I1 - Infectious Disease

Chapter I : Infectious Disease

Prepared by Peter Ting

Disease

•  Weakness •  Alters/Loss of function •  Loss good quality of life •  Death


Infectious Disease

•  Involves pathogen •  Transmissible •  Involves suitable environment •  Dormant/Inactive


Outbreak

•  Occurrence in excess than what is expected in a community

•  Can be restricted to a geographical area or extend across countries/continents

•  May last a few days, weeks or even years •  Emergence from a long absent from a

population


What leads to an outbreak? •  Endemic - Present in a community at all times but in

relatively low frequency. Something that is endemic is typically restricted or peculiar to a locality or region. Has high risk of becoming an epidemic. E.g. Dengue fever in Puerto Rico.

•  Epidemic - The occurrence of more cases of a disease than would be expected in a community or region during a given time period. A sudden severe outbreak of a disease such as SARS. E.g. SARS in Hong Kong-China.

•  Pandemic – An epidemic (a sudden outbreak) that becomes very widespread and affects a whole region, a continent, or the world. E.g. H1N1 swine flu


Name of disease

Type of causative organism

Name of causative organism

Method of transmission

Symptoms Drug/Treatment

Eradication/Control

Cholera bacteria Vibrio cholerae

Contaminated water / food Contact through densed population

Acute diarrhea

ORS – to treat symptoms

Effective sanitation

Malaria

protoctist Plasmodium falciparum / vivax

Vector borne, Anopheles mosquitoe

Severe anemia

Antimalarial drug, NOT antibiotic Artemisinin-based combination therapy (more than one drug) – why? Chloroquine (more resistance)

Monitor water reservoir

Tuberculosis bacterium Mycobacterium tuberculosis / bovis

Aerosol through coughing/sneezing Contact through densed population

Cough with sputum and blood

Combination of antibiotics DOTS (directly observed treatment, short course)

Take great care of immune system

AIDS virus HIV Unprotected sex Sharing needle Breastfeeding

Weak against most infections

ART – antiretroviral therapy

Practise safe sex

Name of disease

Type of causative organism

Name of causative organism

Method of transmission

Symptoms Drug/Treatment

Eradication/Control

Cholera

Malaria

Tuberculosis

AIDS


Cholera

Vibrio cholerae •  Bacterium •  Breed in the small intestine •  Secrete a toxin that reduces the ability of the

epithelial cells of the intestine to absorb salts and water into the blood

•  Toxin activates an efflux pump through a cascade of reactions that pumps out ions and water into lumen of intestines

•  Leads to severe loss of fluid through diarrhea. Prepared by Peter Ting

Cholera

Transmission •  Contaminated water / food supply •  Eating raw/uncooked food, esp. seafood,

accumulates the bacterium from seawater.


Cholera

Eradication / Control •  Use clean and safe water •  Ensure the process of preparing and cooking the

food is clean and thorough •  Improve sewage system/treatment •  Education/Awareness •  Strict rules/penalty on food provider •  Oral vaccines •  Clinics and treatment centre must be ready and

available Prepared by Peter Ting

Cholera

Problems with Eradication / Control (seeta – social, economic, environmental and technological implications and applications of biology)

•  Costs issues •  Communities who are poor are still getting

water from polluted source •  Antigenic concealment •  Antigenic variation •  Hard to control pollution – dumping waste

into open sea Prepared by Peter Ting

Cholera

Treatment •  ORS – Oral Rehydration Salt •  Antibiotics


Cholera

Problems with Treatment •  ORS only treats symptoms •  Antibiotic resistance •  Too late to receive treatment.


Malaria

Plasmodium falciparum / vivax •  Protoctist •  Unicellular eukaryote and parasite •  Several life cycle


Malaria

Transmission •  Vector - An organism responsible for the transmission or

spread of a pathogen. In the case of malaria, the female mosquito is the vector as it transmits the Plasmodium parasite from human to human. Sexual reproduction occurs here

•  Host: An organism inside which the reproduction of another organism occurs. Humans act as a host for the malarial parasite. Asexual reproduction occurs here


Malaria Transmission •  Mosquito bites usually occur during feeding. As the

infected mosquito feeds, it injects saliva containing an anticoagulant to prevent the blood from clotting.

