Abstract
Respiratory illnesses have caused significant mortality in African great ape populations. While much effort has been given to identifying the responsible pathogens, little is known about the factors that influence disease transmission or individual susceptibility. In the Kanyawara community of wild chimpanzees, respiratory illness has been the leading cause of mortality over 31 years, contributing to 27% of deaths. Deaths were common in all age groups except juveniles. Over 22 years of health observations, respiratory signs were rare among infants and most common among older adults of both sexes. Respiratory signs were also common among males during the transition to adulthood (ages 10–20 years), particularly among those of low rank. Respiratory signs peaked conspicuously in March, a pattern that we could not explain after modelling climatic factors, group sizes, diet or exposure to humans. Furthermore, rates of respiratory illness in the chimpanzees did not track seasonal rates of illness in the nearby village. Our data indicate that the epidemiology of chimpanzee respiratory illness warrants more investigation but clearly differs in important ways from humans. Findings on individual susceptibility patterns suggest that respiratory signs are a robust indicator for investigating immunocompetence in wild chimpanzees.
Highlights
Recent reports from across Africa suggest that epidemics of infectious disease can have profound impacts on chimpanzee populations [1], with respiratory illness implicated in approximately 18 deaths during 3 outbreaks at Mahale, Tanzania [2,3], 17 deaths during 3 outbreaks at Taı, Cote d’Ivoire [1], 5–6 individuals during 2 outbreaks at Bossou, Guinea [4], at least 28 deaths during 7 outbreaks at Gombe, Tanzania [5,6], 5 deaths during one recent outbreak at Kanyawara, Uganda [7] and several during an outbreak at Ngogo, Uganda [8]
Molecular analyses of respiratory syncytial virus (RSV), MPV and RV-C indicate that the viruses found in chimpanzees are reverse zoonoses, it is unclear whether successive outbreaks arise from reinfections by local human populations or single introduction events followed by chimpanzee-tochimpanzee transmission [2,7,9,14]
Respiratory illness was a factor in more than half (58.6%) of the deaths. These include 2 individuals for which necropsies were able to confirm respiratory illness as the cause of death, and 13 individuals observed with serious respiratory signs prior to their disappearance
Summary
Recent reports from across Africa (table 1) suggest that epidemics of infectious disease can have profound impacts on chimpanzee populations [1], with respiratory illness implicated in approximately 18 deaths during 3 outbreaks at Mahale, Tanzania [2,3], 17 deaths during 3 outbreaks at Taı, Cote d’Ivoire [1], 5–6 individuals during 2 outbreaks at Bossou, Guinea [4], at least 28 deaths during 7 outbreaks at Gombe, Tanzania [5,6], 5 deaths during one recent outbreak at Kanyawara, Uganda [7] and several during an outbreak at Ngogo, Uganda [8]. The aetiologies of these outbreaks often remain unknown, metapneumovirus (MPV), respiratory syncytial virus (RSV) and rhinovirus-C (RV-C) have been detected in affected individuals [2,7,9], with co-infection by Streptococcus or Pasteurella bacteria in several lethal cases [1,9,10,11,12]. While feeding on provisioned bananas predicted respiratory signs, other exposures to humans (follow time, proximity to research camp) did not, nor did group size or exposure to baboons [15]. Individualistic factors, such as age, sex, dominance rank and SIVcpz infection status did not predict rates of respiratory signs [16]. Across all reports of chimpanzee respiratory outbreaks, several outbreaks have caused mortality only in infants, whereas others have disproportionately killed older individuals (table 1)
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