Why is HIV not vector-borne?

Many pathogens of humans are blood borne, including HIV, Malaria, Hepatitis B and C, West Nile virus, Dengue, and other viral hemorrhagic fevers. Although several of these pathogens are transmitted by blood-feeding arthropods, HIV is not. A number of properties of HIV and its life cycle have been identified as proximate explanations for the absence of arthropod transmission, but little consideration has been given to why HIV has not evolved this form of transmission. We consider the empirical evidence for arthropod transmission, and suggest that mechanical transmission has not evolved in HIV because such strains would induce a faster onset of AIDS during infection, which would thereby limit their ability to spread. On the other hand, it is not as clear why biological transmission has not occurred. Available data suggests that a lack of appropriate genetic variation in HIV is one explanation, but it is also possible that a conflict between natural selection occurring within and between infected individuals has prevented its evolution instead. We discuss the potential significance of these ideas, and argue that taking such an evolutionary perspective broadens our understanding of infectious diseases and the potential consequences of public health interventions.