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Introduction to

Communicable diseases

Dr.Rajkumar PatilProfessor, Community Medicine, MGMCRI

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Learning objectives

At the end of the class student should be able to:• Define communicable disease• Describe the basic differences between communicable and non-communicable diseases• Enumerate the modes of transmission of communicable

disease

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Definitions

• Infection• Contamination and Pollution• Infestation• Communicable disease• Infectious disease• Contagious disease

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INFECTION• “The entry and development or multiplication of an infectious agent in the body of man or animals”

• An infection does not always cause illness.

• Levels: a. Colonization(e.g.- S. aureus in skin & normal nasopharynx)b. Subclinical or inapparent infection (e.g.- polio)c. Latent infection (e.g.- virus of herpes simplex)

d. Manifest or clinical infection.

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CONTAMINATION AND POLLUTION

• “The presence of an infectious agent on a body surface; also on or in clothes, beddings, toys, surgical instruments or dressings, or other inanimate articles or substances including water, milk and food”

• Contamination on a body surface does not imply a carrier state.

• Pollution is distinct from it & implies the presence of offensive, but not necessarily infectious matter.

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INFESTATION

• “For persons or animals the lodgement, development and reproduction of arthropods on the surface of the body or in the clothing, e.g., lice, itch mite”

• Invasion of the gut by parasitic worms, e.g., ascariasis.

• Infested premises: Those which harbour or give shelter to animal forms, especially arthropods and rodents.

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INFECTIOUS DISEASE

“A clinically manifest disease of man or animals resulting from an infection”

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COMMUNICABLE DISEASE

“An illness due to a specific infectious agent or its toxic products capable of being directly or

indirectly transmitted from man to man, animal to animal, or from the environment (through air, dust,

soil, water, food, etc.) to man or animal”.

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Communicable diseases

• Respiratory Infections• Intestinal Infections• Arthropod borne infections• Zoonotic diseases(Rabies,Leptospirosis,JE etc.)• Surface infections(Trachoma,Leporsy,STD etc.)• Hospital acquired infection

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What are the basic differences between Communicable and Non-communicable diseases?

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CONTAGIOUS DISEASE

“A disease that is transmitted through contact”. Examples: scabies, trachoma, STD and leprosy.

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MODES OF DISEASE TRANSMISSION

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A) DIRECT TRANSMISSION

• Direct contact • Droplet infection

• Contact with soil

• Inoculation-skin or mucosa

• Transplacental (vertical)

B) INDIRECT TRANSMISSION

• Vehicle-borne

• Vector-borne a. Mechanical b. Biological

• Air-borne Droplet nuclei Dust

• Fomite -borne

• Unclean hands & fingers

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DIRECT TRANSMISSION

(1) Direct contact

• Skin - skin, mucosa - mucosa, or mucosa-skin of the same or another person.

• From the reservoir or source to a susceptible individual, without an intermediate agency

• Examples: skin-to-skin contact as by touching, kissing or sexual intercourse

• Diseases transmitted - STD and AIDS, leprosy, leptospirosis, skin and eye infections.

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(2) Droplet infection

• Direct projection of a spray of droplets of saliva & naso-pharyngeal secretions during coughing, sneezing, or speaking, talking.

• Examples: Resp. infection, eruptive fevers, TB, meningococcal meningitis, etc.

• Upon the conjunctiva, oro-respiratory mucosa

• Particles: 5 micrometers & less reach the alveoli. • Limited to distance of 1-2 ft between source & host.

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(3) Contact with soil

• Direct exposure of susceptible tissue to the disease agent in soil, compost or decaying vegetable matter.

• Examples: hookworm larvae, tetanus, mycosis etc.

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Hookworm

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(4) Inoculation into skin or mucosa

• Rabies virus by dog bite, Hep. B virus through contaminated needles and syringes etc.

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(5) Transplacental (or vertical) transmission

• TORCH agents, varicella virus, syphilis, Hep-B, Coxsackie B and AIDS.

• Some of the non-living agents (e.g., thalidomide, diethylstilbestrol) can also be transmitted vertically.

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Indirect transmission

• Traditional 5 F's - "flies, fingers, fomites, food and fluid".

• Infectious agent survives outside retaining its basic properties of pathogenesis & virulence.

