Age and Racial Disparities Persist for Gonorrhoea and Chlamydia in the United States

Amy Parnell, University of Warwick

Abstract

Sexually Transmitted Infections (STIs) are primarily spread through sexual activity. In recent years, STIs have been on the rise globally. In the United States, chlamydia^[2] and gonorrhoea^[8] are the most prevalent bacterial STIs and have been for the past decade. Both these infections infect the same tissues, have similar modes of transmission^[26], clinical presentations, and can be treated by the same antibiotics. Yet, the epidemiologies and forecasts appear to be different. This paper identifies vulnerable populations specific to gonorrhoea and chlamydia and assesses factors that are likely driving these disparities. Publicly available surveillance^[23] data from The Centre for Disease Control and Prevention (CDC) was examined to identify vulnerable populations for both diseases. These findings show that there are sex-specific differences in risk of gonorrhoea and chlamydia, and that young females carry an increased risk of both. Also, there is an increased risk of both infections among the Black/African American population. Understanding risk and risk-drivers is essential to targeting these vulnerable populations for the interest of public health.

Keywords: Sexually transmitted infections (STIs); Sexual health; Gonorrhoea; Chlamydia; Risk factors; Adolescent sexual health; Ethnic disparities; Social determinants of health; prevalence.            

Introduction

Epidemiology^[6]

The United States is experiencing a rise in sexually transmitted infections^[22] (STIs), reaching an epidemic status, and posing a significant public health threat (Nelson et al., 2021). For the past decade, chlamydia and gonorrhoea have remained the most prevalent STIs in the United States, with 1,649,716 and 648,056 cases reported in 2022, respectively (Nelson et al., 2021).

Figure 1A - Rates of infection are dependent on historical events.

Figure 1: Rates of gonorrhoea and chlamydia, in the United States 1941–2022. A. Incidence. White, gonorrhoea; grey, chlamydia. White, arrowhead, introduction of penicillin (Hook and Kirkcaldy, 2018); grey arrowhead, start of the free love and the hippie movement (Goldman, 1998; Johnson Lewis, 2019); black arrowhead, impact of condoms during HIV^[9]/AIDS epidemic (Kershaw, 2018; Boti Sidamo et al., 2021). Blue dashed line, threshold resistant to public health interventions. B. Future projections. Dotted lines, exponential extrapolations. Data from CDC, https://www.cdc.gov/sti-statistics/datavis/table-sticasesrates.html.

In recent years, rates of reported chlamydia and gonorrhoea in the United States have risen (CDC, 2024a). In 2022, the rates of reported cases of chlamydia were more than double that of gonorrhoea (Figure 1B) (CDC, 2024a). Curiously, they infect the same tissues, have similar modes of transmission, clinical presentations and can be treated by the same antibiotics (CDC 2021a; CDC, 2021b; CDC, 2024a; Quillin and Seifert, 2018). Yet, the epidemiologies and forecasts appear to be different. Considering the effect of the COVID-19 pandemic^[13] on reporting, estimates were measured prior to the pandemic; these forecasts presented chlamydia doubling every 18 years and gonorrhoea doubling every 7 years (Figure 1B) (Sentís et al., 2021). This strongly suggests that transmission of these infections is driven by different populations. The aim of this project was to identify the risk groups and to explore key determinants that are driving these differences, such as biological, behavioural, cultural and social factors.

Transmission

Transmission of gonorrhoea is highly efficient from males to their sexual partners through ejaculates as N. gonorrhoeae attaches to sperm (James-Holmquest et al., 1974). However, the efficiency of transmission from females to their partners remains unclear (Ketterer et al., 2016). Chlamydia transmission can occur during contact with infected genitalia, regardless of ejaculation (NHS, 2017).

Clinical manifestations and sequala

Clinical manifestations of gonorrhoea and chlamydia are similar but often go unnoticed (Quillin and Seifert, 2018). Typically, symptoms^[24] are more apparent in males, such as dysuria^[4] and a purulent discharge^[16] from the penis, whereas females are more likely to experience inconspicuous symptoms, such as vaginal discharge, which can be mistaken for hormonal fluctuations and typical variability (Quillin and Seifert, 2018).

Timely treatment is important to reduce the severity of sequela and prevent further transmission (NHS, 2017). Importantly, if left untreated, chlamydia and gonorrhoea can result in PID, ectopic pregnancies^[5] and irreversible infertility in females and urethritis^[27], epididymitis^[7] and proctocolitis^[15] in males (Jennings and Krywko, 2023; Stamm et al., 1984).

