Abstract
The seasonal availability of food resources is an important factor shaping the life-history strategies of organisms. During times of nutritional restriction, physiological trade-offs can induce periods of immune suppression, thereby increasing susceptibility to infectious disease. Our goal was to provide a conceptual framework describing how the endemic level bovine brucellosis (Brucella abortus) may be maintained in Yellowstone bison based on the seasonality of food resources and the life-history strategies of the host and pathogen. Our analysis was based on active B. abortus infection (measured via bacterial culture), nutritional indicators (measured as metabolites and hormones in plasma), and carcass measurements of 402 slaughtered bison. Data from Yellowstone bison were used to investigate (1) whether seasonal changes in diet quality affect nutritional condition and coincide with the reproductive needs of female bison; (2) whether active B. abortus infection and infection intensities vary with host nutrition and nutritional condition; and (3) the evidence for seasonal changes in immune responses, which may offer protection against B. abortus, in relation to nutritional condition. Female bison experienced a decline in nutritional condition during winter as reproductive demands of late gestation increased while forage quality and availability declined. Active B. abortus infection was negatively associated with bison age and nutritional condition, with the intensity of infection negatively associated with indicators of nutrition (e.g., dietary protein and energy) and body weight. Data suggest that protective cell-mediated immune responses may be reduced during the B. abortus transmission period, which coincides with nutritional insufficiencies and elevated reproductive demands during spring. Our results illustrate how seasonal food restriction can drive physiological trade-offs that suppress immune function and create infection and transmission opportunities for pathogens.
Highlights
The acquisition and allocation of food resources shape the life-history strategies of organisms (Boggs 1992, 2009)
Our findings suggest that active B. abortus infection is influenced by nutritional condition, with seasonal food restriction assisting infection of immature bison, which become the primary transmission source during early reproductive ages
Yellowstone bison seek emerging forage on low elevation ranges outside the park but are frequently pushed back onto high elevation ranges within the park where snow cover delays the growth of new vegetation
Summary
The acquisition and allocation of food resources shape the life-history strategies of organisms (Boggs 1992, 2009). During periods of food restriction, individuals may face trade-offs in the allocation of limited internal resources toward growth, maintenance, and reproduction (Stearns 1992; Zera and Harshman 2001). Investment in immune defense, which has high nutritional costs, may be reduced (Martin et al 2008). The susceptibility to infectious diseases can increase when seasonal food insufficiencies overlap with periods of high nutritional demands (Jolly and Fernandes 2000; Buehler et al 2010). Individuals in poor nutritional condition may be more susceptible to infection, have higher infection intensities, and serve as an important a 2015 The Authors.
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