Organisms Affecting Human Health Part 3
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;
- Penicillin attacks a narrow range of bacteria and are therefore called narrow spectrum antibiotics.
- Cephlosporins are useful against bacteria which have developed resistance to penicillin.
- Tetracyclines act against a variety of bacteria. They are broad spectrum antibiotics.
- Erythromycins are useful against bacteria which have developed resistance to penicillin.
Fig. 20.18 penicillium chrysogenum
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