32. DMD Epidemiology 2025

Epidemiology of Infection

Objectives of the Lecture

• Understand the concept of epidemiology of infections and the roles of:

  • Inoculum

  • Virulence

  • Host resistance

• Describe the transmission of viruses and bacteria.

• Discuss the seasonality of common infections.

• Analyze patterns of bacterial and viral infections including terms such as:

  • Acute

  • Self-limiting

  • Latent

  • Reactivation

  • Chronic

  • Commensal

  • Opportunistic

• Overview of the epidemiology of herpes viruses:

  • Herpes Simplex

  • Varicella virus

  • Cytomegalovirus (CMV)

  • Epstein-Barr Virus (EBV)

• Define:

  • Virulence of microbes

  • Endemic and epidemic infections

  • Prevalence and incidence

• Briefly describe the standard approach to outbreak investigation.

Fundamental Concepts of Epidemiology

Definition

• Epidemiology is the study of the occurrence, determinants, distribution, and control of disease in populations.

• 5W

• Infection risk is influenced by:

  • Individual susceptibility

  • Level of disease in the population

  • Herd immunity

  • Communicable period

  • Infectiousness

Patterns of Distribution in Epidemiology

• Distribution can be analyzed by:

  • Age

  • Gender

  • Occupation/Hobby

  • Nutritional status

  • Season

  • Geography

  • Living conditions/Social behavior

Disease Metrics

Prevalence vs. Incidence

• Prevalence: Cases per unit population at a given time. total compare to now

• Incidence: Number of new cases per unit population.

• Example: Acute self-limiting infections like influenza have high incidence (in winter) but low prevalence, whereas chronic infections like hepatitis B have high prevalence with low incidence (now have vaccination).

Factors Influencing Infection Risk

Key Influencers

• Factors determining likelihood of infection include:

  1. Inoculum (dose) - number organism

  2. Virulence - ability to cause disease

  3. Host resistance

• Generally, a high dose coupled with high virulence and low host resistance increases the likelihood of disease.

Sources of Infection

Exogenous Infections

• An exogenous infection is an infection caused by a microorganism from outside the body, such as the environment or another person

• Most common sources are:

  • Human-to-Human

  • Zoonotic (from animals to humans)

  • Rarely from the environment.

Endogenous Infections

• Infections arising from normal flora, including:

  • E. coli in gut → causing urinary tract infections

  • Oral streptococci → causing endocarditis

  • Staph aureus from the nose → causing wound infections.

  • Mixed oral flora including anaerobes → dental abscess

Commensals

• (Commensal bacteria are microorganisms that live on the body's surfaces without causing harm. They are also known as normal microflora)

• Many bacteria serve as normal flora and may protect against infection.

• In certain conditions, commensals can become opportunistic pathogens/ endogenous infection leading to clinical infections such as:

  • Mucosal damage

  • Reduced immunity due to disease.

Opportunistic Infections by commensals

• Infections that take hold due to reduced host immunity, often occurring in patients with:

  • Chemotherapy

  • Transplantation

  • Diseases like AIDS.

• Examples include:

  • CMV causing systemic infections in transplant patients

  • EBV causing oral hairy leukoplakia in AIDS patients.

Transmission of Infections

Types - vertical and horizontal

• Vertical Transmission: Mother to baby, routes include:

  • In utero (e.g., syphilis)

  • At birth (e.g., HIV)

  • Post-birth (e.g., HTLV).

Horizontal Transmission

• Direct Transmission: Through activities like kissing, touching, or sexual contact.

• Airborne: Inhalation of droplets from an infected individual (e.g., influenza).

• Fomite: Contact with infected surfaces or instruments (e.g., dental equipment).

• Vector-borne: Transmission via arthropods such as ticks or mosquitoes (e.g., Lyme disease, malaria).

Infectious Parameters

Definitions

• Incubation Period: Time from inoculation (exposure to the infectious agent) to symptom onset.

• Influenza has a short incubation period (1-4 days), while diseases like HIV can have much longer incubation periods.

• Infectious Period: Time during which an infected person can spread the infection.

• This may start before symptoms appear and last even after symptoms resolve, depending on the disease.

• Systematic period The duration during which the infected individual experiences symptoms

• infectiousness A measure of how easily a disease spreads from an infected individual to others. This depends on factors like the mode of transmission (e.g., airborne, droplet, contact), the dose required to establish infection, and host factors (e.g., immunity).

Case Study - Chickenpox

• Timeline of infection stages:

  • Inoculation to shedding (day 13

  • Shedding to symptom onset (day 15)

• Later reactivation can occur as shingles (zoster).

