Disease Outbreak Investigation

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Disease Outbreak Investigation:

Disease Outbreak Investigation Dr. Maria Fidelis C. Manalo , MSc . Epidemiology

Learning Outcomes:

Learning Outcomes When you have studied this session, you should be able to: Define and use correctly all of the key words in disease outbreak investigation, such as outbreak, epidemic, etc. Describe the different types of epidemics. Describe the different types of epidemic curves. Describe the purpose and steps of an epidemic investigation. Explain in outline the basic principles of epidemic management.

What is an outbreak and an epidemic? :

What is an outbreak and an epidemic? If there is an increase in cases of a disease compared with the expected number, but it lasts for only a short time, or it occurs only in a limited area (e.g. in a few nearby households), the rise may be referred to as an outbreak . An epidemic is also an excess of cases compared with the number expected . However, an epidemic is more general than an outbreak, the increase in the number of cases continues far longer (possibly months or even years), and the cases are distributed across a wider area. For example, it may be that during January to March there are normally fewer than 10 cases of tuberculosis (TB) in your barangay . If you found 30 cases of tuberculosis in a particular January, followed by 39 cases in February and 45 cases in March, then you would strongly suspect that there was an epidemic of TB in your community. You would then need to find out why TB had suddenly increased.

Types of epidemics:

Types of epidemics Epidemics are classified into different types according to the source of infection and modes of transmission. Two main modes of transmission of communicable diseases: direct modes of transmission, such as from mother to child, or from fecally contaminated hands into the mouth; and indirect modes of transmission, such as through vectors, contaminated air, water, food or objects such as cooking bowls and utensils.

Types of epidemics:

Types of epidemics Based on criteria such as this, epidemics are classified into three types: Common source outbreaks Point source Continuous common source Propagated or progressive epidemics Mixed epidemics

1. Common source outbreaks:

1. Common source outbreaks Occur when the rise in cases of an infection occurs after a group of people all came into contact with the same unsafe source of infection (the common source), such as contaminated food or water.

Common source outbreaks:

Common source outbreaks For example, imagine a wedding where food was prepared in the morning to serve to wedding guests in the evening. If the prepared food was left outside on a hot day under the sun until evening, bacteria might multiply in the food. If this food was served to the guests in the evening without reheating it thoroughly, many of the guests might fall ill from eating the contaminated food. This kind of epidemic is called a common source outbreak because the affected guests all ate the same contaminated food at the wedding. Guests eating contaminated food during the wedding become ill with diarrhea and vomiting; the most severely affected are too ill to get up.

 Common source outbreaks:

Common source outbreaks After the food had been prepared in the morning, explain what should have been done to make it safe to serve to the wedding guests in the evening: Bacteria like moisture and warmth, and they multiply quickly in food left in hot sun for many hours. After the food had been prepared, it should have been kept covered in a refrigerator or in a cool place inside the house, away from direct sun. The food should also have been reheated thoroughly before being served to the guests. Refrigerating food reduces the risk of a common source outbreak of a foodborne infection.

PowerPoint Presentation:

A point source outbreak is a common source outbreak where the exposure period (e.g. the time at which the contaminated food was eaten) is short. This means that all cases who fall ill after eating the food (the common source) also have the same incubation period , i.e. the period between infection (eating the contaminated food) and the appearance of the first symptoms. 1. Common source outbreaks: a. Point source outbreak

PowerPoint Presentation:

In a continuous common source epidemic , exposure to the source is prolonged over an extended period of time and may occur over more than one incubation period 1. Common source outbreaks: b. Continuous Common Source

2. Propagated or progressive epidemics :

2. Propagated or progressive epidemics Propagated or progressive epidemics occur when the infection spreads from person to person , in which one or more of the first wave of cases serves as a source of infection for subsequent cases, and those subsequent cases, in turn, serve as sources for later cases. The infectious agents causing the disease pass from one host to another, either directly from person to person (e.g. via hand shaking or kissing), or indirectly via vectors (e.g. mosquitoes in the case of malaria), or in water, food or another medium.

