Hello everyone, welcome back to CommEd Made Easy and I hope you all are doing very well. Before we begin, let me remind my dear students that the contents of this channel are only to supplement your knowledge, not to replace the regular online and offline classes in your institution. So please attend your classes and do not miss them.
Also, if you like our contents, please subscribe to our YouTube channel, like the videos and share these videos with your friends, with your batchmates, with your juniors. Also, if you are a teacher, then with your students. Please follow our Facebook page and the link will be given in the description.
In first part, we discussed the steps and uses of descriptive epidemiology. Today in part 2, we shall talk about the time. place and percent distribution of diseases. We shall start with time distribution.
So a disease can be described by the time of its occurrence that means when the disease is starting. The time trends can be either short-term fluctuations example being epidemic, periodic fluctuations and long-term fluctuations also known as secular trends and we shall discuss individual types separately. First the short-term fluctuations.
The most common example is epidemic. What is epidemic? It is defined as the occurrence in a community or region of cases of an illness or other health related events clearly in excess of normal expectancy. So that means suppose in a given or in a defined population we are expecting that there will be a certain number of case from a disease in a particular year.
So this is our expectation based on previous year's records. Now sometimes what happens, there are a lot more number of cases that occurs in the same population and the number clearly exceeds what we are expecting. So this is known as epidemic. Epidemic can be common source epidemic which again can be single exposure or multiple exposure. It can be propagated epidemics or it can be slower modern epidemic.
We shall discuss the first one in details. This is known as epidemic curve where we show the time distribution and the number of cases. So this is y-axis, this is x-axis.
X-axis denotes the time, y-axis denotes the number of cases. So you can see after exposure here, there is a little bit of time until the first case develops. So this is where the first case develops. With time, the number of cases increases and at a certain point of time, we have the maximum number of cases somewhere around here and then with progression of time the number of cases gradually decreases and ultimately it becomes zero.
So, this is when the first case appears and this is when the last case appears. This is known as epidemic curve a very commonly asked short note in theory paper. First one the common source single exposure epidemic also known as point source epidemic. So, if In your question paper, you see they are asking about point source epidemic.
You understand it is basically the common source single exposure epidemic. So, lot of things are written here. I shall try to explain with an example.
For example, suppose in a hospital or rather in a medical college like you people stay in, there is a hostel, right? Lot of students stay in the hostel and they take food from the mess. Say one of the food items gets contaminated somehow.
Now lot of the students will consume that food because they don't know about the contamination. And because of that many of them will develop food poisoning. So what is happening here?
Let's talk about the source. The source is the contaminated food. Now it was a single food item that was consumed by everyone.
Almost everyone. who stays in the hostel and takes food from the mess so that is why this is a common source because thus there is a single source and single exposure because they consume the food only one right because of that what happens within a short period of time lot of cases appear cases of food poisoning so you can see in the epidemic curve here this is when the first case appears maybe within a matter of hours a lot of students become sick and maybe in about three to four hours or maybe six hours there is no more cases so this is the last case to occur so this is known as common source single exposure epidemic so there is only one source infection and all the susceptible population that means the students in the hostel they are exposed to the source of infection only once because they are consuming that contaminated food only once right another example can be Bhopal gastroenterology whatever what is the characteristics of this epidemic curve there is sudden rise and sudden fall so lot of cases appear in a very short period of time and also disappear like this. So, there is a sudden fall of the curve.
There are no secondary curves because we have only one peak. Large number of cases occur within a narrow interval of time. Of course, whatever is the incubation period of food poisoning which is very short that will occur in a very short period of time and all these cases will occur within that incubation period which is the point number 4. all cases develop within the incubation period so this is about common source single exposure epidemic common source multiple exposure epidemic also known as continuous epidemic right so that means we still have only one source that is why common source but multiple exposure another example suppose in a village there is a well okay and the people from the village drinks water or uses water from that well for some reason maybe the water in the well becomes contaminated now what happens the people in the village will keep using the water of the well because probably they do not have any alternative source of water and Because of that they will be exposed to that contaminated water Multiple times okay, so continuous exposure that will lead to maybe food poisoning or other skin diseases or whatever so as you can see sometimes the exposure from the same source which is the well here may be prolonged and continuous because they are consuming the water or using the contaminated water multiple times not necessarily at the same time or place okay waterborne cholera is a familiar example so yes if there is an contamination of water and people keeps on consuming the water then they will have waterborne cholera and that will occur continuously so the epidemic curve reaches the sharp peak so it rises very fast but tails up gradually over a longer period of time it takes a long time to come to zero here because people keep getting exposed continuously it will take a long time before they stop taking the contaminated water or using the contaminated water that means they stop getting exposed to the cause so that is why the rise is very sharp but the fall is quite slow or gradual this is common source multiple exposure epidemic coming to the propagated epidemic which is of course of infectious origin and results from person to person transmission can also be from animal to person or environment to person transmission like epidemics of Hepatitis A and polio right transmission continues till susceptible individuals are exposed to infected person speed of spread depends on herd immunity so if i give you this example this will be better to understand suppose this is the first case of covid 19 in our country so this was the person who introduced the infection in our country. Maybe he infected three more people who came in close contact to him. This individual person will again contaminate many other people who will develop disease.
