BACKGROUND:
Transmission of drug-resistant (DR) strain is associated with treatment failure in newly infected individuals and represents an important therapeutical challenge. Incidence of DR transmission in newly infected individuals may reflect the DR prevalence in chronically infected population. As antiretroviral drug discontinuation is associated with reversion to wild-type virus we should expect a decrease in DR among newly infected individuals after 2001, where therapeutic recommendations have been modified. The objectives of this study were: 1) to monitor the changes in DR prevalence in the Montreal Primary HIV Infection Cohort; and 2) to assess if decrease in DR prevalence may be explained by postponing or interruption of antiretroviral therapy among chronically infected patients followed in HIV clinics in Montreal.

METHODS:
From May 1996 to December 2002, 159 newly infected patients were studied for DR using genotyping (Visible Genetics TRUGENE) and phenotyping analyses (Virologic PhenoSense HIV assay). DR prevalence was compared before and after 2001, when the therapeutic recommendations were introduced. Assessments of antiretroviral usage were performed in chronically infected patients followed in two downtown HIV clinics, together representing an estimated 50% of the total followed Montreal HIV population

RESULTS:
Patient characteristics such as age, gender, risk factors and time elapsed between infection and study enrolment remained similar over time. Patients harbouring any mutation by genotypic analysis [either nucleoside reverse transcriptase inhibitors (NRTIs), non-NRTIs (NNRTIs), protease inhibitors (PIs)] before January 2001 represented 17% (n=20) of the cohort as opposed to 8% (n=3) of such patients after 2001 (P=0.2). Similarly, no significant decrease in DR prevalence for each drug class was observed [NRTIs (4 vs 2.6%), NNRTIs (11.0 vs 2.6%), PI (8.7 vs 2.6%) and two or three classes multi-DR (11 vs 3%)]. However, when risk factors were analysed a significant decrease in DR prevalence was evidenced only among men having sex with men (MSM) (16 vs 0%, P=0.05), injecting drug users (IDU) (20 vs 9%, P=0.4) and heterosexuals (8 vs 40%, P=0.12). Among patient characteristics only IDU predicts DR (OR=2.5 [95% CI=1.4–4.5], P=0.002) and this higher ratio may be explained in part by low drug compliance and by free drug assess in Canada. Likely, decrease in DR prevalence observed after 2001 is not explained by an abrupt change in the proportion of chronically infected patients (n=2560) who discontinued therapy or were drug-naive for antiretroviral in the two Montreal clinics.