Infection Patterns

Categories

• Acute-Self Limiting: E.g., Influenza.

• Chronic: E.g., HBV, HCV, HIV, TB

• Latent with Reactivation: E.g., Herpes viruses, VZV, TB.

General Infection Dynamics

• Various outcomes from an acute infection can lead to:

  • acute infection →Viral clearance

  • acute infection →Chronic infection.

  • acute infection → Latency followed by reactivation

Herpes Virus Epidemiology

ALL HERPES VIRUS HAVE LATENCY

Varicella-Zoster Virus (VZV)

• Initially causes chickenpox, followed by latency by sensory ganglion and potential reactivation as shingles.

  • 95% of adults are seropositive for VZV.

  • Approximately 40% experience shingles.

Herpes Simplex Virus (HSV)

• Initially causes gingivostomatitis , followed by latency by trigenminal ganglion and potential reactivation as cold sore (herpes labialis)

  • 85% of adults are seropositive.

  • ~20% develop recurrent cold sores.

Cytomegalovirus (CMV)

• Infection incidence is ~1% annually; 20% of young adults seropositive.

• Transmission through salivary contact, usually asymptomatic except in pregnancy or immunosuppression.

• retinitis, pneumonia, encephalitis

• incurabe - Life long infection - shed in saliva

Epstein-Barr Virus (EBV)

• Salivary transmission; most people are IgG positive by age 25.

• Can cause infectious mononucleosis (glandular fever) in teenager, viral meningitis, nasopharyngeal cancer, often asymptomatic in children.

Dynamics of Diseases

Outbreak Classifications

• Endemic Infections: Stable incidence within a population.

• Outbreaks: Clusters of cases or increases in incidence.

• Epidemic: Increased incidence over a larger region.

• Pandemic: Spread across multiple continents.

Reproductive Rate (Ro) and R

• R₀ – Basic Reproduction Number

  • It’s the average number of people that one sick person will infect in a completely susceptible population (meaning no one has immunity — no vaccine, no prior infection).

  • It shows how contagious a disease is in ideal conditions.

  Examples:

  • Measles has a high R₀ (~12–18): super contagious.

  • COVID-19 (original strain) had an R₀ around 2–3.

  R – Effective Reproductive Number

  • This is the actual number of people one sick person infects in real-world conditions (where some people are vaccinated, recovered, or immune).

  • So R can be less than R₀ if immunity or control measures (like masks, distancing, etc.) are in place.

  Key point:

  • If R > 1 → the disease is spreading.(epidemic)

  • R=1 endemic - disease is stable

  • If R < 1 → the outbreak is shrinking, reduce transmission

  Term	Meaning	Conditions
  R₀	How contagious a disease is without any immunity or control	Idealized situation (no protection)
  R	How contagious a disease is right now in the real world	Includes effects of immunity, measures

Sentinel GP Consultation Data

Trends

• Consultation rates for flu-like illnesses are tracked from 2010 to 2013, showing fluctuations over the weeks each year.

• allow hospital to prepare and plan

Herd Immunity

Concept

• Resistance of a population to the spread of an infectious agent based on a high proportion of immune individuals.

• Critical for discussions on vaccine effectiveness and strategies.

Disease transmission R0 Herd immunity threshold

Seasonality of Infections

General Patterns

• Compare

  • UTI incidence remains stable throughout the year.

  • Higher respiratory infections in winter.

• Example of seasonal variations:

  • Summer: faeco oral infection , Enteroviruses, Hepatitis A. Barbecue weather?

  • Winter: respiratory infection , Influenza.

  • Spring: Varicella (chicken pox)

• Reason for seasonality may include:

  • Environmental factors (humidity UV) affecting survival of pathogens.

  • Indoor crowding effects.

  • weather related mucosal change

  • Vectors - survival in different weather

Outbreak Control Steps

Procedures

1. Determine if an outbreak is occurring.

2. Form an Outbreak Control Team (OCT) if necessary.

3. Develop case definitions for identification.

4. Collect and analyze data to create epidemic curves.

5. Conduct analytical studies of the outbreak.

6. Implement control measures to mitigate the outbreak such as:

• Removing the source of infection.

• Protecting at-risk individuals.

• 7. come up plans to prevent recurrence

Tools for Infection Control

Management Strategies

• Infection control strategies include:

  • Preventing direct transmission for infection control

  • Treatment or prophylaxis with antimicrobials.

  • Utilizing vaccines for prevention.

  • Applying immunoglobulin where necessary.

  • Conducting environmental cleaning.