Thus far, there are no studies that include recent data released by the CDC that explore the US population. To improve sexual health, we must understand both distinct and common influences on risk and identify populations that are disproportionately at risk of these infections so we can consider public health interventions to target the populations that are disproportionately affected.

Materials and methods

Source of data

Datasets were from the Sexually Transmitted Infections Surveillance Report, published by The Centre for Disease Control and Prevention (CDC), https://www.cdc.gov/sti-statistics/data-vis/index.html.

ꭓ2 tests

The CDC surveillance data includes unrecorded sex, similarly unknown age and ethnicity. Since this data is uninterpretable, it was redacted for analysis. Multiracial data is counterintuitive due to the absence of details within this population, making it challenging to explore the social determinants affecting risk in this population.

Expected values considered the null hypothesis; there is no proportionate difference in sex of the population and observed number of STI diagnoses and these values were calculated from the proportion of the population (using US census data) (Duffin, 2022; BMJ, 2019). When observed frequencies were compared with expected frequencies of a single variable – for example, sex – a ꭓ² Goodness of Fit test was used. An example calculation from 2017 is shown in Table 1. The gender ratio in the United States has remained steady since 2013 so the same ratio was used for analyses (Duffin, 2022).

When examining the relationship between two independent variables, such as sex and age, comparisons were made using a ꭓ² contingency test (BMJ, 2019). Expected values were derived from ꭓ² contingency considerations and calculated using the proportion of ethnicity (using US census data) and observed cases (BMJ, 2019). Risk groups, those with proportionately more or fewer cases than expected, were identified by the largest contribution to the ꭓ² value (‘splitting’ of ꭓ²) (BMJ, 2019). Methods are described in full in BMJ Statistics at square one (BMJ, 2019). A worked example calculation from 2017 is shown in Table 2. Calculation outputs are shown in Table 3.

^a observed cases; ^b expected cases (proportions from US census in 2010); ^c degrees of freedom; ^d probability value.

Table 1: Testing incidence of gonorrhoea in 2017.

^a observed cases; ^b expected cases (proportions from US census in 2010); ^c degrees of freedom; ^d probability value.

Table 2: Testing incidence of gonorrhoea in 2017.

^a observed cases; ^c degrees of freedom; ^d probability value.

Table 3: Chi-squared test calculation outputs.

Results

Males carry the burden of gonorrhoea; females, chlamydia

As a first step towards identifying high- and low-risk groups, diagnosed cases were separated by sex, and these observed numbers were compared with χ² expectations based upon the proportion of males and females in US census records (see Table 1) (Duffin, 2022; U.S. Census Bureau, 2021). Over the past six years where data is available, males have consistently carried a greater burden of diagnosed gonorrhoea than females, with rates ranging from 28 per cent to 42 per cent higher and an average difference of 34 per cent (Figure 2).

Figure 2: Males carry a larger burden of diagnosed gonorrhoea (2017–2022). Columns, diagnoses; red crosses, expected values derived from the proportion of males and females in the US population (Duffin, 2022). Significance was determined by χ² goodness of fit tests: an example calculation from 2017 is shown in Table 1. Data from CDC, https://www.cdc.gov/std/statistics/2022/tables/15.htm.

In marked contrast, females have carried a consistently higher burden of diagnosed chlamydia than males over the same period, averaging 59 per cent higher with rates ranging from 54 per cent to 64 per cent (Figure 3). Thus, despite having the same transmission route and infecting the same tissues, there are sex-specific differences for the two infections. Of note, these are diagnosed cases, which may not reflect the true burden of disease due to factors such as avoidance of, or limited access to, regular sexual health screening^[17].

Figure 3: Females carry a larger burden of diagnosed chlamydia (2017–2022). Columns, diagnoses; red crosses, expected values derived from χ² considerations. Data from CDC, https://www.cdc.gov/std/statistics/2022/tables/6.htm.

Young females are high risk of gonorrhoea; males are low risk of chlamydia

Having established the influence of sex on the burden of infection, the data was divided into age and sex, examining by χ² contingency tests (see Table 2). Despite there being an average of 34 per cent greater burden of gonorrhoea in males, young females (age 15–24) carry a high proportional risk and have done for the last six years (Figure 4). Notably, older males (age 45–54) also carry a high proportional risk, suggesting transmission from older males to young females.

