INFECTIOUS DISEASE

EPIDEMIOLOGY

Maj Fawad

Topics to be covered in this lecture

Definitions Dynamics of Infectious Disease

Transmission Control of Infection

1. Disease• Manifest / Overt• Inapparent / Asymptomatic/

Subclinical / Covert• Latent• Colonization

3. Contamination4. Pollution

2. Communicable disease / Contagious disease

Terms

Infection

Bacteria

Virus

Fungi

Parasite

Infection

The entry and development or multiplication of an infection agent in body of man or animal is called infection. The infectious agent can be virus, bacteria, fungi, parasite etc.

1. Disease (infection)

This results from the infectious agent when it produces signs and symptoms.

1.1.Manifest

Synonymous with disease.

1.2 .Inapparent, asymptomatic or sub- clinical infection Where infectious agent fails to produce signs and symptoms but infectious agent survives and multiplies in the body of man or animal and is capable of being transmitted to another susceptible individual or animal..

1.3 .Latent

When infection is present in the body but does not Produce any sign and symptoms and at the same timecannot be transmitted to a susceptible host during the period of latency i.e. Herpes simplex, Syphilis.

Latent period:This period is the interval of subclinical

infection during which the previously active infectious agent becomes dormant in the host.

Example:Subsequent to the appearance of genital

lesions induced by a herpes simplex type II infection, patients often experience period of remission, after which reactivation of the virus elicits the reappearance of lesions. This interval of remission is referred to as the latent period of the virus.

1.4 Colonization

In the case of Commensal or low grade pathogen, the multiplication may cause little or no harm to the host and may best be described as colonization.

2. Communicable disease / Contagious Disease:

An illness due to a specific infectious agent or its toxic products. This arises through transmission of that agent or its products from an infected person, animal, or inanimate reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment.

Reservoir(Man, animal,

Inanimate)

SusceptibleHost

(Man, animal)

Direct

Vehicle

Vector

Inanimate

Intermediate host

Spread of Communicable disease

Direct:______Indirect:………

Indirect

Airborne

Mode of transmission

Direct:

1. Close physical contact e.g. sexually transmitted disease (AIDS) ,skin to

skin contact (leprosy):2. Droplet infections3. Contact with the soil4. Vertical transmission

Indirect :

1. Vehicle-borne : These infection are transmitted through

the agency of water, food, ice, blood, serum, plasma, and other biological products e.g. tissues and organs.

2. Vector –borne :These infection are transmitted by an

arthropod or a living invertebrate carrier such as snails or Cyclops.

Indirect (contd.)

3. Air –borne: (Droplet Nuclei & Dust )

4. Fomite – borne:fomites refer to inanimate objects such as handkerchiefs, bed linen, towels, books, spoons, forks, etc., which have been soiled with infective material. Freshly soiled fomites are likely to spread the agents of infection.

3. Contamination:

This refers to the presence of organic material or micro – organisms on the body surfaces, articles or inanimate object.

4. Pollution:

Pollution refers to the presence of both inorganic and organic matter such as offensive material like trade effluvia or industrial pollutants.

Reservoirs of infection

patients carriers

Living Non living

ArthropodsBirdsAnimalsHuman beings

soil substances

Reservoir

Any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent normally lives and multiplies, on which it depends primarily for a prolonged survival and where it reproduces itself in such a manner that it can be transmitted to a susceptible host.

Human reservoirs

Patients Carriers

A person or animal that harbours a specific infectious agent in the absence of discernable clinical disease and serves as a potential source of infection.

CarrierCarrier

Based on the duration

Based on the portal of exit of infection

Based on the stage of disease

Classification of carriers

1.Based on the stage of disease

a. Incubatory carriers e.g. Hepatitis

b. Convalescent carrier e.g. typhoid

c. Healthy carrier e.g. Typhoid

a. Incubatory CarrierIt is that type of the carrier which transmits micro-organism during incubation period of the disease.

b. Healthy Carrier.An individual with an infection that is in- apparent throughout its course but can transmit infection to others, is commonly known as a healthy carrier.

c. Convalescent carrier

This carrier transmits micro-organisms during the period of convalescence or recovery.

2. Based on the portal of exit of infection

a. Nasal e.g. Streptococcalb. Oral e.g. Meningococcalc. Urinary tract e.g. Typhoidd. Intestinal e.g. Cholerae. Genital e.g. HIV

3. Based on the durationa. Temporary (duration<12 months) carrierb. Chronic (duration> 12 months) carrier

Host

Types

of host

Immunity Resistance

Susceptible

Host

A person or another living animal including birds and arthropods that affords subsistence and lodgment to an infectious agent under natural conditions.

