Infectious Disease Epidemiology

Outline What is Infectious disease epidemiologySelected definitions specifically used in infectious disease epidemiologyDynamics of disease transmissionSource or reservoirModes of transmissionSusceptible host

What is infectious disease epidemiology?EpidemiologyDeals with one populationRisk caseIdentifies causes

1. Two or more populationsHumansInfectious agentsHelminths, bacteria, fungi, protozoa, viruses, prionsVectors Mosquito (protozoa-malaria), snails (helminths-schistosomiasis)Blackfly (microfilaria-onchocerciasis)AnimalsDogs and sheep/goats EchinococcusMice and ticks Borrelia

What is infectious disease epidemiology?

2. A case is a risk factor Infection in one person can be transmitted to others

What is infectious disease epidemiology?

3. The cause often knownAn infectious agent is a necessary cause

What is infectious disease epidemiology?

Outline What is Infectious disease epidemiologySelected definitions specifically used in infectious disease epidemiologyDynamics of disease transmissionSource or reservoirModes of transmissionSusceptible host

INFECTION:The entry and development or multiplication of an infectious agent in the body of man or animals. Different levels of infectionColonization: (e.g. S. aureus in skin & nasopharynx Sub-clinical/ inapparent infection: (e.g. Polio)Latent infection: (e.g. virus of Herpes Simplex)Manifest/ clinical infection

CONTAMINATION:Presence of an infectious agent on a body surface, or clothes, beddings, toys, surgical instruments, dressings, or other inanimate articles or substances including water, milk, and food. Pollution implies the presence of offensive, but not necessarily infectious matter in the environment.

INFESTATION:The lodgment, development, and reproduction of arthropods on the surface of the body (of human being or animal) or in the clothing : e.g. lice, itch miteAlso invasion of gut by parasitic worms

HOST:A person or animal, including birds and arthropods that affords subsistence or lodgment to an infectious agent under natural conditions (opposed to experimental conditions)DEFINITIVE HOST: in which parasite attains maturity or passes its sexual phaseINTERMEDIATE HOST: parasite is in larval or asexual stage

INFECTIOUS DISEASE:A clinically manifest disease of man or animals resulting from an infection

CONTAGIOUS DISEASE:A disease that is transmitted through contact. E.g. scabies, trachoma, STD

COMMUNICABLE DISEASE:An illness due to a specific infectious agent or its toxic products capable of being directly or indirectly transmitted from man to man, animal to animal, or from the environment (through air, dust, soil, water, food, etc.) to man or animal

DefinitionsInfectious diseasesCaused by an infectious agentCommunicable diseasesTransmission directly or indirectly from an infected personTransmissible diseasesTransmission through unnatural routes from an infected person

NoteInfections are often subclinical infections vs infectious diseases!Antonyms not well-defined Non-communicable diseases virus involved in pathogenesis of ca cervix?Chronic diseases HIV? TetanusMeaslesvCJDInfectious Disease

EPIDEMIC:(Epi=upon; Demos=people)Unusual occurrence in a community or a region of disease, specific health related behavior, or other health-related events clearly in excess of expected occurrence. What constitutes significant excess???OUTBREAK: small or localised epidemicModern/ slow epidemics: CHD, cancer

ENDEMIC:en= in, demos= peopleConstant presence of a disease or infectious agent within a given geographic area or a population group, without importation from outsideRefers to usual/expected frequencyHyperendemic: The disease present in a constant high incidence/ prevalence rateHoloendemic: high level of infection beginning early in life & affecting most of child population (e.g. malaria)

SPORADIC:Sporadic =scattered aboutThe cases occur irregularly, haphazardly from time to time, & generally infrequentlyCases show little or no connection with each otherE.g. polio, tetanus, herpes zoster, and meningococcal meningitisA sporadic case may start an epidemic

PANDEMIC:An epidemic usually affecting a large proportion of the population, occurring over a wide geographic area such as a section of a nation, entire nation, a continent or the world.E.g. inflenza pandemic 1918 & 1957, acute haemorrhagic conjunctivitis in 1971 & 1981

Endemic - Epidemic - PandemicEndemicTransmission occur, but the number of cases remains constantEpidemicThe number of cases increases PandemicWhen epidemics occur at several continents global epidemicR = 1R > 1R < 1

EndemicEpidemicNumber of Cases of a DiseaseTimeEndemic vs Epidemic

EXOTIC:Diseases which are imported into a country in which they do not otherwise occur, e.g. rabies in UK

