STI Spread &
Infection Transmission

Sexually transmitted infections (STIs) are infections that spread from one person to another through sexual contact. They can be caused by bacteria, viruses, or parasites and are a significant public health concern globally. Common STIs include chlamydia, gonorrhea, syphilis, HIV/AIDS, and genital herpes.

Modes of Transmission

Infectious diseases can spread through various modes of transmission, each influenced by the nature of the pathogen and the circumstances of exposure. By recognizing how pathogens spread through direct and indirect routes, healthcare professionals and the public can take steps to reduce transmission, protect vulnerable populations, and promote overall community health and well-being.

Direct Contact

Direct contact transmission occurs when there is physical contact between an infected individual and a susceptible host.

·         Touching: Contact with skin lesions, body fluids, or mucous membranes of an infected person.

·         Sexual Contact: Transmission through sexual intercourse (vaginal, anal, oral) with an infected individual.

·         Droplet Spread: Transmission via respiratory droplets produced when an infected person coughs, sneezes, or talks (typically within close proximity, usually within 6 feet).

Examples of Diseases: Influenza, COVID-19, common cold, sexually transmitted infections (STIs) like HIV, herpes, and syphilis.

Indirect Contact

Indirect contact transmission occurs when the pathogen is transmitted via intermediary objects or surfaces that have been contaminated by an infected individual.

·         Fomite Transmission: Transfer of pathogens via contaminated surfaces or objects, such as doorknobs, utensils, toys, and medical equipment.

·         Vehicle-borne Transmission: Transmission via contaminated food, water, blood, or other biological products.

Examples of Diseases: Norovirus (via contaminated food or surfaces), Clostridium difficile (via contaminated surfaces in healthcare settings).

Airborne Transmission

Airborne transmission occurs when infectious agents are transmitted through inhalation of small particles or droplet nuclei that remain suspended in the air for long periods.

·         Airborne Droplet Nuclei: Transmission of pathogens in fine particles that can remain suspended in the air for prolonged periods and travel distances beyond the immediate vicinity of the infected individual.

·         Aerosols: Transmission through smaller respiratory droplets or aerosols that can linger in the air, especially in enclosed spaces with poor ventilation.

Examples of Diseases: Tuberculosis, measles, chickenpox (varicella), COVID-19 (under certain conditions).

Vector-borne Transmission

Vector-borne transmission occurs when pathogens are transmitted through the bite of an infected arthropod vector (such as mosquitoes, ticks, fleas) that carries and transmits the pathogen from one host to another.

·         Mosquito-borne: Transmission of malaria, dengue fever, Zika virus.

·         Tick-borne: Transmission of Lyme disease, Rocky Mountain spotted fever.

·         Flea-borne: Transmission of plague (Yersinia pestis).

Vertical Transmission

Vertical transmission occurs when pathogens are transmitted from a pregnant woman to her fetus or newborn during pregnancy, childbirth, or breastfeeding.

·         Intrauterine Transmission: Transmission of pathogens across the placenta during pregnancy.

·         Perinatal Transmission: Transmission during childbirth or through contact with maternal genital secretions.

·         Postnatal Transmission: Transmission through breast milk or close contact postnatally.

Examples of Diseases: HIV, hepatitis B, syphilis.

Fecal-Oral Transmission

Fecal-oral transmission occurs when pathogens are ingested via contaminated food, water, or hands that have been contaminated with fecal matter containing infectious agents.

·         Contaminated Food or Water: Transmission of enteric infections like cholera, typhoid fever, norovirus.

·         Poor Hygiene Practices: Transmission through inadequate handwashing after using the restroom or changing diapers.

Examples of Diseases: Hepatitis A, E. coli infections, rotavirus.

Prevalence and Impact

Prevalence refers to the proportion of individuals in a population who have a specific disease or condition at a particular time, often expressed as a percentage or per capita rate. Prevalence is measured by determining the number of existing cases of a disease within a defined population at a specified point in time or over a specific period. It provides insight into the extent and distribution of a disease within a population, helping to assess disease burden and inform public health interventions. Prevalence can be categorized as point prevalence (at a specific point in time) or period prevalence (over a defined period).

Impact refers to the consequences or effects of a disease on individuals, communities, healthcare systems, economies, and society at large. Includes morbidity (illness or disability caused by the disease) and mortality (death caused by the disease), as well as the severity and duration of health effects. Encompasses direct costs (medical expenses, treatment) and indirect costs (lost productivity, economic disruptions) associated with the disease. Considers social consequences such as stigma, discrimination, and disruptions to daily life, as well as humanitarian impacts on vulnerable populations.

Infectious diseases have significant global prevalence and profound impacts on public health, economies, and societal well-being. STIs affect people of all ages, but young people are particularly at risk due to higher rates of sexual activity. They can lead to serious health consequences if untreated, such as infertility, chronic pain, and increased risk of HIV transmission.

Prevalence and Impact of Infectious Diseases

Infectious diseases affect populations worldwide, with varying prevalence rates depending on factors such as geography, climate, healthcare infrastructure, and socioeconomic conditions.

