Infection Reference Materials

INFECTION Infection and immunity involve interaction between the animal body (host) and the infecting microorganisms. The lodgement and multiplication of a parasite in or on the tissues of a host constitute infection. It does not result in disease. In fact, disease is rare consequence of infection, which is a common natural event. INFECTION CLASSIFIED IN VARIOUS WAYS Primary infection Initial infection with a parasite in a host is termed primary infection. Reinfection Subsequent infection by the same parasite in the host are termed reinfection. Secondary infection When a new parasite sets up an infection in a host whose resistance is lowered by a pre-existing infectious disease, is termed secondary infection. Focal infection A condition where due to infection or sepsis at localised sites such as appendix or tonsils, generalised effects are produced. Cross infection When in a patient already suffering from a disease a new infection is set up from another host or another external source is termed cross infection. Nosocomial infection Cross infection occurring in hospitals are called nosocomial infection. Iatrogenic infection Physician induced infections resulting from investigative, therapeutic or other procedures. Endogenous or exogenous infections Depending on whether the source of infection is from the host’s own body or from external sources, infections are classified as endogenous or exogenous respectively. Atypical infection It is one in which the typical or characteristic clinical manifestations of the particular infectious disease are not present. Latent infection Some parasites, following infection, remain in the tissues in a latent or hidden form proliferating and producing clinical disease when the host resistance is lowered is termed latent infection. Sources of infection Human beings The commonest source of infection for human beings are human beings themselves. The parasite may originate from a patient or carrier. Carrier: A carrier is a person who harbours the pathogenic microorganism without suffering from any ill effect because of it. Different types of carrier Healthy carrier: is one who harbours the pathogen but has never suffered from the disease caused by the pathogen. Convalescent carrier: is one who has recovered from the disease and continues to harbour the pathogen in his body. Temporary and chronic carrier: depending on the duration of carriage, the temporary carrier lasts less than six months, while chronic carrier may last for several years and sometimes even for the rest of one’s life. Contact carrier: a person who acquires the pathogen from a patient. Paradoxical carrier: a carrier who acquires the pathogen from another carrier. Animal Infectious disease transmitted from animals to human beings are called zoonoses. Zoonotic diseases may be bacterial (plague from rats), viral (rabies from dogs), protozoal (toxoplasmosis from cats), helminthic (hydatid disease from dogs) or fungal (zoophilic dermatophytes from cats and dogs). Insects Blood sucking insects may transmit pathogen to human beings. The disease so caused are called arthropod borne diseases. Insects such as mosquitoes, ticks, mites, flies, fleas and lice that are transmit infections are called vectors. Mechanical vectors: transmission may be mechanical (for example, transmission of dysentery or typhoid bacilli by the domestic fly) Biological vectors: the pathogen multiplies in the body of the vector, often undergoing part of its developmental cycle in it. (For example Aedes aegypti mosquito in yellow fever) Extrinsic incubation period The interval between the time of entry of the pathogen into the vector and the vector becoming infective is called the extrinsic incubation period. Soil and water Some pathogens survive in soil very long periods (for example spores of tetanus bacilli remain viable in soil for several decades and act as source of infection). Water may act as the source of infection due to contamination with pathogenic microorganisms (cholera vibrio, infective hepatitis virus) or due to presence of aquatic vectors. Food Contaminated food act as source of infection. The presence of pathogens in food due to external contamination (food poisoning by staphylococcus) or due to pre- existent infection in meat or other animal products (salmonellosis). Methods of transmission of infection Contact Infection may be acquired by contact which is direct or indirect. Direct contact infections spread when disease causing microorganisms pass from the infected person to the healthy person via direct physical contact with blood or body fluids. Examples direct touching, sexual contact, kissing, contact with oral secretions and body lesions. Indirect contact transmission occurs when there is no direct human- to-human contact. Contact occurs from a reservoir to contaminated surfaces or objects or to vectors. Inhalation Respiratory infections are transmitted by inhalation of pathogen. Such microbes shed by patients into the environment, in secretions from the nose or throat during sneezing, speaking or coughing. Pathogen resistant to drying may remain viable in the dust and act as a source of infection. Ingestion Intestinal infections are generally transmitted by the ingestion of food or drink contaminated by pathogens. Inoculation Pathogens may be inoculated directly into the tissues of the host. Examples are tetanus spores implanted in deep wounds, rabies virus deposited subcutaneously by dog bite and arbovirus injected by insect vector. Insect Insects may act as mechanical or biological vectors of infectious diseases. Congenital Some pathogens are able to cross the placental barrier and infect the fetus in utero. This is called vertical transmission. This may result abortion, miscarriage or still birth. Intrauterine infections with the rubella virus, may interfere with organogenesis and lead to congenital malformation. Such infections are known as teratogenic infections. Iatrogenic and laboratory infections Infections may be transmitted during administration of injections, lumbar puncture and catheterisation. Modern methods of treatment like exchange transfusion, dialysis, and heart and transplant surgery have increased possibilities for iatrogenic infections. Laboratory personnel handling infectious material are at risk and special care should be taken to prevent the laboratory infection.