Modes of Transmission and Susceptible Host

Modes of Transmission

The transmission of aninfectious agent from the reservoir or source of infection to a susceptible individual occurs in many ways. The mode of transmission of infectious disease is classified as:

a. Direct transmission

1. Direct contact
2. Droplet infection
3. Contact with soil
4. Inoculation into skin or mucosa
5. Transplacental (vertical)

b. Indirect transmission

1. Vehicle-borne
2. Vector-borne
3. Air borne
4. Fomite borne
5. Unclean hands and fingers

a. Direct transmission

i. Direct contact

It involves the transmissionof theinfectious agent by skin to skin, mucosa to mucosa, and mucosa to skin. In direct contact, there is no intermediate agency between areservoir and asusceptible individual. For example. STDs, AIDS, Leprosy, eye infections etc.

ii. Droplet infection

Droplet causes direct projection during coughing, sneezing, speaking, talking. The expelled droplets impinge directly upon conjunctiva, skin, oro- respiratory mucosa. These droplets contain millions of bacteria and viruses, which can be a source of infection to others. Disease transmitted by droplets spread includes respiratory infections, whooping cough, tuberculosis etc.

iii. Contact with soil

In this way, the infectious agent present in thesoil, compost or decaying matters in which it normally leads a saprophytic existence, may come in direct contact upon exposure to the susceptible tissue. Disease transmitted by this route inculdeshookworm larvae, tetanus, mycosis etc.

iv. Inoculation into skin and mucosa

In this method, the infectious agent is inoculated directly into skin or mucosa. The best-known example of the disease transmitted by this mean includes rabies by dog bites. Other example includes hepatitis B through contaminated needle, syringes.

v. Transplacental transmission

It includes syphilis, hepatitis B, AIDS etc. transmitted through theplacenta. Some pathogens that cross placenta are as follows:

Fig. Pathogens capable of crossing placenta

b. Indirect transmission

Indirect transmission of aninfectious agentfrom reservoir or source of infection to thesusceptible host includes an intermediate agency whether living or non-living so unlike direct transmission, in case of indirect transmission the infectious agent may not be immediately transferred. The essential requirement for indirect transmission is the infectious agent must be capable of surviving outside the human host in the external environment retain its basic properties of pathogenesis and virulence till it finds anew host.

i. Vehicle-borne transmission

It implies transmission of the infectious agent through water, food, ice, blood, serum, plasma or biological products such as tissues and organs e.g. typhoid fever, polio, food poisoning etc. The epidemiological features of vehicle transmission are:

a. If the dose of contamination is heavy, the outbreak may be explosive.

b. Cases are initially confined to those who are exposed to the contaminated vehicle.

c. When secondary case occur, the primary case may be obscured.

d. The distance travelled by an infectious agent may be a great e.g. outbreak of the food poisoning.

e. It is always not possible to isolate the infectious agent in the incriminated vehicle.

f. When the vehicle is controlled or withdrawn, the epidemic subsides.

g. The common source of infection is traceable.

ii. Vector-borne transmission

Vector is defined as any arthropod or any living carrier (e.g. snail) that transports an infectious agent to the susceptible individual. Transmission by a vector may be mechanical or biological.

a. Mechanical transmission

The infectious agent is mechanically transmitted by a crawling or flying arthropods though soiling of its feet or proboscis. There is no development or multiplication of the infectious agent on or within the vector.

b. Biological transmission

The infectious agent undergoes replication or development in vector and requires an incubation period before vector can transmit . Biological transmission of three types:

Propagative: The agent multiplies in vector but no change in form, for example, plague bacilli in rat fleas.

Cyclo propagative: The agent changes in form and number, for example, malarial parasite in mosquito

Cyclo development: The agent undergoes only development but no multiplication, for example, microfilaria in mosquito.

Classification of vector-borne transmission

1. By vector

a. Invertebrate type

i. Diptera- flies and mosquitoes

ii. Siphonaptera- fleas

iii. Orthoptera- cockroaches

iv. Anoplura- sucking lice

v. Hemiptera- bugs

vi. Acarina- ticks and mites

vii. Copepoda- cyclops

b. Vertebrate type- mice, rodent, bats etc.

2. By transmission chain

a. Man and non-vertebrates host

i.Man-arthropod-man (malaria)

ii. Man-snail-man (Schistosomiasis)

b. Manand another vertebrate host, and a non-vertebrate host

i. Mammal-arthropod-man (Plague)

ii. Bird-arthropod-man (Encephalitis)

c. Man and 2- intermediate hosts

i. Man-snail-fish-man (Clonorchis sinensis)

ii. Man-cyclops-fish-man (Fish tape worm)

iii. Man-snail-crab-man (Paragonimiasis)

3. By method in which vectors transmit agent

a. Biting

b. Regurgitation

c. Scratching in/ of infective faeces

d. Contamination of host with body fluids of vector

4. By the method in which vectors are involved in the transmission and propagation of parasites

iii. Airborne

a. Droplet nuclei

These are 1-5µm diameter particles, which are dried residue of droplets. These are produced either by evaporation of droplets coughed or sneezed in air or generated by a variety of atomizing devices.

They may also be formed accidentally on microbiology laboratory, rendering plants or autopsy rooms.

Droplet nuclei are easily drawn into the alveoli of lungs and may be retained there e.g. tuberculosis, influenza, measles etc, are transmitted by droplet nuclei.

b. Dust

These are larger than thedroplet, which are expelled during sneezing, coughing, and talking. In hospital wards, there are various infection agents in dust.

iv. Fomite- borne

Fomites are inanimate articles except water or food. Fomites contain infectious discharge from patient and capable of harboring and transferring the infectious agent to a healthy person. Fomites include utensils, books, towels etc. Diseases transmitted by fomites include diphtheria, typhoid fever etc.

v. Unclean hands and fingers

Hands are the most common medium by which pathogenic agents is transferred to food from the skin, nose, bowel, etc. The transmission is directly or indirectly. For example, Staphylococcal or streptococcal infection.

Susceptible Host

For all infections, it requires the following:

1. The infectious agent must find a portal of entry by which it may enter the host.
2. The organisms must reach the "site of infection" where it may find optimum condition for its multiplication and survival.
3. The disease agent must get aportal of exit to reach anew host.
4. After leaving the human body, it must survive in the external environment.

References

Gordis, L. Epidemiology. third edition. 2004.

Joshi, Banjara. Fundamentals of Epidemiology. Kathmandu: Quality Printing Press, 2007

Park, K. Park's Text Book of social and prevention Medicine. 18th edition. 2008.