Investigating Disease Investigating Disease OutbreaksOutbreaks

MED 6301- Chp. 3MED 6301- Chp. 3

Dr. John KowalczykDr. John Kowalczyk

Spring 2009Spring 2009

Infectious Disease ProcessInfectious Disease Process

““Chain of Infection”Chain of Infection”

Infectious Disease ProcessInfectious Disease Process

Results from the interactions of the 3 Results from the interactions of the 3 components of the epidemiological triangle:components of the epidemiological triangle: AgentAgent HostHost EnvironmentEnvironment

Chain of infection includes 6 chain links which all Chain of infection includes 6 chain links which all must happen in succession- break any link in the must happen in succession- break any link in the chain, the disease spread will ceasechain, the disease spread will cease

Chain of InfectionChain of Infection

1.

Chain of infection-1.Chain of infection-1.Causative Agent:Causative Agent: The entity capable of causing diseaseThe entity capable of causing disease Agents are one of three types:Agents are one of three types:

1.1. Biological- protozoa, metazoa, bacteria, viruses, Biological- protozoa, metazoa, bacteria, viruses, fungi, rickettsiafungi, rickettsia

2.2. Chemical- pesticides, additives, industrial chemicalsChemical- pesticides, additives, industrial chemicals3.3. Physical- heat, light, ionizing radiation, noise, Physical- heat, light, ionizing radiation, noise,

vibrationsvibrations

*note- most diseases are caused by biological *note- most diseases are caused by biological agents; chemical and physical agents cause agents; chemical and physical agents cause chronic long term illnesseschronic long term illnesses

Chain of infection-2.Chain of infection-2.

Reservoir of AgentReservoir of Agent

The normal habitat in which an infectious The normal habitat in which an infectious agent lives, multiplies, and growsagent lives, multiplies, and grows

These habitats include humans, animals, These habitats include humans, animals, and environmentand environment

ReservoirReservoir

2 major categories of human sources of 2 major categories of human sources of infection:infection:Acute clinical casesAcute clinical cases

Less likely to cause transmission due to early Less likely to cause transmission due to early diagnosis and treatment (going to doctor)diagnosis and treatment (going to doctor)

Carriers- person who harbors infection but Carriers- person who harbors infection but has no overt signs or symptomshas no overt signs or symptoms

Why is a carrier dangerous?Why is a carrier dangerous?

Chain of infection-3.Chain of infection-3.

Portal of exit of agent from HostPortal of exit of agent from HostThere are 5 means of exit from the There are 5 means of exit from the

human body:human body:1.1. Respiratory tract- very common but Respiratory tract- very common but

difficult to controldifficult to controlCommon coldCommon coldInfluenzaInfluenzaTuberculosisTuberculosisAirborne dropletsAirborne droplets

Portal of exit of agent from Portal of exit of agent from HostHost

2.2. Genito-urinary tract- urine, semenGenito-urinary tract- urine, semenSyphilisSyphilis

GonorrheaGonorrhea

Schistosomiasis (blood flukeworm inside body, Schistosomiasis (blood flukeworm inside body, eggs leave thru urine- urinate in the lake; eggs leave thru urine- urinate in the lake; eggs enter skin while wading in water, eggs enter skin while wading in water, enters bloodstreamenters bloodstream

Portal of exit of agent from Portal of exit of agent from HostHost

3.3. Alimentary tract- mostly related to food- by Alimentary tract- mostly related to food- by mouth; also vomitus, feces, salivamouth; also vomitus, feces, saliva

Rabies- dog biteRabies- dog bite

Cholera-intestinal tractCholera-intestinal tract

4.4. Skin- Skin- Superficial wounds- cuts, gashes, burns, puncture Superficial wounds- cuts, gashes, burns, puncture

woundwound

smallpoxsmallpox

Portal of exit of agent from Portal of exit of agent from HostHost

5.5. Transplacental mode (mother to fetus)- Transplacental mode (mother to fetus)- pregnant woman passes disease thru pregnant woman passes disease thru placenta-umbilical cord to babyplacenta-umbilical cord to baby

RubellaRubella

SyphilisSyphilis

Hepatitis BHepatitis B

HIV+HIV+

Chain of infection-4.Chain of infection-4.

