Epidemiology of Communicable Diseases DR. ZaherNazzal MD, ABCM
Epidemiology of Communicable Diseases Introduction • Vaccination and control of communicable diseases are two of the great public health achievements of the last century • Infectious diseases are now the world's biggest killer of children and young adults; both are vital age groups. • They account for more than 13 million deaths a year - one in two deaths in developing countries
Introduction • About 32 percent of all deaths worldwide are caused by communicable diseases, maternal and perinatal conditions and nutritional deficiencies. • Almost 90 percent of these deaths are caused by pneumonia, tuberculosis (TB), diarrheal diseases, malaria, measles, and the HIV. • It is a fundamental part of the whole of Epidemiology.
Epidemiology of Communicable Diseases Epidemiology • “The study of the distribution and determinants of a health-related event and application of this study to control of health problems” (John Last, 1988). • That branch of medical science which treats epidemic (Parkin,1873) • Epidemiology is the study of epidemics and their prevention (Kuller LH)
Epidemiology of Communicable Diseases Definitions • Infection: The entry and development or multiplication of an infectious agent in a human or an animal with the response of the body to defend it self against that invader. An infection doesn’t always cause illness. There are several level of infection • Colonization: (e.g., S. aureus in skin and normal nasopharynx) • Subclinical or in apparent infection: (e.g., polio) • Latent infection: (e.g., herpes simplex) • Manifest or clinical infection:
Epidemiology of Communicable Diseases Definitions……cont’d • Infectious disease A clinically manifest disease of man or animals resulting from an infection. • Infectious agent An organism (virus, bacteria, rickettsia, fungus, protozoan, or helminthes) that is capable of producing infection or infectious disease .
Epidemiology of Communicable Diseases Definitions……cont’d • 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 inanimate reservoir to a susceptible host; either directly or indirectly. • Contagious disease A disease that is transmitted through direct contact
Epidemiology of Communicable Diseases Definitions……cont’d • Reservoir Any person, animal, arthropod, plant, soil, or substance ; in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that it can be transmitted to a susceptible host. • Host A person or animal ( including birds and arthropods) that afford subsistence or lodgment to an infectious agent under natural conditions.
Epidemiology of Communicable Diseases Definitions……cont’d • Susceptible A person or animal not possessing sufficient resistance against a particular pathogenic agent to prevent contracting infection or disease when exposed to the agent. • Carrier A person or animal that harbors a specific infectious agent without apparent clinical disease and serves as potential source of infection.
Epidemiology of Communicable Diseases Definitions……cont’d • Transmission of infectious agents Any mechanism by which an infectious agent is spread from a source or reservoir to a person.
Epidemiology of Communicable Diseases Definitions……cont’d • Infectivity: Capacity of agent to enter and multiply in a susceptible host (hence produce infection/disease) (polio and measles have high infectivity) • Pathogenicity: Capacity of agent to cause clinical disease in the infected host (measles has high pathogenicity)
Epidemiology of Communicable Diseases Definitions……cont’d • Virulence The degree of pathogenicity of an infectious agent. i.e. the ability of the agent to invade and damage tissues of the host causing severe manifestations or death. • Toxigenicity: The ability of a microbe to produce bio chemicals, known as toxins that disrupt the normal functions of cells or are generally destructive to human cells and tissues
Epidemiology of Communicable Diseases Definitions……cont’d • Resistance: Ability of agent to survive adverse environmental conditions (hepatitis agents generally very resistant whereas influenza viruses are typically fragile). Note: “resistance” is also applied to the host. • Antigenicity: Ability of agent to induce antibody production in the host (e.g. re-infection with measles virus is very rare).
Epidemiology of Communicable Diseases Endemic, Epidemic and Pandemic • Endemic The constant presence of a disease or infectious agent within a given geographic area or population group, without importing from outside. • Epidemic The unusual occurrence of a disease (in excess of the expected) in a community or region.
Epidemiology of Communicable Diseases • Outbreak: The unusual occurrence of a disease(in excess of the expected) in a localized small area. • Pandemic: It is an epidemic that spreads over several countries or continents, affecting a large number of people.
