MICROBIOLOGY 101/102 INTERNET TEXT

CHAPTER XIV: PUBLIC HEALTH AND THE PREVENTION OF DISEASE

UPDATED: 02/10/00

GLOSSARIES

MICROBIOLOGY |  MEDICAL | CDC MORTALITY AND MORBIDITY REPORT | CDC DATA REPORTS 

THE YEAR IN MICROBIOLOGY

Hand Washing and Good Health--How Important?

Be an Epidemiologist and Solve These Medical Mysteries?

Flu Map; Improved Diagnosis for Flu

DNA vaccines; a New Era in Immunization

Environmental Changes May Spur Asthma Rise

Foods and Cancer Prevention: Tomatoes; Potatoes

Fighting Super Bugs

TABLE OF CONTENTS

EPIDEMIOLOGY

OBJECTIVES

To understand the principles of epidemiology:

To understand how infectious diseases spread.

To understand the concept of disease reservoirs.

To understand the public health management of diseases.

HISTORY OF EPIDEMIOLOGY

Our ancient ancestors recognized that diseases spread from one person to another. The idea that people with certain diseases are "unclean" is ancient, and is still with us (e.g. HIV+). These people, usually marked by some highly visible condition like leprosy or other skin condition, were (& they still are) shunned by their fellows. Lepers were often required to wear bells or to continually cry "UNCLEAN" in a loud voice so as to warn people of their presence; failure to follow these rules resulted in severe punishment or even death.  During outbreaks of highly infectious diseases like the bubonic plague, smallpox and flu people fled in terror from the foci of the infections, which were often deemed "unclean". Of course this almost always had the effect of SPREADING infectious diseases further afield; desirable from the standpoint of the infectious agent, but not fun for the new victims. (Sci. Am. Feb. pg. 118, 1988)

Even today disease is frequently attributed to "evil spirits, demons, witches & devils" or the "punishment of God". It may seem foolish to us living in this enlightened age (where the daily #horoscope is published in every newspaper), but considering that microbes were not discovered until the 1600s and not commonly known until the 20th century, how were people to explain diseases that struck without warning and seemingly killed at random? Does the idea of calling the HIV virus a "devil" really seem all that odd to you? After all millions of people today claim to have spoken to angels or to have been kidnapped by aliens.

The SCIENCE OF EPIDEMIOLOGY or the study of the origin, cause and spread of disease, got its start during the #cholera epidemic of 1854-55 in London. A physician, John Snow, reasoned that cholera was an infectious disease and that its spread was related to unsanitary conditions, particularly the #LACK OF SEWAGE DISPOSAL. To support his contention, he collected a wealth of data on the cholera victims, including their addresses, their major source of water, professions etc. He plotted this information on a map of London and noted that the majority of cholera cases were located in a certain area. He further noted that most of the victims obtained their water supply from a local pump known as the "BROAD-STREET-PUMP". After failing to convince people to stop drawing their water from this source he halted the epidemic by the simple process of removing the handle from the pump. His study was so well done that it is still used today to teach epidemiologists how to collect and interpret data on the spread of disease. He further went on to show that local water companies that obtained their water from regions of the Thames river, which were heavily polluted with untreated sewage also contributed to the cholera epidemic, whereas people who used the water from water suppliers that obtained their water from an UNPOLLUTED section of the river rarely contracted cholera.

The recently published book Level 4, #Virus Hunters of the CDC (listed in the syllabus for extra credit) is a story of modern epidemiologists and how they study diseases in today's world.

Studies of subsequent events, including outbreaks of cholera in the late 1800s in Germany, that repeated Snow's epidemiological findings, along with the development of the #germ theory of disease and the general recognition that disease, poverty and filthy living conditions contributes to the spread of diseases like cholera and typhoid, gradually converted people to the quaint ideas that:

(1) the water supply, as well as the general environment, had to be cleaned up and;

(2) that statistical evidence could provide information that was useful in stopping or preventing epidemics. Some people think that this lesson has to be relearned by new generations. Have we learned this lesson today when it comes to sexually transmitted diseases, smoking, overweight etc?

As educated citizens it is our responsibility, as well as being in our own best interest, to understand and appreciate the usefulness of epidemiological studies and the role of public health agencies in maintaining community health. Pathogens do not recognize the boundaries between wealth and poverty or between those who are educated and those who are not. It is true that many pathogens affect those who live in poverty and filth disproportionately, but others who are better off are not spared the ravages of epidemics they just lie on clean sheets).

