ByProf: Eman Darwish

Community Medicine Department

A model used to understand the infection process is the chain (cycle) of infection , a circle of links, each representing a component in the cycle. Each link must be present and in sequential order for an infection to occur.

Infectious process

Microbiological Agents A pathogen that is capable of producing an infection.

Viruses: Influenza, Mumps, chicken pox, poliomyelitis

Bacteria: Cocci (strep., Staph., diplo), Bacilli (diphtheria, salmonella), Vibrio (Classical, Eltor).

Rickettsia: Typhus group of fever Fungi: candida, Aspergillus Protozoa: Entamaeba, Plasmodia,

Leishmania Parasites: Shistosoma, Ancylostoma

Mechanisms of disease production

Invasiveness:

The ability of the organisms to invade the tissues and multiply. E.g: Treponema pallidum, Typhoid and paratyphoid.

ExotoxinIndotoxin

-Released by living organisms

-Destroyed rapidly by heating

60oC

-Highly immunogenic

-Converted to antigenic, non-toxic toxoid

-Not produce fever

- E.g. Neurotoxin of Tetanus, Botulism

Erythrogenic toxin of Scarlet fever

Enterotoxin of staph. Food poisoning

-Released after disintegration of the organisms

-highly stable (withstand heating

60oC

-Weekly immunogenic

-Not converted to toxoid

-Usually produce pathophysiologic effects as fever, hypotension, hypoglycaemia, shock

Toxicity

It is an allergic state of the host following exposure to certain antigens of microorganisms (e.g. Mycobacterium tuberculosis and Mycobacterium leprae)

Hypersensitivity

Factors affecting disease production in relation to agents

Pathogenecity and Virulence Antigenic power Period and ease of communicability Dose of infection (inoculums) Tissue selectivity (Tropism) Host specificity Spore formation Resistance of the organism Susceptibility to chemotherapy

Pathogenicity: is the ability of the organisms to produce specific clinical reaction after infection has occurred, however, it does not refer to severity of reaction.

It can be measured by :Ratio of clinical to subclinical

cases = Clinical cases Subclinical cases

Virulence: Is the ability of organisms to produce severe pathological reaction, it refers to severity of the reaction.

It can be measured by :Case fatality rate =

No. of deaths due to particular disease in an area and yearX100

Total no. of cases of this particular disease in the same area and year

Antigenic power of the microorganisms

The ability to initiate the development of antibodies or antitoxin and associated immunity.

It can be measured by:- Second attack frequency: In certain diseases

second attacks are rarely recorded as in case of measles, mumps and Chicken pox. In other diseases re-infection occurs as in case of common cold, upper respiratory infections, syphilis and gonorrhea.

Age specific attack rate: In diseases caused by microorganisms of high antigenic power as measles there is a drop of the attack rate after young age.

Period and ease of communicability

It can be measured by: secondary attack rate, which is the

number of secondary cases, occurring within the accepted incubation period following exposure to a primary case, calculated as a percentage of the number of exposed susceptible.

Dose of infection ( inoculums ): The higher the dose of infection, the

more liability of having an apparent illness, and the severer will be the diseases.

Tissue selectivity ( tropism ): Is the inherent capacity of the pathogen to invade some particular tissues. This is the factor that gives each disease its characteristic symptoms and signs.

Host specificity: Some pathogens infect man only as relapsing fever, other infect animals only, while some others infect both man and animal such as zoonotic diseases.

Spore formation: The ability of some bacteria to change to a resistant form under unsuitable conditions and these spores remain viable for long periods. When these spores get the chance of coming into contact with a susceptible host-under favorable conditions-they change to vegetative forms, they cause the disease as in case of tetanus and anthrax.

Viability of the organism ( the resistance of the organism ):

Is the ability to live outside the body, the longer the duration, the more the chance to come into contact with a new host transmitting the infection to them.

Susceptibility of the pathogen to chemotherapy: The degree of sensitivity to antibiotics differs from one pathogen to the other and even from one strain of the same pathogen to another.

Reservoir of infection

A place within which microorganisms can survive, grow and reproduce and from which infection is transmitted to the susceptible host. For example, microorganisms thrive in

human beings (Most common) animals, plants and inanimate objects such as soil (Tetanus,

anthrax, ancylostomiasis), water, table tops, and doorknobs.

