Original Research Article Implication of Ariaal Sexual Mixing on Gonorrhea C. CONNELL MCCLUSKEY, 1 ERIC ROTH, 2 * AND P. VAN DEN DRIESSCHE 3 1 Department of Mathematics and Statistics, McMaster University, Hamilton, Ontario L8S 4K1, Canada 2 Department of Anthropology, University of Victoria, Victoria, British Columbia V8W 3P5, Canada 3 Department of Mathematics and Statistics, University of Victoria, Victoria, British Columbia V8W 3P4, Canada ABSTRACT Recent research on sexual mixing in populations of sub-Saharan Africa raises the question as to whether STDs can persist in these populations without the presence of a core group. A mathematical model is constructed for the spread of gonorrhea among the Ariaal population of Northern Kenya. A formula for the basic reproduction number R 0 (the expected number of secondary infections caused by a single new infective introduced into a susceptible population) is determined for this population in the absence of a core group. Survey data taken in 2003 on sexual behavior from the Ariaal population are used in the model which is formulated for their age-set system including four subpopulations: single and married, female and male. Parameters derived from the data, and other information from sub-Saharan Africa are used to estimate R 0 . Results indicate that, even with the elevating effect of the age-set system, the disease should die out since R 0 < 1. Thus, the persistence of gonorrhea in the population must be due to factors not included in the model, for example, a core group of commercial sex workers or concurrent partnerships. Am. J. Hum. Biol. 17:293–301, 2005. # 2005 Wiley-Liss, Inc. 1. INTRODUCTION Mathematical models (see, e.g., Garnett, 2002; Ovaskainen and Grenfell, 2003; Yorke et al., 1978) can aid in quantifying the spread of sexually transmitted diseases (STDs). In a seminal paper on mathematical models of STDs, Yorke et al. (1978) introduced the con- cept of a ‘‘core group,’’ that is, a small subpo- pulation that contributes disproportionately highly to the persistence of STDs because it features large numbers of partners and recur- rent periods of infectiousness. Modeling the mid-twentieth-century epidemic of gonorrhea in the United States, their work showed that a core group was important to the persistence of this disease. Since this inaugural work, the concept of core groups has retained a promi- nent place in the epidemiology of STDs and related public health policies (Aral, 2000; Brunham, 1997; Thomas and Tucker, 1996). However, recent studies from West Africa (Messerschmidt et al., 2000) suggest that sub-Saharan cultural patterns featuring high levels of polygyny, partner change, extramar- ital unions, and concurrent sexual relation- ships among the general population may suffice to maintain STDs at an endemic level without necessitating a core group. Recent fieldwork with an East African population, the Ariaal of the Marsabit District, in northern Kenya, provided an opportunity to assess the importance of a core group on the spread of gonorrhea. Specifically, sexual behavior surveys con- ducted in the Ariaal community of Karare in 1996, 1999, and 2003 revealed a high degree of disassortative (single with mar- ried) sexual mixing and a young age of sexual debut for females (Roth et al., 1999, 2001). Both of these factors are well recognized as elevating the transmission efficiency of STDs (Anderson, 1998; Aral, 1992). In addi- tion, during focus group discussions in 2003, men reported gonorrhea as a commonly recognized disease with a long history among the Ariaal. While lacking systematic ß 2005 Wiley-Liss, Inc. Contract grant sponsors: SSHRC; National Geographic Society; University of Victoria; NSERC *Correspondence to: Eric Roth, Department of Anthropology, University of Victoria, P.O. Box 3050 STN CSC, Victoria, British Columbia V8W 3P5, Canada. E-mail: ericroth@uvic.ca Received 30 July 2004; Revision received 5 January 2005; Accepted 26 January 2005 Published online in Wiley InterScience (www.interscience. wiley.com). DOI: 10.1002/ajhb.20123 AMERICAN JOURNAL OF HUMAN BIOLOGY 17:293–301 (2005)