•  Malarial parasites known as sporozoites are injected along with the saliva and enter the human bloodstream where they migrate to the liver. The mosquito is therefore said to act as a vector of the malarial parasite.

•  In the liver cells the sporozoites multiply asexually, very rapidly increasing in number.

•  The liver cells burst open releasing many merozoites, which then invade the red blood cells.


Malaria Transmission •  These merozoites reproduce asexually again inside the red blood

cells, causing the red blood cells to burst, releasing more merozoites which cause the characteristic fever and other symptoms of the disease.

•  Some of these merozoites develop into gametocytes (the male and female forms of the parasite) which are ingested by the female mosquito during feeding, and so complete the cycle of transmission between human and mosquito.

•  Once inside the female mosquito’s gut, the male and female gametocytes fuse to form sporozoites which migrate to the salivary glands ready to be injected into a new human host at the next blood meal.



Malaria

Eradication / Control •  Eliminate water drainage •  Spreading oil over water surfaces •  Biological control – Rear fish to eat larvae, •  Use mosquito nets •  Sleeping with animals •  Vaccination •  Herd Immunity Prepared by Peter Ting

Malaria

•  Problem with Control (seeta – social, economic, environmental and technological implications and applications of biology)

•  Mosquito will breed in the tinniest bodies of water •  Insecticides may harm human health and cause

resistance •  Civil war and unrest •  Densed population •  Tropical area/climate •  Antigenic concealment •  Antigenic variation


Malaria

Treatment •  Use antimalarial drugs •  Must be in combination with artemisinin

(most effective) •  Why combination?


Malaria

Problems with Treatment (seeta – social, economic, environmental and technological implications and applications of biology)

•  Antibiotic resistance •  Drug cost •  Not likely to make available to all

communities


Tuberculosis

Mycobacterium tuberculosis / bovis •  rod – shaped bacterium •  Infects the lungs / respiratory tract •  Phagocytes take in by phagocytosis •  Phagocytes line together and forms a small

tissue, granuloma. •  This is seen as initial infection, usually in

children (primary infection) and most of the time, the granuloma resolves and no spread of infection


Tuberculosis

Mycobacterium tuberculosis / bovis •  Secondary tuberculosis: seen mostly in

adults as a reactivation of previous infection (or reinfection), particularly when health status declines. The granulomatous inflammation is much more active and widespread. Typically, the upper lung lobes are most affected, and cavitation can occur.


Tuberculosis

Transmission •  When infected person coughs or sneezes

aerosol / droplets and inhaled by uninfected •  Can also infected by drinking unpasteurized milk •  Infects the lung tissue first (rich in O2) •  Usually stay dormant for years •  If immune system weakens, the bacteria awakes

when lung tissue releases the content of the granuloma


Tuberculosis

Eradication / Control •  Do not go to over-populated places •  Take good care of the immune system •  Quarantine •  Skin testing (before leaving country and before

returning back to country) •  Use of respiratory protective equipment •  Herd immunity •  Vaccination Prepared by Peter Ting

Tuberculosis

Problems with Eradication / Control •  Difficult to control as people are free to move

and choose •  Infection •  Difficult to trace infected people in remote places •  People will not obey and do skin testing •  Negligence to carry protective equipment •  Antigenic concealment •  Antigenic variation Prepared by Peter Ting

Tuberculosis

Treatment •  Combination of four antibiotics - ISONIAZID,

RIFAMPICIN, PYRAZINAMIDE and ETHAMBUTOL

•  Taken in a long period of time


Tuberculosis

Problems with Treatment •  Antibiotic resistance •  Antigenic variation


AIDS

Human Immunodeficiency Virus (HIV) •  Retrovirus •  Binds to CD4 receptors on t-helper cells •  Lead to weakened immune system and

many other opportunistic infections


AIDS

Human Immunodeficiency Virus (HIV)

1.  The glycoprotein on the outer surface of the virus attaches to receptors on the host cell 2.  The lipid bilayer of the virus then fuses with the host cell’s membrane and the virus

enters the host cell 3.  The viral enzyme reverse transcriptase stimulates the cell to make viral DNA from the

viral RNA template 4.  This viral DNA is then inserted into the host cell's chromosomes 5.  The viral DNA then codes for the production of thousands of new viruses 6.  Eventually, these burst out of the cell, killing it 7.  These viruses then infect other cells 8.  These include helper T – cells. These are part of the immune system. The immune

system is weakened and opportunistic infections result 9.  Thus, AIDS is not a single disease, but a descriptive term for the opportunistic infections 10. People who develop AIDS often die from Karposi’s sarcoma, a rare cancer itself caused

by a virus.