Figure 4: Young females carry a larger proportional burden of diagnosed gonorrhoea (2017–2022). Columns, diagnoses; orange, high-risk female groups; red crosses, expected values derived from χ² contingency considerations: an example calculation from 2017 is shown in Table 2. Data from CDC, https://www.cdc.gov/std/statistics/2022/tables/15.htm.

Conversely, even though females carried an average of 59 per cent greater burden of diagnosed chlamydia, young males (age 15–24) have the lowest proportional risk of chlamydia and have done for the last six years (Figure 5), indicating that the burden of infection is age dependent.

Figure 5: Young males carry the lowest proportional burden of diagnosed chlamydia (2017–2022). Columns, diagnoses; green, low-risk male groups; red crosses, expected values derived from χ² contingency considerations. Data from CDC, https://www.cdc.gov/std/statistics/2022/tables/6.htm.

Black/African American population are high risk of gonorrhoea and chlamydia; White population is low risk

Having identified the impact of age and sex, the impact of ethnicity on risk was investigated by χ² goodness of fit. The Black/African American population were revealed to have a proportionately high risk of gonorrhoea, and the White population are proportionately low risk (Figure 6). Specifically, Black/African American males are ten times more likely to be diagnosed with gonorrhoea than White males, while Black/African American females are almost nine times more likely than White females.

Females were also identified as lower relative risk of gonorrhoea compared to their male counterparts, except in the American Indian/Alaska Native population. Despite the burden of diagnosis being higher among the Black/African American population, there is a notable shift in the Hispanic/Latino population, where Hispanic/Latino males are at a proportionately higher risk of infection and females at a proportionately low risk. The Asian population have the lowest relative risk. American Indian/Alaska Native have a relatively high risk of infection.

Figure 6: The US Black/African American population carries a disproportionate burden of gonorrhoeal cases in 2022. Columns, diagnoses; orange, highest risk group; green, lowest risk group; crosses, expectation. Inset, RR, relative risk (observed/expected). B/AA, Black/African American; Hispanic, Hispanic/Latino; AI/AN, American Indian/Alaskan Native; NH/PI, Native Hawaiian/Pacific Islander. Data from CDC, https://www.cdc.gov/std/statistics/2022/tables/16.htm.

Like gonorrhoea infection, Black/African American population carry the highest proportional risk of chlamydia cases (Figure 7). Black/African American males are more than eight times as likely to be diagnosed with chlamydia, and females are over six times as likely, compared to their White counterparts. However, White males have the highest proportional contribution to χ², carrying the lowest proportional risk. Remarkably, Hispanic/Latino females have a high proportional risk, and males have a low proportional risk, despite the inverse being observed for gonorrhoea. A similar trend was observed in the Native Hawaiian/Pacific Islander population. Like gonorrhoeal infection, the Asian population are associated with low risk whereas American Indian/Alaska Native are associated with high risk.

While these results indicate that ethnicity influences the risk of these infections, social and cultural factors also play a role in these disparities.

Figure 7: The US Black/African American population carries a disproportionate burden of chlamydia cases in 2022. Columns, diagnoses; orange, highest risk group; green, lowest risk group; crosses, expectation. Inset, RR, relative risk (observed/expected). B/AA, Black/African American; Hispanic, Hispanic/Latino; AI/AN, American Indian/Alaskan Native; NH/PI, Native Hawaiian/Pacific Islander. Data from CDC, https://www.cdc.gov/std/statistics/2022/tables/7.htm.

Young females are high risk of gonorrhoea; young males are low risk of chlamydia

Next, the risks associated with age within ethnic groups was examined by χ² contingency tests. Overall, risk groups were similar across ethnicities, with adolescent and young adult females (age 15–24) having the highest proportional risk of gonorrhoea (Figure 8). Interestingly, American Indian/Alaskan Native individuals formed a distinct risk group, with young adult males (aged 15–19) exhibiting a high proportional risk. This was attributed to underreporting, which may mask the true number of cases in this population. Nevertheless, the uniform risk across the three largest populations suggests factors, other than cultural, are driving infection.

