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1. Disease management along the boundaries of wildlife reserves is a growing conservation problem worldwide, as infected wildlife can migrate outside protected areas and pose a threat to livestock and human health. The bison Bison bison population in Yellowstone National Park has long been infected with Brucella abortus, but culling of Yellowstone bison to prevent transmission to cattle has been ineffective at reducing brucellosis infection. This management strategy is negatively affecting long-term bison conservation because of difficulties in diagnosing actively infected animals.
2. We integrated age-specific serology and B. abortus culture results from slaughtered Yellowstone bison to estimate probabilities of active brucellosis infection using a Bayesian framework. Infection probabilities were associated with age in young bison (0–5 years old) and with elevated antibody levels in older bison (>5 years old). Our results indicate that Yellowstone bison acquire B. abortus infection early in life but typically recover as they grow older.
3. A tool was developed to allow bison management to better reflect the probability that particular animals are infective, with the aim of conserving Yellowstone bison while reducing the risk of brucellosis transmission to cattle. Combining selective removal of infectious bison with additional management practices, such as vaccination, has the potential to advance an effective brucellosis reduction programme.
4. Synthesis and applications. We conclude that active B. abortus infection in Yellowstone bison is age dependent, which allows true infection probabilities to be estimated based on age and quantitative diagnostic tests. These findings have important application to disease management worldwide where accurate diagnostic tests for wildlife are unavailable. Estimation of true infection probabilities can replace culling practices that conflict with wildlife conservation. The ability to identify infective individuals can improve management practices that support conservation, particularly when human health is at risk or endangered wildlife species are involved.
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The increasing number of wildlife diseases transmissible to humans has raised worldwide concerns regarding free-ranging wildlife as a source of emerging human pathogens (Daszak, Cunningham & Hyatt 2000). In particular, agents of infectious diseases, such as bovine tuberculosis, brucellosis and salmonellosis, which can establish persistent infections in wild ungulates, are especially difficult to manage (Renter et al. 2006; Cross et al. 2009; White et al. 2011). The limitation of diagnostic tests to accurately identify infectivity of persistent bacterial diseases in wild ungulates has led to disease management practices that are not aligned with wildlife conservation. Traditional test-and-slaughter programmes, which have been effective for livestock management, may not be realistic or socially acceptable for wildlife (Bienen & Tabor 2006). However, the sharing of diseases at the human, livestock and wildlife interface is a global problem (Böhm et al. 2007), which requires management practices that advance wildlife conservation as well as human and animal health.
During the past century, the livestock industry and wildlife managers in the Greater Yellowstone Area in the western United States have been concerned about the infectious disease brucellosis caused by the bacterium Brucella abortus. This zoonosis can infect the reproductive organs of several ungulate species during pregnancy and can lead to the induction of late-term abortions of infected hosts. Aborted foetuses are highly infectious and can serve as a transmission source for the disease (Thorne 2001). This non-native disease was most probably introduced to Yellowstone bison Bison bison by European cattle (Bos spp.)(Cheville, McCullough & Paulson 1998) nearly a century ago (Mohler 1917). In 1934, a nationwide brucellosis eradication programme was initiated and has since resulted in the elimination of B. abortus in most of the United States, with the exception of free-ranging wildlife in the three states surrounding Yellowstone National Park (Yellowstone): Montana, Idaho and Wyoming. Cattle industries in these states have additional economic expense if they lose their brucellosis-free status, as has occurred in all three states because of multiple brucellosis exposures in the past decade (Montana Department of Livestock 2008). As such, this remaining pocket of brucellosis has led to decades of conflict among wildlife managers, environmental groups and the livestock industry over management of Yellowstone bison.