Reservoir of Infection

Man Animal

Case Carrier Zoonotic

Diseases Through:

*Animal Products

*Direct Contact *Arthropods

Human reservoirs

Cases: Clinical-in apparent=sub clinical

Inapprent infection: The presence of infection without recognizable clinical S&S

Identified by Lab. Or positive skin tests

From the public point of view: sub clinical cases are MORE IMPORTANT than clinical cases as spread of infection is unrecognized.

Carriers A person or an animal who harbors the

infectious agents and is apparently free from clinical manifestations of the disease. He serves as a potential source of infection.

Carriers are dangerous because: 1. They don’t show any clinical manifestations2. The carrier and his contacts are not aware of

their condition3. It is difficult to discover them4. It is not always possible to deal with them

Classification of carriers

PlaceDurationSpectrum

Upper respiratory

FecalUrinary

SkinBiliary

TransientTemporary(+)

ChronicPermanent

IncubatoryContact

Convalescent

Animal Reservoirs

Zoonotic diseases:

An infectious disease transmissible under natural conditions from vertebrate animals to man. E.g.

DiseaseAnimal reservoir

Bovine TBBrucellosis

RabiesPlague

Yellow feverSalmonella

CattleGoatsDogs

RodentsMonkeys

Mice, rodents,

ducks, cows

Portal of Exit

A place of exit providing a way for a microorganism to leave the reservoir. For example:

Respiratory tract Gastrointestinal tract Urinary tract Skin and mucous membrane Insect bite Syringes and taking blood from donors Transplacental

Mode of transmission

Method of transfer by which the organism moves or is carried from one place to another. The hands of the health care worker may carry bacteria from one person to another. It is of great importance for prevention and control of diseases.

1- Contact transmission Direct (Scabies, Venereal diseases) Indirect (Surgical wounds) Droplet (Measles, Meningitis) Perinatal (Tran placental) Syphilis, AIDS

2- Common vehicle transmission Ingestion (Typhoid, food poisoning) Inoculation (Viral hepatitis, malaria) Deposition (Swimming pool diseases)

3 -Vector transmission

Mechanical - Direct (purulent conjunctivitis) - Indirect (Typhoid, Dysentery) Biological - Propagative (plague, yellow fever) - Cyclopropagative (malaria) - Cyclodevelopmental (filaria)

4- Air borne transmission Droplet nuclei Dust nuclei (TB, Streptococci)

PORTAL OF ENTRY

An opening allowing the microorganism to enter the host. Portals include

body orifices, mucus membranes, or breaks in the skin. Portals also result from tubes placed in body

cavities, such as urinary catheters, or from punctures produced by invasive procedures such as intravenous fluid replacement.

Incubation period (IP)

The time interval between exposure to an infectious agents and appearance of the first sign and symptom of the disease.

Types: Min-Max, Extrinsic-intrinsic, International

Length of IP depends on: - Agent (Organism-Toxin)- Virulence of the organism- Dose of infection- Host resistance

Knowledge of IP is important to:

Surveillance and isolation of contacts

Apply preventive measures (Measles vaccine)

Identify the source of infection (Typhoid)

SUSCEPTIBLE HOST

A person who cannot resist a microorganism invading the body, multiplying, and resulting in infection.

The host is susceptible to the disease, lacking immunity or physical resistance to overcome the invasion by the pathogenic microorganism.

Resistance

Total body mechanisms which act as barriers to invasion or

multiplication of infectious agents or their damaging effects of their toxins.

Types of resistance Natural barriers (inhere ant, non-

specific) Acquired immunity (specific)

Natural barriersDefensive barriers include

the skin (mechanical protection, decontamination by shedding of outer flakes and addition of glandular secretions, and maintenance of normal flora),

Lining of mouth and pharynx (mechanical barrier, decontamination by saliva, lymphoid tissue such as tonsils, and normal flora),

Gastro-intestinal tract (acidity of gastric environment, peristaltic movements, mucus, antibodies in intestinal secretions, lymphoid tissue, presence of normal flora and fauna in large intestine),

Airways (mucus secretions moved by ciliated lining, antibodies in secretions, macrophages),

Urinary tract (mechanical barrier, movement of urine), Conjunctiva of eye (mechanical, flow of tears containing

bacteriostatic substances), and Vagina (mechanical, normal flora).