ZOONOSIS:An infection or infectious disease transmissible under natural conditions from vertebrate animals to man. Enzootic/ epizooticE.g. rabies, plague, bovine TB, anthrax, brucellosis, salmonellosis, endemic typhus, hydatidosis, etcEPIZOOTIC: an outbreak of disease in an animal populationEPORNITHIC: outbreak of disease in bird population

NOSOCOMIAL INFECTION:(Hospital acquired) is an infection originating in a patient while in a hospital/ health care facility. It denotes a new disorder (unrelated to the patients primary condition) associated with being in a hospitalE.g. infection of surgical wounds, hepatitis B, and UTI

IATROGENIC DISEASE:(Physician induced) any untoward or adverse consequence of a preventive, diagnostic, or therapeutic regimen or procedure, that causes impairment, handicap, disability or death resulting from a physicians professional activity or from the professional activity of other health professionalsE.g. reaction to penicillin and immunizing agents, aplastic anaemia following chloramphenicol use, childhood leukemia due to prenatal X-ray, hepatitis B following blood transfusion etc.

OPPORTUNISTIC INFECTION:Infection by an organism(s) that takes the opportunity provided by a defect in host defense to infect the host and hence cause disease. The organism normally not pathogenicE.g. Herpes simplex, CMV, Toxoplasma, MTB, M avium intracellularae, etc

SURVEILLANCE:The continuous scrutiny of the factors that determine the occurrence and distribution of disease and other conditions of ill health (to detect changes in trend)

ERADICATION:Termination of all transmission of infection by extermination of the infectious agent through surveillance & containment. (Small pox, Guinea worm, polio, measles, diphtheria)

Outline What is Infectious disease epidemiologySelected definitions specifically used in infectious disease epidemiologyDynamics of disease transmissionSource / reservoirModes of transmissionSusceptible host

Horton & Parker: Informed Infection Control Practice Chain of Infection

Chain of TransmissionPortal of exitPortal of entryAgentSusceptible HostMode of transmissionReservoirPerson to person transmission

Dynamics of transmission: time line

Cases Index the first case identified Primary the case that brings the infection into a population Secondary infected by a primary case Tertiary infected by a secondary case

Transmission

1. Reservoir of infection: any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent 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

Reservoir1. Human Cases (Person with symptomatic illness) Carriers: Asymptomatic/ Healthy IncubatingTemporary vs chronic Convalescent2. Animal: zoonosis3. Environmental (non-living): soil, plant, water

Two types of reservoirHomologous reservoir: when another member of the same species is the victim. E.g. man is the principal reservoir of vibrio cholerae Heterologous reservoir: when the infection is derived from a reservoir other than man. E.g. birds and animals infected with salmonella

Source of infection:person, animal, object or substance from which an infectious agent passes or disseminated to the hostReservoir and source may be different

Examplesin hookworm infection, reservoir is man, but source of infection is the soil contaminated with infective larvaein tetanus, reservoir and the source are the same, i.e. soilin typhoid fever, the reservoir of infection may be a case or carrier, but the source of infection may be faeces or urine of patients or contaminated food, milk or water

Peters: Tropical Medicine and Parasitology, 2001 (www)Infection Cycle of Schistosomiasis

Lipoldova & Demand, 2006 (www)Infection Cycle of Leishmaniasis

Arthropod VectorsPathogen - VectorViruses (Arbovirus) - MosquitoesBacteria (Yersinia) - FleasBacteria (Borrelia) - TicksRickettsias (R. prowazeki) - Lice, ticksProtozoa (Plasmodium) - MosquitoesProtozoa (Trypanozoma) -Tsetse fliesHelminths (Onchocerca) - Simulium flies

2. Mode of TransmissionDirectDirect contactDroplet spreadContact with soilInoculation into skin or mucous (through Secretions, Blood, Faeces/urine)Transplacental (vertical)

IndirectVehicle borne (Food/ water)Vector borne (mechanical/ biological)Air borne (Dust/ droplet nuclei)Fomite-borneUnclean hands and fingers

Portal of exit Human/animal Respiratory tractGenito-Urinary tract G.I. tract: Faeces, Saliva Skin (exanthema, cuts, needles, blood-sucking arthropods) Conjunctival secretions PlacentaEnvironmentalCooling towers