AIDS

Transmission •  Through sexual contact -- including oral, vaginal,

and anal sex •  Through blood -- through blood transfusions,

accidental needlesticks, or needle sharing •  From mother to child -- a pregnant woman can

transmit the virus to her fetus through their shared blood circulation, or a nursing mother can pass it to her baby in her breast milk


AIDS

Eradication / Control •  Avoid drugs •  Avoid unprotected sexual activities •  Avoid donating blood (+ve person) •  Trace contact •  Strict communities/families •  Education/Awareness •  Usage of sterile needles •  Avoid promiscuous relationships Prepared by Peter Ting

AIDS

Problems with Eradication / Control •  Tracking can be expensive and difficult •  Asymptomatic for the first 10 years – long

incubation time •  Community practices open culture •  Family disruption lead to rebellious behavior •  People are just too ignorant about safe sex and

good education


AIDS

Treatment •  Antiretroviral therapy •  Check CD4 cell count


AIDS

Problems with Treatment •  Antigenic variation •  Drug resistance


Antibiotics •  Chemicals produced by microorganism

which are capable of destroying or inhibiting the growth of other microorganisms.


•  They can be synthetic (isoniazid) or derived from living organisms.

•  Antibiotics are selective toxins, killing or disabling the pathogen without harming the host.


Antibiotic’s action

mechanism

Synthesis of bacterial walls

Plasma membrane

function

Protein synthesis

Enzyme function

•  Antibiotics interfere with some aspects of growth or metabolism of the target microorganism:


The sites of action of antibiotics in bacteria Prepared by Peter Ting

•  Different diseases are treated with different antibiotics.

•  Some kinds of bacteria are completely resistant to particular antibiotics

•  E.g. Mycobacterium tuberculosis is resistant to penicillins

•  Other bacteria have certain strains that are resistant.


Broad spectrum antibiotics •  Antibiotics that are effective against a wide

range of bacteria.

•  Narrow spectrum antibiotics •  Antibiotics that are active only against a

few.


Penicillin

•  In 1928, bacteriologist Alexander Fleming made a chance discovery from an already discarded, contaminated Petri dish.

•  The mold that had contaminated the experiment turned out to contain a powerful antibiotic, penicillin.

•  Though Fleming was credited with the discovery, it was over a decade before someone else turned penicillin into the miracle drug for the 20th century.


Penicillin

•  Penicillin is a group of antibiotics derived from Penicillium fungi.



•  Bacteria that attempt to divide in the presence of penicillin fail to do so and end up shedding their cell walls in the process.


•  Penicillin and other β-lactam antibiotics act by inhibiting penicillin-binding proteins, which normally catalyze cross-linking of bacterial cell walls.


Penicillin’s Mode of Action

•  Penicillins functions by preventing the synthesis of the cross links between the peptidoglycan polymers in the cell walls of bacteria.

•  They are only active against bacteria and only when they are growing.

•  Many types of bacteria have enzymes for destroying penicillins (penicillinase) and are therefore resistant to these antibiotics.


•  Result of an antibiotic sensitivity test carried out on a pathogenic strain of the human gut bacterium Escherichia coli. Various antibiotics are absorbed onto discs of filter paper and placed on the agar plate.

•  The most effective antibiotics are chosen based on the diameter of the inhibition zones.



•  Antibiotics should be chosen carefully •  Screening antibiotics against the strain of

the bacterium or fungus isolated from sufferers ensures that most of the effective antibiotics can be used in treatment.


•  An example of this is MRSA, (Methicillin Resistant Staphylococcus aureus) which is a bacteria that is resistant to four of the most popular antibiotics due to their inappropriate use and people not finishing their course of antibiotics – leaving antibiotic resistant bacteria to grow and spread.


•  It is advisable to keep some antibiotics for use as the last resort when everything else has failed to lessen chances of more such resistant organisms.



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I1 - Infectious Disease