Foreign tick smuggling rickettsia evades Australian border control

Tick-borne infectious diseases, including rickettsial infections, acquired in Australia or after international travel remain a diagnostic challenge.1

A 68-year-old man presented with umbilical pain 10 days after returning from a 2-month camping trip through the south-west of the United States (ie, Texas, New Mexico, Arizona, Colorado and Utah). On examination, a live tick was detected and removed from the patient’s umbilicus (Box).

The umbilical pain resolved after tick removal. There was no development of fevers, constitutional symptoms, or rash to suggest a tick-borne illness. Laboratory investigations were unremarkable. He was educated about the signs and symptoms of tick-borne illnesses and prescribed a single dose of doxycycline 200 mg for prophylaxis due to his high risk exposure.

The tick was identified as Dermacentor andersoni (Rocky Mountain wood tick), which is endemic to North America and not known to occur in Australia.2 D. andersoni adult ticks are principal vectors of Rickettsia rickettsii (the cause of Rocky Mountain spotted fever), and are associated with transmission of other pathogens to humans, including Colorado tick fever virus and Francisella tularensis (the cause of tularemia).2,3 Although isolated from D. andersoni ticks, transmission of Coxiella burnetii (the cause of Q fever) is uncommon. D. andersoni is not known to transmit Lyme disease.2

Analysis of the tick for rickettsial DNA was positive. No Borrelia DNA was detected. Rickettsia was isolated in cell culture and identified as Rickettsia peacockii based on sequencing of the 17kDa, OmpB, gltA and Sca4 genes. R. peacockii is a member of the spotted fever group of rickettsiae.3,4 The presence of R. peacockii in ticks is correlated with reduced prevalence of R. rickettsii.2,3 R. peacockii is closely related to R. rickettsii, and deletion or mutation of genes, possibly resulting in loss of virulence in R. peacockii, have been identified.3 R. peacockii is not known to be a pathogen of humans or other animals.3,4

Rickettsial serology 10 weeks after the tick bite showed detectable antibodies (titre, 1/256), predominantly to the spotted fever group of Rickettsia, compatible with exposure to R. peacockii identified in the tick. Unfortunately, definitive seroconversion or a rising antibody titre was not able to be demonstrated as no earlier sera were available for parallel testing. Pre-existing antibodies from a distant rickettsial exposure from his tick-prone lifestyle (history of extensive international camping trips) cannot be excluded. The patient remains asymptomatic 9 months later and is still an avid traveller.

Tick-borne rickettsial infections in Australia include Queensland tick typhus (Rickettsia australis), Flinders Island spotted fever and Australian spotted fever (Rickettsia honei) and Q fever transmitted by ticks including Ixodes spp., Amblyomma triguttatum and Bothriocroton hydrosauri.1

With increasing international travel, recognition of tick-borne rickettsial diseases is becoming more important. Dermacentor ticks have been detected on livestock exported from North America into Europe.5 This case shows the ability of human ectoparasites, and their potentially pathogenic bacteria, to bypass stringent Australian quarantine controls. Further studies of Australian and imported tick-borne infections are required to increase understanding of these emerging infectious diseases.

Box – Dermacentor andersoni removed from the patient’s umbilicus