Human T-Lymphotropic Virus type 1 infection in an Indigenous Australian population: epidemiological insights from a hospital-based cohort study

Abstract

Background: The Human T Lymphotropic Virus type 1 (HTLV-1) subtype C is endemic to central Australia where each of the major sequelae of HTLV-1 infection has been documented in the socially disadvantaged Indigenous population. Nevertheless, available epidemiological information relating to HTLV-1c infection is very limited, risk factors for transmission are unknown and no coordinated program has been implemented to reduce transmission among Indigenous Australians. Identifying risk factors for HTLV-1 infection is essential to direct strategies that could control HTLV-1 transmission.

Methods: Risk factors for HTLV-1 infection were retrospectively determined for a cohort of Indigenous Australians who were tested for HTLV-1 at Alice Springs Hospital (ASH), 1st January 2000 to 30th June 2013. Demographic details were obtained from the ASH patient management database and the results of tests for sexually transmitted infections (STI) were obtained from the ASH pathology database.

Results: Among 1889 Indigenous patients whose HTLV-1 serostatus was known, 635 (33.6 %) were HTLV-1 Western blot positive. Only one of 77 (1.3 %) children tested was HTLV-1 infected. Thereafter, rates progressively increased with age (15-29 years, 17.3 %; 30-49 years, 36.2 %; 50-64 years, 41.7 %) reaching 48.5 % among men aged 50-64 years. In a multivariable model, increasing age (OR, 1.04; 95 % CI, 1.03-1.04), male gender (OR, 1.41; 95 % CI, 1.08-1.85), residence in the south (OR, 10.7; 95 % CI, 7.4-15.6) or west (OR, 4.4; 95 % CI, 3.1-6.3) of central Australia and previous STI (OR, 1.42; 95 % CI, 1.04-1.95) were associated with HTLV-1 infection. Infection was acquired by three of 351 adults who were tested more than once during the study period (seroconversion rate, 0.24 (95 % CI = 0.18-2.48) per 100 person-years).

Conclusions: This study confirms that HTLV-1 is highly endemic to central Australia. Although childhood infection was documented, HTLV-1 infection in adults was closely associated with increasing age, male gender and STI history. Multiple modes of transmission are therefore likely to contribute to high rates of HTLV-1 infection in the Indigenous Australian population. Future strategies to control HTLV-1 transmission in this population require careful community engagement, cultural understanding and Indigenous leadership.

Keywords: Australia; Epidemiology; HTLV-1; Indigenous; Sexually transmitted infections.

Fig. 1

a. Map showing the area served by Alice Springs Hospital (yellow), which includes the Anangu Pitjantjatjara Yankunyatjara (APY) lands of South Australia (A), the Ngaanyatjarraku shire of West Australia (B) and the Central Desert Shire and MacDonnell Shires of the Northern Territory (C). b. Detailed map of central Australia showing the location of the two major population centres, Tenant Creek and Alice Springs

Fig. 2

Flow diagram showing reasons for excluding patients from analysis. 1945 subjects were initially screened for HTLV-1 infection using serological tests. In 56 cases (Group 1, 48; Group 2, 4; Group 3, 4), initial serological screening tests were positive, but subjects were excluded because confirmatory testing could not be performed. In 19 cases confirmatory Western blot tests could not be performed because insufficient sample remained after the initial screening tests. In a further 37 cases, Western blots were indeterminate, but an appropriate sample for HTLV-1 PCR could not be collected because patients had returned to their remote communities before the Western blot result was available to clinicians. Thus, 1889 subjects were included in the final analysis. Abbreviations: HTLV-1+, positive HTLV-1 serological screening test; HTLV-1 WB+, positive HTLV-1 Western blot test; PCR, polymerase chain reaction; WB, Western blot

Fig. 3

Graph of HTLV-1 seropositivity rates for Indigenous males and females according to age among 1889 patients tested at Alice Springs Hospital