Rickettsial Illnesses as Important Causes of Febrile Illness in Chittagong, Bangladesh

Abstract

We conducted a yearlong prospective study of febrile patients admitted to a tertiary referral hospital in Chittagong, Bangladesh, to assess the proportion of patients with rickettsial illnesses and identify the causative pathogens, strain genotypes, and associated seasonality patterns. We diagnosed scrub typhus in 16.8% (70/416) and murine typhus in 5.8% (24/416) of patients; 2 patients had infections attributable to undifferentiated Rickettsia spp. and 2 had DNA sequence-confirmed R. felis infection. Orientia tsutsugamushi genotypes included Karp, Gilliam, Kato, and TA763-like strains, with a prominence of Karp-like strains. Scrub typhus admissions peaked in a biphasic pattern before and after the rainy season, whereas murine typhus more frequently occurred before the rainy season. Death occurred in 4% (18/416) of cases; case-fatality rates were 4% each for scrub typhus (3/70) and murine typhus (1/28). Overall, 23.1% (96/416) of patients had evidence of treatable rickettsial illnesses, providing important evidence toward optimizing empirical treatment strategies.

Keywords: Bangladesh; Orientia tsutsugamushi; Rickettsia felis; Rickettsia typhi; bacteria; murine typhus; rickettsia; rickettsial disease; scrub typhus; undifferentiated febrile illness; vector-borne diseases.

Figure 1

Geographic distribution of scrub typhus (A) and murine typhus (B) cases, Chittagong, Bangladesh, August 2014–September 2015. Inset shows location of enlarged area (red box).

Figure 2

Seasonality of scrub typhus and murine typhus–related hospital admissions, Chittagong Medical College Hospital, Chittagong, Bangladesh, August 2014–September 2015. We observed a biphasic pattern in scrub typhus, with an increase of cases in the cooler months and a smaller peak before the rainy season.

Figure 3

Phylogenetic analysis of pathogens contributing to rickettsial illnesses, Chittagong Medical College Hospital, Chittagong, Bangladesh, August 2014–September 2015. A) Phylogenetic dendrogram based on the nucleotide sequence of the partial open reading frame of the 56-kDa TSA gene (aligned and cropped to ≈450 bp), depicting Orientia tsutsugamushi strains in relationship with reference and other strains. O. tsutsugamushi genotypes in Bangladesh included Karp, Gilliam, Kato, and TA763 strains, with a predominance of Karp-like strains. B) Rickettsia spp. as characterized by 17-kDa gene sequencing (aligned and cropped to 314 bp). The predominant pathogen identified was R. typhi. Of note, 2 R. felis infection cases were identified, including 1 systemic bloodstream infection and 1 scrub typhus case with eschar swab positivity in patient no. SW211ES (Technical Appendix Table), whose blood specimen was negative for R. felis, suggesting possible skin colonization of R. felis. Scale bar indicates nucleotide substitutions per site; branches shorter than 0.002 are shown as having a length of 0.002.