Acquired resistance

Passive Active

Natural Artificial Natural Artificial

Ideal immunizing agent:

Antigenically stable Give durable immunity Have minimal side effects Easy in administration Of reasonable cost Available Keeping quality is good

Types of vaccines

I. Live attenuated vaccines:A) Attenuated vaccine: e.g. Sabin vaccine of polio, measles,

German measles and mumps vaccines. B) Variant forms of living organisms vaccine: smallpox vaccine

using cow pox virus and BCG vaccine using bovine tubercle bacilli.

II. Non living vaccines:-A) Killed or inactivated: - Killed bacterial organisms Typhoid, whooping cough vaccines.- Inactivated virus as Salk vaccine of polio virus.(B) Products of organisms (Toxoid):Diphtheria and tetanus toxoid.(C) Part of organisms:- The subunit of hepatitis B surface antigen (HBsAg) prepared from

plasma of HBsAg positive carriers or by genetic engineering.-Part of polysaccharide capsule of Nisseria meningitides used as a

vaccine against meningeococcal meningitis.

Herd immunity:

It is the state of immunity within the community.

The factor that decides the epidemiological pattern of any infectious disease among that community.

The incidence of a disease rises at times when the number of susceptibles in the population is highest and the herd immunity is lowest.

The best example is measles epidemic in Fidjii in 1975 in which the attack rat approached 100% and fatality rate was excessively high.

The herd immunity could be produced artificially by immunization, or naturally after infection.

Factors play a role on community protection namely:

1. The extent of coverage of the immunization program.

2. The degree of resistance to infection afforded by the vaccine

3. Duration and degree of infectivity of the organism.

4. Past experience with different infections.

5. Overcrowding and environmental sanitation.

Levels of prevention

Primary prevention. Secondary prevention. Tertiary prevention

(Rehabilitation).

Examples of primary prevention:

Health education Environmental sanitation Nutritional interventionsSpecific protection Immunization Chemoprophylaxis Use of specific nutrients Protection from carcinogens and allergens Protection against accidents Protection against occupational hazards

An example of secondary prevention

Mass treatment: an approach is used to control of certain diseases as trachoma, malaria, bilhariziasis

It is a critical factor in the interruption of disease transmission

Examples of Rehabilitation

Special schools for blind pupils Provision of aids for crippled Reconstructive surgery for leprotics Occupational rehabilitation Modification of life for TB or cardiac

patients

Control measures

1- Measures applied to the agent: sterilization, disinfection

2- Measures applied to the reservoir of infection

a) cases: Case finding and early detection Reporting Isolation Treatment (Chemotherapy)

Physical methods of disinfection and sterilization

moist heat (boiling 10-15 minutes) pasteurization (63 C for 30 minutes

or 72 C for 15 seconds) sterilization with steam ( 121 C for 20

minutes @ 15 lbs/sq in) freezing ultraviolet radiation ionizing radiation ultrasound filtration

Examples of chemical disinfectants

Phenolic compounds: Lysol (phenol + ethanol) 

Hypochlorites: Clorox, household bleach (5.25% sodium hypochlorite)

Halogens: Betadine, Formaldehyde (37% solution)

b) Measures applied to carriers

Detection (Proportion in the community, occupation)

Exclusion from job Treatment

c) Measures applied to animal reservoir Killing (rabies) Slaughtering (bovine TB) Immunization (brucellosis) Careful husbandry and sterilization of animal products (anthrax)

3 -Measures applied to contacts

Enlistment Surveillance Isolation Increasing resistance Active immunization (measles) Passive immunization (diphtheria) Chemoprophylaxis (INH, Rifampicin,

tetracycline, penicillin)

4 -Measures applied to the environment

Environmental sanitation Chlorination of water Sanitary refuse and sewage disposal Rodent and vector control Food sanitation

5 -measures applied to the host

Non-specific Health education Good nutrition Provision of adequate housing Provision of adequate recreation facilities Provision of suitable working conditions Periodic selective examination Personal hygiene Protection against accidents

Specific Measures

Passive and active immunization chemoprophylaxis