Microbiology, Lecture 13

8/7/2019 Microbiology, Lecture 13

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Sun 27/3/2011

Microbiology Lec # 13

Slides: (Lec 13, Epidemiology and Public Health) & (Lec 14 + 15,

Microbial Pathogenesis and Virulence)

Done By: Laith Jamal & Moh'd Rafay3a

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Today, for the first part of this lecture we will talk quickly about

epidemiology & public health, and in the second part of this lecture we

will talk about microbial pathogenesis and virulence which is the first

part of the next time lecture enshallah.

** Lec 13, Epidemiology and Public Health **

Ok, so today we will have many definitions, so Ill read some of them

and describe the other in my words. So epidemiology can be loosely

defined as the study of disease, and the people who perform

epidemiological studies are called epidemiologists.

So these people study the factors that determine the frequency,

distribution, and determinants of diseases in human populations, and

also these are the people who develop ways to prevent, control or

eradicate diseases in populations. So if you have, for example,salmonella in the person, who investigates where its coming from and

how to control it, is epidemiologist.

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Ok, here are a few epidemiologic terminologies

A Communicable disease is an infectious disease that can be

transmitted from one person to another, whereas a Contagious

disease is a communicable disease that is easily transmitted from

person to another. So for example common cold or influenza are

considered contagious diseases where as for example diarrhea (not

sure) is not considered as a contagious disease, it is considered to be a

communicable disease.

Ok, diseases can be acquired from many sources , if the disease

source is an animal then the net disease is called a Zoonotic disease,

and we also referred to zoonotic disease as zoonoses, and we will talk

about this later in this lecture.

Lets talk about a few other terms. So whenever we say Incidence of

disease that means the number of new cases that occurred in a

particular population in a particular year or a particular time period. So

we can say there were 100 new cases for example of tuberculosis in

Jordanian population in 2009.

Whereas when we mentioned The Morbidity Rate, it is basically the

same as incidence but we refer to it as a percentage. So we can say we

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have 100 people out of 1000 people in Jordan are infected and so on.

(number of cases per population number)

In contrast to incidence and morbidity rate we have Prevalence. It

divides to 2 divisions:

- The period prevalence: it's the number of cases in total,

whether this cases occurred in this year or in a previous time.

So we can say that in 2009 for example, Jordan had 300

cases of HIV. But that does not tell you that this infection

occurred in 2009 or in a previous year.

- The point prevalence: it mentions the rate of infection

that's currently in population in a specific period of time.

So we can say that every year we have 20 people get infected

in Jordan with HIV.

Mortality/death rate: it's refers to percentage of how many people

died due to get infected with a particular disease. So out of 100 people

who get the disease let's say maybe 90% of them died, so the mortality

rate will be 90%.

Sporadic disease: it's the disease that happens pretty much randomly

in the population without any particular pattern, such as the tetanus.

Endemic disease: is one that is always present within the population of

a particular geographic area. For example in Egypt we have Pelharsya,

so Pelharsya is an Endemic disease in Egypt but its endemic in Jordan.

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An Epidemic disease is defined as a greater than the usual number of

cases of a disease in a particular region, usually within a short period of

time. For example, the Legionnaires disease epidemic of 1976 (a

severity infection of the lower respiratory tract). So usually some of the

diseases happen very infrequent but suddenly the number of the cases

in a particular area increases so it's considered as an epidemic disease.

Legionnaire is a severe infection of the lower respiratory tract.

Pandemic is a disease that is occurring in epidemic proportions in

many countries at the same time. The most recent example is the

H1N1 (swine flu), you still see many people in the world getting

infected with this flu in many locations in the world in the same time.

And the very famous example was the Spanish flu pandemic in 1918,

which killed more than 20 million people around the world, and

basically infected all the countries around the world.

HIV, Tuberculosis and Malaria are also examples about pandemic

diseases.

Now, if you get exposed to a particular microbe or a particular

pathogen, there are many factors which determine whether you are

actually going to develop a disease or not due to expose to this

microbe or pathogen.

* Factors related to the pathogen itself:

- The very important factor is the virulence of the

pathogen (we are going to talk about this next lecture)

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- Mood of entry: for example a specific virus which causes

a GI disease by entering through the GI tract, so if it gets to

your skin it will not cause the disease.

- Number of organisms: you have to have a particular

number to enter your body in order to cause a disease. And the

number can be very high or can be very low. For example if you

get only one Bacillus of Tuberculosis you can be infected with

TB. Whereas you have to be injected with maybe a

million of cholera Bacillus in order to be infected with cholera.

So there is a huge variation in the infection dose in causing disease.

* Factors related to the host itself: (e.g., health status, nutritional

status, hygiene, age, travel, Lifestyle, etc.). So it depends

particularly on the immune system status, if you have a good

immune system then the probability of getting the

disease is low, on the other hand if you have a suppressed

immunity then your chance of getting the disease is high.

* Factors related to the environment surrounding the pathogen and the

host (e.g., physical factors such as climate, season,

geographic location, availability of appropriate reservoirs;

sanitary and housing conditions and availability of clean water).