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Respiratory tract Mouth (faecal-oral transmission) Skin Mucous membranes Blood

Portal of Entry

Factors influencing disease transmission

AgentHostEnvironment Age Sex Genotype Behaviour Nutritional status Health status Infectivity Pathogenicity Virulence Immunogenicity Antigenic stability Survival Weather Housing Geography Occupational setting Air quality Food

Factors influencing disease transmission

3. Susceptible hostA. susceptible parasitism (4 stages)Portal of entrySite of election & multiplicationPortal of exitSurvival in environment

Predisposition to Infections(Host Factors)GenderGeneticsClimate and WeatherNutrition, Stress, SleepSmokingStomach Acidity Hygiene

Susceptible host (contd..)B. Incubation period: the time interval between invasion by an infectious agent and appearance of the first sign or symptom of the disease in questionLatent period: in non-infectious diseasesImportance of IPTracing the sourcePeriod of surveillanceImmunizationIdentification of point sourcePrognosis

Susceptible host (contd..)C. Serial intervalthe gap in time between the onset of the primary case and the secondary case

Susceptible host (contd..)D. Generation timethe interval of time between receipt of infection by a host and maximal infectivity of that hostRoughly equals to I.P. Exceptions: Mumps: maxim communicability 48 hrs before swelling of salivary glandsGT refers to transmission of infection whether clinical or subclinical

Susceptible host (contd..)E. Communicable periodthe time during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected animal to man or from an infected person to an animal, including arthropodsMeasure of communicability: SAR

Susceptible host (contd..)F. Secondary attack ratethe number of exposed persons developing the disease within the range of the incubation period, following exposure to the primary caseSAR= No. of exposed persons developing the disease within the range of IPtotal number of exposed/ susceptible contacts

Ecological Factors in InfectionsAltered environment{Air conditioning}Changes in food production & handling{intensive husbandry with antibiotic protection; deep-freeze; fast food industry}Climate changes{Global warming}DeforestationOwnership of (exotic) petsAir travel & Exotic journeys / Global movementsIncreased use of immunosuppressives/ antibioticsAmerican Museum of Natural History Exhibition: Epidemic! The World of Infectious Disease

What is infectious disease epidemiology then used for? Identification of causes of new, emerging infections, e.g. HIV, vCJD, SARSSurveillence of infectious diseaseIdentification of source of outbreaks Studies of routes of transmission and natural history of infectionsIdentification of new interventions

Types of ScreeningMass: Screening whole population or subgroups.Very popular in the past. Now, they feel it is useless unless followed by treatment.

High Risk:

More returns can be expected if applied to high risk groups (based on Epidemiological Studies). Now, screening for risk factors- as these precede onset of diseases

Multi phasic: combination of many tests for many conditions in large no. of people. It can be a combination of questionnaire, clinical exam and investigations.There is a lot of doubt about the practice and its benefits.

DiseasedNot diseased

TestResults DiagnosisTotalPositive(a) TP(b) FPa + bNegative(c) FN(d) TNc + dTotala + cb + da + b+c + d

Sensitivity: Ability of the test to identify correctly those who have the disease. a/ a+c x100.Specificity: Ability of the test to identify correctly those who do not have the disease d/b+d x100.An ideal Screening test should be 100% sens. & 100% speci.

Predictive accuracy: Predictive value reflects the diagnostic power of a screening test.The predictive value of a positive test indicates the probability that a person with positive result has the disease in question a/a+b X100.As the prevalence of disease declines, the PvPT becomes low.

False Negative: Means the result is negative in those who actually have the disease in question c/a+c X100.False Positive: Means the result is positive in those who actually do not have the disease in question b/b+d X100.

Yield: The amount of previously undiagnosed cases that are diagnosed as a result of screening test. This depends on many factors like sensitivity, specificity, prevalence & participation.

Screening Test resultsLung cancer Total Diseased Not diseasedPositive a [40]b [20] a+b[60]Negative c [100]d [9840]c+d[9940]Totala+c[140]b+d [ 9860]a+b+c+d[10000]

Sensitivity = ( 40/ 140) x 100=28.57%Specificity = ( 9840 / 9860) x 100=99.79%Predictive value of a positive test= [ 40/ 60]x100=66.66%Predictive value of a negative test = [ 9840/ 9940]x100=98.9%Percentage of false negatives = [ 100 / 140] x100=71.4%Percentage of false positives = [ 20 / 9860] x 100 =0.20%