Chapter 8 Infectious Diseases and Outbreak Investigation Learning

Chapter 8 Infectious Diseases and Outbreak Investigation

Learning Objectives • Describe modes of transmission of communicable diseases • Name three microbial agents associated with infectious diseases • Describe the epidemiology of two infectious diseases • State procedures for investigating infectious disease outbreaks

Introduction • Infectious diseases are important causes of morbidity and mortality in the U. S. and worldwide. – Category of influenza and pneumonia was eighth leading cause of death in U. S. in 2005. – Infectious/parasitic diseases account for almost 20% of mortality worldwide. – Respiratory infections account for 7% of mortality worldwide.

Infectious (Communicable) Disease • “An illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment. ”

Parasitic Disease • An infection caused by a parasite, which “…is an organism that lives on or in a host organism and gets its food from or at the expense of its host. ” • Example: amebiasis

Epidemiologic Triangle • One of the long-standing models used to describe the etiology of infectious diseases • Includes three major factors: agent, host, and environment

Agent • “A factor—such as a microorganism, chemical substance, or form of radiation—whose presence, excessive presence, or (in deficiency diseases) relative absence is essential for the occurrence of a disease…. ”

Host • “A person or other living animal, including birds and arthropods, that affords subsistence or lodgment to an infectious agent under natural conditions. ”

Environment • The domain in which disease-causing agents may exist, survive, or originate

The Epidemiologic Triangle Source: Reprinted from Friis RH, Sellers TA. Epidemiology for Public Health Practice. 4 th ed. Sudbury, Massachusetts: Jones and Bartlett Publishers; 2009: 439.

Infectious Disease Agents • With respect to infectious and communicable diseases, agents include specific microbes and vectors involved in the cycle of disease transmission. – Examples of infectious agents: bacteria, rickettsia, viruses, fungi, parasites, and prions

Infectivity • The capacity of an agent to enter and multiply in a susceptible host and thus produce infection or disease

Virulence • Refers to the severity of the disease produced, i. e. , whether the disease has severe clinical manifestations or is fatal in a large number of cases.

Toxin • Some infectious disease agents, instead of acting directly, produce a toxin that causes illness. • A toxin usually refers to a toxic substance made by living organisms. – Examples of illness: foodborne intoxications such as botulism.

Four infectious disease agents Upper left, bacillus anthracis bacteria Lower left, dermatophyti c fungus (causes ringworm infections of the skin and fungal infections of the nail bed Upper right, herpes simplex virions Lower right, Cryptosporidium parvum oocysts Source: Reprinted from Centers for Disease Control and Prevention. Public Health Image Library, ID# 10123 (upper left); ID# 10230 (upper right); ID# 4207 (lower left); ID# 7829 (lower right). Available at: http: //phil. cdc. gov/phil/details. asp. Accessed July 3, 2008.

Host Characteristics • Immunity • Other factors related to host responses: – Incubation period – Subclinical illness – Generation time – Carrier status

Immunity • Refers to the host’s ability to resist infection by the agent. • May be either active or passive

Active Immunity • Immunity that the host has developed as a result of natural infection with a microbial agent • Also can be acquired from an injection of a vaccine that contains an antigen – Antigen: A substance that stimulates antibody formation, e. g. , a microbial agent • Usually of long duration (measured in years)

Passive Immunity • Immunity acquired from antibodies produced by another person or animal • Example 1: newborn infant’s natural immunity conferred transplacentally from its mother • Example 2: immunity conferred by injections of antibodies contained in immune serums • Usually of short duration (measured in days to months)

Herd Immunity • The resistance of an entire community to an infectious agent as a result of the immunity of a large proportion of individuals in that community to the agent

Incubation Period • Time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease

Subclinical Infection • An infection that does not show obvious clinical signs or symptoms • Also called an inapparent infection • Example: hepatitis A infections among children

Generation Time • The time interval between lodgment of an infectious agent in a host and the maximal communicability of the host

Carrier • “A person or animal that harbors a specific infectious agent without discernible clinical disease, and which serves as a potential source of infection. ”

Index Case • Used in an epidemiologic investigation of a disease outbreak to denote the first case of a disease to come to the attention of authorities

Environment and Infectious Diseases • The external environment comprises these components: – Physical – Climatologic – Biologic – Social – Economic

Endemic • Refers to a infectious disease agent that is habitually present in an environment (either geographic or population group) • Example: Plague is endemic among certain species of rodents in the western U. S.

Reservoir • A place where infectious agents normally live and multiply • Can be human beings, animals, insects, soils, or plants

Zoonosis • “An infection or infectious agent transmissible under natural conditions from vertebrate animals to humans. ”

Means of Transmission of Infectious Disease Agents • Direct transmission – Person to person • Indirect transmission – Vehicle-borne infections – Airborne infections – Vector-borne infections

Portal of Exit • For an infected person, a portal of exit is the site from which the agent leaves that person’s body. • Examples: – Respiratory passages – Alimentary canal – Genitourinary system – Skin lesions

Person to Person (Direct Transmission) • “Direct and essentially immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place…. ”

Portal of Entry • Site where the agent enters the body • Example: a skin wound

Indirect Transmission • Involves intermediary sources of infection such as vehicles, droplet nuclei (particles), and vectors.