Epidemiology of Communicable Diseases Nosocomial infections • Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility. • It has to be a new disorder unrelated to the patient’s primary condition. • Examples include infection of surgical wounds, hepatitis B and urinary tract infections.
Epidemiology of Communicable Diseases Opportunistic infection • This is infection by organisms that take the opportunity provided by a defect in host defense (e.g. immunity) to infect the host and thus cause disease. • Opportunistic infections are very common in AIDS. (Herpes simplex, cytomegalovirus, M. tuberculosis….etc)
Epidemiology of Communicable Diseases Zoonosis, epizootic and enzootic • Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis….. • An Epizotic is an outbreak (epidemic) of disease in an animal population, e.g. rift valley fever. • An Enzotic is an endemic occurring in animals, e.g. bovine TB.
Epidemiology of Communicable Diseases Eradication and Elimination • Eradication: termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment. • Eradication is an absolute process, an “all or none” phenomenon, restricted to termination of infection from the whole world. • Example: smallpox, polio • Elimination: it is sometimes used to describe eradication of a disease from a large geographic region. Disease which is amenable to elimination in the meantime is measles.
Dynamic of Diseases Transmission Dr.ZaherNazzal MD, ABCM
Epidemiology of Communicable Diseases Chain of Infection A process that begins when an agentleaves its reservoiror host through a portal of exit, and is transported by some modeoftransmission, then enters through an appropriate portal of entryto infect a susceptible host.
Epidemiology of Communicable Diseases Chain (Cycle) of infection Agent Susceptible Host Reservoir IP PC Portal of Inlet Portal of Exit Mode of transmission
Epidemiology of Communicable Diseases The requisites (essentials) for the perpetuation of communicable diseases: The elements of the cycle of infection: • Presence of microbiological agent. • Presence of reservoir. • Portal of exit. • Mode of transmission. • Portal of entry (inlet). • Presence of susceptible host.
Epidemiology of Communicable Diseases Incubation Period • The period from exposure to infection to the onset of symptoms or signs of infectious disease. The length of incubation period depends on: • The portal of entry. • The rate of growth of the organism in the host. • The dosage of the infectious agent. • The host resistance.
Epidemiology of Communicable Diseases Period of Communicability • The time during which an infectious agent my be transmitted directly or indirectly from an infected person to a susceptible person or animal. • Its length varies from one disease to another
Epidemiology of Communicable Diseases 1. Agent • Microorganisms are responsible for disease production (viruses, bacteria, protozoa, parasites, fungi,.. • Agent factors that affect disease transmission: • Infectivity • Pathogenicity • Virulence • Aantigenicity
Epidemiology of Communicable Diseases Infectivity: The ability of an agent to invade and multiply (produce infection) in a susceptible host. How to measure (Infectivity);ease & spread of infection? Secondary Attack Rate • The proportion of exposed susceptible persons who become infected. • Examples: High infectivity: Measles, Chickenpox Low infectivity: Leprosy
Epidemiology of Communicable Diseases Pathogenicity: Is the ability of the organisms to produce specific clinical reaction after infection It refers to the proportion of infected persons who develop clinical disease. Examples: • High pathogenicity: Measles, Chickenpox • Low pathogenicity: Polio, Tuberculosis, Hepatitis A, Meningitis, AIDS It can be measured by: Ratio of clinical to sub-clinical case=
Epidemiology of Communicable Diseases Virulence: It refers the ability of organisms to produce severe pathological reaction. It is the proportion of persons with clinical disease who become severely ill or die. Examples: Rabies, Hemorrhagic fevers caused by Ebola and Murberg viruses. Case fatality rate
Epidemiology of Communicable Diseases Mechanisms of disease production (pathogenesis): • Invasiveness:(Pneumococcosis, measles). • Toxicity: (Tetanus, Botulism). • Hypersensitivity: (Tuberculosis). • Others: Immune suppression: (AIDS).
