The isolation of Orientia tsutsugamushi and Rickettsia typhi from human blood through mammalian cell culture – a descriptive series of 3,227 samples and outcomes in the Lao PDR

In the Lao People's Democratic Republic (Laos) rickettsial infections, including scrub and murine typhus, account for a significant burden of fevers. The Mahosot Hospital Microbiology Laboratory in Vientiane, Laos, routinely performs rickettsial isolation from hospitalised patients with suspected rickettsioses using mammalian cell culture systems. We review the clinical and laboratory factors associated with successful Orientia tsutsugamushi and Rickettsia typhi isolations over a period of six years between 2008 and 2014 from this laboratory.The overall isolation success was 7.9% for all samples submitted, and 17.3% for samples for which the patient had positive O. tsutsugamushi or R. typhi RDT, serology or PCR. The frequency of successful isolation was highest for samples submitted in November, at the end of the wet season (28.3%). A longer median duration of reported illness and a positive result for a concurrent Orientia or Rickettsia spp. quantitative PCR, and the use of antibiotics by the patient in the week before admission were significantly associated with isolation success (p<0.05). Buffy coat inoculation and a shorter interval between sample collection and inoculation in the laboratory were associated with a higher frequency of isolation (both p<0.05). This frequency was highest if cell culture inoculation occurred on the same day as blood sample collection.Factors related to the initial rickettsial bacterial concentration are likely the main contributors to isolation success. However, modifiable factors do contribute to the rickettsial isolation success, especially delays in inoculating patient samples into culture.