Investigating Disease Outbreaks MED Chp. 3 Dr. John Kowalczyk Spring 2009

Infectious Disease Process “Chain of Infection”

Infectious Disease Process Results from the interactions of the 3 components of the epidemiological triangle: Results from the interactions of the 3 components of the epidemiological triangle: Agent Agent Host Host Environment Environment Chain of infection includes 6 chain links which all must happen in succession- break any link in the chain, the disease spread will cease Chain of infection includes 6 chain links which all must happen in succession- break any link in the chain, the disease spread will cease

Chain of Infection 1.

Chain of infection-1. Causative Agent: The entity capable of causing disease The entity capable of causing disease Agents are one of three types: Agents are one of three types: 1. Biological- protozoa, metazoa, bacteria, viruses, fungi, rickettsia 2. Chemical- pesticides, additives, industrial chemicals 3. Physical- heat, light, ionizing radiation, noise, vibrations *note- most diseases are caused by biological agents; chemical and physical agents cause chronic long term illnesses

Chain of infection-2. Reservoir of Agent The normal habitat in which an infectious agent lives, multiplies, and grows These habitats include humans, animals, and environment

Reservoir 2 major categories of human sources of infection: 2 major categories of human sources of infection: Acute clinical cases Acute clinical cases Less likely to cause transmission due to early diagnosis and treatment (going to doctor) Carriers- person who harbors infection but has no overt signs or symptoms Carriers- person who harbors infection but has no overt signs or symptoms Why is a carrier dangerous?

Chain of infection-3. Portal of exit of agent from Host There are 5 means of exit from the human body: 1. Respiratory tract- very common but difficult to control Common cold InfluenzaTuberculosis Airborne droplets

Portal of exit of agent from Host 2. Genito-urinary tract- urine, semen SyphilisGonorrhea Schistosomiasis (blood flukeworm inside body, eggs leave thru urine- urinate in the lake; eggs enter skin while wading in water, enters bloodstream

Portal of exit of agent from Host 3. Alimentary tract- mostly related to food- by mouth; also vomitus, feces, saliva Rabies- dog bite Cholera-intestinal tract 4. Skin- Superficial wounds- cuts, gashes, burns, puncture wound smallpox

Portal of exit of agent from Host 5. Transplacental mode (mother to fetus)- pregnant woman passes disease thru placenta-umbilical cord to baby RubellaSyphilis Hepatitis B HIV+

Chain of infection-4. Mode of transmission of agent to new host Direct transmission (person to person)- touching, biting, kissing, sexual intercourse Direct transmission (person to person)- touching, biting, kissing, sexual intercourse Indirect transmission- Indirect transmission- Vehicle borne (inanimate) blood, water, food, surgical equipment, eating utensils, clothing, etc Vehicle borne (inanimate) blood, water, food, surgical equipment, eating utensils, clothing, etc Vector borne- feces, eggs, biting (saliva) Vector borne- feces, eggs, biting (saliva)

Chain of infection-5. Portal of entry into new Host Basically the same as #3- portal of exit Mouth, nose SkinGenital/analtransplacental

Chain of infection-6. Host susceptibility Depends upon: Genetic factors Genetic factors General resistance General resistance Specific acquired immunity (natural or thru vaccinations) Specific acquired immunity (natural or thru vaccinations) +skin toughness -malnutrition +gastric acidity -poor health +cough reflex -repressed immune

Summary

Surveillance of Disease Entire process of collecting, analyzing, interpreting, and reporting data concerning incidence of disease, injuries, or death Entire process of collecting, analyzing, interpreting, and reporting data concerning incidence of disease, injuries, or death USA- Centers for Disease Control (CDC) federal agency responsible for surveillance activities of acute and infectious diseases USA- Centers for Disease Control (CDC) federal agency responsible for surveillance activities of acute and infectious diseases Local and state health departments also responsible on local and state level for surveillance Local and state health departments also responsible on local and state level for surveillance

Baseline Data Endemic levels of disease- usual level of disease in a community Endemic levels of disease- usual level of disease in a community Evaluation of time trends: Evaluation of time trends: Long term secular trends- over time period a disease is looked at to determine trends Long term secular trends- over time period a disease is looked at to determine trends Seasonal variation trends- based on route of spread; i.e. respiratory route for influenza is during winter and early spring; insect vectors in summer time for Lyme or West Nile; Seasonal variation trends- based on route of spread; i.e. respiratory route for influenza is during winter and early spring; insect vectors in summer time for Lyme or West Nile;

Epidemic Disease outbreak at an unusual or unexpected frequency; above and beyond usual endemic level Disease outbreak at an unusual or unexpected frequency; above and beyond usual endemic level Pandemic- epidemic outbreak affecting several countries or continents; pandemic flu will likely be pandemic over many countries in the world when it happens Pandemic- epidemic outbreak affecting several countries or continents; pandemic flu will likely be pandemic over many countries in the world when it happens Epizootic- ‘upon animals’- unusual pattern of disease in animal populations Epizootic- ‘upon animals’- unusual pattern of disease in animal populations

Procedures- 5 steps 1. Define the problem and establish diagnosis: determine from outset whether this epidemic (outbreak) is real. What needs to be looked at? determine from outset whether this epidemic (outbreak) is real. What needs to be looked at? Surveillance Surveillance Endemic levels Endemic levels

Foodborne Outbreak Definition Two or more persons (except for botulism and chemical poisoning it may be only one person affected) Two or more persons (except for botulism and chemical poisoning it may be only one person affected) Who all ate the SAME food at the same location….. Who all ate the SAME food at the same location….. AND ALL became ill at about the SAME time, and with the SAME symptoms. AND ALL became ill at about the SAME time, and with the SAME symptoms.