VOCABULARY

FAQ: "Why should a nonscience major be required to learn things like this in order to graduate from a university?"

ANSWER: As a science major undergraduate, sharply focused on my goal of research and teaching, I asked this question myself about non-science courses. But, after much bitching and complaining about the waste of my time & money, I took the required art, history and world religion GERs of my day. Strangely enough I actually learned later that not everyone is as wild about science as I was, that there were people who actually like (I know that this is hard to believe, but I swear that it's true) opera and others that actually visit art museums and go to concerts. In fact one of those people turned out to be me: not opera though--there are just some things I still resist.

Life has taken some interesting turns in my 66 years (like marrying a wife who adores opera and art) and I was actually glad I'd been required to take GER courses. Anyway, it turns out that pathogens don't just munch on science majors, nor do science majors set public health policies that determine how many pathogens we let roam around our environment, so it is important, one might even say a citizen's responsibility, that citizens of any society (including politicians), regardless of their strange tastes in music, science and art, know something about the beasts that threaten them and those they care for. Now for the terminology:

CATEGORIES OF DISEASE RESERVOIRS:

RESERVOIR = This refers to the place in nature where a disease NORMALLY LIVES or is always found in significant numbers; that is the place you would go if you wanted to commit suicide by catching, say the EBOLA VIRUS. When we discuss the role of public health, the importance of knowing the reservoir of a disease will become clear.

ZOONOSES = Reservoirs of diseases in animals (usually other than humans however, don't be too offended but epidemiologists consider humans just another animal), that can be transmitted to humans (e.g. Ebola, Lassa fever, Lyme disease and probably the original HIV; now the HIV reservoir is humans). These are the most common types of reservoirs since pathogens are pathogens because they metabolize other living organisms who fail to appreciate their activities. Rabies is a classical case of a nonhuman animal reservoir.

INANIMATE = With few exception, inanimate reservoirs are secondary or accidental reservoirs. For example, you can catch a cold by handling a virus-laden Kleenex or a doorknob infected by someone's spit or snot with the flu virus, but the main reservoir of cold and flue are infected persons. The soil serves as a reservoir for pathogens like #Clostridium perfringens, Clostridium tetani and Bacillus anthrax spores, but the ultimate reservoirs again are the hosts they reproduce in. #Clostridium botulinum, as well as some fungal diseases, have their reservoirs in the soil and mostly live in the soil, probably on dead matter; that is, living organisms are rare, accidental hosts.

DISEASE TRANSMISSION:

MODE OF TRANSMISSION = This refers to how the disease is spread between hosts. Obviously in combating any infectious disease it is crucial to know how it is spread, but this also applies to hereditary diseases. For if this is known it follows that you are more likely to be able to stop or prevent a disease's spread. There are many ways infectious agents are spread.

DIRECT CONTACT = This refers to direct PERSON-TO-PERSON or RESERVOIR-TO-PERSON contact. For

example, the common cold viruses seem to be spread mainly through hands contaminated with the virus being placed in the mouth or rubbed in the eyes. Further, direct exposure to liquid droplets from coughing or sneezing from an infected person close to you also spread the cold and flu viruses. Many other viruses and bacterial infections are also spread this way. The etiological agents of TUBERCULOSIS, measles, and chicken pox are resistant to drying and are easily spread through the air; TB is readily spread in closed in areas like classrooms, subway cars, airplanes, movie theaters, sports arenas and similar places where people are confined to close quarters. This is why I made the statement above about the #catholic nature of pathogens.

FOMITES = When an infectious agent is spread by an INANIMATE OBJECT such as a doorknob, dirty glass, towel, bedding, polluted food/water, or a shared beer, the infectious object is called a FOMITE. Diseases that are commonly spread by fomites include anthrax (rusty nails, thorns, dirt etc.), respiratory viruses (door knobs, chair arms etc.), numerous #water borne diseases (about which we will hear much more later), and #food borne diseases (about which we will hear much more later).

VECTORS = When infectious agent are spread by an insect, such as a mosquito, flea, lice, biting fly or tick, they are referred to as VECTORS. Examples of infectious agents spread in this way include bubonic plague (flea), Lyme disease (tick), dengue fever (mosquito) and malaria (mosquito). 

#CARRIERS = These are humans or other animals in which the infectious agent is growing and reproducing with little or no visible harm to the current host; that is carriers may be "as healthy as a HORSE". #HIV positive people appear healthy for ~8 or more years while being able to spread the virus. The new treatments allow those infected with HIV to maintain a "healthy" phenotype for long periods, although they remain capable of spreading the virus. Do you see a problem here?