Epidemiology of Communicable Diseases Source or Reservoir
Epidemiology of Communicable Diseases 2. Source or Reservoir • The reservoir of an agent is the habitat in which an infectious agent normally lives, grows, and multiplies. • “any person, animal, arthropod, plant, soil, or substance, or a combination of these, in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host. It is the natural habitat of the infectious agent.”
Reservoir Human reservoir Animal reservoir Non-living reservoir Epidemiology of Communicable Diseases Types of reservoirs
Epidemiology of Communicable Diseases Pathogen Reservoirs Humans are the most important reservoir of human infectious disease.
Human reservoir Cases Carriers Epidemiology of Communicable Diseases Human reservoir
Epidemiology of Communicable Diseases Cases A case is defined as “a person in the population or study group identified as having the particular disease, health disorder, or condition under investigation”
Epidemiology of Communicable Diseases Cases Cases are classified as • Primary case • Index case • Secondary cases According to spectrum of disease: • Clinical cases: (mild/severe-typical/atypical) • Sub-clinical cases • Latent infection cases
Epidemiology of Communicable Diseases Carriers • A person or animalwithout apparent disease who harbors a specific infectious agent and is capable of transmitting the agent to others. • It occurs either due to inadequate treatment or immune response, or the disease agent is not completely eliminated, leading to a carrier state.
Epidemiology of Communicable Diseases Carriers are dangerous because: • They do not show any clinical manifestation so they carry normal life. • The carrier and his contacts are not aware of their condition so, they take no precautions. • It is difficult to discover them. • It is not always possible to deal with them. • They can remain infectious for a long time leading to repeated introduction of the disease to contacts.
Epidemiology of Communicable Diseases Types of Carriers: • Asymptomatic (In-apparent) carrier: The carrier state that may occur in an individual with an infection that is in-apparent throughout its course Examples: Polio virus, meningococcus, hepatitis A virus • Incubatory, Convalescent, Post-Convalescent carriers: The carrier state may occur during the incubation period, convalescence, and post convalescence of an individual with a clinically recognizable disease. Examples of Incubatory carrier: Measles, chickenpox
Epidemiology of Communicable Diseases Examples of convalescent carriers: Salmonella typhi, Diphtheria, hepatitis B virus • Healthy (chronic) carriers: They continue to harbour an agent for an extended time (months or years) following the initial infection. Examples: Hepatitis B virus, S. typhi
Human reservoir • Type: • Incubatory • Convalescent • Healthy Cases Carriers • Primary case • Index case • Secondary cases • Portal of exit: • Urinary • Intestinal • Respiratory • others • According to spectrum of disease: • Clinical cases • (mild/severe-typical/atypical) • Sub-clinical cases • Latent infection cases • Duration: • Temporary • Chronic Epidemiology of Communicable Diseases Human reservoir
Epidemiology of Communicable Diseases Animal reservoirs • Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis….. • There are over a 100 zoonotic diseases that can be conveyed from animal to man. • brucellosis (cows and pigs), • anthrax (sheep), • plague (rodents), • rabies (bats, dogs, and other mammals).
Epidemiology of Communicable Diseases Zoonoses are Human Diseases with Animal Reservoirs. Zoonoses
Epidemiology of Communicable Diseases Toxoplasmosis Zoonoses
Epidemiology of Communicable Diseases Reservoir in non-living things • Water, Soil and inanimate matter can also act as reservoir of infection. For example • Soil may harbor agents that causes tetanus, anthrax. • Pools of water are the primary reservoir of Legionnaires’ bacillus.
Epidemiology of Communicable Diseases 3. Portal of exit • Portal of exit is the path by which an agent leaves the source host. • Examples: • Respiratory tract • GIT • Skin and mucous membrane
Epidemiology of Communicable Diseases Modes of transmission
Epidemiology of Communicable Diseases 4. Modes of transmission • Many different ways • As a rule only one route for each pathogen • Typhoid fever ----- vehicle transmission • Common cold---------direct contact • But there are others that may be transmitted by several route • AIDs, Salmonellosis, Hepatitis B, brucellosis, Q fever,…..etc.