Risk assessment and management of brucellosis in the southern greater Yellowstone area (II): Cost-benefit analysis of reducing elk brucellosis prevalence.

Recent cases of bovine brucellosis (Brucella abortus) in cattle (Bos taurus) and domestic bison (Bison bison) of the southern Greater Yellowstone Area (SGYA) have been traced back to free-ranging elk (Cervus elaphus). Several management activities have been implemented to reduce brucellosis seroprevalence in elk, including test-and-slaughter, low-density feeding at elk winter feedgrounds, and elk vaccination. It is unclear which of these activities are most cost-effective at reducing the risk of elk transmitting brucellosis to cattle. In a companion paper, a stochastic risk model was used to translate a reduction in elk seroprevalence to a reduction in the risk of transmission to cattle. Here, we use those results to estimate the expected economic benefits and costs of reducing seroprevalence in elk using three different management activities: vaccination of elk with Brucella strain 19 (S19), low-density feeding of elk, and elk test-and-slaughter. Results indicate that the three elk management activities yield negative expected net benefits, ranging from -$2983 per year for low-density feeding to -$595,471 per year for test-and-slaughter. Society's risk preferences will determine whether strategies that generate small negative net benefit, such as low-density feeding, are worth implementing. However, activities with large negative net benefits, such as test-and-slaughter and S19 vaccination, are unlikely to be economically worthwhile. Given uncertainty about various model parameters, we identify some circumstances in which individual management activities might generate positive expected net benefit.

Effects of management, behavior, and scavenging on risk of brucellosis transmission in elk of western Wyoming.

Brucellosis is endemic in elk (Cervus elaphus nelsoni) using winter feedgrounds of western Wyoming, USA presumably because of increased animal density, duration of attendance, and subsequent contact with aborted fetuses. However, previous research addressed antibody prevalence rather than more direct measures of transmission and did not account for elk behavior or scavenging in transmission risk. Throughout March and early April 2005-07, we monitored 48 sets of culture-negative, pseudoaborted elk fetuses, placentas, and fluids (fetal units, FUs) on one winter free-ranging (WFR) location and four sites (Feedline, High Traffic, Low Traffic, Adjacent) associated with four feedgrounds. "At-risk" elk (total elk within 5 m of FU) and proportions of elk sniffing and contacting FUs were highest on Feedlines and decreased toward Low Traffic sites. We did not observe elk investigating FUs Adjacent to feedgrounds or on the WFR location. At-risk elk on Feedline and High Traffic sites decreased throughout the sampling period, whereas proportions of elk investigating FUs were correlated positively to at-risk elk among all sites within feedgrounds. At-risk elk and proportions of elk investigating FUs were correlated with total feedground elk density and population only on High Traffic and Low Traffic sites. Proportions of sex/age groups (female, juvenile, male) investigating FUs did not differ from background populations. Females, however, spent more time (mean [SE], 21.07 [3.47] sec) investigating FUs than juveniles (14.73 [3.53] sec) and males (10.12 [1.45] sec), with positive correlation between total investigations and time spent investigating per female. Eight species of scavengers consumed FUs, removing FUs faster on feedgrounds than WFR locations and reducing proportions of elk that investigated FUs. Our results suggest that 1) reduction of elk density and time attending feedgrounds, particularly on Feedlines; and 2) protection of scavengers on and adjacent to feedgrounds would likely reduce intraspecific transmission risk of brucellosis.

Effects of management and climate on elk brucellosis in the Greater Yellowstone Ecosystem.

Every winter, government agencies feed approximately 6000 metric tons (6 x 10(6) kg) of hay to elk in the southern Greater Yellowstone Ecosystem (GYE) to limit transmission of Brucella abortus, the causative agent of brucellosis, from elk to cattle. Supplemental feeding, however, is likely to increase the transmission of brucellosis in elk, and may be affected by climatic factors, such as snowpack. We assessed these possibilities using snowpack and feeding data from 1952 to 2006 and disease testing data from 1993 to 2006. Brucellosis seroprevalence was strongly correlated with the timing of the feeding season. Longer feeding seasons were associated with higher seroprevalence, but elk population size and density had only minor effects. In other words, the duration of host aggregation and whether it coincided with peak transmission periods was more important than just the host population size. Accurate modeling of disease transmission depends upon incorporating information on how host contact rates fluctuate over time relative to peak transmission periods. We also found that supplemental feeding seasons lasted longer during years with deeper snowpack. Therefore, milder winters and/or management strategies that reduce the length of the feeding season may reduce the seroprevalence of brucellosis in the elk populations of the southern GYE.