Propagated or progressive epidemics :

Propagated or progressive epidemics The distribution of malaria cases is a good example of a propagated epidemic, because increased numbers of malaria cases occur again and again at different times. Propagated epidemics last longer than the common source outbreaks. This is because malaria will continue to spread in the community, as long as mosquitoes are present in the environment and there are people who carry the parasite.

Propagated or progressive epidemics :

Can you think of any epidemic-prone diseases that spread quickly in overcrowded conditions where there is poor sanitation and personal hygiene? Example: typhoid fever, cholera, shigellosis (bacterial dysentery), louse-borne relapsing fever and typhus. Propagated or progressive epidemics

3. Mixed epidemics :

3. Mixed epidemics Show characteristics of both common source and propagated epidemics. So a mixed epidemic can start with a common source, and be followed by a propagated spread. Mixed epidemics are often caused by food-borne infectious agents. Typhoid fever can easily spread and become a propagated epidemic. Can you remember how the typhoid bacteria are transmitted from person to person? Typhoid bacteria are transmitted from infected people to new susceptible hosts via contaminated food or water.

Mixed epidemics :

Mixed epidemics The organism that causes typhoid ( Salmonella typhi ) can survive in sewage for 14 days and in water for up to seven days. Water polluted by fecal matter is therefore the main source of infection for typhoid. If the whole community drinks water from the same water source, which has been contaminated with Salmonella typhi, there will be a common source outbreak of typhoid fever.

Mixed epidemics :

Mixed epidemics The epidemic may continue to spread through fecal matter passing from person to person, if the people in the affected community do not improve their standards of personal hygiene, or if the water is not treated and made safe to drink. This type of spread of typhoid is called a propagated epidemic of typhoid. Drinking water collected from the same unsafe source can expose a whole community to waterborne infection and lead to a common source outbreak, followed by a propagated spread.

Interpretation of Epidemic Curves During an Active Outbreak:

Interpretation of Epidemic Curves During an Active Outbreak

Epidemic Curve :

Epidemic Curve Epidemic curves tell you more than just whether or not there is an outbreak. The epi curve shows progression of an outbreak over time. The horizontal axis represents the date when a person became ill, also called the date of onset . The vertical axis is the number of persons who became ill on each date. These are updated as new data come in, and thus are subject to change. The epi curve is complex and incomplete.

Understanding Epidemic Curves :

Understanding Epidemic Curves There is an inherent delay between the date that an illness starts, and the date that the case is reported to public health authorities. It typically takes 2-3 weeks for Salmonella infections. That means that someone who got sick last week is very unlikely to have been reported yet, and someone who got sick three weeks ago may just be reported now.

Understanding Epidemic Curves :

Some background cases of illness are likely to occur that would have occurred even without an outbreak.  This makes it difficult to say exactly which case is the first in an outbreak. Epidemiologists typically focus on the first recognized cluster or group of cases rather than the very first case. Due to the inherent reporting delay, the cluster is sometimes not detected until several weeks after the persons became ill. Understanding Epidemic Curves

Understanding Epidemic Curves :

For some cases, the date when they became ill is not known because it takes time before someone from the health department can do an interview to ask for this information. Sometimes the interview never occurs.  If the date that the person brought the specimen to be tested to the laboratory is known, we may estimate the date of illness onset as three days before that. Understanding Epidemic Curves

Understanding Epidemic Curves :

It can be difficult to determine when cases start to decline because of the reporting delay.  This can become clearer as time passes. It can be difficult to say when the outbreak is over, because of the reporting delay. The delay means that the curve for the most recent three weeks always looks like the outbreak could be ending even during an active outbreak. The full shape of the curve is only clear after the outbreak is over. Understanding Epidemic Curves

What Type of Epidemic Curve? :

What Type of Epidemic Curve? What kind of an epidemic curve is this? A point source epidemic. A continuous point source epidemic. A propagated source epidemic. None of the above.

Take another look at the epidemic curve::

Take another look at the epidemic curve: Note that the number of cases rises very quickly, peaks, and then begins to fall off. Also bear in mind that the average incubation period is about 28 days, with a range of 15 to 50 days. So far all of the cases are contained within a single incubation period. Then, put this together with a) what you already know about the biology and the epidemiology of hepatitis A and b) the ages of the cases on the line listing. Hepatitis A is transmitted via the fecal-oral route , that is, virus particles in the stool of an infected person somehow get ingested. This can occur as a result of travel to a developing country with suboptimal sanitation, sexual contact (especially MSM), in a child day-care setting where fecal contamination and children putting things into their mouths are common elements, or when food is contaminated by an infected food handler.