* The Chain of InfectionThere are 6 components in the infectious disease process:

_ A pathogen

_ A source of the pathogen (which we called a reservoir)

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_ A portal of exit from the host

_ A mode of transmission

_ A portal of entry to a new host

_ A susceptible host

To get this point clear let's take an example, assume that we have a

host (reservoir ) with a virus of the common cold ( pathogen ), so he's

shedding the virus from nasal secretion ( portal of exit ) then he may

touch his nose with his hand an then he shakes the hand (mode of

transmission " skin to skin contact" ) with another person (susceptible

host), so now he may eventually touch his eyes or nose ( portal of entry

to a new host ) . (Look at The next picture)

* Reservoirs of Infection

The sources of microorganisms that cause infectious diseases are

many and varied, they are known as reservoirs of infection or simply

reservoirs.Living reservoirs: humans, pets, farm animals, insects, arachnids

(spiders).

Human carriers:

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- Passive carriers: the person who transmits the virus without

getting infected with that virus. For example if I shake the hands

with a host then I shake with another person, I will transmit the virus

without getting infected.

- Incubatory carriers: a person who is developing a disease but

now he is in the incubation phase, so at this time the virus is

replicating and shedding in a small number from his body, but

regardless to the small number he still able to transmit the infection

to the other people.

- Convalescent carriers: A person, who is recovering from the

disease or in the final stages, so although there will not be any

symptoms of the disease, they may shed the virus in small numbers

and infects other people.

- Active carriers: a person completely recovered from disease

but still carry the pathogen for a long time, so he still able to

transmit the infection to other people. Their immune system was not

able to completely clear from the pathogen.

- Animals:

So we refer to the infections or diseases obtained from animals

as zoonoses (Plural, its singular is zoonosis).

And we can obtain animal diseases either with direct contact or

indirect contact.Indirect contact: for example food materials or through the bite

of infected insect.

Examples: Rabies, Lyme disease.

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- Arthropods:

They can be either reservoirs of infection, which means that

some of the infections maybe reserved in arthropods in the

environment. Alternatively they can be vectors of the infections, so

they can transmit the infection from the animals to humans. And

they can be reservoirs and vectors at the same time. So they

are considered as the best carriers for the infections.

Examples of arthropod-borne diseases: Lyme disease, Malaria

(reservoirs and vectors at the same time).

- Nonliving reservoirs: Air, soil, dust, contaminated water and foods,

and fomites.

- Fomites: inanimate objects capable of transmitting pathogens

(e.g., bedding, towels, eating and drinking utensils, hospital equipment,

telephones, computer keyboards, etc.) [Slide 8]

Now, we have many methods to transmit the diseases, they can be

direct or indirect methods.

- Direct skin-to-skin contact

- Direct mucous membrane-to-mucous membrane contact by

kissing or sexual intercourse

- Indirect contact via airborne droplets of respiratory secretions,

usually produced by sneezing or coughing.- Indirect contact via food and water contaminated by fecal

matter

- Indirect contact via arthropod vectors

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- Indirect contact via fomites

-Indirect contact via transfusion of contaminated blood or blood

products or by parenteral injection or Using non-sterile syringes or

needles.

[Slide 9]

Public Health Agencies

You have to read this slide, so maybe I would ask about this in the

exam.

So how can a public health agencies such as a WHO prevent and

control the spread of epidemics?

- Increase host resistance through the development and

administration of vaccines that induce active immunity and maintain

it in susceptible persons.

Ensure that persons exposed to a pathogen are protected

against the disease e.g. passive immunization (Premade antibodies

against the pathogen).

Segregate, isolate and treat those who have contracted a

contagious infection to prevent the spread of the pathogen to others.

Identify and control potential reservoirs and vectors of

infectious diseases.

* Bioterrorism and Biological (Warfare Agents): self reading

required.

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Now we finished from part 1, so we are going to go through some

terms in microbial Pathogenesis.

** Lec 14 15 Microbial Pathogenesis and Virulence **

Pathogenicity means the ability to cause disease (Pathogenic

Microbe).

It refers to the steps or mechanisms involved in the development of a

disease.

So, any organism that causes human disease is called a pathogen and

the disease caused by this pathogen is called an infectious disease.

In contrast, the word infection itself does not necessarily mean

development of a disease. For example you maybe get infected with a

certain microbe but it may not develop any disease or symptoms.

However, this distinction is usually not very common in the field, only

microbiologists use it.

(I usually do not use this distinction)

Now, you might get exposed to a pathogen or a microbe, but do not

develop a disease, why?

- The microbe may land at an anatomic site where it is unable to

multiply (for example a respiratory disease microbe lands on your skin)

- Many pathogens must attach to specific receptor sites before theyare able to multiply and cause damage.

- Antibacterial factors may be present at the site where the pathogen

lands.

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- Indigenous microflora of that site may inhibit growth of the foreign

microbe (i.e., microbial antagonism).

- The indigenous microflora may produce antibacterial factors (i.e.,

bacteriocins) that destroy the pathogen.

