Men taking antiretroviral drugs to treat HIV infection may be at an increased risk for syphilis, a new study has warned.
The availability of highly active antiretroviral therapy (HAART) for the treatment of human immunodeficiency virus (HIV) infection has meant that the infection is no longer the automatic death sentence that it once was, researchers said.
The absence of the fear factor is thought to have prompted higher rates of sexually transmitted infections as a result of risky, unprotected sex. Researchers from the University of British Columbia in Canada and African Institute for Mathematical Sciences in South Africa said it is not clear why rates of syphilis among gay and bisexual men should be so much higher than those of chlamydia or gonorrhoea, especially as HAART boosts immune system activity and so would be expected to lower susceptibility to infections.
They reviewed the available research on the impact of HAART on behavioural and immune system change to come up with a numerical analysis to explore which might affect the prevalence of the infection the most.
They used two risk models to test the likelihood of syphilis infection: one (lower risk) which compared HAART with no treatment in an HIV-positive partner; and the other (higher risk) which compared existing infection with no infection in a partner who was either HIV negative or positive.
Behaviour change was taken to mean that HAART would result in more sexual partners; and immune system changes were taken to mean that HAART would boost susceptibility to Treponema pallidumm – a spirochaete bacterium with subspecies that cause treponemal diseases such as syphilis.
The calculations showed that either factor could produce outbreaks of syphilis that would be substantially higher than expected, but that both factors combined produced a peak in the number of infections that was greater than that associated with either factor alone – and equivalent to the sorts of figures seen in the current outbreak.
This suggests that there is an interplay between behavioural change and immune system changes, researchers said, offering a possible explanation for a biological effect on the immune system.
The body’s clearance of T pallidum relies on an increase in the number of an immune system cell called a T cell and a cascade of chemicals that stimulate an inflammatory response. HAART dampens these activities. Clearance of chlamydia and gonorrhoea is less reliant on these processes. Researchers also refer to other relevant associations, such as the increased risk of certain types of cancer that have been linked to HAART.
“Overall, these findings suggest a possible link between HAART and an increased risk for selected diseases of infectious and non-infectious origin, a potential unforeseen consequence that warrants further study,” they added.
The study appears in the journal Sexually Transmitted Infections.