These black circles they are basically the people who got infected and sometimes we also have red circles those people who did not get infected even after coming in. close contact with a known person who is infected this can occur when there are if this person already had developed covid 19 or maybe he received the vaccine of covid 19 so you will not develop the disease anymore so as you can see the number of cases gradually increases and each infected person keeps transmitting the disease to others so this is known as propagated epidemic as The infection is being propagated from a person to another person. So initially, there are a lot of susceptible people because Most of them have never developed COVID-19 or have not received vaccine yet. With time, there will be lot of people who are not susceptible, that means were protected, maybe because of the infection or maybe because they already took the vaccine.
And after a long period of time, you will see most of the people either already developed the disease or received the vaccine and because of that, there are only very few susceptible people like this. Black circles as you can see in this part is quite few in number. So, this is propagated epidemic. Okay, the curve looks somewhat like this.
It rises gradually, reaches its peak and then goes down very slowly. So, if you remember in common source the rise is first. very swift but here we have very slow rise and then gradual decline coming to periodic fluctuation which can be seasonal trends or cyclic trends seasonal trends means some diseases which occur at a particular season or at least the number of cases increases during that season for example measles chickenpox in early spring season that means after the winter urti or upper respiratory tract infection during the winter seasons and diarrheal diseases during summer and also during the monsoon season.
Mosquito borne diseases can occur during and after the monsoon season when there is stagnation of water. This is seasonal trend. Cyclic trend means certain diseases appear in cycles which may be spread over short period of time like days, weeks, months or years.
So maybe if you look at the example epidemic of measles in every 2 to 3 years and influenza once in 7 to 10 years. This was before the vaccination era when the vaccines were not available. So what do we see?
We will see that there is an epidemic of measles and maybe next 2 years or 3 years there is no epidemic and then they will again appear. So this is known as cyclic trend. Coming to the third type that is long-term fluctuation also known as secular trends.
Again, this is a short note. If you get secular trend in your short note in your theory paper, you have to write this thing. So, changes in the occurrence of disease over a long period of time, generally several years or decades.
So, the number of cases or the progression of disease keeps changing over a long period of time. And by long period, I don't mean weeks or months. Rather, I mean many years or even decades.
It could be increasing or decreasing. as you can see the number of cases over period can be gradually increasing or decreasing it will not happen like it will increase at a point of time then it will decrease again increase it's not like that so it is either increasing or decreasing it can be real or apparent so sometimes the increase or decrease may not be real okay and it can be communicable disease or a non-communicable disease applicable for everything so downward trend for plague and cholera because Now, especially in developed countries, they have controlled any kind of infection very well. So, that is why infectious disease they have decreased over a long period of time. Upward trend means when the number of cases gradually increases or at least they are diagnosed more in number.
For example, diabetes mellitus, chronic heart disease, lung cancer, etc. These are non-communicable diseases. Maybe the number of cases of these non-communicable diseases are actually increasing then it is a real increase or maybe that diagnosis is being done more right now that means earlier maybe we did not have much diagnostic procedures with high sensitivity and specificity so because of that we could not diagnose those people who had diabetes or heart disease or cancer now we can so even though the actual number remains the same Now they are being diagnosed even more.
So then it will be an apparent increase. So this is an example. Yearly cases of tetanus in USA for 45 years. So as you can see around 1955 there are about 700 cases in USA.
With time it has gradually decreased. So around 2000 we have less than 100 maybe less than 50 cases of tetanus in USA. So this is just an example.
So, that was all about time distribution, next we shall talk about place distribution. So, place distribution is basically the geographical pattern which provides important source of clues about the causes of the disease. So, if we find there are lot of cases being reported from a geographical area, then we have to visit that area and have to look for the causes behind that.
Maybe the place, the geographical location has some role in the causation of the disease. So, there can be. Multiple types of press distribution like international variation. That means some countries reporting more cases and some countries reporting less cases of any particular disease.
Malaria, leprosy in hot and humid climate like Africa, South America, etc. Carcinoma of stomach, Japan. Cancer oral cavity that is and cancer cervix in India. So as you can see in certain countries we have.
more number of cases as compared to other countries for certain diseases. National variation means difference of case reporting or occurrence of disease within a single country. So, endemic goiter, latharism, malaria, filaria, leprosy these diseases can have national variation.
Rural urban variations like tetanus, organophosphorus poisoning, zoonotic diseases are common in rural because People are more exposed to the environment because of their professional occupation. So, tetanus can come from exposure to the dust or soil. Organophosphorus poisoning occurs because of the contact with different kind of pesticide, insecticide, etc.
Again, in urban areas, there are lot of cases of road traffic accident, drug abuse, diabetes mellitus, cancer, mental stress, etc. So, as you can see. We can also have rural and urban variation in diseases and local distribution means we can have a spot map for a given area and we can try to find out the number of cases in that area where we have a lot of cases as compared to the other areas. I will give you an example. So, this is an example of a smart map.
