Epidemiology of Communicable Diseases

Epidemiology of Communicable Diseases
DR. Zaher Nazzal MD, ABCM

4/16/2017 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.

4/16/2017 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)

4/16/2017 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: Contamination: the presence of an infectious agent on a body surface; also on or in clothes, beddings, toys, surgical instruments or dressings

4/16/2017 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 .

4/16/2017 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

4/16/2017 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. Reservoir is the natural habitate in which the m.o metabolize and replicates. Host Obligate host: e.g., human for measles and typhoid fever Primary or definitive host: e.g., parazite passes its sexual cycle Secondary or intermediate host; e.g., a sexual stage Transport host: e.g., a carrier in which the the m.o. remains alive but doesn’t under go development

4/16/2017 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.

4/16/2017 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.

4/16/2017 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)

4/16/2017 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

4/16/2017 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).

Endemic, Epidemic and Pandemic
4/16/2017 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.

4/16/2017 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.

Nosocomial infections
4/16/2017 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.

Opportunistic infection
4/16/2017 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)

Zoonosis, epizootic and enzootic
4/16/2017 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.

Eradication and Elimination
4/16/2017 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. Zaher Nazzal MD, ABCM

4/16/2017 Epidemiology of Communicable Diseases Chain of Infection A process that begins when an agent leaves its reservoir or host through a portal of exit, and is transported by some mode of transmission, then enters through an appropriate portal of entry to infect a susceptible host.

Chain (Cycle) of infection
4/16/2017 Epidemiology of Communicable Diseases Chain (Cycle) of infection Susceptible Host Agent Reservoir Mode of transmission Portal of Exit Portal of Inlet IP PC

4/16/2017 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.

4/16/2017 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.

Period of Communicability
4/16/2017 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

4/16/2017 Epidemiology of Communicable Diseases agent

4/16/2017 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

4/16/2017 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

4/16/2017 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=

4/16/2017 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

Mechanisms of disease production (pathogenesis):
4/16/2017 Epidemiology of Communicable Diseases Mechanisms of disease production (pathogenesis): Invasiveness:(Pneumococcosis, measles). Toxicity: (Tetanus, Botulism). Hypersensitivity: (Tuberculosis). Others: Immune suppression: (AIDS).

4/16/2017 Epidemiology of Communicable Diseases Source or Reservoir

4/16/2017 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.”

4/16/2017 Epidemiology of Communicable Diseases Types of reservoirs Reservoir Human reservoir Animal Non-living

4/16/2017 Epidemiology of Communicable Diseases Humans are the most important reservoir of human infectious disease. Pathogen Reservoirs

4/16/2017 Epidemiology of Communicable Diseases Human reservoir Human reservoir Cases Carriers

4/16/2017 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”

4/16/2017 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

4/16/2017 Epidemiology of Communicable Diseases Carriers A person or animal without 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.

Carriers are dangerous because:
4/16/2017 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.

4/16/2017 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

4/16/2017 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

4/16/2017 Epidemiology of Communicable Diseases Human reservoir Human reservoir Cases Carriers According to spectrum of disease: Clinical cases (mild/severe-typical/atypical) Sub-clinical cases Latent infection cases Primary case Index case Secondary cases Type: Incubatory Convalescent Healthy Duration: Temporary Chronic Portal of exit: Urinary Intestinal Respiratory others

4/16/2017 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).

Zoonoses Zoonoses are Human Diseases with Animal Reservoirs. 4/16/2017
Epidemiology of Communicable Diseases Zoonoses are Human Diseases with Animal Reservoirs. Zoonoses

4/16/2017 Epidemiology of Communicable Diseases Toxoplasmosis Zoonoses

Reservoir in non-living things
4/16/2017 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.

4/16/2017 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

4/16/2017 Epidemiology of Communicable Diseases Modes of transmission

4/16/2017 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.