Although brucellosis-infected elk Cervus elaphus have been responsible for disease transmissions to cattle (Beja-Pereira et al. 2009), Yellowstone bison have long been the focus of brucellosis management in the northern portion of the Greater Yellowstone Area. Yellowstone bison management operates under an Interagency Bison Management Plan (IBMP) aimed at conserving wild bison while reducing the risk of brucellosis transmission to Montana livestock (USDI and USDA 2000). Bison management practices used to prevent brucellosis transmission to local cattle conflicts with the goal of conserving bison and the processes that sustain them (e.g. migration). Severe winter conditions encourage bison movement to low-elevation ranges outside Yellowstone (Geremia et al. 2011) where they are not tolerated because of the risk of transmitting brucellosis to cattle. In some years, large numbers of migrating bison are captured and tested for brucellosis, with seropositive animals being shipped to slaughter. Approximately 3200 Yellowstone bison were shipped to domestic slaughter facilities between 2001 and 2010, with 899 shipped during 2006 and 1434 shipped during 2008.
These large-scale bison removals have not been random, because bison social structure and the reproductive demands of pregnancy predispose female bison and their recent offspring (i.e. male and female calves and yearlings) to culling as they move onto low-elevation winter ranges outside the park. The effects of several large, nonrandom culls during the past decade have contributed to a skewed sex ratio in favour of male bison, gaps in the population’s age structure and reduced productivity that, if continued over time, could reduce the potential of Yellowstone bison to respond to future challenges (White et al. 2011). Additionally, boundary culling has not contributed to a measurable reduction in brucellosis infection in the bison population. The proportion of seropositive adult female bison has increased slightly since 1985 or remained constant at c. 60% (Hobbs et al. 2009).
Removing brucellosis-infected bison is expected to reduce the level of population infection, but test and slaughter practices may instead be removing mainly recovered bison. Recovered animals provide protection to the overall population through the effect of herd immunity (John & Samuel 2000), thereby reducing the spread of disease. Identifying recovered bison is difficult because serologic tests (i.e. blood tests) detect the presence of antibodies, indicating exposure but cannot distinguish active from inactive infection. In bison, B. abortus antibodies are long lived (Rhyan et al. 2009); thus, seroprevalence overestimates the level of active infection (Roffe et al. 1999) by failing to distinguish between infected and recovered animals (i.e. bison that have cleared the bacteria). Although it is highly probable that bison can have serologic titres and not be infected (Cheville, McCullough & Paulson 1998), all seropositive Yellowstone bison have been treated as actively infected because of potentially chronic or latent forms of the disease.
The ability of B. abortus to establish undetectable latent infections in young animals (e.g. heifer syndrome) has been documented in cattle (Lapraik et al. 1975; Wilesmith 1978; Catlin & Sheehan 1986), although the occurrence of latency is infrequent (Ray et al. 1988; Rhyan et al. 2009). Latently, infected animals are typically exposed as calves and do not react on serologic tests until they have calved or aborted their pregnancy following infection. The proportion of adult bison that develop chronic infections (i.e. persistent infection of lymphatic tissue) following acute disease (i.e. reproductive tract infections) is unknown. However, the epidemiology and pathogenesis of brucellosis in chronically infected bison is similar to chronically infected cattle (Rhyan et al. 2009), and most infected cattle recover by clearing the infection and exhibiting life-long immunity (Ficht 2003).
The epidemiology of brucellosis in Yellowstone bison appears typical of an endemic disease, with a reduced portion of seropositive animals being actively infected and immune protection increasing with age. B. abortus has been isolated from 46% of seropositive Yellowstone bison, with young bison and high antibody titred animals predominantly infected (Roffe et al. 1999). Thus, the relationship between bison demography and quantitative serologic responses may be an important association for identifying actively infected animals. We integrated age-specific serology and B. abortus culture results from Yellowstone bison shipped to slaughter in 2008 to estimate probabilities of active infection. We then applied this information to brucellosis risk management for the purpose of identifying high-risk animals (i.e. those contributing to brucellosis maintenance) and low-risk animals (i.e. recovered bison contributing to herd immunity). This approach has important application to the growing problem of disease management along the boundaries of protected reserves (Newmark 2008). The ability to target-specific individuals that disproportionately contribute to the maintenance of infectious disease can improve the effectiveness of disease management programmes while supporting long-term conservation efforts.
