Why is HIV not vector-borne?

Abstract

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.

Keywords: AIDS; disease; evolutionary medicine; insect-borne transmission; mosquito.

Figure 1

Sexual transmission rate (solid line) and vector transmission rate (dashed line) as a function of viremia, ε. The resulting per capita growth rate based on equation (4) is also plotted (growth rate is negative where is falls below the horizontal axis, meaning that such strains can never increase in number). Letters ‘A’ and ‘B’ denote the sexually transmitted and vector transmitted genotypes considered in model (5) of the text. Parameter values: δ₀ = 1/65, a = 1, v = 100, x = 5, μ = 15. Panel (a) – vector transmission is selectively disadvantageous. , , b₂ = 0.945. Figure is drawn with the growth rate of ‘B’ less than that of ‘A’ but still positive for illustrative purposes only. Negative values of r_B also readily occur with only slight changes in parameters values. Panel (b) – vector transmission is selectively advantageous. , , b₂ = 1.1.

Figure 2

The factor by which vectors must increase the overall transmission rate of HIV in order for a vector transmissible virus to increase in relative frequency by a factor of K, as a function of the amount of time over which evolution occurs (between 30 and 50 years for HIV in humans). Solid lines assume that the increased viremia caused by the vector-transmissible virus decreases the time until the development of AIDS from τ_A = 8 to τ_B = 5 years. Dashed lines assume a reduction from τ_A = 8 to τ_B = 1 year. The parameter K has very little effect over several orders of magnitude, meaning that the benefit of vector transmission required for it to evolve over 30–50 years is determined largely by the value of τ_B.