Indirect transmission
4/16/2017 Epidemiology of Communicable Diseases Mode of transmission Direct transmission Indirect transmission Direct contact Droplet infection Contact with soil Inoculation into skin or mucosa Trans-placental (vertical) Vehicle-borne Vector-borne: Mechanical biological Air-borne Fomite-born Unclean hands and fingers propagative Cyclo-prop. Cyclo-develop.

4/16/2017 Epidemiology of Communicable Diseases Modes of Transmission

Pathogen Transmission
4/16/2017 Epidemiology of Communicable Diseases Pathogen Transmission

4/16/2017 Epidemiology of Communicable Diseases Direct transmission Direct Contact There is essentially immediate transfer of the agent from a reservoir to a susceptible host by direct contact or droplet spread. Direct contact occurs through: Skin-to-skin contact, kissing, and sexual intercourse. Direct contact refers also to contact with soil or vegetation harbouring infectious organisms. STDs, AIDs, leprosy, Skin and Eye infections

4/16/2017 Epidemiology of Communicable Diseases Contact Transmission Rhinovirus?

4/16/2017 Epidemiology of Communicable Diseases Direct transmission Droplet Infection Direct projection of a spray of droplets of saliva and naso-pharyngeal secretions Produced by sneezing, coughing, or even talking. Particles <5mmm can penetrate deeply and reach the alveoli. Usually limited to a distance of max 1 meter (30-60cm). Close proximity Overcrowding Lack of ventilation Common cold, TB, Meningococcal Meningitis, Whooping cough.

4/16/2017 Epidemiology of Communicable Diseases Droplet Transmission Less than one meter Measles?

4/16/2017 Epidemiology of Communicable Diseases Direct transmission Contact with soil Direct exposure of the susceptible tissues to the disease agent in soil E.g. Tetanus, Hookworm. Inoculation into skin or mucosa The agent is directly inoculated into skin or mucosa. E.g. rabies, hepatitis B, Trans-placental TORCH, Varicella, Hepatitis B, AIDs

Indirect Transmission
4/16/2017 Epidemiology of Communicable Diseases Indirect Transmission From a reservoir to a susceptible host by: Vehicle borne: inanimate vehicle Vector borne: animate vector Airborne transmission: suspended air particles The infectious agent must be capable of surviving outside the human host in the external environment. Inanimate= non-living

4/16/2017 Epidemiology of Communicable Diseases Vehicle borne: An infectious agent is carried from a reservoir to a susceptible host by an inanimate intermediary. Vehicles include: Contaminated food and water: typhoid, paratyphoid, food poisoning, dysentery and cholera. Biologic products (blood): Hepatitis B, AIDS, Syphilis. Fomites (inanimate objects such as: door knobs, toys, handkerchiefs, bedding, or surgical instruments). Influenza.

Foodborne Transmission
4/16/2017 Epidemiology of Communicable Diseases Hepatitis A Foodborne Transmission

Waterborne Transmission
4/16/2017 Epidemiology of Communicable Diseases Waterborne Transmission Cryptosporidium parvum?

Common-Source Outbreak
4/16/2017 Epidemiology of Communicable Diseases The Broad Street Pump. Cholera! Common-Source Outbreak

Indirect-Contact Transmission
4/16/2017 Epidemiology of Communicable Diseases Indirect-Contact Transmission Methicillin-resistant Staphylococcus aurius (MRSA)? Beddings are an example of a Fomite, an inanimate object that can transmit pathogens between people.

Indirect-Contact Transmission
4/16/2017 Epidemiology of Communicable Diseases Indirect-Contact Transmission Influenza virus? Door knobs are another good example of a fomite.

4/16/2017 Epidemiology of Communicable Diseases Vector-borne Vector is an arthropod or any living carrier (e.g. snail) that transport an infectious agent to a susceptible individual. Arthropod include flies and mosquitoes, fleas, cockroaches, ticks and mites, sucking lice, eyc. Mechanical transmission: the agent does not multiply or undergo physiologic changes in the vector. For example, flies carry Shigella on appendages. Biologic transmission: When the agent undergoes changes and/or multiplication within the vector before it is transmitted.