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Brucellosis seroprevalence in bison shipped untested to slaughter (n = 237) increased with age in bison <6 years old and decreased in older age classes (Table 1). Active B. abortus infection measured by culture prevalence increased rapidly in bison <3 years old, closely matching seroprevalence in young bison, and then decreased in age classes following reproductive maturity (Table 1). The highest culture prevalence was observed in 2·75-year-old bison, just before the age of first parturition. The portion of sampled seropositive bison that were actively infected was highest in calves and decreased with age (Fig. 1). FPA values in seropositive bison were more variable across ages for culture-positive bison compared with culture-negative animals (Fig. 2), indicating that seropositive bison that were culture negative may maintain reduced levels of antibodies, while active infection raises this level because of repeated exposure to B. abortus antigen.
Table 1. Age-specific seroprevalence and culture prevalence of Brucella abortus in Yellowstone bison shipped untested to slaughter during the winter of 2008
|Bison age||Culture prevalence||Seroprevalence||Seroconversion rate*|
|0||0·11 (3/27)||0·10 (3/29)||0·10|
|1||0·28 (17/61)||0·37 (22/59)||0·21|
|2||0·43 (6/14)||0·46 (6/13)||0·19|
|4||0·16 (4/25)||0·48 (11/23)||0·12|
|5||0·27 (6/22)||0·81 (17/21)||0·24|
Figure 1. Age distribution of active Brucella abortus infection in seropositive Yellowstone bison shipped to slaughter during 2008. Values above bars indicate the proportion of culture-positive bison for each specified age.
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Figure 2. Age-stratified fluorescent polarization assay values (mP) based on Brucella abortus isolation from culture assays.
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The magnitude of estimated parameters in a simple form of the Ricker model, which included only bison age as a predictor variable, indicated the probability of active B. abortus infection increased in young bison (median a = 0·29, 95% credible interval = 0·19–0·43) and peaked at 2·6 years of age (median 1/b = 2·63, credible interval = 2·08-3·45), which coincided with middle to late gestation during the age of first pregnancy (Fig. 3). These results suggest that active B. abortus infection in Yellowstone bison occurs early in life and peaks during the time when female bison would have their first opportunity to transmit the bacteria.
Figure 3. Estimated posterior probabilities of active Brucella abortus infection based on shape parameter (p) estimates of a Bernoulli distribution. The shape parameter is the probability of observing active infection with bison age related by . The solid line represents the median, and dotted lines represent the 95% credible interval. Points represent observed proportions of culture-positive bison for specified ages.
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Posterior distributions from the process model which included covariate values for age and FPA allowed for testing whether higher antibody levels were associated with active infection in older bison. Posterior probabilities indicated that active infection was greatest in young bison while active infection in older bison was associated with higher FPA values (Fig. 4). The posterior distributions of model parameters suggested that active infection increased with age in young bison (median a = 0·32, 95% credible interval = 0·20–0·49) and peaked just before age of first parturition (median 1/b = 2·22, 95% credible interval = 1·72–3·03). FPA antibody levels had a positive effect on our ability to culture B. abortus (median c = 0·32, 95% credible interval = −0·04–0·64). There was a high probability (0·82) that the interaction of age and FPA was positively associated with active infection (median d = 0·05, 95% credible interval = −0·05–0·15), indicating the probability of active infection increased at higher FPA values in older bison.
Figure 4. Estimated posterior probabilities of active Brucella abortus infection based on shape parameter (p) estimates of a Bernoulli distribution. The shape parameter is the probability of observing active infection with age and fluorescent polarization assay related by . The solid line represents the median, and dotted lines represent the 95% credible interval.
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We developed a management tool that forecasts the true infection status of bison based on age (0·75–14·75 years old) and net FPA values (−25–250 in 25 mP increments). Posterior distributions of the probability of infection for each age and net FPA combination were recovered. We used the empirical cumulative distribution function to determine age and net FPA combinations such that the probability of infection was >95%, >85%, >75%, >65% and >55% (Table 2).
