Updated: 10/31/96
Updated: 10/31/96
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.
Disease is frequently attributed to "evil spirits, demons & devils" or the "punishment of God". It may seems foolish to us living in this enlightened age (where the daily horoscope is published in every newspaper), but considering that microbes were not known to exist until the 1600s and not commonly known until the 20th century, how were people to explain diseases that struck without warning and killed at random? Does the idea of calling the HIV virus a "devil" really seem that odd to you?
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 noticed 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 one of the local water companies that obtained its water from a region of the Thames river, which was heavily polluted with untreated sewage also contributed to the cholera epidemic, whereas people who used the water from another company that obtained its water from an UNPOLLUTED section of the river rarely contracted cholera.
Studies of subsequent events, including outbreaks of cholera 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 contributed to the spread of diseases like cholera, gradually converted people to the ideas that:
(1)
the water supply 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.
As educated citizens it is your 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.
FAQ: "Why should a nonscience major be required to learn things like this in order to graduate from a university?"
ANSWER: I asked this question myself (about non-science courses) as a science major sharply focused on my goal of research and teaching. But, with 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 63 years (like marrying a wife who likes 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 that educated citizens (including politicians), regardless of their strange tastes in music, science and art, know something about the beasties that threaten them and those they care for. Now for the terminology:
RESERVOIR
= This refers to the place in nature where a disease NORMALLY
LIVES or is 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 living creatures. 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 Kleenex or
a doorknob infected by someone with the cold virus, but the main reservoir
is still infected persons. The soil does serve as a reservoir for things
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 accidental hosts.
MODE
OF TRANSMISSION = This refers to how the disease is spread between
hosts. Obviously in combating any disease it is crucial to know how it
is spread. 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 IA 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 eye. Further, direct exposure to liquid droplets from coughing
or sneezing from an infected person close to you can spread the cold virus.
Many other viruses and bacterial infections are spread this way. The agent
of TUBERCULOSIS is very resistant to drying and is easily spread through
the air; TB is readily spread in closed in areas like classrooms, subways
cars, 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 IA is spread by an INANIMATE OBJECT such as a doorknob, dirty
glass, polluted water, or a shared beer bottle, the object is called a
FOMITE. Diseases that are commonly spread by fomites include anthrax, respiratory
viruses, numerous water
borne diseases (about which we will hear much more later), and food
borne diseases (about which we will hear much more later).
Let
me post a warning here. Don't ever try to tell your parents, mate, significant-other
etc. that you caught a STD from a fomite;
The
'ol toilet seat story), because it is virtually impossible and you'll
have to end up in the medical text books before they will buy that story.
VECTORS
= When IA are spread by an insect, such as a mosquito, flea or tick, they
are referred to as VECTORS. Examples
of IA spread in this way include bubonic plague, Lyme disease, dengue fever
and malaria. Click
here for a cool pictures of a vector.
CARRIERS
= These
are individuals or animals in which the IA 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 of HIV carriers will allow them to
stay "healthy" longer while being able to spread the virus. Do you see
a problem here?
CHRONIC
CARRIERS = Organisms that usually have had the disease but from
whom the IA has not been completely purged. Such carriers may release the
organisms for days, months or even a life time (see Typhoid Mary
story). Many of us are chronic carriers of
POTENTIAL
PATHOGENS. We ill isolate some of these from ourselves and our
lab mates 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) and those
who are RECOVERING from the disease, but who are feeling well enough to
enter the world once again. Remember those occasions when you were feeling
"punk" but you went to class anyway and the next day we came down with
a terrible cold etc., or we felt we'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 (POE) = These are the ways an IA must enter a host
to establish the disease. The major POE are the gastrointestinal
tract (eating it) or anal intercourse;
the respiratory tract (breathing it); the
urogenital tract (sexual intercourse, lack
of cleanliness, and use of wrong sanitary napkins); and breaks
in the skin or entry through the
hair follicles
or eyes. All pathogens have to enter the host
via the correct POE 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 would have loved to have been swallowed. MOMMIE was right again!
NOSOCOMIAL
= Infections contracted during a HOSPITAL
STAY involving an organism that is a "hospital strain" of the IA These
infections are usually picked up from the personnel transporting the IA
from one patient to another (see Semmelweis),
from instruments, often inserted into sensitive parts of the body (e.g.
catheters, needles, protological scopes 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 hospital patients
in the US ACQUIRE A NOSOCOMIAL INFECTION
during their stay.
COMPROMISED
HOST (CH) = These are individuals whose general condition of
health renders them MORE
SUSCEPTIBLE
to IA and opportunistic IA gaining a foothold in their bodies. CH include
people who have suffered serious trauma and injury by accident or intention
(an operation); those undergoing treatment with immunosurpressant drugs
or anticancer drugs; the aged; those suffering extreme stress; those suffering
or recovering from a serious illness; those suffering from diabetes; and
those suffering from childbirth. I probably should have said "anyone
admitted to the hospital" & I would have been complete enough.
ENDEMIC
= Refers to the fact that a particular disease is NORMALLY
FOUND IN A PARTICULAR AREA. For example, AIDS in 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
IA 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 (>5.6 billion), pandemics are likely to be increasingly
in our future.
MORTALITY
AND MORBIDITY (MM) = 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) provides
weekly reports on MM (requires a machine that contains an Adobe Acrobat
reader).
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. 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. 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 identify the etiological agent, find the 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 Center 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 (WHO) 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. See the movie "Jaws" for an illustration of this type of concern. Also see Microbes in the News.
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 previous congress (1995-6) 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 even 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 journal dedicated to "Emerging Diseases" and look at some of the titles of the articles to see what you might have to deal with in your lifetime.
INDENTIFICATION
of those with the disease and their medical treatment.
ELIMINATION
of the reservoir,
Making
the host resistant: IMMUNIZATION
PREVENTING
TRANSMISSION to new hosts: quarantine, travel restrictions.
Setting
up of HYGENIC CONDITIONS,
training of local personnel etc., purification of the water, clean food
supply etc.
DOING
NOTHING: Probably not acceptable; public reaction to young children
dying usually require action.
ELIMINATING
THE VECTOR MOSQUITO: This is 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. 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 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 hemorragic response upon
the following exposure MORE VIRULENT AND LETHAL. Therefore, a vaccine might
NEVER be found that works.
TREATMENT:
There is NONE other than TLC. 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, 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 that I haven't thought of?
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. 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 were the disease has struck the 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 to get some idea of the new diseases facing the world.
SCIENCE HALL, ROOM 440CA
PHONE: 509-335-5108
FAX: 509-335-1907
E-MAIL: hurlbert@mail.wsu.edu or hurlbert@wsu.edu hurlbert@pullman.com
OFFICE HOURS: M,W 1:30 to 3:30 PM.
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