average typhus cases in southern California in recent years. The typhus outbreak has prompted local officials, including the mayor of Los Angeles, to form a task force aimed at controlling the outbreak. Steps include cleaning the downtown area of accumulated trash, corralling stray animals that harbor infected fleas, and educating the public to take preventative actions, including treating their pets for fleas and disposing of trash properly.

There is reason to suspect, however, that a growing number of flea-borne typhus outbreaks may occur in endemic areas in the years to come. Rising rates of homelessness, the trend toward urbanization, and climate change may contribute to spreading typhus, as well as other diseases known to disproportionately affect the urban poor and homeless populations. Other cities in the US, especially those located in sub-tropical climates (eg, cities in Florida and other southern states), may need to be on the lookout for the emergence of typhus. As with the spread of hepatitis A across the country, other cities with characteristics similar to Los Angeles (eg, housing/homeless crisis, rising temperatures, migration of urban animals to the suburbs) could potentially face similar risks from typhus.

Improved and expanded preventative efforts will be needed to tackle these underlying factors, such as addressing the homeless and housing crises. Physicians in endemic areas will also need to adopt higher clinical suspicion to treat typhus cases at an early stage before costly hospitalization is required, while working closely with public health departments to ensure accurate epidemiology and surveillance for this centuries-old disease.

Photo: Flea-borne typhus is spread to humans via contact with rats, cats, and opossums, such as the one pictured, which may harbor fleas capable of transmitting the infection. Photo courtesy of royguisinger/Pixabay.

Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly to allow others to learn from these experiences in order to improve global outbreak response capabilities.