Since the mid-1990s, genetic fingerprinting of HIV strains has helped convict suspects who transmitted the deadly virus to their victims.
A genetic “match” between the viral strain infecting rapist and victim is frequently presented to juries and TV audiences with certitude, as a molecular smoking gun. That seemingly compelling piece of evidence is “increasingly determining convictions by criminal courts,” according to six European virologists in the current issue of The Lancet Infectious Diseases.
But calling the comparison of HIV strains’ genes “fingerprinting” — calling to mind the more-familiar matching of human suspects’ DNA to blood at a crime scene — is dangerously misleading, they warn.
“By calling such investigations HIV fingerprinting, scientists raise unrealistic expectations” about the method’s accuracy among juries and judges, the write. “Unlike for (human) DNA fingerprinting, where a likelihood can be calculated for a full match between the evidential DNA and the suspect’s DNA, there is never a full match between the RNA or the DNA of HIV in two samples, even within an individual.”
That is partly because HIV strains are constantly evolving, they note.
“Proper identification of the transmission source would require two major assumptions: that a phylogenetic tree can flawlessly reconstruct a true epidemic history and that strains from all patients ever infected with HIV are available as controls,” the authors write. “Both assumptions are unrealistic.”
Worse, since the full range of local genetic variation and the relatedness of local HIV strains are unknown, the probability that one individual’s HIV infection came from another specific individual simply cannot be quantified, the authors note.
“Because the full transmission tree is unknown, no likelihood can be attached to the a priori hypothesis of direct transmission,” they write.
Indirect routes of infection through third parties can never be ruled out, and the rapid evolution of strains could inaccurately clear a real rapist, they caution.
“Phylogenetic analysis is a powerful technique that can, if properly used, provide valuable circumstantial evidence in forensic investigation for cases of HIV transmission,” they conclude. “However, scientists should be aware of the limitations of this analysis, and should emphasize that courts must use other evidence to achieve a conviction.”
The authors recommend six best practices to avoid misuse of viral phylogenetic data in criminal cases, including the use of two samples from different points in time from each party under investigation as close in time to the alleged transmission, to analyze more than one region of the viral genomes, and for testifying scientists to make no claims or estimates of the statistical probability of direct transmission.
The authors declared that they have no conflicts of interest.
Abecasis AB, et al. Science in court: the myth of HIV fingerprinting. Lancet Infectious Diseases. 2011(Feb);11:78-79.
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Shankarappa R, et al. Consistent viral evolutionary changes associated with the progression of human immunodeficiency virus type 1 infection. Journal of Virology. 1999;73:10489-104502.