- Diseases are transmitted from the source of infection to susceptible host. The medical model of infectious disease transmission consists of three interlinked chains.
- They are:
- Sources of reservoir
- Modes of transmission
- Susceptible host
I. Sources or reservoir:
- A source of infection is defined as the person, animal, objects or substance from which an infectious agent passes or is disseminated to the host.
- Reservoirs are sites in which viable infectious agents remain alive and from which infection of individuals may occur.
- Reservoirs may be either animate or inanimate.
- Reservoirs is the natural habitat in which the microorganisms metabolizes and replicates.
- Example: in typhoid fever the reservoir of infection is human but the source of infection may be faeces or urine of patients or contaminated food or water.
- In hookworm infection, the reservoir is a man, but the source of infection is the soil contaminated with infective larvae.
- In tetanus, the reservoir and source are the same, which is the soil.
- There are three types of reservoir:
- Human reservoir
- Animal reservoir
- Reservoir in non-living things
1. Human reservoir:
- Human himself is the important source or reservoir of infections.
- Human maybe a case or a carrier.
Cases:
- A case is defined as ‘ a person in the population or study group detected as having the particular disease, health disorder or condition under investigation.’
- The cases are identified by clinical, biochemical and laboratory tests.
- The cases maybe clinical, sub-clinical or latent.
- The clinical case may be mild or moderate, typical or atypical, severe or fatal.
- The mild cases are more important source of infection as compared to severe cases.
- The subclinical cases are also called inapparent cases.
- They are important source of infection because they do not show any signs and symptoms of disease and shed infectious agent continuously.
- Subclinical infection can be detected by lab test, isolation of micro-organisms, antibody assay, biochemical and skin sensitivity tests.
- The host does not shed the infectious agent in latent infection.
Carrier:
- A carrier is defined as ‘ an infected person or animal that harbors a specific infectious agent in the absence of the clinical disease and serve as a potential source of infection for others’.
- Epidemiologically, carriers are more infectious because they are not recognized and live as normal in community.
- The Typhoid Mary is an example of a carrier.
- Characteristics to be a carrier:
- There must be the presence of disease agent in the body.
- Absence of noticeable signs and symptoms of disease.
- Shedding of the disease agents in the excretions.
- Classification of carriers: basis of classification
- I. On the basis of Type:
- Incubatory carrier:
- They shed the infectious agents during the incubation period of disease.
- They are capable of infecting others before the onset of disease.
- Example: measles, polio, influenza, hepatitis B etc.
- Convalescent carrier:
- These carriers continue to shed the infectious agents during recovery.
- Convalescent carriers pose threat to the unprotected household member for example Typhoid fever in which patients may excrete the bacilli for 6-8 weeks.
- Healthy carrier:
- These are subclinical cases which have developed carrier state without suffering from disease and shed the disease agent. For ex- poliomyelitis, salmonellosis and diphtheria.
- Opportunistic infection also may occur in persons with normal defences following autoinoculation, the transfer of microbes indigenous to one area of the body to another arear where they may cause disease. Thus a person maybe an endogenous reservoir of his/her own infectious disease.
- Incubatory carrier:
- On the basis of Duration of infection:
- Temporary carrier:
- They are those who shed the infectious agents for short period of time.
- Incubatory, convalescent and healthy carriers are temporary carriers.
- Chronic carrier:
- Chronic carriers excrete the infectious agents for indefinite period of time.
- The longer the carrier state, the greater is the risk to the community.
- Some chronic carriers excrete the infectious agents intermittently and some continuously.
- Temporary carrier:
- On the basis of portal of exit:
- Carriers are also classified according to the portal of exit to the infectious agents.
- Therefore, there are urinary carrier, intestinal carriers, nasal carrier etc.
- In typhoid fever, urinary carrier is more dangerous than an intestinal carrier.
2. Animal reservoir:
- The animal reservoir may also be case or carrier.
- The disease which are transmitted from animal to man are called zoonotic diseases, e.g. Rabies, Yellow fever, influenza.
- There are more than 100 zoonotic diseases.
- Pigs and ducks serve as reservoir for the transmission of influenza virus.
- Pigeons can transmit Chlamydia.
- Histoplasmosis is carried by birds.
- Mosquitos are responsible for transmitting many diseases as well.
- The reservoir of plague, for example, is usually the rodent population.
- Rabies is carried by many domestic and wild mammals.
3. Environment reservoir ( Reservoir in non-living things):
- Soil and inanimate matter can also act as reservoirs on infection.
- Soil also serves as a permanent habitat for a few virulent human pathogens.
- These dangerous microbes include Clostridium tetani, Clostridium botulinum, and fungi responsible for opportunistic pathogen.
- Water is an important reservoir of human disease when it is contaminated by raw sewage.
- Faeces from infected persons are the most common source of water contamination.
II. Modes of Transmission:
- Different pathogens have different modes of transmission, which are usually related to the habitats of the organisms in the body, For instance, respiratory pathogens are generally airborne, whereas intestinal pathogens are spread by food or water.
- Transmission involves three stages:
- Escape from the host
- Travel
- Entry into a new host
- Hence, the transmission of infectious agent from the reservoir to a susceptible individual occurs in many ways.
- The mode of transmission of infectious disease is classified as:
- Direct transmission:
- Direct contact
- Droplet infection
- Contact with soil
- Inoculation into skin or mucosa
- Transplacental
- Indirect transmission:
- Vehicle-borne
- Vector borne
- Mechanical
- Biological
- Fomite-borne
- Unclean hands and fingers
1. Direct transmission:
i. Direct contact:
- Direct transmission is the immediate transfer of the infectious agent from the reservoir to a new host with no intervening agent.