The smart map you can see we have this geographical area here. and these green dots they represent individual cases so we have one isolated case here two isolated cases here three here and then we have a cluster of many cases in this area so that means there must be something of importance in this geographical area that is causing the clustering of the disease so smart map showing the geographical distribution of cases so if A researcher or investigator who is trying to know about the disease itself or the reasons behind that, he should definitely go to this area because the number of cases in this area is quite high. Migration study.
Again, this is a very commonly a short note. Here, we try to find out whether the environmental difference in two areas account for variations in the occurrence of disease in a question. So.
If a person migrates from one place to another and he develops the disease, whether this disease is because of the new environment or something else, maybe his genetic factor, we have to find out. And for that we conduct migration study. So this is an example.
Suppose we have area A and area B and in each area we have 10 people living. So these are the yellow people and this is the orange people. we have also assumed that the environmental condition of area a and area b are completely different all right so what happens now three orange people from area b migrates to area a and three yellow people from area a migrates to area b so as you can see we have in area now seven yellow people remaining and three three orange people have migrated from area b similarly In area B, we have 7 orange people who have stayed back here and also 3 yellow people who have migrated from area A. Now, what we can do?
We have to look for the incidence rate of disease. Suppose, how frequently this disease is occurring in these people? We have to calculate that. From previous video, we should already have an idea about what is incidence rate, okay?
Say, If the incidence rate of disease in orange pupil in area A is same as the incidence rate of disease in orange pupil in area B. So, that means, even though they are exposed to two different environmental condition and still have the same incidence rate of the disease. That means the environment does not have any role in causation of the disease. Okay. If the incidence rates were different, so that may be because they were exposed to two different environments.
So, probably environment had some role in the causation of the disease. All right. Now, we can ignore any area. Suppose we are ignoring area B for the time being and we are looking at area A. Next we have to do is find the incidence rate of disease among the orange people these three orange people and also incidence rate of disease among the yellow people if these two incidence rates are same That means there is role of environment Because they are all of them these three orange men and these three yellow men They were exposed to the same environmental condition and also they have the same incidence rate so probably environment has some role in the causation of the disease all right if the incidence rate among the orange men and the yellow men were different so that means even after being exposed to the same kind of environment they showed different incidence rate that means the disease was probably caused by some genetical factor So, this is about migration study.
We can compare the incidence rate of the migrated people with the people who stayed back at their actual area of area of living that that is the first thing we can do. Also we can compare the incidence rate of the migrated people to the original people who are native to the area that means who are actual habitant of the area. So, this is migration study.
Now coming to person distribution where we try to find out different characteristics of human population which can be associated with disease. So disease is further characterized by defining person who develop the disease by age, sex, occupation, marital status, habits, social class and other factors. So all these factors can have some role in causation of disease.
These factors do not necessarily represent the etiological factor. So, they may not directly contribute to the causation of the disease, but they can have some indirect role in causation of the disease. And also important for us to understand the natural history of disease.
So, that means how a disease progresses. Some of the characteristics have been explained with example. For example, age. So, different.
Diseases are very common in different age group. For example, measles in childhood is very common, cancer in middle-aged and atherosclerosis in old people. What about gender? Diabetes, hyperthyroidism and obesity are common in women.
On the other hand, some diseases like lung cancer, coronary heart disease are less frequent among women. So that means male are more commonly affected. Ethnicity, Jews have higher prevalence of the gene for Tay-Sachs disease. So that means Jews, since the Jews have higher prevalence of the gene, they will have a higher prevalence of this Tay-Sachs disease, the genetical disorder among them. So ethnicity can also have some role in causation of disease.
Marital status, mortality rates are lowest among married followed by single, widowed, divorced. in that order irrespective of sex or gender. So as you can see, mortality rate can vary based on the marital status of a person. Occupation, of course, there are a lot of occupations which leads to exposure to different kind of risk factors or pathogens and thus leading to different diseases. For example, people who work in the coal mine are more likely to develop silicosis, which is one kind of pneumoconiosis or lung disease.
Social class individuals from upper social classes have a longer life expectancy and better nutritional status because they can afford good quality food. Also, they can attain healthcare seeking behavior because they are from upper social classes, but they also have shown higher prevalence of coronary heart disease, hypertension, diabetes, because since they are from upper social class. They are involved in those kind of profession which does not involve much physical activity. Also, maybe they will take. Those foods which are high in calorie.
So all these things can lead to coronary heart disease, hypertension, diabetes, all these non-communicable diseases. Behavior like cigarette smoking, sedentary lifestyle, overeating, drug abuse can also lead to different diseases. Stress is an important factor for mental illnesses.
Migration as you already explained. When a person migrates to a new place. that means he is exposed to new kind of threats also when he is moving to a new place he also brings the infection from his native place to a new place right and there can be other factors of course so that's all about time place and person distribution in part three we shall discuss few types of descriptive epidemiological studies Please subscribe to our channel, like the video and share this video with your batchmates, with your juniors and maybe with your friends from other colleges.
We also have our Facebook page and the link is given in the description. Please follow it. Take care and we shall see you in our next video.