Personal and Public Health Impact

Infectious diseases can cause a wide range of health effects, from mild illness to severe complications and death. In some regions, endemic diseases (persistently present) contribute to a substantial burden of illness and mortality, particularly in low- and middle-income countries where access to healthcare and preventive measures may be limited. They may lead to chronic conditions, long-term disabilities, and decreased quality of life for affected individuals.

Outbreaks of infectious diseases pose challenges to public health systems, requiring rapid response, surveillance, and containment measures. They necessitate coordination among healthcare providers, government agencies, international organizations, and communities to mitigate spread and reduce transmission.

Social and Economic Impact

Infectious diseases can exacerbate social inequalities and disrupt communities, particularly in vulnerable populations. Stigma and discrimination associated with certain diseases (e.g., HIV/AIDS) can further impact affected individuals and communities. Infectious diseases impose substantial economic burdens on individuals, healthcare systems, and economies at large. Costs include direct medical expenses for treatment, hospitalization, and pharmaceuticals, as well as indirect costs related to lost productivity and economic disruptions during outbreaks.

Emerging and Re-emerging Diseases

The emergence of new infectious diseases or the re-emergence of previously controlled diseases poses ongoing challenges to global health security. Factors such as urbanization, globalization, climate change, antimicrobial resistance, and changes in human behavior contribute to the spread and resurgence of infectious diseases.

Contact Tracing in Public Health

Contact tracing is a fundamental public health practice that involves identifying, assessing, and managing individuals who have been exposed to a contagious disease to prevent further transmission. Contact tracing has three main purposes.

Early Detection facilitates early detection of cases, enabling prompt medical intervention and treatment to reduce severity and complications. Contact tracing aims to interrupt the chain of transmission by identifying and isolating potentially infected individuals before they can spread the disease to others. By identifying and monitoring contacts of confirmed cases, public health officials can prevent localized outbreaks from becoming larger epidemics or pandemics.

How Contact Tracing is Performed

Public health officials interview infected individuals to determine their recent contacts. Contacts are then notified, tested if necessary, and advised on preventive measures like quarantine or treatment.

Public health authorities identify individuals diagnosed with a contagious disease (index cases) through laboratory testing and clinical diagnosis. Trained public health professionals interview the index case to gather information about their recent activities, places visited, and individuals they have had close contact with during their infectious period. Contacts are identified based on the type and duration of exposure to the index case. Close contacts, such as household members, intimate partners, and individuals in close proximity (within 6 feet for an extended period), are prioritized.

Contacts are notified of their potential exposure to the disease, educated about symptoms to watch for, and provided with instructions for self-quarantine or self-isolation. They may be advised to monitor symptoms daily, undergo testing for the disease, and seek medical care promptly if symptoms develop. Public health agencies provide support to contacts throughout the monitoring period, ensuring access to healthcare services, mental health support, and resources needed for quarantine or isolation.

Technologies and Tools

Contact tracing helps control outbreaks of infectious diseases, including STIs, by identifying and containing transmission pathways. It plays a vital role in preventing widespread community transmission and reducing the overall burden of disease. Digital Contact Tracing utilizes mobile apps and digital platforms to enhance the efficiency and accuracy of contact tracing efforts. Uses Bluetooth technology or GPS data to track proximity and interactions between individuals, alerting users if they have been in close contact with someone who later tests positive. Public health agencies employ epidemiological tools and algorithms to analyze data, identify transmission patterns, and prioritize contacts for follow-up based on risk assessment.

Challenges

Timeliness and delays in testing results or reporting can hinder timely identification and notification of contacts. Balancing the need for public health intervention with protecting individual privacy rights remains a significant challenge, especially with digital contact tracing technologies. Contact tracing requires substantial resources, including trained personnel, technology infrastructure, and community engagement, which may be limited in some regions.

Basic Reproduction Number (R₀)

The basic reproduction number, often denoted as R₀ (pronounced "R naught"), is a fundamental epidemiological metric used to quantify the transmission potential of an infectious disease within a susceptible population. R₀, or basic reproduction number, estimates the average number of secondary infections caused by one infected individual in a susceptible population.

It depends on factors such as the infectiousness of the pathogen, contact rates between individuals, and population immunity. R₀ helps predict the potential spread of an infectious disease. If R₀ is greater than 1, the infection is likely to spread in the population; if less than 1, the infection may die out.

Factors Influencing R₀

·         Infectiousness of the Pathogen: The ability of the pathogen to infect susceptible individuals, replicate within the host, and shed from the infected host plays a crucial role in determining R₀. Pathogens with higher infectiousness (e.g., measles virus) tend to have higher R₀ values.

·         Contact Rate (β): The frequency and intensity of contacts between infectious and susceptible individuals influence the transmission dynamics. Factors such as population density, social interactions, and behavior patterns affect the contact rate and consequently impact R₀.

·         Duration of Infectiousness (D): The length of time an infected individual remains infectious and capable of transmitting the disease to others affects the potential for secondary infections. Diseases with longer infectious periods generally have higher R₀ values.

Calculation of R₀

R₀ is calculated using mathematical models that incorporate parameters such as infectious period (D) and contact rate (β).

