Organisms Affecting Human Health Part 3

Organisms Affecting Human Health Part 3

20.8.1 Vectors of diseases

Germs are transmitted from an infected person to a healthy person by means of animals (for example; by many insects). These animals are called vectors. Common vectors are housefly, sand fly, mosquitoes, fleas, cockroaches, dogs, mice etc. (fig 20.15).

Fig 20. 15 diagrams summarizing the spread of infections

Housefly as a vector of disease

Housefly is a mechanical vector. It feeds on decaying matter and dung which is rich source of germs causing typhoid fever, typhus, cholera, dysentery, tuberculosis and anthrax along with some tapeworm and roundworm eggs. As flies feed their feet, bodies and digestive system becomes contaminated with germs and eggs.

If the flies then settle on human food, the germs and eggs are deposited on the food. By eating this contaminated food persons become ill.

Fig 20.16 housefly

Mosquitoes as vectors

Mosquito is a biological vector because the pathogen which is responsible for causing the disease under goes a cycle of development in it. Mosquito is a vector for transmitting malaria and yellow fever.

In case of malaria, pathogen is a protozoan plasmodium. Plasmodium under goes two stages in its life cycle, one in man and one in the mosquito. In man, the parasite enters through the bits of an infected female anopheles mosquito. One stage of its life cycle is completed in man, as a result gametocytes are formed which are sucked along with blood by a mosquito. Second stage of its life cycle is complete in mosquito (fig. 20.17).

Fig. 20.17 life cycle of plasmodium

The mosquito in its life cycle is an intermediary host. Without this intermediary stage. The disease would not be passed from patient to healthy person.
Antibiotics and their importance
Antibiotics are chemicals that are extracted from one living organism and which destroy pathogens in another living organism.
Antibiotics are used to kill pathogenic bacteria but they do not harm viruses. Some of the common antibiotics are;

1. Penicillin attacks a narrow range of bacteria and are therefore called narrow spectrum antibiotics.
2. Cephlosporins are useful against bacteria which have developed resistance to penicillin.
3. Tetracyclines act against a variety of bacteria. They are broad spectrum antibiotics.
4. Erythromycins are useful against bacteria which have developed resistance to penicillin.

Fig. 20.18 penicillium chrysogenum