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Pertussis is a vaccine-preventable respiratory disease caused by Bordetella pertussis [1]. Globally, it is estimated that pertussis vaccines prevented approximately 700 000 deaths in 2008, attesting to the ostensible success of the vaccines [2]. However, despite vaccine coverage approaching 90% [3], 16 million cases of pertussis occur annually [2], resulting in 195 000 deaths in children <5 years of age [4]. Most of these cases are in lowand middle-income countries. Although the morbidity and mortality numbers are lower in high-income countries, pertussis remains endemic in the face of high vaccination coverage [5, 6]. For example, in the United States, despite a 92.2% reduction in the number of cases and a 99.3% reduction in deaths since the introduction of pertussis vaccines in the 1940s [7], B. pertussis continues to circulate [8], causing outbreaks such as the 2010 epidemic in California, which resulted in 7824 cases and 10 deaths in infants [9, 10]. Outbreaks occur every 2–5 years, and this cyclical nature of pertussis, which has not changed since the introduction of vaccines [8], is attributed to the immune status of the population. Immunity lasts 4–20 years after infection and 4–12 years after vaccination [11]. When immunity is high as a consequence of natural infection or vaccination, the number of cases dips; when immunity wanes, the number of cases rises. Although other factors may play a role [8, 10, 12, 13, 14], the epidemiology of pertussis is ultimately framed by the transmission dynamics of B. pertussis, which is influenced by waning immunity and the ability of the immune status of the population to be boosted either by vaccination or by natural infection [11, 15, 16, 17]. The reason why immunity to pertussis wanes even after natural infection is probably linked to the ability of B. pertussis to evade and modulate both innate and adaptive immune responses [18], although the mechanisms by which it does so are incompletely understood. There are also many questions regarding how B. pertussis is transmitted and whether specific virulence or survival factors are involved in transmission. B. pertussis is restricted to humans and has no known environmental reservoir [1]; thus, survival of this pathogen relies on strategies that aid transmission between those who are infected and those who have no or low levels of protective immunity. Pertussis is highly contagious [1]. Direct inoculation with as few as 140 bacteria is sufficient to cause disease in naive children [19], and epidemiological studies have shown that transmission is associated with close contact, primarily among family members [20, 21, 22]. Because pertussis is a respiratory illness with distinctive symptoms [1], airborne transmission via respiratory droplets has been postulated, but, surprisingly, such transmission has not been formally demonstrated, in part owing to the difficulties in ruling out contact between infectious and uninfected individuals. Studies to address airborne transmission are more tractable in animal models; animals can be kept segregated more easily, and experimental parameters can be better controlled. Mice, rabbits, rats, and pigs are some of the animals used to study pertussis [23]; however, transmission has not been demonstrated in these models. The Merkel laboratory thus revisited the idea of using a nonhuman primate to model pertussis, reasoning that a model using a species more closely related to humans would be more likely to reproduce all aspects of pertussis pathogenesis, including disease and transmission [24]. They found that monkeys had been used previously to study pertussis, but that the results were inconsistent. It was noted that the Taiwan macaque (Macaca cyclopsis) was more susceptible to pertussis compared to the rhesus macaque (Macaca mulatta); however, experiments with M. cyclopsis were not feasible because they are endangered. Received and accepted 29 June 2012; electronically published 17 July 2012. Correspondence: Rachel Fernandez, Department of Microbiology & Immunology, University of British Columbia, 25592350 Health Sciences Mall, Vancouver, BC, Canada V6T 1Z3 (rachelf@mail.ubc.ca). The Journal of Infectious Diseases 2012;206:808–10 © The Author 2012. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals. permissions@oup.com. DOI: 10.1093/infdis/jis444

Airborne Transmission of Bordetella pertussis Demonstrated in a Baboon Model of Whooping Cough