The formula for R₀ is typically expressed as:

R₀ = 𝛽 × 𝐷

·         β (beta) is the average contact rate between infectious and susceptible individuals per unit time.

·         D (delta) is the average duration of infectiousness (this is sometimes problematic, due to the complexity of how to calculate delta).

Interpretation and Implications

If R₀ is greater than 1 (R₀ > 1), each infected individual, on average, transmits the disease to more than one other person. This suggests that the disease may spread and cause an epidemic.

If R₀ is less than 1 (R₀ < 1), the infection is likely to die out over time as each infected individual, on average, transmits the disease to fewer than one other person.

·         Control Strategies: R₀ helps public health officials devise strategies to control disease spread, such as vaccination campaigns, quarantine measures, and promotion of preventive behaviors.

·         Comparison of Diseases: Comparing R₀ values across different diseases provides insights into their relative transmissibility and informs prioritization of public health interventions.

Lab Experiment

Objective

To simulate the spread of sexually transmitted infections (STIs) using a fluid exchange activity and to observe the transmission pattern through a color change reaction. This activity will also explore the concepts of contact tracing and the basic reproduction number (R₀).

Materials

·         Test tubes (one per student)

·         Pipettes (one per student)

·         Phenolphthalein solution

·         Sodium hydroxide (NaOH) solution

·         Distilled water

·         PPE

Protocol

1.       Preparation

a.   Students should wear gloves, safety goggles, and a lab coat.

b.   Each student receives a test tube containing 10 mL of distilled water. One randomly selected test tube will contain 10 mL of 0.1M NaOH instead of water. The identity of the student with the NaOH solution should remain unknown to the participants.

2.       Fluid Exchange

a.   Students will pair up with another student and write down their name (keep the names in order of contact).

b.   Using their pipette, each student will transfer 1 mL of their fluid into their partner's test tube.

c.        After transferring, gently swirl the test tube to mix the contents.

d.   Repeat the process, ensuring each student transfers fluid into their partner's test tube and receives fluid in return.

e.   Repeat steps 2a-2d, two more times.

                                                               i.      Students will find a new partner (not the same as previous exchanges).

                                                             ii.      Repeat the fluid exchange process as described in steps 2a-2d.

3.       Testing for NaOH Presence

a.   After the three rounds of fluid exchanges, each student should return to their original workspace.

b.   The instructor will add 2-3 drops of phenolphthalein solution to each student's test tube.

c.        Observe the color change in the test tubes. A pink color indicates the presence of NaOH “infection”.

4.       Analysis

a.   Record the number of students whose test tubes turned pink.

b.   Discuss the pattern of "infection" spread. Determine how many students were "infected" (test tubes turned pink) by the end of the activity.

c.        Compare this to the initial number of infected individuals (one student with NaOH).

Contact Tracing:

1.       Working together, map out the potential transmission pathways by identifying who exchanged fluids with whom.

2.       Identify the original infected individual and trace the subsequent spread through the group.

a.   Create a list of all students with positive test results (pink test tubes).

b.   Each infected student will review their logs to identify all partners they exchanged fluids with during each round.

c.        Determine which partners were already infected before the exchange and which partners were newly infected after the exchange.

                                                               i.      Start from the students who tested positive and trace back through their exchange partners.

                                                             ii.      For each infected student, draw lines to indicate their partners in each round.

                                                            iii.      Use different colors or symbols to indicate primary, secondary, and tertiary infections.

                                                            iv.      Look for patterns in the map that might indicate the source of the initial infection.

                                                              v.      Pay special attention to the first instances where a student became infected.

3.       In the space below draw the contact tracing map.

Basic Reproduction Number (R₀):

1.       R₀ is defined as the average number of secondary infections produced by a single infected individual in a fully susceptible population.

2.       Identify the number of new infections caused by the initial infected individual in each round.

3.       Calculate the average number of secondary infections (R₀).

a.   R₀ = Total number of new infections in a round / Number of initially infected individuals in that round

4.       Look up R₀ values for known STIs (e.g., HIV, HPV, chlamydia, etc….)

5.       Record below the R₀ value of the “STI” that was spread in lab and the R₀ value of real STIs.

Review Questions

1.       How does this activity simulate the spread of sexually transmitted infections?

2.       What factors in real life could influence the rate of STI spread that were not accounted for in this simulation?

3.       How effective was the tracing process in identifying the transmission paths?

4.       How might this process be more challenging in a real-world scenario with a larger population and less controlled environment? (timing, digital contact-tracing, etc…)

5.       How would incomplete or inaccurate information affect real-world contact tracing efforts?

6.       What could happen if more partners were included (fourth, fifth, or more rounds)?

7.       Explain the importance of preventive measures such as safe practices, regular testing, and public health interventions in reducing the spread of infections.

8.       How does the calculated R₀ in this activity compare to known R₀ values for various STIs? (e.g., HIV, HPV, chlamydia)

9.       What factors could influence R₀ in real-world scenarios, and how can interventions alter this number? (e.g., reducing the number of partners, increasing the use of condoms/protections, vaccinations)

10.    Why is it important to understand the transmission pathways of infections when dealing with public health issues?