A. Types of Host1. Definitive Host.

Hosts in which parasite attains maturity or passes its sexual stage, are primary or definitive hosts.

2. Intermediate Host:Hosts in which the parasite is in larval orasexual stage, are secondary or intermediate Hosts.

3. Propagative Host:A transport or propagative host, is the one in which, the organism remains but does not undergo development.

4. Obligate Host:This term refers to the only host of infections such as man in measles

Host parasite relationship(Symbiosis)

Parasitism Harmful

Mutualism Beneficial

Commensalisms Neither beneficial Nor harmful

Mutualism:It is a type of symbiosis in which there is reciprocal benefit to both the host and parasite.

Commensalisms: It is a relationship where the host gives the food and shelter to the parasite. It can either move to the mutualism or parasitism. It is neither good nor bad.

Parasitism:It is of unilateral benefit to the parasite only and harmful for the host.

Phases in infections

Infective period Communicability period Incubation period External Incubation period Prodromal Period Convalescence period Generation time Serial interval

Communicable period/Infective period:

The period during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected person to an animal, including arthropods.

Incubation Period

The time interval between initial contact with an infectious agent and the appearance of first symptom of the disease in question, or in a vector, if The first time transmission is possible (Extrinsic incubation period).Example:

After exposure to the measles virus and before the onset of symptoms, a child is considered to be highly contagious due to continued attendance at school and social encounters with other children.

Importance of Incubation Period:

1. Quarantine regulations2. Tracing the source of infection3. Determining the type of epidemic4. For immunization, active as well as

passive5. Predicting prognosis

Prodromal period:It is the interval between the onset of symptoms & appearance of characteristic clinical manifestations, for example as in measles, the interval between the onset of symptoms such as fever and coryza and appearance of rash.

Convalescent Period

This is the period of recovery when the clinical symptoms have subsided.

Generation timeThe interval of time between receipt of infection by a host and maximum communicability / infectivity of that host.

Serial IntervalThe gap in time between the onset of primary case and the secondary case is called the serial interval.

Disease spread

Source

Contact

Suspect

Reservoir

Fomites

Source:The person, animal, object or substance from which an infectious agent passes to a host.

Contact:A person or animal that has been in an association with an infected person or animal or a contaminated environment and might provide an opportunity to acquire the infective agent.

Suspect:A person whose medical history and symptoms suggest that he or she may have or be developing some communicable disease.

Fomites:It refers to inanimate

object such as handkerchiefs, bed lines towels, books, spoons, forks, etc. Which have been soiled with infective material. Freshly soiled fomites are likely to spread the agents of infection.

Opportunistic

Nosocomial

Cross-infection

Infestation

Epizootic EnzooticZoonosis

Exotic

Infections

Nosocomial infection:An infection occurring in a patient in a hospital or other health-care facility and in whom it was not present or incubating at the time of admission or residual of an infection during the period of previous admission. It includes infection (unrelated to the primary condition) acquired in the hospital but appearing either before or after discharge and also such infection among the staff of the facility. Various types of the nosocomial infections may be.

1.Exogenous: Which are acquired from contaminated hospital environment air, water, food, equipment, infected staff, infectious patients, etc.)

2.Endogenous:Which are acquired from patient’s normal

flora……

Nosocomial infection:

Before admission Hospital stay After Discharge

3

Case#1 Not nosocomial

infection

Case #2 &3Nosocomial

infection

12

Infection

symptoms

Cross infection:This is the infection acquired by an admitted patient from the other patients during the period of hospitalization.

Infestation:For persons or animal, the lodgment,

development and reproduction of arthropods on the surface of the body or in the clothing. Infested articles or premises are those which harbor or give shelter to animal forms, especially arthropods and rodents.

Zoonosis:It is an infection or infectious disease transmissible under natural conditions from vertebrate animal to man. It may be:

1.Enzootic:It is identical to endemic in man.

2.Epizootic: It is identical to epidemic. Outbreaks that affect

large number of animals are referred to as epizootic.

Exotic:It is an infection that is not usually present in the given country but introduced from abroad.

Iatrogenic:It is an adverse

consequence of a preventive, diagnostic or therapeutic regimen or procedure..

Opportunistic:An infection caused by the micro-organism which

are not normally pathogenic but can assume pathogenecity, when the immune status is compromised.

Distribution.

Pandemic

Endemic

1 2

3 4

Epidemic

Sporadic

1.Pandemic:Global occurrence of a

disease.Pandemics are widespread

epidemics that achieve large geographic proportions.Example :during the influenza pandemic of the 19th century, millions of people across the continents were affected. Today, AIDS is considered to be the most alarming pandemic of the century.