Table 2. Minimum net fluorescent polarization assay values for corresponding probabilities of active infection and bison age
|Bison age||Probability of active Brucella abortus infection|
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The data supported our hypothesis that B. abortus in bison behaves much like an endemic disease, with infection occurring primarily in young animals and recovery increasing with age. Bison age was an important predictor of active infection, in which active infection increased rapidly in young bison and peaked during the age of first pregnancy. These findings are in agreement with Rhyan et al. (2009), which found seroconversion rates in Yellowstone bison to be highest in calves and juveniles (20%) compared to adult females (10%). Bison social structure may predispose newborns and reproductively immature bison to B. abortus exposure. In Yellowstone, pregnant and barren females tend to associate with females in similar states of pregnancy (Rutberg 1984). Young bison in these groups have been observed licking birth tissues shed by calving females (Jones, Treanor & Wallen 2009). The high seroprevalence observed in reproductively immature bison may result from close associations with infectious, pregnant females. Additionally, neonates born to infected mothers may become infected at birth or through B. abortus in milk; nursing calves receive maternal antibodies, but these wane after 5–6 months of age (Rhyan et al. 2009). In our study, calves were c. 9 months old when sampled, and antibodies were highly associated with active infection (Table 1), suggesting that active B. abortus infection begins early in life.
The highest probability of active infection was in bison c. 2·75 years old. Seroprevalence (0·46) and culture prevalence (0·43) were also similar in this age class, which may be caused by systemic infection following reproductive maturity. Ingestion of B. abortus into the gastrointestinal tract is the most common route of exposure from where infection spreads to local lymph nodes before colonization of the uterus during pregnancy (Carvalho Neta et al. 2010). The proliferation of Brucella, necessary for transmission, is influenced by the stage of gestation (Nicoletti 1980), with increasing replication in placental cells during late gestation (Carvalho Neta et al. 2010). Female bison in Yellowstone typically experience their first fertile oestrous early (27 months of age) as 2 years old. In the present study, all reproductively mature female bison sampled were in middle to late stages of gestation, suggesting that this large, recently infected age group may be the primary source of infection for the overall population.
Seroprevalence tracked culture prevalence in young animals but diverged after reproductive maturity (age 2, Table 1). Similarly, the proportion of seropositive bison found to be culture positive decreased as age increased, with low levels of active infection prevalence after the age of five (Fig. 1). These results suggest that bison exposed to B. abortus early in life may begin to recover from acute infection after their first pregnancy following seroconversion. However, B. abortus exposure, measured in seroprevalence, increased until bison were older than 5 years of age. Rhyan et al. (2009) found the greatest potential for positive tissue culture occurred within 2 years following seroconversion, which suggests a high risk of transmission during this period. In our study, seroconversion began early in life, prior to reproductive maturity, and active infection peaked at the age of first parturition with a steady decrease in the probability of active infection beyond the age of three. This suggests that young bison (<3 years old) may be a vulnerable age class to B. abortus infection in comparison with older animals, where a large proportion has experienced B. abortus infection earlier in life.
The protective effect of the humoral immune response (i.e. serum antibodies) in brucellosis is mild or questionable (Carvalho Neta et al. 2010) because the Brucellae are intracellular pathogens able to hide from humoral defences. Accordingly, the positive association between bacterial isolation and high serologic titres (Roffe et al. 1999; Thorne 2001) may indicate reactivation of persistent infection or recent exposure. In the absence of B. abortus antigen, antibody levels are expected to decrease. Yellowstone bison identified with low antibody titres have converted back to seronegative status (Rhyan et al. 2009). This finding may explain the delayed decline in seroprevalence after the age of five in comparison with declines in culture prevalence after the age of three. If bison are able to clear the infection with age, then we may observe a slower decrease in seroprevalence because of circulating antibodies (i.e. long-lived humoral IgG responses), high test sensitivity and spikes in antibodies resulting from re-exposure.
We found a positive relationship with FPA values and active infection (Figs 2 and 4), supporting the view that active infection is associated with increased antibody production. Bison that have cleared the infection will still react positive on serologic tests until antibodies have decreased below detectable levels. The FPA is a sensitive test for detecting antibodies specific to B. abortus, and the high rates of seroprevalence in older bison include animals that seroconverted at younger ages because of antibody persistence. These bison may have undergone acute infection early in life and have developed some level of immunity against re-exposure.