- The individuals nutritional and overall health status often influences

the outcome of the pathogen-host encounter.

- The person may be immune to that particular pathogen.

- Phagocytes present in the blood may destroy the pathogen.

Now, if you exposed to a particular microbe, it will get through 4

phases:

1 - The incubation period: the Microbes are multiplying in very high

numbers, without any symptoms of the disease. (Incubatory carrier)

2- The prodromal period: the microbe now reaches a high enough

number to cause some damage to the host, and we start experiencing

some vital symptoms of the infection.

3- The period of illness: when you have the characteristic signs and the

symptoms of any disease. Such as fever, vomiting, muscle aches and

so on.

4- The convalescent period: after that, you end up with 3 options, if the

disease is very severe and you survive you might end up with some

tissue damage or organ damage, or if it's for example a common cold

virus (very minor) you will completely recover. and also if it's verysever and the host couldn't survive, he would die.

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Localized Versus Systemic Infections

Localized: Once an infectious process is initiated, the disease may

remain localized; examples of localized infections are pimples, boils

and abscesses.

System: When the infection spreads throughout the body it is said to

have become a systemic or generalized infection; an example is miliarytuberculosis caused by Mycobacterium tuberculosis (it spreads from

the lungs to the tissues of the body).

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Regarding to the duration of the disease, we have acute, chronic and

subacute infections.

Acute disease: is the disease that begins quickly and disappeared

quickly by a rapid recover stage (2 3 weeks).

Examples: measles, mumps, and influenza.

Chronic disease: is a disease that has long time to show symptoms,

so it has a slow onset, and it lasts for a very long time (months or

years). Examples are tuberculosis, leprosy, and syphilis.

Subacute disease: is one that comes on more suddenly than a

chronic disease, but less suddenly than an acute disease; an example

would be bacterial endocarditis.

Also we have a 4th type of disease; it's the latent infection or latent

diseases. In latent disease you first get exposed to the pathogen, some

symptoms of the disease appear, then the symptoms eventually go

away and you will think that you treat the infection and you no longer

have the pathogen in your system, but actually the pathogen enters

the latent stage and now it's hiding somewhere in your tissues. at a

later time in your life, this latent infection will be activate, and now we

will start shedding the virus or the microbe again, and the new

symptoms of the disease start appear.

An example is syphilis; it's a chronic and a latent infection. So when

you exposed to syphilis, then you start suffering symptoms of primarysyphilis, then secondary syphilis, after that the symptoms disappear

and you go through a latent stage for months or years or even a life

time. Then at some point in your life, you might get a reactivation of

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syphilis, and the symptoms of the tertiary stage start appearing (And

do not want you to memorize the contents of these boxes, just now if

you see syphilis, it's an example of the latent infection, and I don't

want you to memorize the stages and so on).

And you also need to differentiate between terms, the symptoms of

disease versus the signs of disease.

so, symptom of disease is the subjective evidence of disease,

something that is not quantified ,it can not be described with numbers,

such as the degree of pain, ringing in the ears, blurred vision, nausea,

dizziness, and so on.

In contrast, the sign of disease is an objective evidence of disease,

something that can be measured in the lab, or in the clinic. Such as

increase in blood pressure, abnormal heart sounds, abnormal pulse

rate, abnormal laboratory results, and so on.

And there's something that I mentioned before, that we have a disease

with symptoms which is called symptomatic disease, and

alternatively we have disease without any symptoms which is called

asymptomatic disease.

I think this is the last slide (finally).

I mentioned this example before, so sometimes; one infection can

commonly follow another infection. When you have this particularscenario, the first infection is a primary infection, and the second

infection is a secondary disease. An example, you might have an upper

respiratory tract infection which is caused by a virus, then the virus will

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damage the ciliated epithelial cells of the respiratory tract, so now you

can get opportunistic bacterial pathogens from the respiratory tract,

leading to the secondary infection (e.g. pneumonia), so the primary

infection will be the viral, the second infection will be the bacterial one.

The end

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shokor 5a9 la kol mn saham fe injaz hatha el3amal :

- Steam Caf

- Semsema Super market

- Mr.Moh'd Sharayri

- Mr.Jehad Tamimi

- Our laptops + 3bood's headphones

- Miss Sara Ibdiwi ( elle jad et7amalt mna kol hata25eer )

W a7la salam lakol shbab edof3a,,5a99a ( Mawkli, Pagera, Asteeka, Anteeka, Barameel &

toto )

w ma bnenker fa9'el "elflafel" 3alaina,,,sbb wjoodna

w ma bnensa a9di8a2na fe (Jam3yt etwa7od) allah yshfeehom

w salamna to our parents in Amman & Aqaba

Laith : mashkoor a5oy w 9adee8e M7mad elRafay3a,,w ya jabal ma yhezzak ree7 ;)

Mohammad: la $okor 3la wajeb 9ade8e laith.....w teslam 3al da3m

Done by :

Laith Jamal

Mo7ammad Rafay3a

Good Luck

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Microbiology, Lecture 13