Outbreak of Rickettsia typhi Infection — Austin, Texas, 2008

Opossum

The November 20, 2009, issue of the CDC’s Morbidity Mortality Weekly Reports includes an account of a murine typhus outbreak in Austin, Texas, reported in 2008.  Murine typus is a fleaborne rickettsial disease caused by the organism Rickettsia typhi.  Environmental investigation suggested that opossums and domestic animals likely played a role in the maintenance and spread of R. typhi, which is normally transmitted by rat or mouse fleas.

Although murine typhus is endemic in southern Texas, only two cases had been reported during the past 10 years from Austin, located in central Texas.  On August 8, 2008, the Austin/Travis County Department of Health and Human Services (ATCDHHS) contacted the Texas Department of State Health Services (TDSHS) concerning a cluster of 14 illnesses with serologic findings indicative of murine typhus.  On August 12, 2008, TDSHS initiated an investigation with assistance from CDC to characterize the magnitude of the outbreak and assess potential animal reservoirs and peridomestic factors that might have contributed to the disease.

According to the TDSHS report, 33 confirmed cases involved illness comparable to that associated with previous outbreaks of murine typus.  Illness ranged from mild to severe, with 73% of patients requiring hospitalization.   Delayed diagnosis and administration of no or inappropriate antibiotics might have contributed to illness severity. Environmental investigation suggested that opossums and domestic animals likely played a role in the maintenance and spread of R. typhi; however, their precise role in the outbreak has not been determined.

The first two case reports associated with this outbreak were received in April 2008. Fourteen more reports were received in May and June. Receipt of eight additional case reports in July prompted ATCDHHS to seek assistance from TDSHS. On August 8, CDC was requested to assist in the investigation. An additional 29 cases were reported during the course of the investigation, which concluded on December 1.

Of the 53 cases reported during 2008, 33 (62%) were laboratory confirmed. Illness onset dates for confirmed cases ranged from March to November, with the highest number of cases occurring in June.  Most confirmed cases occurred during May–August (70%). The most commonly reported symptoms in confirmed cases were fever (100%), malaise (76%), headache (73%), chills (61%), myalgia (61%), anorexia (58%), nausea (52%), rash (46%), vomiting (42%), and diarrhea (36%). No deaths were attributed to murine typhus; however, 73% of the confirmed patients were hospitalized, and 27% were admitted to intensive-care units.

Confirmed patients were clustered in central Austin.  Two patients resided north of Austin but worked or engaged in recreational activities in central Austin.  Among the 33 confirmed cases, only two patients (6%) noted flea bites or flea exposure during the two weeks before illness onset. Recent close exposure to opossums or rats was reported by 18% and 15% of patients, respectively.

Opossum Tracks

Most patients with confirmed cases resided in homes with yards bordered by thick vegetation; 79% owned a dog or cat, but only 42% reported regularly administering flea or tick preventatives. Nineteen (95%) of the 20 households assessed had obvious evidence of wildlife or wildlife attractants on the property (e.g., pet food or water dishes outside the home or unsealed outdoor garbage containers). Among the 57 animals assessed, only 33%  had evidence of active murine typhus infection by serology. Antibodies to R. typhi were detected in three feral cats, four domestic dogs, and 12 opossums; none of four wild rats or nine raccoons tested positive. None of the animal tissue or flea specimens tested positive for R. typhi or R. felis DNA by PCR.  Seropositive animals were from five different postal code areas. Most seropositive animals (68%) were found in the two contiguous postal code areas where the most human cases (36%) were reported.

In response to this outbreak, ATCDHHS increased public awareness of fleaborne rickettsiosis via alerts posted on its Internet website. The alerts included a definition of murine typhus and its symptoms in addition to descriptions of how the disease is transmitted, treated, and prevented. Recommended prevention and control measures included using dog and cat flea preventatives, exterminating household rodents, eliminating rodent habitats in or near homes, using pesticides to limit flea infestations, avoiding wild animals (including feral cats and opossums), and using insect repellents containing DEET.

These findings underscore the need to increase awareness of murine typhus and communicate appropriate treatment and prevention measures through the distribution of typhus alerts before and throughout the peak vector season of March–November.

Source: CDC’s Morbidity Mortality Weekly Report November 20, 2009 / 58(45);1267-1270

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