ETH Zurich researchers discover HIV transmission chains

Since the emergence of the first AIDS cases in the early 1980s, an infectious wave has spread over Switzerland. Every year, several hundred people are infected with the HI-virus which triggers the acquired immunodeficiency syndrome, AIDS. Does the infection spread according to a particular pattern? Do these patterns vary among the transmission groups such as drug addicts, homosexual or heterosexual persons?

These questions were explored by researchers funded by the Swiss National Science Foundation (SNSF) using the latest molecular methods. In a study  just published in The Journal of Infectious Diseases, they focus on HIV-1 subtype B, which accounts for around 70 percent of all cases in Switzerland. They defined the HIV genome of 5,700 persons infected with the virus between 1981 and 2007. Their idea was that the greater the similarity between the viruses of two patients, the greater the likelihood that one infected the other. This allowed the researchers and their colleagues from ETH Zurich to establish transmission chains showing how the virus was passed from one patient to the next.

Two different transmission paths

The researchers discovered a total of 60 different transmission chains in which at least ten persons each were infected with HIV. All of these chains belonged to one of only two chain types: Either the transmission chains comprised mainly addicts who inject heroin or other drugs into their veins and heterosexuals, or they were a chain type in which the virus predominantly spread among homosexual men. The chains with drug addicts and heterosexuals comprise on average 144 patients; in the largest transmission group as many as 1,051 people were infected. Principal investigator, Huldrych Günthard from the University Hospital of Zurich ascribes this primarily to the more common spread of the virus in the drug scene at the beginning of the epidemic. «Initially, a few infected persons swapped contaminated syringes. This led to the virus spreading rapidly from the blood of one drug addict to that of the next.» By contrast, in the case of homosexuals where the virus is transmitted by sexual intercourse, it is spread in much smaller chains. In fact, in homosexual transmission chains, fewer partners, on average 29 people, infected each other.

Key role of drug-related prostitution

«Interestingly, we didn't find any transmission chains in which the virus was predominantly transferred by heterosexuals», says Günthard. This means that the driving factors for the spread of the epidemic in Switzerland were clearly the infection chains among homosexuals and drug addicts on the one hand, and infections acquired abroad on the other. The virus, however, tends to move back and forth from one group to another. Every ninth HI-virus in heterosexuals for instance, originates from homosexual transmission chains. Whereas infection scarcely occurs between homosexuals and drug addicts, the infection of heterosexuals by drug addicts was very frequent, especially in the early 1980s. «Drug-related prostitution certainly played a significant role in this», says Günthard.

The figures, however, show that infections between drug addicts and heterosexuals dropped dramatically later on. This was primarily due to the fact that the epidemic could be contained by the introduction in 1986 of sterile needle distribution to drug addicts in exchange for used ones. This also resulted in fewer heterosexual people being infected with HIV. The syringe exchange scheme therefore protected society as a whole. Günthard regards this finding to be of international relevance. Although needle exchange programs exist in 77 countries worldwide, these schemes remain controversial in many countries and have still not been introduced, for example, in Iceland, Turkey or Kosovo.