Figure 8: Independent of ethnicity, young females carry a disproportionately high burden of gonorrhoeal cases in 2022. Columns, diagnoses; orange, highest risk group; green, lowest risk male groups; red crosses, expected values derived from χ² contingency considerations. B/AA, Black/African American; Hispanic, Hispanic/Latino; AI/AN, American Indian/Alaskan Native; NH/PI, Native Hawaiian/Pacific Islander. Data from CDC, https://www.cdc.gov/std/statistics/2022/tables/16.htm.

Similarly, risk groups for chlamydia are consistent across ethnicities. Notably, young males (age 15–24) have the lowest proportional risk of chlamydia (Figure 9). Distinct risk groups were identified in Multiracial and Native Hawaiian/Pacific Islander ethnicities, although this is likely justified by the relatively small population size.

Figure 9: Independent of ethnicity, young males carry a disproportionately low burden of chlamydia cases in 2022. Columns, diagnoses; orange, highest risk group; green, lowest risk male groups; red crosses, expected values derived from χ² contingency considerations. B/AA, Black/African American; Hispanic, Hispanic/Latino; AI/AN, American Indian/Alaskan Native; NH/PI, Native Hawaiian/Pacific Islander. Data from CDC, https://www.cdc.gov/std/statistics/2022/tables/7.htm.

Discussion

Sex-specific risk

Given that STIs are on the rise in the United States, I sought to identify risk groups and explore potential drivers of infection for gonorrhoea and chlamydia (Nelson et al., 2021). When comparing observed cases with ꭓ² expectations, I noticed that there are different burdens of diagnosis between sexes and have been since 2017. Males carry the burden of gonorrhoea diagnosis and females carry the burden of chlamydia diagnosis.

Female biological vulnerability

It is recognised that females are biologically more susceptible to STIs due to the vulnerability of the vaginal membrane (Van Gerwen et al., 2022). The vaginal mucosa is thin and easily penetrated by an array of pathogens, including N. gonorrhoeae and C. trachomatis (Van Gerwen et al., 2022). In tandem, there is an increased efficiency of transmission from males to females, compared to females to males (Hooper et al., 1978; Platt et al., 1983). Recent studies indicate that following a single sexual encounter^[18] exposing an individual to gonorrhoea, a female is 60–90 per cent likely to become infected whereas a male is only 20–30 per cent likely, due to a greater exposure in females due to pooled semen in the vagina in conjunction with trauma to vaginal tissue during intercourse (Platt et al., 1983; Hooper et al., 1978). In addition, females are more likely to carry less decision-making power over sexual relationships, which is associated with the ability to ensure consistent condom use (Ford and Lepkowski, 2004; Tschann et al., 2002). Therefore, females are more biologically susceptible and have less empowerment over protecting themselves, which may partly explain the increased risk observed in females for chlamydia.

Sex-specific health-seeking behaviours

Biological susceptibility does not explain the difference observed in gonorrhoea, where males carry the burden of diagnosis. Instead, the cryptic nature of these infections may explain this disparity (Quillin and Seifert, 2018). Both infections often go unnoticed, but chlamydia infection appears to be more inconspicuous. Chlamydia infection is asymptomatic^[1] in up to 70 per cent of females and 50 per cent of males while gonorrhoea is estimated to be 10 per cent of males and 50 per cent of females (NHS, 2019). This suggests that asymptomatic screening likely plays a substantial role in reported chlamydia (CDC, 2024b). In contrast, symptomatic^[25] testing, more common in males, is responsible for the majority of reported gonorrhoea cases. This concept was supported by a study associating sex with different health-seeking behaviours; specifically, young females are more engaged in health services, such as regular Pap testing, which link them to sexual health services (Knight et al., 2016). Hence, females are more likely to go for asymptomatic testing.

In marked contrast, young males practice self-monitoring of symptoms and tend to avoid formal diagnosis (Knight et al., 2016). When young males did access sexual health services, they did so reactively: after engaging in a high-risk sexual encounter or when experiencing symptoms (Knight et al., 2016). Taken together, the sex differences in cases of gonorrhoea and chlamydia can be explained by diverging health-seeking behaviours, such that females tend to be proactive so are more likely to detect asymptomatic infections, which are most common in chlamydia, while males tend to be reactive, seeking testing when they experience symptoms, which are more common in gonorrhoea (Knight et al., 2016).