Although B. abortus antibodies may not be protective, they do play an important role in decreasing the number of bacteria upon subsequent exposure. Opsonized Brucellae, which are extracellular bacteria marked for destruction by host antibodies, are less likely to survive and establish intracellular infections (Carvalho Neta et al. 2010). Once B. abortus establishes intracellular infection, bacterial clearance requires effective cell-mediated immune responses induced by activation of specialized T cells (Oliveira, Soeurt & Splitter 2002). Memory T cells persist after the infection has been cleared and allow for secondary responses to low concentrations of B. abortus antigen that are faster and of greater magnitude than the primary response (Bernard & Tough 2002). Effective cell-mediated immune responses may clear the initial infection and quickly respond to secondary infections, thereby reducing antibody levels as a result of eliminating B. abortus from bison tissues. Thus, high antibody levels may indicate ineffective cell-mediated immune responses, and conversely, effective cell-mediated immune responses may clear bacteria and reduce B. abortus antibody production.
A limitation of our work may be that we failed to detect low-level chronic infections in adult bison. Brucella abortus is known for its intracellular hiding ability which can facilitate long-term persistence (Spera et al. 2006). Consequently, the association of high serologic responses with active infection in older bison may indicate recent exposure or recrudescence of chronic infection. Chronically infected female cattle have been observed to periodically shed B. abortus in genital infections (Manthei, DeTray & Goode 1950; Lambert et al. 1960). However, in our study, the relationship between antibody levels and active infection in older bison suggests that low serologic responses may be indicative of recovery rather than chronic infection. The reactivation of infection is expected to occur from stress and favourable conditions for B. abortus proliferation during late gestation (Cheville, McCullough & Paulson 1998). In 2008, bison were sampled in middle to late gestation during one of the most stressful years in Yellowstone’s recorded history (based on high bison population density and winter severity). Thus, we would expect a high rate of recrudescence (i.e. active infection) in older animals if chronic infection was the typical progression of brucellosis in Yellowstone bison.
The objective of the present study was not to identify every actively infected bison, but to provide managers with a reliable tool for identifying FPA cut-off values based on specified active infection probabilities and bison age (Table 2). This allows managers to take a conservation approach to bison management by targeting infectious animals based on active infection probabilities, which can be adjusted to different phases of a brucellosis reduction programme. Currently, brucellosis seroprevalence is high (0·45, White et al. 2011), with bison diagnosed as seropositive when net FPA values typically exceed 10–20 mp. This tool allows managers to reduce removals when seroprevalence is high by identifying infectious bison (e.g. probability of active infection >0·95) with a high level of certainty. Similarly, as immune protection increases against B. abortus through vaccination and seroprevalence declines, removal efforts can focus on the majority of B. abortus-exposed bison (e.g. probability of active infection >0·55) that are less likely to be actively infected. This targeted approach allows for removing high-risk individuals and increasing herd immunity through vaccination, while promoting bison conservation without large-scale culling.
Brucellosis in the greater Yellowstone area is one of the most challenging issues facing wildlife managers, livestock producers and the concerned public in the western United States. High genetic diversity, unique alleles and lack of introgression of domestic cattle genes (Halbert 2003) make the Yellowstone bison population critical to the conservation of the species in North America. Because definitive brucellosis tests do not currently exist, the threat of spreading brucellosis has prevented translocation of Yellowstone bison for purposes of genetic augmentation of smaller herds elsewhere in North America. Yellowstone bison management has long relied on serologic tests, which overestimate the level of brucellosis infection, the continuation of which may lead to management decisions that reduce bison numbers and reduce long-term viability of this unique population. We have presented a novel method for estimating active brucellosis infection in bison that will allow managers to better assess the relative risk of individual animals and influence culling decisions in a way that should reduce annual losses from this practice. Our approach has broad application to balancing disease management with wildlife conservation, such as situations where migratory wildlife poses a risk to livestock and human health or when endangered wildlife may be impacted by infectious disease. The ability to identify infectious individuals, which disproportionately contribute to disease maintenance, increases management options beyond traditional culling practices.