Heterogeneity in disease transmission: Nematodes and tuberculosis in juvenile and adult African buffalo

Disease models predict that heterogeneities in disease transmission can have drastic effects on the dynamics of pathogen and parasite populations. Evaluating the magnitude and importance of such transmission heterogeneities in natural host-parasite systems has been hampered by a dearth of sufficiently large and detailed wildlife disease datasets. We examined age-prevalence patterns of bovine tuberculosis (TB) and gastrointestinal nematodes (worms) in free-ranging African buffalo (Syncerus caffer). Worm infections are far more prevalent and more severe among juvenile buffalo than adults. By contrast, TB prevalence increases up to the age of 5 years, but remains constant in adults thereafter. When the relative abundance of buffalo of different ages is taken into account, these patterns imply that the majority of worm infective larvae originate from juvenile buffalo under the age of 2 years, while young adults (4-8yrs) may contribute disproportionately to TB transmission. Both behavioural and immunologcal differences between adult and juvenile buffalo may underlie these patterns. Juveniles spent more time in close proximity to their nearest neighbours than adults; and differed from adults in regulation of their immune defenses against worms and TB. We conclude that age-related heterogeneities in disease transmission, mediated by behavioural and immunological mechanisms, may play an important role in shaping patterns and dynamics of infection in wildlife populations.