CHRONIC CARRIERS = Organisms that usually have had the disease but from whom the infectious agent has not been completely purged or a virulent pathogen for one species may be a harmless component of the natural flora of another species. Such carriers may release the organisms for days, months or even a life time (book on #Typhoid Mary story is on extra credit list). Many of us are chronic carriers of POTENT PATHOGENS (e. g. hemolytic streptococcus, staphylococcus etc.). We will isolate some of these from ourselves in Lab Exercises #18-20.

TRANSIENT CARRIERS = Are organisms that are COMING-DOWN with a disease, but who haven't shown identifiable symptoms (e.g. Hepatitis A) and those who are RECOVERING from the disease, but who are feeling well enough to roam free in the world once again. Remember those occasions when you were feeling "punk" but you went to class anyway and the next day you came down with a terrible cold etc., or the time you felt you'd recovered enough from a bout with the flu to go back to Dr. Hurlbert's fantastic, scintillating lecture because you were suffering from "withdrawal pains" due to not hearing his voice? Well, it is likely that you contributed to the spread of whatever diseases you were feeding.

PORTALS OF ENTRY = These are the ways or places an infectious agent must enter a host to establish the disease. The major portals of entry are:

• The gastrointestinal tract (eating/drinking it).
• Anal intercourse.
• The respiratory tract (breathing it).
• The urogenital tract (sexual intercourse, lack of cleanliness, and improper use of sanitary napkins).
• Breaks in the skin
• Entry through the hair follicles or eyes.

All pathogens have to enter the host via the correct portals of entry in order to set up an infection. Do you recall your MOTHER telling you not to eat that piece of candy that had dropped on the ground? Well it may have fallen into an old piece of doggie or bird doo doo containing an intestinal pathogen which couldn't pass through your skin but which has to be swallowed. MOMMIE was right again!

NOSOCOMIAL = Infections contracted during a #HOSPITAL STAY involving an organism that is a "hospital strain" of the infectious agent. These infections are usually picked up from the personnel transporting the infectious agent from one patient to another (see #Semmelweiss), from instruments, often inserted into sensitive parts of the body (e.g. catheters, needles, proctoscopes etc.), on the equipment (e.g. bed pans, beds, sheets, dishes, flowers etc.). Nosocomial bacteria are often very resistant to antibiotics having been exposed to a variety of antibiotics while in the hospital. It is estimated that 2 to 10% of US-hospital patients ACQUIRE A NOSOCOMIAL INFECTION during their stay.

COMPROMISED HOST = These are individuals whose general condition of health renders them MORE SUSCEPTIBLE to infectious agent and opportunistic infectious agent gaining a foothold in their bodies. Compromised hosts include:

• People who have suffered serious trauma and injury by accident or intention (an operation).
• Those undergoing treatment with immunosuppressive drugs or anticancer drugs.
• The aged.
• Those suffering extreme stress (e.g. students before difficult exams).
• Those suffering or recovering from a serious illness.
• Those suffering from diabetes.
• Those who have recently experienced childbirth.

I probably should have said "anyone admitted to the hospital" & I would have just about covered everyone in the Western world.

PATTERNS AND EXTENT OF DISEASES:

ENDEMIC = Refers to the fact that a particular disease is NORMALLY FOUND IN A PARTICULAR AREA. For example, AIDS is endemic now in most countries in the world; respiratory infections are endemic in the world; malaria is endemic in the southern hemisphere; Lyme disease is endemic in a number of states in the US. With the ability of people to rapidly travel around the world, we are nearing the day when we can say that every infectious agent is ENDEMIC TO PLANET EARTH.

EPIDEMIC = a relatively SUDDEN INCREASE in the number of cases of a particular disease in a PARTICULAR PLACE OR AREA. For example, we can have a local epidemic of the Palouse trots or the WSU flu, or we may have an epidemic of flu in the US or a cholera epidemic in Bolivia and Mexico etc. The area and the number of increased cases are always part of the description of any epidemic.

PANDEMIC = An epidemic that encompasses the ENTIRE WORLD. AIDS is a pandemic; the flu is often pandemic. In the past pandemics were rare, but through the combination of world wide rapid travel and the burgeoning human population (~6 billion), pandemics are likely to be increasingly in our future.