2.Endemic:The constant presence of a disease or

infectious agent within a given geographic area, may also refer to the usual prevalence of a given disease within such area.

3.Sporadic :it refers to the scattered presence of the disease in the given population (a case here and a case there). This indicates that the number of the susceptible is less than the number of resistant in the population

4.Epidemic :This is occurrence, in a community or region, of

cases of an illness (an outbreak) clearly in excess of expected occurrence. This indicates that the number of susceptible is more than the resistant in the community.

Epidemic are classified as follows:

1. Common source epidemics2. Point source epidemics3. Propagative (progressive) Epidemics

.

Disease control

1. Surveillance 2. Eradication3. Control4. Quarantine5. Segregation6. Isolation7. Disinfections8. sterilization

Surveillance:Surveillance of disease is the continuous scrutiny of all aspects of occurrence and spread of disease / other condition of ill health that are pertinent to effective control.

Isolation:As applied to patients, it represents separation for the period of communicability to prevent or limit the direct or indirect transmission of the infectious agent from those infected to those who are susceptible or who may spread the disease to others.

Segregation:This refers to the separation of a healthy person from an infected person.

Quarantine:Restriction of the activities of well person or animals who have been exposed to a case of communicable disease during its period of communicability (i.e. contacts) for maximum incubation period to prevent disease transmission during the incubation period, if the infection should occur.

Eradication:It means ending the transmission of the disease and elimination of reservoirs of infection, in a campaign limited in time and carried out to such a degree of perfection that when it comes to an end, there is no resumption of transmission.

Control :It means to reduce the prevalence of the disease to such a low level that it no longer remains a big public health problem.

Disinfection:It refers to the destruction of the pathogenic micro- organisms.

Sterilization:It refers to the destruction of pathogenic as well as non-pathogenic microorganisms and it includes destruction of spores also.

a. Chain of infection

Infectious agent Reservoir / source of infection Exit of infectious agent Transmission (mode) of infection Entry into a new host Host susceptibility

Summary of disease transmission and control

Infectious Agent:

An organism (viruses, rickettsia, bacteria, fungus, protozoa or helminthes) that is capable of producing infection or infectious disease.

Exit of infectious agent

Avenues of escape are:1. Respiratory tract-droplet nuclei (continuous emission)

2. Intestinal tract-discharge of faeces (discontinuous emission)

3. Urinary tract.

4. Open wounds (escape from the lesions on to clothes)

5. Mechanical escape (biting or sucking insects e.g. malaria spread by mosquitoes)

6. Contact (close physical contact)

Transfer of infection to new host:Direct transmission (person to person)Indirect transmission (requires a vehicle)

Classification of vehicle:Animate refers to as vectors e.g. housefly,

flea. Mosquito.Inanimate-water, milk, food, air soil and fomites (e.g. clothes, door knobs, moneyetc.), blood transfusion.

Entry into new susceptible host

Portals of entry are:• Respiratory tract- contaminated food or

drink.• Gastro-intestinal tract- contaminated food

or drink.• Direct infection of membranes e.g.

diphtheria, venereal diseases.• Percutaneous infection passage through

skin via bite, for rabies and malaria, direct penetration by infectious agent e.g. schistosomiasis, hookworm.

A person or animal presumably not possessing sufficient resistance against a particular pathogenic agent to prevent contracting infection or disease if or when exposed to the agent.

Host susceptibility:

b. Chain of control

1.Notification2.Early diagnosis3.Isolation4.Destruction of infecting agent5.Investigation of an attack of illness6.Immunization7.Health education

Chain of control Notification:It means the immediate intimation of the

occurrence of every case of infectious disease to the healthy officers.

Early diagnosis:The first step in the control of a communicable

disease is its rapid identification for..a) The treatment of patientsb) For epidemiological investigationsc) To study the time, place and person

distribution.

Isolation:Separation for the period of communicability of infected person or animals from others in such places and in such conditions as to prevent or limit the direct or indirect transmission of infectious agent from those infected to those who are susceptible or who may spread the agent to others.

Destruction of the infecting agent (disinfection)

Destruction of pathogenic microorganism is called disinfection.

Investigation of an attack of an illnessBroadly the investigation covers the identification of the source of infection and of the factors influencing its spread in the community.

Immunization:Increasing the resistance of the susceptible host.

Health education:Health education is the process by which individuals and groups of people learn to behave in a manner conducive to the promotion, maintenance or restoration of health