4/16/2017 Epidemiology of Communicable Diseases Modes of Transmission

4/16/2017 Epidemiology of Communicable Diseases Airborne Airborne transmission occurs by particles that are suspended in air. There are two types of these particles: - dust and - droplet nuclei. Dust particles: -result from re-suspension of particles that have settled on floor or bedding, - infectious particles blown from the soil by the wind. Example: Fungal spores.

4/16/2017 Epidemiology of Communicable Diseases Droplet nuclei They represent the dried residue of droplets that have been coughed or sneezed into the air. They are very tiny particles less than 5 µ (microns) in size and may remain suspended in the air for long periods. Examples: Tuberculosis is transmitted more often indirectly, through droplet nuclei, than directly, through direct droplet spread. Legionnaires’ disease and histoplasmosis also spread through airborne transmission.

Airborne Transmission
4/16/2017 Epidemiology of Communicable Diseases Airborne Transmission More than one meter Mycobacterium tuberculosis?

4/16/2017 Epidemiology of Communicable Diseases 5. Portal of entry An agent enters a susceptible host through a portal of entry. -The portal of entry must provide access to tissues in which the agent can multiply or a toxin can act. - Often, organisms use the same portal to enter a new host that they use to exit the source host.

4/16/2017 Epidemiology of Communicable Diseases 73 THANK YOU THANK YOU

4/16/2017 Epidemiology of Communicable Diseases Susceptible host

4/16/2017 Epidemiology of Communicable Diseases 6. Susceptible host A susceptible host is the final link in the chain of infection. The host is a person or other living organism that can be infected by an infectious agent under normal conditions. An infectious agent seeks a susceptible host aiming “successful parasitism”. Four stages are required for successful parasitism: Portal of entry Site of election inside the body Portal of exit Survival in external environment

Susceptibility of a host depends on
4/16/2017 Epidemiology of Communicable Diseases Susceptibility of a host depends on Genetic factors General factors Host defense (Specific acquired immunity)

General factors which defend against infection:
4/16/2017 Epidemiology of Communicable Diseases General factors which defend against infection: The skin, Mucous membranes, Gastric acidity, Cilia in the respiratory tract, The cough reflex, and Nonspecific immune response.

General factors that may increase susceptibility
4/16/2017 Epidemiology of Communicable Diseases General factors that may increase susceptibility Malnutrition, Alcoholism, and Disease or therapy which impairs the immune response (Cortisone, cytotoxic drugs, ...

4/16/2017 Epidemiology of Communicable Diseases Host Defence Host defence against infection is at once Local and systematic Specific and no-specific Humoral (B cells) and cellular (T cells) Most of infectious agents stimulate multiple host defence mechanisms.

Specific acquired immunity:
4/16/2017 Epidemiology of Communicable Diseases Specific acquired immunity: It refers to protective antibodies that are directed against a specific agent. It is of two types: Active immunity: Resistance developed in response to stimulus by an antigen either; Naturally by infecting agent or Artificially by vaccine and usually characterized by the presence of antibody produced by the host.

4/16/2017 Epidemiology of Communicable Diseases Passive immunity: Immunity conferred by an antibody produced in another host and may be acquired; Naturally by an infant from its mother (3- 6 mths) Artificially by administration of an antibody containing preparation (antiserum or immune globulin).

4/16/2017 Epidemiology of Communicable Diseases Herd Immunity It is the state of immunity of a group or a community. Also it is; “The resistance of a group or a community to an infectious agent, based on the immunity of a high proportion of individual members of the group”.

4/16/2017 Epidemiology of Communicable Diseases Source: Timmreck, 1998

4/16/2017 Epidemiology of Communicable Diseases Herd Immunity If the herd immunity is sufficiently high, the occurrence of an epidemic is highly unlikely High level of immunity (by high vaccination coverage) makes elimination of a diseases possible. It was crucial in polio and diphtheria Herd immunity may be determined by serologic survey

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