Oh, yeah.. The Poo-Poo Story!

Procedures- 5 steps- cont’d 2. Appraise the existing data: evaluate known distribution of cases with respect to person, place, and time evaluate known distribution of cases with respect to person, place, and time Case identification- find all potential cases involved in the outbreak; start with index case- first person to get sick and transmit to others; how did it spread? Case identification- find all potential cases involved in the outbreak; start with index case- first person to get sick and transmit to others; how did it spread? Clinical observations- record number, types, and patterns of symptoms associated with disease Clinical observations- record number, types, and patterns of symptoms associated with disease Tabulation & spot maps- cases of disease are plotted on a map (Dr. John Snow- Cholera); these can be done by date/time of disease onset of symptoms, geographical clustering; graphing can show time of onset over period of time Tabulation & spot maps- cases of disease are plotted on a map (Dr. John Snow- Cholera); these can be done by date/time of disease onset of symptoms, geographical clustering; graphing can show time of onset over period of time Plot an epidemic time curve graph Plot an epidemic time curve graph Identification- determine the responsible agent (biological, chemical, physical)- How? Identification- determine the responsible agent (biological, chemical, physical)- How?

Procedures- 5 steps- cont’d 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 disease) What is likely method of transmission? (spread of disease) What is best approach to control the outbreak? What is best approach to control the outbreak? 4. Test the hypothesis: collect data needed to confirm or refute your initial suspicions; collect data needed to confirm or refute your initial suspicions; continue to look for more cases; evaluate alternative sources of data; continue to look for more cases; evaluate alternative sources of data; begin laboratory investigations begin laboratory investigations

Procedures- 5 steps- cont’d 5. Initiate Control Measures and Draw conclusions to formulate practical applications Sanitation Sanitation Prophylaxis Prophylaxis Diagnosis and treatment Diagnosis and treatment Control of disease vectors Control of disease vectors based on results of investigation, likelihood of new programs, policies, or procedures will need to be implemented based on results of investigation, likelihood of new programs, policies, or procedures will need to be implemented long term surveillance and prevention efforts to prevent recurrence of similar outbreaks long term surveillance and prevention efforts to prevent recurrence of similar outbreaks

Measures of Disease Outbreaks Attack rate- same as ‘incidence rate’; looking at number of new cases of disease per unit of population per unit of time. Attack rate- same as ‘incidence rate’; looking at number of new cases of disease per unit of population per unit of time. Occurrence of disease in population increases greatly over a short period of time, often related to specific exposure Occurrence of disease in population increases greatly over a short period of time, often related to specific exposure Attack rate= # ill persons x 100% Attack rate= # ill persons x 100% # ill + # well persons

Attack Rate Table- to find specific food responsible for outbreak A (ate the food) A (ate the food) ill not ill ‘A’ totalattack rate% ex: % B (did not eat food) B (did not eat food) ill not ill ‘B’ total attack rate% ex: %

Identifying Food that Caused Outbreak 1. List all foods consumed at event 2. Persons involved in outbreak: A (ate food) A (ate food) B (did not eat the food) B (did not eat the food) 3. Calculate attack rates for each food item with well and ill persons 4. After calculating AR, find difference in attack rates A-B between those who ate and those who did not eat

Identifying Food that Caused Outbreak- cont’d 5. Repeat process for each food item suspected in outbreak 6. Foods that have greatest difference in attack rates may be the foods that were responsible for the illness

Secondary Attack Rate S.A.R.= Number of cases of an infection that occur among contacts within the incubation period following exposure to a primary case in relation to the total number of exposed contacts S.A.R.= Number of cases of an infection that occur among contacts within the incubation period following exposure to a primary case in relation to the total number of exposed contacts Denominator is restricted to susceptible contacts when these can be determined Denominator is restricted to susceptible contacts when these can be determined Is a measure of contagiousness and useful in evaluating control measures Is a measure of contagiousness and useful in evaluating control measures

Secondary Attack Rate Formula SAR%= # 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-primaries Index case- case that first comes to attention of public health officials Co-primaries- cases related to index case so closely in time- same generation of cases

Epidemic Curve Defined as a graph in which cases of a disease that have occurred during an epidemic period are graphed according to the time of onset of illness in the cases Defined as a graph in which cases of a disease that have occurred during an epidemic period are graphed according to the time of onset of illness in the cases Provides a simple visual display of the outbreak’s magnitude and trends Provides a simple visual display of the outbreak’s magnitude and trends

How to Draw an Epidemic Curve You must first know time of onset of illness You must first know time of onset of illness Number of cases plotted on the y-axis (vertical axis) Number of cases plotted on the y-axis (vertical axis) Unit of time on x-axis (hortizontal axis) Unit of time on x-axis (hortizontal axis) Time is usually based on hours Time is usually based on hours Show pre and post epidemic period on your graph Show pre and post epidemic period on your graph

Interpreting Epidemic Curve Consider overall shape- this will determine pattern of epidemic- common source or person- to-person transmission Consider overall shape- this will determine pattern of epidemic- common source or person- to-person transmission Curve with steep up  slope and gradual down slope indicates a point source epidemic where people were exposed to same agent over brief period of time Curve with steep up  slope and gradual down slope indicates a point source epidemic where people were exposed to same agent over brief period of time Person-to-person transmission spread will have series of progressively taller peaks one incubation period apart Person-to-person transmission spread will have series of progressively taller peaks one incubation period apart Cases that stand apart are called ‘outliers’ and also should be looked at closely Cases that stand apart are called ‘outliers’ and also should be looked at closely