•Infectivity (ability to infect)- (number infected / number susceptible) x 100•Pathogenicity (ability to cause disease)-(number with clinical disease / number infected) x 100•Virulence (ability to cause death) -(number of deaths / number with disease) x 100All are dependent on host factors

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Fomites

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Vehicle-borne • water, • food, • raw vegetables and fruits, • milk, • ice, • blood

Food poisoning example:In order to grow and multiply germs need

HeatFoodMoistureTime

Remember it like thisToo Many Flies Waiting

These bacterial cells are beginning to divide into two

After 10 minutesAfter 20 minutesAfter 30 minutesAfter 40 minutes

Time : 9.30 Bacteria : 0 Time : 9.40Time : 9.50

Time : 10.00

Bacteria : 12,000Bacteria : 24,000Bacteria : 48,000

Time : 10.10Time : 10.20Time : 10.30Time : 10.40Time : 10.50

Bacteria : 96,000Bacteria : 192,000Bacteria : 384,000Bacteria : 768,000Bacteria : 1.5 million

From 0 to 1,536,000 in only 80 minutes !!!!!!

Knife contaminated by blood

cooking chicken to a core temperature of 75°C should kill most of the bacteria

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Epidemiological features of vehicle transmission

• Heavy dose of contamination – explosive outbreak e.g., cholera & Hep-A epidemics • When sec. cases occur, primary may be obscured • Distance travelled may be great, e.g., food poisoning • Its not always possible to isolate the infectious agent in the vehicle, e.g., typhoid bacilli in contaminated water • When the vehicle is controlled or withdrawn, the epidemic subsides, e.g., epidemics of cholera• The common source of infection is often traceable

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Vector-borne

• “vector is defined as an arthropod or any living carrier (e.g., snail) that transports an infectious agent to a susceptible individual”. • Transmission -mechanical or biological.

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Epidemiological classification of vector-borne diseases

I. By vector : Invertebrate type : Arthropod vectors fall into seven orders largely (1) Diptera - flies and mosquitoe (2) Siphonaptera - fleas (3) Orthoptera - cockroaches (4) Anoplura - sucking lice (5) Hemiptera - Bugs, including kissing bugs (6) Acarina - ticks and mites (7) Copepoda - cyclops

Vertebrate type: Mice, rodents, bats

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II. By transmission chaina) Man and a non-vertebrate host

1. Man-arthropod-man (malaria) 2. Man-snail-man (schistosomiasis)

b) Man, another vertebrate host, and a non-vertebrate host 1. Mammal-arthropod-man (plague) 2. Bird-arthropod-man (encephalitis)

c) Man and 2 intermediate hosts

1. Man-cyclops-fish-man (fish tape worm) 2. Man-snail-fish-man (Clonorchis sinensis) 3. Man-snail-crab-man (Paragonimiasis)

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III. By methods in which vectors transmit agent:

• Biting • Regurgitation • Scratching

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IV. By methods in which vectors are involved in the transmission and propagation of parasites:

(a) Mechanical transmission :

Crawling or flying arthropod through soiling of its feet or proboscis; or by passage of organisms through its GIT and passively excreted.

There is no development or multiplication of the infectious agent on or within the vector.

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(b) Biological transmission: The infectious agent undergoing replication or development or both in vector and requires an incubation period before vector can transmit.

Three types: (i) Propagative: The agent merely multiplies in vector, but no change in form, e.g., plague bacilli in rat fleas (ii) Cyclo-propagative : The agent changes in form and number, e.g., malaria parasites in mosquito (iii) Cyclo-developmental : The disease agent undergoes only development but no multiplication, e.g., microfilaria in mosquito.

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Airborne transmission

(1) Droplet nuclei• Tiny particles (1-10 mic) represent dried residue of

droplets. Formed by droplets coughed or sneezed into the air.

• Diseases spread : TB, influenza, chickenpox, measles, Q fever, Resp. infections.

• Spread of toxic air pollutants including "smog" results in air pollution epidemics.

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(2) Dust: Droplets expelled during talking, coughing or sneezing, settle down & become part of the dust.

• Infectious agents (e.g., streptococci, other pathogenic bacteria, viruses, spores) have been found in the dust of hospital wards

• Some (e.g., TB bacilli) survive in dust for considerable periods

• During the act of sweeping, dusting & bed-making, the dust is released into the air.

• Transmission is most common in Hospital-acquired (nosocomial) infection.

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Fomite-borne

• Inanimate articles/ substances other than water or food contaminated by the infectious discharges from a patient and capable of harbouring and transferring to

a healthy person.

• Example: soiled clothes, towels, linen, handkerchiefs, cups, spoons, pencils, books, toys

• Diseases transmitted: diphtheria, typhoid fever, bacillary dysentery, hepatitis A, eye and skin infections.

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Unclean hands and fingers

• Most common medium ,transferred to food from the skin, nose, bowel, etc.

• Transmission: Directly (hand-to-mouth)& Indirectly, e.g.- staph./strepto. infections, typhoid fever.

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Thank you.