MORTALITY AND MORBIDITY = Morbidity refers to the number of people who catch a disease, whereas mortality refers to the number of people who die from a disease. This data is usually presented in terms of numbers per 100,000. For example, chickenpox has a high morbidity, but a very low mortality. Conversely AIDS has a low morbidity, but a high mortality. The Centers for Disease Control (CDC; be prepared to describe the general activities of the CDC) provides weekly reports on mortality and morbidity (requires a machine that contains an Adobe Acrobat reader).

THE EFFECT OF WEATHER ON THE SPREAD OF DISEASE:

HOW DOES ONE DO EPIDEMIOLOGY?

The best way to view an epidemiologist is as a PATHOGEN DETECTIVE; a Sherlock Holmes if you will. As a good detective the epidemiologist must search out clues, follow false leads and suffer long periods of frustration during which the etiological agent of a disease continues to evade its pursuers and ruthlessly kill and maim new victims. These duties are often carried out in a heated political and social atmosphere where the epidemiologist's actual & professional life may be at stake. Often the investigations are performed under primitive and dangerous conditions; conditions of chaos due perhaps to political or economic upheaval where epidemiologists must deal with uneducated, superstitious populations, unstable governments run by uneducated dictators and soldiers who are suspicious of all outsiders. Epidemiologists must travel to foreign lands, far from civilization and days or weeks from modern medical treatment, where they are in danger, not only from the very diseases they may be investigating, but from a host of equally murderous pathogens lurking in the environment. It is not unusual for epidemiologists to become victims of diseases under these conditions and a number have died from illnesses contracted in the field.

On the other hand, much epidemiological work consists of routine data collection and analysis. Usually this data is collected in the field and sent back to laboratories for analysis, although modern advances in miniaturization, computers and rapid communication have made it possible to do more work in the field. Epidemiologists today work as members of HIGH-TECH TEAMS which often includes microbiologists, molecular biologists, entomologists, sociologists, linguists, doctors, nurses, military personnel and government officials. There are no EPIDEMIOLOGIST COWBOYS in today's world. An epidemiological team must first determine if there is a serious problem. It then must determine how serious the problem is and how quickly, and to where, it is likely to spread. The team must then try to identify the etiological agent, find its reservoir, define the major means of transmission and develop a way of stopping or limiting the spread of the infection. Sometimes they are able to do this in a few weeks or months, but in other cases it takes longer; all the while the death toll continues to mount.

Every state and local departments of health have a person or persons whose responsibility it is to detect & study local or regional "epidemics". The local health departments have the responsibility of calling in federal help where a serious disease problem is recognized. The Centers for Disease Control or CDC is the US federal organization charged with the responsibility of identifying and dealing with serious disease outbreaks in the US and ANYWHERE ELSE IN THE WORLD that are deemed a threat to our national interest. Since we are the world's leading power and richest nation, this means that our CDC teams go anywhere in the world and study serious disease outbreaks. The World Health Organization and health organizations in other countries have groups concerned with epidemiology. Generally these organizations work together, but there are occasions when they do not. For example, the political and economic situation in a country may be such that the local leaders do not want it KNOWN that there is an epidemic in their country. This has been the case where tourism is important to a country's economy; tourists have a tendency to cancel a trip to a place infested with the bubonic plague for example. View the movie "Jaws" for an illustration of this type of concern.

QUESTION: "Do we really have a need for such an expensive program like the CDC runs, when, after all, science has come so far and we know so much about how to deal with most infections?"

ANSWER: We citizens have to resolve this question. It is not just a scientific decision, but also a political and economic one. The 1995-6 congress initially suggested significant cuts be made in the budget of CDC, but later the budget was actually increased. The scientific community is UNANIMOUS in the belief that we are more than ever in need of the CDC's epidemiological efforts; indeed most biologists believe that the CDC's efforts need to be expanded. There is growing concern that diseases like the Ebola virus are just the beginning of a whole new group of deadly diseases that will be striking the human population in the next generation. What is your opinion (remember it may result in increased taxes down the road)? Click here for a site dedicated to "Emerging Diseases" and look at some of the articles to see what you might have to deal with in your lifetime.

BREAKING THE CYCLE OF INFECTION

Once a disease situation has been characterized decisions must be made as to how to deal with it in both the short and long term. The basic approach to breaking the cycle of infection involves:

IDENTIFICATION of those with the disease and treating them.

ELIMINATION or control of the reservoir.

Making the host resistant: IMMUNIZATION.

PREVENTING TRANSMISSION to new hosts: quarantine, travel restrictions, mass immunizations, application of sanitary procedures, killing of vectors.

Setting up of HYGIENIC CONDITIONS, training of local personnel etc., purification of the water, clean food supply etc.

