Anthrax Surveillance and the Limited Overlap Between Obligate Scavengers and Endemic Anthrax Zones in the United States

Abstract

Anthrax is a zoonosis caused by the spore-forming bacterium Bacillus anthracis, with potential for high fatality rate, especially in herbivores. Upon host death, spores can enter the soil surrounding the carcass and be ingested by other animals feeding in the same location. Accordingly, surveillance to quickly identify and decontaminate anthrax carcasses is crucial to outbreak prevention. In endemic anthrax areas such as Texas and Africa, vultures are used as a surveillance tool for identifying presence and location of dead animals. However, many anthrax outbreaks in the United States have occurred in areas outside the ranges of both black and turkey vultures. Here, we used a longitudinal camera trap survey at carcass sites in southwestern Montana to investigate the utility of facultative avian scavengers on disease and carcass surveillance in a reemerging anthrax risk zone. From August 2016 to September 2018, camera traps at 11 carcass sites were triggered 1996 times by avian scavengers. While the majority were facultative avian scavengers such as corvids and eagles, our results suggest that facultative scavengers cannot replace vultures as a surveillance tool in this ecosystem due to their absence during the anthrax risk period (June to August), reduced search efficiency, or low flight patterns. We found that the conditions in Montana likely parallel systems elsewhere in the continental United States. Using ecological niche models of B. anthracis distribution overlaid with relative abundance maps of turkey vultures, we found that much of North Dakota, South Dakota, Minnesota, Wyoming, Nebraska, and Iowa have areas of anthrax risk, but low or absent turkey vulture populations. Without vultures in these areas, surveillance capacity is reduced, and it becomes more difficult to identify anthrax cases, meaning fewer carcasses are decontaminated, and consequently, outbreaks could become more frequent or severe.

Keywords: anthrax; corvids; obligate scavengers; turkey vultures; vultures; zoonosis.

FIG. 1.

A map showing turkey vulture relative abundance in the continental United States, based on the mean number of birds seen on North American Breeding Bird Survey routes from 2011 to 2015. The magnified inset shows the boundaries of our study site within southwestern Montana and the carcass camera trap locations.

FIG. 2.

Images of avian species captured during a camera trap study in southwestern Montana: golden eagle (A), turkey vulture (B), bald eagle (C), black-billed magpie (D), and Corvus spp. (E).

FIG. 3.

The number of (A) golden eagles, (B) bald eagles, (C) turkey vultures, (D) magpies, and (E) Corvus spp. captured by camera traps at carcass sites from 1 to 5 days after animal death. All avian scavengers were present in the initial days after death (days 1–3) apart from turkey vultures, which is unusual and is a finding unique to this ecosystem.

FIG. 4.

The counts of four avian species (A) bald eagles, (B) turkey vultures, and (C) Corvus spp. captured by camera traps at carcass sites from 1 to 15 days after animal death, when the animal died during the anthrax risk period (June to August). Corvus spp. (ravens and crows) are present in the largest numbers. Turkey vultures are absent in the initial days after death and are present only in low numbers from 4 days after death to 7 days after death. Magpies and golden eagles were absent; surprisingly, magpies are present at carcass sites during the anthrax risk period but were only seen 15 or more days after animal death.

FIG. 5.

Monthly totals of the number of (A) golden eagles, (B) bald eagles, (C) turkey vultures, (D) magpies, and (E) Corvus spp. present at carcass sites. Magpies and Corvus spp. are present throughout the year, and Corvus spp. have higher numbers in late summer and early fall (August to October). Turkey vultures are only present from April to August, a reflection that this location is likely part of their summer breeding grounds.

FIG. 6.

Predicted overlap of turkey vulture (Cathartes aura) and Bacillus anthracis distribution in the continental United States based on three turkey vulture relative abundance cutoffs. (A) Prediction of B. anthracis suitability without vulture overlay; (B) prediction based on turkey vulture relative abundance of 10 individuals or higher; (C) prediction based on relative abundance of 5 individuals or higher; and (D) prediction based on relative abundance above 0.