There are microorganisms that contaminate a living body or an inanimate substance with their presence either on or in it. They belong to either of the following categories: viruses, rickettsiae, bacteria, fungi, protozoa, metazoal parasites and ectoparasites. They are all parasitic in nature because they live at the expense of the host they invade. They replicate in the tissue of the host and thereby cause an infection. The detectable clinical manifestation of this infection is the disease. For example, the virus that causes influenza enters the body of the host (a man) and multiplies by replicating itself thereby spreading influenza infection in the host. This infection manifests itself by a rise in body temperature (called fever), head and body ache, a running nose and sneezes. This is diagnosed as the disease influenza.
Three major factors have to interact with each other for a disease or infection to occur. These are :
a) The agent of infection
b) The mode of transmission of the infection
c) The host
The agent is the very cause of the disease or infection. The second link in the chain of infection is transmission i.e., the process by which the agent passes to the host. The final link in this chain of infection is the host. This is where the microorganism carrying the infection enters, lodges, multiplies, and causes the disease. The presence of an infective agent on a body surface, or in an inanimate object or substance is termed 'contamination'. You can get contaminated by the influenza virus; the food can get contaminated by microorganisms causing cholera; in the process of cleaning the sores of a patient your hands can get contaminated or the towel you use for cleaning can get contaminated.
It is, however, important to know that the presence of an infectious agent on or in a human does not necessarily confirm the presence of disease. It only means that there is a potential for disease to occur. We will discuss this in more detail a little later on.
In this block, we are going to talk about the agents that pass infection around, or contaminate a body or substance, thereby causing the transmission of disease. We will also discuss the various modes of transmission of infection and what can be done to prevent it. Anything that can break this chain which links the agent of infection to the host will prevent disease from spreading.
Let us now look at these agents of infection (also called pathogens), individually.
Viruses : These are the smallest of all pathogens. They can reproduce and multiply only inside a living cell and they do so entirely by making use of the machinery of the host cell itself - they borrow from the cell the enzymes and nutrients necessary for reproduction. In this process they accomplish two important things -they deplete or destroy the cells which ultimately leads to organic dysfunction, and in a way they hide behind the cell's own machinery for reproduction, thereby protecting themselves from attack by antibiotics. Antibiotics are designed to attack pathogenic microorganisms and not the host cells; they destroy the pathogen by attacking its reproduction cycle at some point. They arc not meant to harm the cells of the host, either man or animal. If they did so the cure would kill the patient itself. Viruses, by making use of the host cell's machinery for reproduction, escape destruction by antibiotics. This is why an infection by a virus is much more serious then infection by any other category of microbial agents of disease. Antibiotics have no effect on viruses. The antibiotics that are given during a virus infection do not act against the virus but protect the body from other bacterial infections to which the body becomes susceptible during the viral infection. Some of the diseases caused by viruses are influenza (flu), polio, smallpox, chickenpox, measles, mumps, German measles, hepatitis herpes, AIDS and rabies.
Rickettsiae: like viruses,they are also parasitic that multiply only inside a living cell, but unlike viruses, they are susceptible to antibiotics. Also unlike viruses, they are visible under a conventional microscope by special staining techniques. They are transmitted to humans by the bites of infected ticks, lice and fleas. Diseases caused by rickettsiae are Q-fever, trench fever, rickettsial pox, rocky mountain spotted fever also called tick fever, and typhus.
Bacteria : Bacteria can multiply outside of living cells. They can therefore be cultivated on artificial media. They can also be seen under a conventional microscope. Bacterial causes of diseases are therefore easy to identify and cure. Diphtheria, whooping cough, tuberculosis, cholera, acute diarrhea diseases like dysentery, typhoid, tetanus, trachoma and other infections of the eye, ear and the throat, and leprosy are examples of bacterial infections. Once the bacteria is identified and the disease diagnosed, it can be cured in many cases.
Fungi : These fall in between the free-living and the host-dependent parasites. In other words, they can live outside a living organism and totally independent of it, as well as on or in a living organism like a parasite. For example, mushroom is a fungus that grows independent of a living host, while there are fungi that grow on the skin of man like a parasite causing various skin infections, or grow in between the fingers or the toes of people whose hands and feet are constantly moist like in the case of washing-women and agricultural laborers. Some fungi even grow inside the oral cavity and cause infection. Fungi can be identified and destroyed by chemotherapeutic agents.
Protozoa : These are single-celled organisms and can be identified under the microscope and destroyed with suitable chemotherapy. Examples of some diseases caused by protozoa are malaria, amoebic dysentery and Kala azar.
Metazoal parasites : Unlike the protozoal parasites, these are multicellular organisms and are capable of adapting their life cycle to the environment. Therefore, they occur in widely varied forms. Some examples of metazoal parasites which can infect man are all kinds of worms that get into our bodies, like hook worm, tape worm, round worm, guinea worm, etc. Filaria is also caused by a metazoal parasite.
Ectoparasites : These are parasites that live on the exterior of animals and humans. Some of them affect only the outer surface of the skin of the host, like the lice and the scabies mites and some manage to get inside the skin like the larvae of some insects.
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