AN EXAMPLE OF A DISEASE ON OUR SOUTHERN BORDER:

DENGUE FEVER (breakbone fever) is now infecting Mexicans living in the towns along the US border. This disease is endemic and epidemic in Central and South America, but it is rare in our country. It is caused by a RNA virus, there is no vaccine for it and it is spread by a mosquito that has been found as far north as Chicago. It is a virulent disease with a significant mortality rate. It initially produces fever, headaches and VERY SEVERE BACK AND LEG PAINS in victims; this has given it the name "breakbone" because it feels like the bones have been broken. It is painful and debilitating with the first infection, but upon reinfection it produces a HEMORRHAGIC FEVER that often kills horribly. There are several varieties of the disease and we're not certain of the virulence and mortality of the various strains, but one study showed an 8% mortality in children. In the Fall of 1995 there was an outbreak in Acuapa Nicaragua that killed 15 people.

ALTERNATIVES (CHOICES) OF DEALING WITH DENGUE FEVER:

DOING NOTHING: Probably not acceptable; public reaction to young children dying usually requires action.

ELIMINATING THE VECTOR MOSQUITO: This is possibly DOABLE, but expensive and would require the extensive use of pesticides in all residential areas (both in the poor as well as the richer communities). The widespread use of pesticides is sure to cause a FIRE-STORM OF PROTEST, both because of their perceived health danger and the cost. To keep the mosquito population down would require putting in place a PERMANENT SYSTEM of spraying and elimination of the mosquito's-breeding habitats. This would require an expansion of the local public health agencies at added cost to the community budget (e.g. the school budget). The vector mosquito is already resistant to many pesticides and would certainly become resistant to any new ones employed to control them.

IMMUNIZATION: First, NO VACCINE is currently available and the development of one might take years and be very expensive in a time of budget limitations. Also, it seems that prior exposure to the virus actually makes the hemorrhagic response upon the following exposure MORE VIRULENT AND LETHAL. Therefore, a vaccine might NEVER be found that works, so we may have to start on an investigation of the molecular nature of the disease before we can understand how to treat/prevent it. Such a course may take many years and millions of dollars.

TREATMENT: There is NONE other than tender loving care. There is currently no way to reduce the severity of the disease, that is, it has to simply run its course. Since this is a disease strongly related to poverty and low incomes, who is going to pay for the hospitalization and expensive care of those who can NOT AFFORD to pay their own health care costs?

RESEARCH INTO NEW TREATMENTS: This might be a long term answer (antiviral drugs), but no promises can be made and it might take 20 to 50 years before a cure or prevention is found. In the meantime the virus is known to mutate rapidly, so no one treatment might ever work.

Do you have any suggestions to mail me that I haven't thought of?

SUMMARY OF EPIDEMIOLOGY

Epidemiologists study the pattern of diseases spreading among populations. An epidemiologist identifies the etiological agent of a disease outbreak and identifies the mode of transmission and the reservoir. Finally an epidemiologist determines the best course of action for stopping or limiting the spread of the infection. The science of epidemiology relies heavily on statistical data to determine, if there is a problem and, if so, how serious is it. This information can point to the disease reservoir and to its major mode of transmission.

Most people are suspicious of statistical data. Statistical data usually means that the nature of the problem is not easily visible or there would not be a need to collect statistical data in the first place. This often translates into public APATHY and even ANTAGONISM towards the advice given by the epidemiologists (e.g. the common attitude "If I can't see it, it's a lot of crap."). This may result in little or NOTHING BEING DONE to correct a problem. For example, it has been found that the Ebola virus is best transmitted by close contact with a victim, but in the African areas where the disease has struck the traditional burial rituals require that family members clean out the organs of the dead with their bare hands before burying the body. Since these organs are rife with Ebola virus it insures that those who perform this religious ritual usually become infected and many will die themselves etc.

Click here and here to get some idea of the emerging diseases in your future; be prepared to name at least one emerging disease.

ADDITIONAL INFORMATION:

http://129.109.136.65/microbook/ch009.htm; Limited discussion of epidemiology.

Click here for a self assessment test of what you have learned.

Copyright © Dr. R. E. Hurlbert, 1999.
This material may be used for educational purposes only and may not be duplicated for commercial purposes.
SCIENCE HALL, ROOM 440CA
PHONE: 509-335-5108
FAX: 509-335-1907
E-MAIL: hurlbert@wsu.edu or hurlbert@pullman.com

Return to Table of Contents