Mode of transmission of agent to new hostMode of transmission of agent to new host Direct transmission (person to person)- Direct transmission (person to person)-

touching, biting, kissing, sexual intercoursetouching, biting, kissing, sexual intercourse

Indirect transmission- Indirect transmission- Vehicle borne (inanimate) blood, water, food, surgical Vehicle borne (inanimate) blood, water, food, surgical

equipment, eating utensils, clothing, etcequipment, eating utensils, clothing, etc Vector borne- feces, eggs, biting (saliva)Vector borne- feces, eggs, biting (saliva)

Chain of infection-5.Chain of infection-5.

Portal of entry into new HostPortal of entry into new Host

Basically the same as #3- portal of exitBasically the same as #3- portal of exit

Mouth, noseMouth, nose

SkinSkin

Genital/analGenital/anal

transplacentaltransplacental

Chain of infection-6.Chain of infection-6.

Host susceptibilityHost susceptibility

Depends upon:Depends upon: Genetic factorsGenetic factors General resistanceGeneral resistance Specific acquired immunity (natural or thru Specific acquired immunity (natural or thru

vaccinations)vaccinations)

+skin toughness+skin toughness -malnutrition -malnutrition

+gastric acidity+gastric acidity -poor health -poor health

+cough reflex+cough reflex -repressed immune -repressed immune

SummarySummary

Surveillance of DiseaseSurveillance of Disease

Entire process of collecting, analyzing, Entire process of collecting, analyzing, interpreting, and reporting data concerning interpreting, and reporting data concerning incidence of disease, injuries, or deathincidence of disease, injuries, or death

USA- Centers for Disease Control (CDC) USA- Centers for Disease Control (CDC) federal agency responsible for surveillance federal agency responsible for surveillance activities of acute and infectious diseasesactivities of acute and infectious diseases

Local and state health departments also Local and state health departments also responsible on local and state level for responsible on local and state level for surveillance surveillance

Baseline DataBaseline Data

Endemic levels of disease- usual level of Endemic levels of disease- usual level of disease in a communitydisease in a community

Evaluation of time trends:Evaluation of time trends:Long term secular trends- over time period a Long term secular trends- over time period a

disease is looked at to determine trendsdisease is looked at to determine trendsSeasonal variation trends- based on route of Seasonal variation trends- based on route of

spread; i.e. respiratory route for influenza is spread; i.e. respiratory route for influenza is during winter and early spring; insect vectors during winter and early spring; insect vectors in summer time for Lyme or West Nile;in summer time for Lyme or West Nile;

EpidemicEpidemic

Disease outbreak at an unusual or Disease outbreak at an unusual or unexpected frequency; above and beyond unexpected frequency; above and beyond usual endemic levelusual endemic level

Pandemic- epidemic outbreak affecting Pandemic- epidemic outbreak affecting several countries or continents; pandemic several countries or continents; pandemic flu will likely be pandemic over many flu will likely be pandemic over many countries in the world when it happenscountries in the world when it happens

Epizootic- ‘upon animals’- unusual pattern Epizootic- ‘upon animals’- unusual pattern of disease in animal populationsof disease in animal populations

Procedures- 5 stepsProcedures- 5 steps

1. Define the problem and establish 1. Define the problem and establish diagnosis: diagnosis:

determine from outset whether this determine from outset whether this epidemic (outbreak) is real. What needs epidemic (outbreak) is real. What needs to be looked at? to be looked at?