- Infection may be transmitted by direct contact from skin to skin, mucosa to mucosa, or mucosa to the skin of the same or another person.
- For instance, through touching, kissing, sexual contact etc.
- Direct transmission is also involved in the transmission of skin pathogens, such as staphylococci and fungi.
ii. Droplet infection:
- Transmission by infectious droplets is the most frequent means by which upper respiratory infections such as the common cold and influenzae are propagated.
- Droplets are formed from the projection of a spray of naso-pharyngeal secretions during sneezing, coughing, speaking and talking.
- The expelled droplets may reside on the conjunctive, oro-respiratory mucosa or skin by close contact.
- These droplets contain thousands of bacteria and virus which can be source of infection.
- Disease transmitted by droplet spread includes respiratory infections, whooping cough, tuberculosis, meningococcal meningitis etc.
iii. Contact with soil:
- The disease agent is acquired by direct touch with soil. For example: Hookworm larvae, tetanus, mycosis etc.
iv. Inoculation into skin or mucosa:
- The disease agent is inoculated directly into the skin.
- For. E.g. Rabies by dog bites, Hepatitis B through contaminated needle and syringe etc.
v. Transplacental (Vertical transmission):
- Vertical transmission is the spread of disease from parents to offspring by an infected sperm or egg, by passage of pathogens across the placenta during fetal development, or during the birth process.
- Gonorrhoea, syphilis, genital herpes, and rubella can all be vertically transmitted from and infected mother to her child causing blindness, congenital malformations or death.
- The common agents include TORCHES ( Toxoplasma, Rubella, Cytomegalovirus, Herpes simplex virus).
- Note: Horizontal transmission is the spread of disease from person to person within a group.
2. Indirect transmission:
- Indirect transmission occurs when the infectious agent is transmitted by an agent either a vehicle or a vector.
- It includes the traditional 5F’s i.e. Flies, finger, fomite, fluid, and food.
i. Vehicle borne:
- Vehicles are non-living materials are capable of transmitting infectious agent.
- If the vehicle is a contaminated inanimate object, it is called a fomite.
- Bedding, clothing, cooking, and eating utensils, surgical instruments are common fomites.
- Vehicles usually become contaminated by contact with either the reservoir or with respiratory droplets, pus, urine, or feces shed from infected person.
- Acute diarrhoeas, typhoid fever, cholera, polio, hepatitis A are water borne diseases.
- Those transmitted by blood include Hepatitis B, malaria, syphilis, trypanosomes etc.
ii. Vector borne:
- Living organisms that are agents in disease transmission are called vectors.
- They are generally arthropods. For example, insects, mites, ticks, or fleas and vertebrates for example, dogs, cats, rodents.
- Arthropod vectors may not be hosts for the disease but simply carry the agent from one host from another.
- Mechanical vectors pick up microorganisms on their feet or other body parts.
- For-example flies carry microorganism on their feet to the food.
- Biological vectors are infected and not merely contaminated.
- Biological vectors may introduce the microbe by biting the host.
- Classification of vector borne diseases:
- By vector:
- Invertebrate type:
- Diptera-flies and msoquitoes
- Siphonaptera-fleas
- Orthoptera-cockroaches
- Acarina-Ticks and mites
- Vertebrate type:
- Mice, rodents, bats.
- Invertebrate type:
- By transmission chain:
- Man and a non-vertebrate host
- Man-arthropod-man (malaria)
- Man-snail-man (Schistosomiasis)
- Man, the other vertebrate host, and a non-vertebrate host
- Mammal-arthropod-man (Plague)
- Bird-arthropod-man (encephalitis)
- Man and 2 intermediate hosts
- Man-cyclops-fish-man (fish tapeworm)
- Man-snail-fish-crab-man (Paragonimiasis)
- By methods in which vectors transmit agent:
- Biting
- Regurgitation
- Scratching in of infective faeces
- Contamination of host with body fluids of vectors
iii. Airborne:
- Air is one of the most common vehicle of disease transmission.
- The droplet released from the infected person is subjected to evaporation forming smaller, lighter particles called droplet nuclei.
- Droplet nuclei are important vehicles in the transmission of respiratory diseases.
- Airborne pathogens organism may also be transmitted by dust particles that are often produced by agitation of soil, contaminated laundry, or surfaces on which micro-organisms have settled.
- Fungal spores are often transmitted by the dust generated when contaminated soils are agitated.
- Particles in the 1-5 micron range are able to go into the alveoli of the lungs and may retain there.
- Diseases such as tuberculosis, influenza, chickenpox, measles, Q fever and many respiratory infections are spread by droplet infections.
- Drop (>10
Droplet (5-10
) and droplet nuclei (<5
)
- Dust: These are larger than droplets which are expelled during sneezing, talking, coughing, etc.
- In the hospital ward the dust may contain various infectious agents which maybe the cause of cross infection.
- During the act of sweeping, dusting and bed-making, the dust is released into the air and becomes once again airborne.
- Streptococci, other pathogenic bacteria, viruses and fungal spores are found in the dust in the hospital ward and in the dust of the street.
III. Susceptible host:
- The infectious agent must find the portal of entry.
- To cause disease an organism that has escaped from its reservoir must enter a new host through a portal of entry.
- These portals of entry are the respiratory tract, alimentary tract, genitourinary tract, mucous membrane, traumatic openings in the epithelium caused by insect bites, surgical wounds, burns, and punctures.
- On gaining entry into the host, the organisms must reach the appropriate tissue in the body of the host where it may find optimum conditions for its multiplication.
- The disease agent must find a way out of the body (portal of exit) in order that it may reach a new host and propagate its species.
- After leaving the human body, the organism must survive in the external environment for sufficient period till a new host is found.