Sexual risk among adolescents

Risk is also age dependent. I revealed that while males have an elevated overall risk of gonorrhoea, young females (aged 15–24) have a high proportional risk of infection. Biological susceptibility is partially responsible for the elevated risk in pubescent females due to increased cervical ectopy and a lower production of cervical mucus (Lee et al., 2006; Shannon and Klausner, 2019). Studies have shown the cervical ectopy is linked with increased risk of gonorrhoea and chlamydia infection (Kleppa et al., 2014). Young females also tend to have a lower production of cervical mucus, which plays a protective role against infection (Wong et al., 2004). Thus, if exposed to an STI, young females are more likely to get infected, which would explain the disparity in risk of gonorrhoea (Kleppa et al., 2014; Lee et al., 2006; Shannon and Klausner, 2019; Wong et al., 2004).

Additionally, behavioural factors, as highlighted in a pilot study, influence risk of STIs (Tzilos et al., 2020). Namely, alcohol consumption while at college was closely linked to engaging in condomless sex (Tzilos et al., 2020). Another study highlighted that young people are more likely to reduce or drop using condoms when in monogamous^[10] relationships, potentially exposing them to STIs (Brady et al., 2009).

Risk paradox

Notably, this difference was not seen in young males (age 15–24), identified as proportionately low risk of gonorrhoea and chlamydia. However, these results seem paradoxical. In fact, according to current literature, all adolescents are at an increased risk of STIs (Maraynes et al., 2017; The Lancet Child & Adolescent Health, 2022). I reasoned that reduced screening is masking true cases in the young male population (Maraynes et al., 2017).

In terms of behaviour, adolescents are typically more likely to engage in sexual activity associated with increased risk of STIs, such as concurrent partners, polygamous^[14] relationships and condomless sex, due to a developing prefrontal cortex (Shannon and Klausner, 2019). The prefrontal context is responsible for executive function, so an undeveloped may somewhat explains poor decision-making typically seen in this age group (Shannon and Klausner, 2019). Furthermore, adolescents perceive themselves as low-risk and are less likely to access sexual health services (Shannon and Klausner, 2019; Tzilos et al., 2020). To address this, an increase in sexual health education prior to sexual debut and continued awareness and access to screening services would be essential to target adolescents. In addition, further efforts are necessary in screening symptomatic young males for a comprehensive understanding of risk in this group.

Sexual risk among different ethnicities

Since my results are driven by Chi-squared expectation, risk groups are revealed if they deviate from the major population – in this case, White (U.S. Census Bureau, 2021). When comparing ethnicities, it is clear there are considerable differences in relative risk of gonorrhoea and chlamydia. Black/African Americans are at elevated risk and Whites are proportionately low risk. I first explored the possibility of behavioural differences driving these diverging risks. A study discovered that Whites and Hispanics are more likely to engage in oral sex^[12] when compared to Black/African Americans (Auslander et al., 2009). The study also revealed that among this cohort, females who had experience of vaginal and oral sex were six times more likely to have a history of STIs, compared to those who had experience of vaginal sex only (Auslander et al., 2009). Another study supported these findings (Salazar et al., 2008). Therefore, despite Black/African American being more likely to participate in behaviour associated with low risk, they remain at increased risk. Alternatively, White females are more likely to engage in risk-associated behaviours yet remain low risk. This provides compelling evidence that risk is not determined solely by sexual practices (Auslander et al., 2009; Salazar et al., 2008).

Neisseria gonorrhoeae ^[11] A bacterium that causes gonorrhoea.

Oral sex ^[12] Using a mouth to stimulate another person’s genitals or anus.

Pandemic ^[13] An infectious disease prevalent over several countries or continents.

Polygamous ^[14] Referring to having more than one sexual partner at one time.

Proctocolitis ^[15] Inflammation of the rectum.

Purulent discharge ^[16] A pus-containing fluid.

Screening ^[17] STI screening can detect for the presence of a sexually transmitted infection.

Sexual encounter ^[18] A single instance of sexual activity, physical intimacy can vary.

Sexual network ^[19] A network of people that are linked through sexual relationships.

Sexual orientation ^[20] A personal pattern of romantic or sexual attraction.

Stigma ^[21] A negative social attitude toward a person or circumstance, can include shame.

Sexually Transmitted Infection (STI) ^[22] A disease that can be transmitted through sexual contact.

Surveillance ^[23] Collecting information about the cases of STIs in a population.

Symptom ^[24] A feature (physical or mental) that indicates a disease.

Symptomatic ^[25] Presence of symptoms.

Transmission ^[26] The spread of something from one person to another.

Urethritis ^[27] Inflammation of the urethra.