SurveillanceSurveillance Endemic levelsEndemic levels

Foodborne Outbreak DefinitionFoodborne Outbreak Definition

Two or more persons (except for botulism Two or more persons (except for botulism and chemical poisoning it may be only one and chemical poisoning it may be only one person affected)person affected)

Who all ate the SAME food at the same Who all ate the SAME food at the same location…..location…..

AND ALL became ill at about the SAME AND ALL became ill at about the SAME time, and with the SAME symptoms.time, and with the SAME symptoms.

Oh, yeah.. The Poo-Poo Story!Oh, yeah.. The Poo-Poo Story!

Procedures- 5 steps- cont’dProcedures- 5 steps- cont’d

2. Appraise the existing data:2. Appraise the existing data: evaluate known distribution of cases with respect to evaluate known distribution of cases with respect to

person, place, and timeperson, place, and time Case identification- find all potential cases involved in the Case identification- find all potential cases involved in the

outbreak; start with index case- first person to get sick and outbreak; start with index case- first person to get sick and transmit to others; how did it spread?transmit to others; how did it spread?

Clinical observations- record number, types, and patterns of Clinical observations- record number, types, and patterns of symptoms associated with diseasesymptoms associated with disease

Tabulation & spot maps- cases of disease are plotted on a map Tabulation & spot maps- cases of disease are plotted on a map (Dr. John Snow- Cholera); these can be done by date/time of (Dr. John Snow- Cholera); these can be done by date/time of disease onset of symptoms, geographical clustering; graphing disease onset of symptoms, geographical clustering; graphing can show time of onset over period of timecan show time of onset over period of time

Plot an epidemic time curve graphPlot an epidemic time curve graph Identification- determine the responsible agent (biological, Identification- determine the responsible agent (biological,

chemical, physical)- How?chemical, physical)- How?

Procedures- 5 steps- cont’dProcedures- 5 steps- cont’d

3. Formulate a hypothesis:3. Formulate a hypothesis: What are possible sources of infection?What are possible sources of infection? What is likely agent?What is likely agent? What is likely method of transmission? (spread of What is likely method of transmission? (spread of

disease)disease) What is best approach to control the outbreak?What is best approach to control the outbreak?

4. Test the hypothesis:4. Test the hypothesis: collect data needed to confirm or refute your initial collect data needed to confirm or refute your initial

suspicions; suspicions; continue to look for more cases; evaluate alternative continue to look for more cases; evaluate alternative

sources of data; sources of data; begin laboratory investigationsbegin laboratory investigations

Procedures- 5 steps- cont’dProcedures- 5 steps- cont’d

5. Initiate Control Measures and Draw conclusions 5. Initiate Control Measures and Draw conclusions to formulate practical applicationsto formulate practical applications SanitationSanitation ProphylaxisProphylaxis Diagnosis and treatmentDiagnosis and treatment Control of disease vectorsControl of disease vectors

based on results of investigation, likelihood of based on results of investigation, likelihood of new programs, policies, or procedures will need new programs, policies, or procedures will need to be implementedto be implemented

long term surveillance and prevention efforts to long term surveillance and prevention efforts to prevent recurrence of similar outbreaksprevent recurrence of similar outbreaks

Measures of Disease OutbreaksMeasures of Disease Outbreaks

Attack rate- same as ‘incidence rate’; Attack rate- same as ‘incidence rate’; looking at number of new cases of disease looking at number of new cases of disease per unit of population per unit of time.per unit of population per unit of time.

