Emerging pattern of rabies deaths and increased viral infectivity

Abstract

Most human rabies deaths in the United States can be attributed to unrecognized exposures to rabies viruses associated with bats, particularly those associated with two infrequently encountered bat species (Lasionycteris noctivagans and Pipistrellus subflavus). These human rabies cases tend to cluster in the southeastern and northwestern United States. In these regions, most rabies deaths associated with bats in nonhuman terrestrial mammals are also associated with virus variants specific to these two bat species rather than more common bat species; outside of these regions, more common bat rabies viruses contribute to most transmissions. The preponderance of rabies deaths connected with the two uncommon L. noctivagans and P. subflavus bat rabies viruses is best explained by their evolution of increased viral infectivity.

Figure 1

Phylogenetic tree of bat-associated rabies cases. Taxa represent 208 rabies virus variants from 27 human rabies cases (formalin-fixed taxa removed) and 98 terrestrial mammals infected with a bat rabies virus, 60 bat samples representing 17 species, and 23 terrestrial mammal outgroup taxa. Each circle represents a case (terrestrial mammal = closed circles, human = open circles) associated with the monophyletic clade in the phylogeny to the left. Numbers at tree nodes indicate nonparametric bootstrap proportions.

Figure 2

Geographic distribution of (A) human (including 5 formalin-fixed samples not shown in Figure 1) and (B) terrestrial mammal cases identified by rabies virus variant isolated. Minimum length polygons delimiting human cases associated with Silver-haired Bats (clade 1) and Eastern Pipistrelles (clade 2) shown in (A) and superimposed in (B). (C) Prevalence in regions delimited by clades 1 and 2. Lasionycteris noctivagans (Ln) and Pipistrellus subflavus (Ps) prevalence in bats was estimated from unpublished state public health department reports that determined the percentage of rabies-positive Silver-haired or Eastern Pipistrelle bats from the total number of bats submitted (7, unpub. data). L. noctivagans and P. subflavus prevalence in terrestrial mammals and humans is estimated as percentage of all spillover cases in each clade region infected with L. noctivagans or P. subflavus.