Vehicle-borne Infections • These infections result from contact with vehicles, which are contaminated, nonmoving objects. • Examples: – Fomites – Unsanitary food – Impure water

Unsafe injection practices and circumstances that likely resulted in transmission of hepatitis C virus (HCV) at clinic A—Nevada, 2007 Source: Reprinted from Centers for Disease Control and Prevention. Acute hepatitis C virus infections attributed to unsafe injection practices at an endoscopy clinic—Nevada, 2007. MMWR. 2008; 57: 516.

Fomite • An inanimate object that carries infectious disease agents • Examples: – Classroom doorknob – Discarded tissues

Airborne Infections • Involve the spread of droplet nuclei (particles) that are present in the air. • Example: infections caused by stirring up dust that carries fungi or microbes

Vector-borne Infections • A vector is an animate, living insect or animal that is involved with the transmission of disease agents. • Transmission of an infectious disease agent may happen when the vector feeds on a susceptible host.

Four vectors of infectious diseases Upper left, a female louse Lower left, a tick Upper right, a female Aedes aegypti mosquito acquiring a blood meal Lower right, a Norway rat. Source: Reprinted from Centers for Disease Control and Prevention. Public Health Image Library, ID# 9250 (upper left); ID# 9255 (upper right); ID# 9959 (lower left); ID# 5445 (lower right). Available at: http: //phil. cdc. gov/phil/details. asp. Accessed July 3, 2008.

Examples of Significant Infectious Diseases • • Sexually transmitted diseases Foodborne diseases Waterborne diseases Vector-borne diseases Vaccine-preventable diseases Zoonotic diseases Emerging infections Bioterrorism-related diseases

Sexually Transmitted Diseases • HIV/AIDS • Gonococcal infections • Chlamydial genital infections

Foodborne Illness • Biologic agents of foodborne illness include bacteria, parasites, viruses, and prions (linked to mad cow disease). • Examples of bacterial agents include: – Campylobacter – Clostridium botulinum – Salmonella

Source: Reprinted from Centers for Disease Control and Prevention. Summary of Notifiable Diseases—United States, 2006. MMWR. 2008; 55: 42.

Vector-borne Diseases • Examples of diseases (name of vector in parentheses) from Table 8 -6: – Bacterial diseases • Lyme disease (tick) – Arthropod-borne (arboviral) diseases • Eastern equine encephalitis (mosquito) – Parasitic diseases • Malaria (mosquito)

Vaccine-Preventable Diseases (VPDs) • Conditions that can be prevented by vaccination (immunization) • Examples include: – Diptheria – Tetanus – Whooping cough – Hepatitis A and B

Zoonotic Diseases • Diseases transmitted from vertebrate animals to human beings • Examples: – Rabies – Anthrax – Avian influenza – Hantavirus pulmonary syndrome – Toxoplasmosis – Tularemia (rabbit fever)

Emerging Infectious Diseases (Emerging Infections) • “An infectious disease that has newly appeared in a population or that has been known for some time but is rapidly increasing in incidence or geographic range. ” • Examples: hepatitis C, E. coli O 157: H 7.

Bioterrorism-Related Diseases • Bioterrorism attack: “…the deliberate release of viruses, bacteria, or other germs (agents) used to cause illness or death in people, animals, or plants…. ” • Example: anthrax

Methods of Outbreak Investigation • • Clinical observations Epidemic curve Incubation period Attack rate Case mapping (used by John Snow) Hypothesis formulation and confirmation Draw conclusions

Clinical Observations • The pattern of symptoms suggests possible infectious agents. • Possible symptoms of an outbreak include: – Fever – Nausea – Diarrhea – Vomiting – Headache – Rashes – Stomach pain

Epidemic Curve • “A graphic plotting of the distribution of cases by time of onset. ” • May reflect a common-source epidemic or a point-source epidemic.

Common-Source Epidemic • An “Outbreak due to exposure of a group of persons to a noxious influence that is common to the individuals in the group. ”

Point-Source Epidemic • A type of common-source epidemic that occurs “When the exposure is brief and essentially simultaneous, [and] the resultant cases all develop within one incubation period of the disease…”

Incubation Period • In conjunction with information about symptoms, provides clues to possible infectious disease agents • Example: The incubation period for foodborne illness caused by Salmonella ranges from 6 to 72 hours.

Attack Rate • Used frequently to describe the occurrence of foodborne illness, infectious diseases, and other acute epidemics • Formula: Attack rate = Ill/(Ill + Well) X 100 during a time period

Final Steps in Outbreak Investigation • Case mapping: Some investigations may use computer software to show the location of cases. • Hypotheses: Using the information that has been gathered, the epidemiologist may formulate a hypothesis regarding the causative agent. • Draw a conclusion: Plan for the prevention of future outbreaks.

Conclusion • Infectious diseases remain a threat for the following reasons: – New types of diseases are constantly evolving and imperiling public health. – Outbreaks caused by bioterrorism are a potential threat. – Some infectious disease agents mutate into forms that resist conventional antibiotic treatment.