Activity Patterns of St. Louis Encephalitis and West Nile Viruses in Free Ranging Birds during a Human Encephalitis Outbreak in Argentina

Abstract

St. Louis encephalitis virus (SLEV) (Flavivirus) is a reemerging arbovirus in the southern cone of South America. In 2005, an outbreak of SLEV in central Argentina resulted in 47 human cases with 9 deaths. In Argentina, the ecology of SLEV is poorly understood. Because certain birds are the primary amplifiers in North America, we hypothesized that birds amplify SLEV in Argentina as well. We compared avian SLEV seroprevalence in a variety of ecosystems in and around Córdoba city from 2004 (before the epidemic) and 2005 (during the epidemic). We also explored spatial patterns to better understand the local ecology of SLEV transmission. Because West Nile virus (WNV) was also detected in Argentina in 2005, all analyses were also conducted for WNV. A total of 980 birds were sampled for detection of SLEV and WNV neutralizing antibodies. SLEV seroprevalence in birds increased 11-fold from 2004 to 2005. Our study demonstrated that a high proportion (99.3%) of local birds were susceptible to SLEV infection immediately prior to the 2005 outbreak, indicating that the vertebrate host population was primed to amplify SLEV. SLEV was found distributed in a variety of environments throughout the city of Córdoba. However, the force of viral transmission varied among sites. Fine scale differences in populations of vectors and vertebrate hosts would explain this variation. In summary, we showed that in 2005, both SLEV and to a lesser extent WNV circulated in the avian population. Eared Dove, Picui Ground-Dove and Great Kiskadee are strong candidates to amplify SLEV because of their exposure to the pathogen at the population level, and their widespread abundance. For the same reasons, Rufous Hornero may be an important maintenance host for WNV in central Argentina. Competence studies and vector feeding studies are needed to confirm these relationships.

Fig 1. Geographic location of sampling sites in Córdoba city and neutralizing antibodies seroprevalence (%) in wild birds for St. Louis encephalitis virus and West Nile virus per site and year.

BG: Bajo Grande, CS: Camino San Carlos, BT: Botanical Garden, VP: Villa Gran Parque. Bar graphs present seroprevalence values expressed as percentages (number of positive sera/number of analyzed sera).

Fig 2. Ordination analyses based on a subset of 6 bird species selected collected and tested in three sites of Córdoba city from January to March 2005.

A) Correspondence analysis ordination bi-plot for bird species abundance derived from frequency of capture in mist-nets. For each axis, the amount of variation explained as a part of the total variation in the model is shown. B) Non-metric multidimensional scaling (NMDS) analysis ordination bi-plot for SLEV seroprevalence. Colored areas represent amount of variation for analyzed variables.

Fig 3. St. Louis encephalitis virus host infection dynamics during January-March 2005.

A) SLEV infection prevalence and the ratio of selected vs non-selected infected bird species. B) Species composition of seropositive birds for SLEV.