Sexually transmitted infections (STI) continue to pose a complex, significant, and constantly evolving public health concern in the United States (US) [1]. In the Midwest from 2016 to 2017, the chlamydia rate increased 5.6%, the gonorrhea rate 19.5%, and the syphilis rate 8.8% [1,2,3]. This increase is attributed to more widespread screening, reporting, and cases altogether [1]. Other factors include more sensitive and accurate diagnostic tests, like nucleic acid amplification tests (NAATs) [1]. A small percentage increase indicates a significant amount of new cases and subsequent healthcare costs as more are being screened and treated [1]. Human Immunodeficiency Virus (HIV) incidence has declined in recent years due to extensive preventative campaigns but has now plateaued as specific high-risk groups such as transgender persons, men who have sex with men, African American, and Latinos, are not being adequately reached, especially in the South [4].

There are serious implications if infected individuals go unscreened and untreated after contracting STIs. The reasons for not seeking diagnosis and treatment can be complex. Marginalized ethnicities, individuals that may experience racism, homophobia, and xenophobic interactions, and impoverished populations, have the lowest access to healthcare and screening tests, placing them at high risk of acquiring STIs [5]. Drug use also increases risk of STI contraction, along with unplanned pregnancies [6]. More specifically, methamphetamine use, a known problem within Missouri, increases libido and risky sex among its users [6]. Finally, group sex or sex with multiple partners can increase risk of transmission [7].

Some STIs can manifest asymptomatically more so in women and cause them to go unscreened and untreated for longer than men who typically develop symptoms faster and more noticeably [8]. For this reason, women often suffer more severe, long-term complications from STIs, some including chronic pelvic pain, ectopic pregnancies, and infertility [8].

Between screening, treatment, and long-term management of STIs, the estimated financial burden in the United States is around $15.6 billion [9]. Chlamydia and gonorrhea account for $516.7 million and $162.2 million respectively, while syphilis accounts for $39.3 million, and HIV for $12.6 billion. HIV accounts for 81% of the total annual cost of STIs in the US. The overall increase in STI cases has not been limited to a single social group, age group, gender, or socioeconomic class [7]. These costs may change as STIs shift, such as the emergence of multi-drug resistant gonorrhea [10]. Although data over the cost of STIs within Missouri is unknown, in 2016, the CDC provided $6.8 million in funding specific for HIV/Acquired Immunodeficiency Syndrome (AIDS) prevention and treatment and an additional $2.2 million in funding specific for other STIs. These values may be subject to change as multi-drug resistant gonorrhea becomes more prevalent [10, 11]. Antibiotic resistant gonorrhea has the potential to become an incurable, chronic disease resulting in disability and death in a previously easily curable infection, highlighting the need to research and intervene in STIs at a population-level now [12].

It is known that increased levels of unemployment can cause lowered income for individuals and families [13]. Increased poverty levels have been linked to increased practice of risky sexual behaviors, like using a condom inconsistently or never, not using oral contraceptives, or other forms of birth control, while also having more sexual partners and ‘one-night stands’ [13]. These behaviors increase risk of both encountering and contracting an STI [13]. Here, it can be seen that population-level variables, or macro effects, can effect STI rates and spread as populations continue to change and encounter different situations.

Social epidemiology, a research methodology and theoretical framework that focuses on social determinants, practices, and health outcomes, provides an appropriate framework to address the factors that impact STI rates. Macro effects on disease and STI transmission are often ignored and instead the focus is put on individual risk factors [7]. Social epidemiology defines three levels to the spread of STIs throughout society: 1) individual components, 2) social components, and 3) structural components [14]. Individual components involve biological susceptibility and risky behaviors. Social components involve networks, communities, and how disease diffuses across populations [14]. The structural components are grouped into 1) cultural context, 2) social networks, 3) neighborhood effects, and 4) social capital. Social epidemiology can be used as a lens to understand the impact of population change and STI rates within a state. Social epidemiology lacks the insight of an individualized point of view on STIs, but the majority of data focuses on individual trends [7]. It instead provides insight into large-scale trends affecting many with STIs, where data may be less extensive but equally compelling due to the vast amount of individuals it can then reach [7].

Given the social complexity and financial cost of STIs, it becomes critical to monitor changing rates of these infections not only throughout static populations but also as moving populations change proportions of age, gender, ethnicity, and sexuality. With these changing dynamics, estimated risks of STI transmission fluctuate as different groups have different risk factors surrounding STIs [7]. As populations grow and move, the incidence of STIs is expected to grow, showing the importance of population-level predictors alongside individual interventions. Moving, changing populations face problems of isolation and loneliness after relocating [12]. It has been observed that moving populations typically experience a higher risk of contracting STIs [15]. Characteristics of a moving population include the social disruption that accompanies geographic relocation and a lack of access to health resources [15]. STIs have remained persistent within society, in part, due to failure to contact trace all sexual partners of STI patients [9]. This is possibly due to geographic or networking barriers, consistent with a moving population. Young people and those seeking secondary education most often have to relocate, particularly from rural to urban areas [16]. In the past, the urban population of the world has increased much more quickly than those of the rural population [12]. This is expected to remain the trend into the future as the United Nations expects the world population to increase to 9.1 billion by 2100 with urban population continuing to increase in population size, and rural populations moving more towards urban areas [12]. Urban populations were considered the only “high-risk” areas for STI spread in the past, but new pockets of widespread infection in less densely populated areas have begun to form [8]. In this study, we analyze the relationship among common STIs and population change at the county level in the state of Missouri from 2008 to 2017.