The Likely Source?:

The Likely Source? Good descriptive epidemiology focuses not only on time, but also person and place. The line listing has limited information regarding personal characteristics, but it does list age and gender. Take another look at the line listing, and consider the age and gender of the cases in conjunction with the information in the epidemic curve. Given this information which of the following you think is the most likely source of the outbreak? Sexual transmission. People returning from foreign travel in developing countries. A day-care facility. An infected food handler

The Likely Source? An infected food handler:

The Likely Source? An infected food handler Absolutely right. The sudden, dramatic increase in the number of cases followed by a decline suggests that a substantial number of people were exposed to hepatitis A from a single source. (In fact, any sudden rise in the number of cases suggests sudden exposure to a common source.) The diverse age and gender distribution of the victims argues against a day-care source, or sexual transmission, or from people returning from foreign travel. This is a classic example of a point source epidemic , in which all of the cases occur within a single incubation period.

PowerPoint Presentation:

Point source outbreaks tend to have epidemic curves with a rapid increase in cases followed by a somewhat slower decline , and all of the cases tend to all fall within one incubation period . 1. Common source outbreaks: a. Point source outbreak This is because point sources outbreaks involve a common source, such as contaminated food, or an infected food handler who contaminated food, water, or beverages that were consumed by a large number of people, and all the exposures tend to occur in a relatively brief period.

PowerPoint Presentation:

This epidemic curve is from a cholera outbreak in London in 1854 that was investigated by Dr. John Snow. Here the source was a community water well. The incubation period for cholera is 1-3 days. Even though many residents fled when the outbreak began, the source of infection remained, and the epidemic continued beyond the 3 day incubation period. If the residents had not fled, there would have been a plateau instead of a peak. 1. Common source outbreaks: b. Continuous Common Source The down slope of the curve may be: very sharp if the common source is removed, or gradual if the outbreak is allowed to exhaust itself.

PowerPoint Presentation:

The shape of the curve usually contains a series of successively larger peaks , which are one incubation apart in which an increasingly large number of cases caused by person-to-person contact, until the pool of susceptibles is exhausted, or control measures are implemented. In reality, most of these epidemics do not produce the classic pattern 2. Propagated or progressive epidemics This outbreak of measles begins with a single index case that infects a number of other individuals. (The incubation period for measles averages 10 days with a range of 7-18 days.) One or more of the people infected in the initial wave infects a group of people who become the second wave of infection. So here transmission is person-to-person, rather than from a common source.

Tips on Epidemic Curves :

Tips on Epidemic Curves Outliers may be informative An early case may be unrelated to the outbreak, may be the source of the epidemic (such as an infected food handler) or a person who was exposed early. Late cases may also be unrelated, but they may also be secondary cases or persons exposed later than most.

Tips on Interpreting Epidemic Curves :

Tips on Interpreting Epidemic Curves Consider the overall shape of the epidemic curve. The shape will be determined by the incubation period and whether it is a common source versus propagated epidemic. Find the peak of the outbreak and count back on the time axis one incubation period. Also identify the earliest case in the outbreak and count back the minimum incubation period. If these dates are at all close, they may identify the time period during which the cases became exposed. In a point source epidemic with a known disease, you can use the epidemic curve to calculate a likely period of exposure. This provides two advantages: You can identify the likely period of exposure, and this will help with directing questions to the cases about possible exposures. It may point to the index case. If the causative organism has not yet been identified, the epidemic curve may help by enabling you to calculate the incubation period.

Epidemic investigation:

Epidemic investigation Epidemic investigation is a set of procedures used to identify the cause, i.e. the infectious agent, responsible for the disease. It is also used to identify the people affected, the circumstances and mode of spread of the disease, and other relevant factors involved in propagating the epidemic. This is especially important if the epidemic has unusual features, if it presents a significant threat to public health, and it is not self-limiting (i.e. it does not end spontaneously without professional intervention).

