Assessing the Epidemic Potential of RNA and DNA Viruses

Abstract

Many new and emerging RNA and DNA viruses are zoonotic or have zoonotic origins in an animal reservoir that is usually mammalian and sometimes avian. Not all zoonotic viruses are transmissible (directly or by an arthropod vector) between human hosts. Virus genome sequence data provide the best evidence of transmission. Of human transmissible virus, 37 species have so far been restricted to self-limiting outbreaks. These viruses are priorities for surveillance because relatively minor changes in their epidemiologies can potentially lead to major changes in the threat they pose to public health. On the basis of comparisons across all recognized human viruses, we consider the characteristics of these priority viruses and assess the likelihood that they will further emerge in human populations. We also assess the likelihood that a virus that can infect humans but is not capable of transmission (directly or by a vector) between human hosts can acquire that capability.

Keywords: DNA viruses; RNA viruses; emerging viruses; epidemic potential; outbreaks; transmission; viruses; zoonoses.

Figure 1

Pathogen pyramid for RNA and DNA viruses. Level 1 indicates viruses to which humans are exposed but which do not infect humans. Level 2 indicates viruses that can infect humans but are not transmitted from humans. Level 3 indicates viruses that can infect and be transmitted from humans but are restricted to self-limiting outbreaks. Level 4 indicates viruses that are capable of epidemic spread in human populations. Transitions between levels (indicated by arrows) correspond to different stages of virus emergence in human populations. Reprinted from Woolhouse et al. (10).

Figure 2

Expected outbreak dynamics for RNA and DNA viruses given a single primary case in a large, previously unexposed host population, as a function of the basic reproduction number R₀. Mean size of outbreak as total number of cases (N) is given by N = 1/(1 − R₀) for R₀<1 (light gray line, left axis). Probability of 0 secondary cases (i.e., outbreak size N = 1) is given by P₁ = exp(−R₀) (black line, right axis). Probability of a major outbreak is given by P_takeoff = 1 – 1/R₀ for R₀>1 (dark gray line, right axis).

Figure 3

Distribution of outbreak sizes for RNA and DNA viruses as plots of outbreak size x (horizontal axis) versus fraction of outbreaks of size >x (vertical axis), both on logarithmic scales. Data are shown for 4 infectious diseases. Squares indicates Andes virus disease in South America (24); diamonds indicate monkeypox in Africa (26); circles indicate Middle East respiratory syndrome in the Middle East (25); and triangles indicate filovirus (all species) diseases in Africa before 2013 (27). For comparison, expected values for the case R₀ = 1, obtained from the expression for the probability of an outbreak of size >x, P(x) = Γ(x – 1/2)/√πΓ(x), are also shown (dashed line). Data for filoviruses are not consistent with expectation for R₀<1.

Figure 4

Phylogenetic trees for simulated emerging infectious disease outbreaks caused by RNA and DNA viruses in a mixed population of 1,000 human and 5,000 nonhuman hosts. Trees were constructed by using a standard susceptible–infected–removed model (6). For each of 3 infection scenarios in nonhuman hosts (black lines), rare zoonotic transmission events (blue lines), human-to-human transmission (red lines), and human cases (red circles) are indicated. For the nonhuman population R₀ = 2 throughout. Transmissibility within the human populations varies from A) spillover: no human−human transmission (R₀ = 0); B) limited human−human transmission with R₀ = 1; and C) epidemic spread within humans (R₀>1). A maximum of 100 infections are randomly sampled from each population in each simulated outbreak.