Occurrence of disease in population Occurrence of disease in population increases greatly over a short period of increases greatly over a short period of time, often related to specific exposuretime, often related to specific exposure

Attack rate= Attack rate= # ill persons # ill persons x 100%x 100%

# ill + # well persons# ill + # well persons

Attack Rate Table- Attack Rate Table- to find specific food to find specific food

responsible for outbreakresponsible for outbreak

A (ate the food)A (ate the food)

illill not ill not ill ‘A’ total‘A’ total attack rate%attack rate%

ex: 10 + 3 13ex: 10 + 3 13 77%77%

B (did not eat food)B (did not eat food)

ill not ill ‘B’ total attack rate%ill not ill ‘B’ total attack rate%

ex: 7 + 4 11 64%ex: 7 + 4 11 64%

Identifying Food that Caused Identifying Food that Caused OutbreakOutbreak

1.1. List all foods consumed at eventList all foods consumed at event2.2. Persons involved in outbreak:Persons involved in outbreak: A (ate food)A (ate food) B (did not eat the food)B (did not eat the food)3.3. Calculate attack rates for each food item Calculate attack rates for each food item

with well and ill personswith well and ill persons4.4. After calculating AR, find difference in After calculating AR, find difference in

attack rates A-B between those who ate attack rates A-B between those who ate and those who did not eatand those who did not eat

Identifying Food that Caused Identifying Food that Caused Outbreak- cont’dOutbreak- cont’d

5.5. Repeat process for each food item Repeat process for each food item suspected in outbreaksuspected in outbreak

6.6. Foods that have greatest difference in Foods that have greatest difference in attack rates may be the foods that were attack rates may be the foods that were responsible for the illnessresponsible for the illness

Secondary Attack RateSecondary Attack Rate

S.A.R.= Number of cases of an infection S.A.R.= Number of cases of an infection that occur among contacts within the that occur among contacts within the incubation period following exposure to a incubation period following exposure to a primary case in relation to the total number primary case in relation to the total number of exposed contactsof exposed contacts

Denominator is restricted to susceptible Denominator is restricted to susceptible contacts when these can be determinedcontacts when these can be determined

Is a measure of contagiousness and Is a measure of contagiousness and useful in evaluating control measuresuseful in evaluating control measures

Secondary Attack Rate FormulaSecondary Attack Rate Formula

SAR%= SAR%= # new cases in grp - initial case(s) # new cases in grp - initial case(s)

# susceptible persons in grp – initial case x100# susceptible persons in grp – initial case x100

Initial case- index case + co-primariesInitial case- index case + co-primaries

Index case- case that first comes to attention of Index case- case that first comes to attention of public health officialspublic health officials

Co-primaries- cases related to index case so Co-primaries- cases related to index case so closely in time- same generation of casesclosely in time- same generation of cases

Epidemic CurveEpidemic Curve

Defined as a graph in which cases of a Defined as a graph in which cases of a disease that have occurred during an disease that have occurred during an epidemic period are graphed according to epidemic period are graphed according to the time of onset of illness in the casesthe time of onset of illness in the cases

Provides a simple visual display of the Provides a simple visual display of the outbreak’s magnitude and trendsoutbreak’s magnitude and trends

How to Draw an Epidemic CurveHow to Draw an Epidemic Curve

You must first know time of onset of illnessYou must first know time of onset of illnessNumber of cases plotted on the y-axis Number of cases plotted on the y-axis

(vertical axis)(vertical axis)Unit of time on x-axis (hortizontal axis)Unit of time on x-axis (hortizontal axis)Time is usually based on hoursTime is usually based on hoursShow pre and post epidemic period on Show pre and post epidemic period on

your graphyour graph

Interpreting Epidemic CurveInterpreting Epidemic Curve Consider overall shape- this will determine Consider overall shape- this will determine

pattern of epidemic- common source or person-pattern of epidemic- common source or person-to-person transmissionto-person transmission

Curve with steep up Curve with steep up slope and gradual down slope and gradual down slope indicates a point source epidemic where slope indicates a point source epidemic where people were exposed to same agent over brief people were exposed to same agent over brief period of timeperiod of time

Person-to-person transmission spread will have Person-to-person transmission spread will have series of progressively taller peaks one series of progressively taller peaks one incubation period apartincubation period apart

Cases that stand apart are called ‘outliers’ and Cases that stand apart are called ‘outliers’ and also should be looked at closelyalso should be looked at closely