Enzootic transmission of yellow fever virus in Peru

Abstract

The prevailing paradigm of yellow fever virus (YFV) ecology in South America is that of wandering epizootics. The virus is believed to move from place to place in epizootic waves involving monkeys and mosquitoes, rather than persistently circulating within particular locales. After a large outbreak of YFV illness in Peru in 1995, we used phylogenetic analyses of virus isolates to reexamine the hypothesis of virus movement. We sequenced a 670-nucleotide fragment of the prM/E gene region from 25 Peruvian YFV samples collected from 1977 to 1999, and delineated six clades representing the states (Departments) of Puno, Pasco, Junin, Ayacucho, San Martin/Huanuco, and Cusco. The concurrent appearance of at least four variants during the 1995 epidemic and the genetic stability of separate virus lineages over time indicate that Peruvian YFV is locally maintained and circulates continuously in discrete foci of enzootic transmission.

Figure 1

A) Peruvian river basins in which yellow fever virus is endemic. B) Annual incidence of confirmed cases of yellow fever in Peru, 1972–2001.

Figure 2

Maximum likelihood phylogeny of prM/E sequences of Peruvian yellow fever isolates constructed using PAUP*, 4.0b4a (17). Horizontal branch lengths represent genetic divergence, and numbers above the branch lengths denote support for individual clades as determined by nonparametric bootstrap analysis with 1,000 replicates (first value) and Bayesian posterior probabilities (second value). Only the values relevant for the interpretation of results are given. The strains used are listed in Table 1.

Figure 3

Amino acid alignment for the 25 Peruvian yellow fever virus. Dots indicate identity with prototype Asibi strain (from Ghana, 1927).

Figure 4

Schematic topographic cross-section through the southern Peruvian Amazon. Areas in red indicate river basins where yellow fever virus is endemic.