Insects and Diseases | Phylum Arthropoda

In this article we will discuss about:- 1. Introduction to Insects 2. Transmission of Disease by Insects 3. Management of Insects 4. Common Insect-Borne Diseases.

Introduction to Insects:

The relation between some of the important diseases and insects is surprisingly intimate. The insects which belong to the phylum ‘Arthropoda’ are one of the most highly organised invertebrates.

Some of the insects carry disease producing organisms only to transmit these to humans or other animals, Others harbour the causative organisms in them and are known as vectors. They are potential sources for continuous transmission of parasites.

A great number of insects cause diseases while some others are entirely beneficial to human beings. Sometimes, the insects themselves are parasities or blood suckers. It is in their capacity as carriers of germs or as intermediate hosts of parasities that they had to be reckoned with the foremost of human foes.

With the advancement of the scientific knowledge in the present century, it has been clearly demonstrated that the most important human and animal diseases are exclusively transmitted by insects. In diseases such as Yaws, Pink-eye, Q fever, Tularemia and Anthrax, ‘Arthropoda’ play an important role while in diseases due to bacterial, protozoan or helminthic infections also their role cannot be ignored.

Transmission of Disease by Insects:

It is chiefly effected in three ways:

1. Mechanical transmission

2. Biological transmission

3. Transovarial transmission

1. Mechanical transmission:

This may be indirect or direct. The simplest method is the indirect mechanical transmission where the insects take of the role of passive carriers of diseases. They only pick up the organisms causing disease, from the bodies or the excretions of man or animals and deposit them on food. The commonest carriers are the house flies and roaches.

A step further towards the specialization of transmission may be found in the direct mechanical transmission method. Here, the insect picks up the germs from the body of a diseased individual and directly injects them into the blood of other animals through the skin or sores or wounds.

Tsetse flies and certain others transmit the blood diseases namely, Anthrax, Fowl pox and some animal Trypanosomiases. The flies that feed on wounds, transmit skin or eye diseases ‘oriental sores’, ‘trachoma’, etc. The organisms of all these diseases do not survive for more than a few minutes to a few days in the vectors.

Organs Concerned with Transmission:

1. Mouth parts, (a) Bitting. In the insects transmitting diseases, specially in plants, the mouth parts are modified for bitting. Example. Thrips.

(b) Sucking. In mosquitos, the mouth parts are modified for sucking. Generally, thypopharynx and epipharynx undergo modifications. The mandible is lost or rudimentary. In fleas, hypo-pharynx, mandible and maxillae are modified to form piercin organs.

Another variety of sucking modification is found in the sponging type, where the organ concerned is popularly known as proboscis. Example. House fly.

2. Legs. In house flies, legs carry the germs of diseases.

3. Antennae. The antennae serve the same purpose in the insects in general.

4. Body. In house fly, the body as a whole acts as the carrier.

2. Biological transmission:

When multiplication or cyclical changes or both take place within the body of the insects, it is known as biological transmission. It may be propagative, cyclopropagative, cyclodevelopmental, and transporting.

3. Transovarial transmission:

This means transmission of disease through infected ovary and the egg, often referred to, as hereditary transmission. The infection is effected specially by mites and ticks. Insect-borne infections are extremely insignificant, although some virus infections may be significant.

Management of Insects:

The harmful insects are managed by various ways.

(i) Chemical Management:

Chemicals used to kill insects are known as insecticides. Injurious insects are killed by certain chemicals, viz. hydrocyanic acid, sulphur dioxide, arsenic, etc. But there are other chemicals which are more specially detrimental to the insects, namely, pyrethrum and a number of chlorinated hydrocarbons, such as D.D.T., benzene hydrochloride (B.H.C.).

Organophosphates are extensively used as systematic poison. The majority of these insecticides are believed to act primarily on the nervous system of insects and paralyse them, some very rapidly. The insecticides are absorbed either through the cuticle or the stomach.

Modes of use of insecticides:

Insecticides are used in the form of dusts, solutions, emulsions, water suspensions and fumigants or these may be mixed up with the food and used as bait.

(i) Dusting:

Applied to protect the health of man or animals against insects.

(ii) Residual sprays:

Chlorinated hydrocarbons can be mixed up usually with kerosene oil, or some other suitable medium and sprayed.

(iii) Space sprays:

These are effective in small indoor disinfections. They may be sprayed in the form of fogs, mists, smokes, etc.

(iv) Water treatment:

Applied to kill aquatic larvae of mosquitoes. Paris green dust is effective only in the case of Anopheles larvae.

(v) Skin application:

Application of insecticides, particularly D.D.T., is effective in case of animals, since it is not absorbed through the skin.

(vi) Fumigation:

Hydrocarbonic acid gas is a powerful fumigant and is extensively used, but it is very toxic to all torms of life.

Repellants:

These are chemicals which may be applied to the skin or clothing to keep off insects from bitting, Indalone, is particularly effective against bitting flies and some mosquitoes.

(ii) Biological Management:

Insects may be controlled by other insects; some insects devour the eggs and adults of others.

Common Insect-Borne Diseases:

(a) The sucking lice. Ectoparasite on mammals, feed on blood; two species are parasite on man. Diseases are transmitted by continuation of scratches in the human body; or by excreta, or by the excreta of the infected louse.

(b) Mosquitoes. Anopheles. Females are carriers of malaria.

Culex. Intermediate hosts for filaria. They are also carrier for the causative agents of dengue fever.

(c) Flies. Carrier of many pathogenic bacteria like those of cholera and typhoid.

(d) Rat flea. Carry bacilli from diseased rat to man. The plague bacilli rapidly mul­tiply in the blood of the flea and when the flea bites a man, the bacilli are released in the blood of the victim.