Epidemic investigation:

Epidemic investigation is a challenging task for health workers. The main purpose of epidemic investigation is to control the spread of the disease before it causes more deaths and illness. As a Public Health Practitioner, the first action you should take is to confirm the existence of an epidemic. To do this, you need to know the average number of cases of that disease during this specific month in your community in previous years, so you can compare that number with the current number of cases. Is there an excess number of cases or deaths from this disease compared to the usual occurrence? If there really are excess cases, you should report your findings to the District/Municipal/City Health Office immediately. Epidemic investigation

Epidemic investigation:

The next steps will be taken by the Epidemic Management Team, which is composed of many different health professionals such as doctors, nurses, environmental experts and others. These steps include confirming the cause (the infectious agent involved), the number of people affected (the cases) and the modes of transmission of the infection from cases to new susceptible hosts. Epidemic investigation

Steps in an epidemic investigation:

Steps in an epidemic investigation Establish the existence of an outbreak Verify the diagnosis or causes Define and identify cases: Use a standard case definition Identify and count cases Perform descriptive epidemiology, i.e. collect data on the age, sex, etc. of the cases and analyse the data to see if useful patterns emerge

Steps in an epidemic investigation:

Develop hypotheses to explain the occurrence of the epidemic: Evaluate the hypotheses Reconsider/refine the hypotheses Carry out additional studies to confirm or reject the explanations for the epidemic: Additional epidemiological studies Other types of studies, e.g. laboratory tests, environmental investigations Implement control and prevention measures Communicate findings to higher levels in the health system, community leaders and other local stakeholders. Steps in an epidemic investigation

Management of epidemics:

Management of epidemics Epidemic management activities include taking appropriate control measures, such as treating those who are ill to reduce the reservoir of infection, and providing health education to limit the transmission of the disease to others. Health professionals at higher levels will require your help in putting into taking any measures needed to control the spread of the disease, such as giving drugs to people in the community and providing health education.

Management of epidemics:

You may be involved in the management of an epidemic once it is confirmed by the health authorities. The type of control measures you need to implement depend on the type of infectious agent, how the disease is transmitted, and any other factors contributing to the disease. Generally, your control measures should target the infectious agent, the source of any infection, and the treatment of those who became ill. Remember, the source of infection could be humans or animals, or non-living things in the environment. Management of epidemics

Management of epidemics:

Management of epidemics If you do not implement the correct control measures, the epidemic may continue to spread in your area. For example, if contaminated food is the source of an outbreak in your community, you will need to control the outbreak by teaching the community about food hygiene, so they are not exposed to contaminated food. If it is caused by contaminated water, you should educate them not to drink the water until it is treated with chlorine. If mosquito breeding sites are the source of a malaria epidemic, you will need to teach the community to clear the breeding sites for mosquitoes.

Summary/Key Points:

Summary/Key Points An epidemic is the occurrence of more cases of a disease than would be expected in the population at that period of time. An outbreak is an increase in cases for a short time in a limited area. A common source outbreak is an epidemic which arises from a single source of infection, and where most people fall ill after the same incubation period. A propagated epidemic occurs when the infection spreads from one person to another, e.g. through the air, via a vector, via contaminated food or water, or during unprotected sexual intercourse.

PowerPoint Presentation:

A mixed epidemic can start with a common source and be followed by a propagated spread. An epidemic investigation is conducted to rapidly identify the cause of an outbreak or epidemic and to take effective actions to contain and prevent the spread of the disease. Epidemic investigation and management involves team work. Your role as a Public Health Practitioner is to report the occurrence of an epidemic, to mobilize and educate the community, and to assist the District/Municipal/City Health authorities in carrying out control and prevention measures as required. Summary/Key Points

References:

References 1. Communicable Diseases HEAT Module http://labspace.open.ac.uk/mod/oucontent/view.php?id=452786&section=1.7 2. Steps of an Outbreak Investigation http://www.cdc.gov/excite/classroom/outbreak/steps.htm 3. E-Training. Hepatitis in Sparta: A disease outbreak investigation. What type of epidemic curve? http://sph.bu.edu/otlt/sparta/html/18.asp

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