Transmission of Diseases by Vectors | Parasitology

In this article we will discuss about the transmission of disease by vectors.

There are three different kinds of transmis­sion for the spread of various diseases:

(a) Direct transmission:

In this case the transmission of dis­ease is occurred through close contact i.e. trans­ferred from man to man through close contact, e.g., scabies.

(b) Mechanical transmission:

In this method, the disease-agent is transferred simply by the carrier-agent. Here the carrier does not har­bour any part of life cycle of parasite i.e. in other words the parasite does not stay or live within the tissue or cells of carrier.

So that the spread of dis­ease agent or parasite occurs only mechanically by different body parts through contamination. The transmission of diarrhoea, typhoid, dysentery etc. by the housefly are examples of mechanical trans­mission of diseases.

(c) Biological transmission:

When the disease-agent (parasite) undergoes multiplication or some developmental changes in carrier-host is known as biological transmission. The organisms or animals which are involved in spreading or transmitting the disease are known as vectors or carriers.

Vectors are noth­ing but vehicles by which the parasites are trans­mitted from one host to another i.e., the animal that acts as a carrier of disease producing germs (parasites) for transmission of disease from one host to another host.

In terminology, the word vector means, “any agent that acts as an interme­diate or alternative host for a pathogenic orga­nism and transmits to a susceptible host, is called vector”. For example—female Anopheles mosquito acts as vector of malarial parasite (Plasmodium sp.).

In strict sense the carrier means “the orga­nism which can spread the parasite by transmitting simply through different external body parts and which does not harbour any part of life cycle of parasite”. Sometimes carriers are termed as mechanical vectors as they spread the parasite mechanically.

For example rat, housefly, cock­roach etc. can spread the parasite simply by dif­ferent external body parts through contamina­tion. Any animal which transmits infection by inoculation into or through the skin or biting or by deposition of infective materials on the skin or on food or other objects, is known as carrier or vector. In strict sense they differ for each other.

Vector:

i. Harbours some part of life cycle of parasite.

ii. Provides place of residing of parasite within tissue or cells.

iii. The animal which is involved in spreading of the parasite after some modifications or change in the body of it through inoculation.

Carrier:

i. It does not harbour any part of life cycle of parasite.

ii. Parasite does not reside or stay within the cells or tissue.

iii. The animal which can spread the parasite simply by different body parts through contamination. 

Types of Vectors:

Vectors may be classified into two groups:

1. Biological vectors are those carrier organisms (invertebrate animals) in which the parasites (disease agents) increase their numbers by multiplication or transformation inside the body of the carrier-organisms. For example, female Anopheles mosquito is regarded as the biological vector of Plasmodium sp. (malarial parasite).

2. Mechanical vectors:

There are certain vectors where the parasites (germs) are attached to the outside of their body, such as in legs and thus transmit the germs or parasites from one host to another without involving any deve­lopmental stages of the parasites in their body. These types of vectors are known as mechanical vectors and are found in house­fly, cockroach etc. So they are known as mechanical vectors.

Mode of Transmission of Disease through Biological Vectors:

Transmission of various diseases is occurred by three ways:

1. Propagative transmission:

When the disease agent or parasite undergoes multiplication within the body of biological vector but no cyclical change is observed, then the trans­mission is said to be propagative.

Example:

Plague bacillin in rat fleas.

2. Cyclo-propagative transmission:

Here the parasite undergoes multiplication in the body of the vector and at the same time cyclical change is also noticed.

Example:

Plasmodium (malarial parasite) in female Anopheles mosquito.

3. Cyclo-developmental transmission:

When the disease agent or parasite undergoes no multiplication in the body of vector but they undergo cyclical changes. Example: Guinea- worm embryo in Cyclops and filarial parasite (Wuchereria) in Culex mosquito (Table 15.1).