Introduction to infections
- Based on relationship to hosts, microorganisms are classified as; Saprophytes and Parasites.
- SAPROPHYTES: The word is from Greek “sapros” meaning decayed and “phyton” meaning plants.
- Free living microbes that subsist on dead and decaying organic matter.
- They are found in soil and water and are important for degradation of organic materials.
- They are incapable of multiplying in living tissues which means they have less relevance for infectious diseases.
- But, saprophytes like Bacillus subtilis may infect devitalized host whose natural resistance is lowered. Such infections are termed Opportunistic infections.
- PARASITES: can establish themselves and multiply in hosts.
- Parasites are either pathogens or commensals.
- PATHOGENS: From Greek words, “pathos” meaning suffering and “gen” meaning produce which roughly translates to “disease-producing”.
- COMMENSALS: From Latin words, “com” meaning with and “mensa” meaning table. These parasites live together with the host without causing any disease causing effects.
- However, many commensals behave as “facultative pathogens” and can produce disease when resistance is lowered.
Classification of Infections
- Primary Infections: initial infections with a parasite.
- Re – Infections: subsequent infections by same parasite.
- Secondary Infections: new parasite infects when resistance is lowered by presence of a previous one.
- Focal Infections: or focal sepsis, infection or sepsis at a localized site like appendix or tonsils which produces generalized effects.
- Cross infections: When a patient is already suffering from a disease, a new infection is set up from another host or another external source.
- Nosocomial infections: Cross- infections occurring in hospitals are called as nosocomial infections. The term is derived from the Greek work “nosocomion” meaning hospital.
- Iatrogenic infections: refers to physician-induced infections resulting from investigative, therapeutic or other procedures.
- Depending on whether the source of infection is within or outside the host’s own body, infections are classified as Endogenous or Exogenous.
- Endogenous- source is outside host’s body; Exogenous- source is inside host’s own body.
- Based on clinical effects, they may be classified as;
- In apparent infection – clinical effects are not apparent. Also called as “Sub-clinical infection”.
- Atypical infection – The typical or characteristic clinical manifestations of the particular infectious disease are not present.
- Latent infection – Some parasites, following infections may remain in the tissues in a latent or hidden form, proliferating and producing disease when the host resistance is lowered.
Sources
of infection
Humans:-
- Commonest source of infection
- Source may be patient or a carrier
- Carrier – A person who harbours the pathogenic microorganisms without suffering any ill effect because of it.
- There are several types of carriers;
- Healthy carriers – One who harbours the pathogen but has never suffered from the disease caused by the pathogen.
- Convalescent carriers – One who has recovered from the disease and continues to harbour the pathogen.
- Depending on duration of carriage, carriers are classified as Temporary or Chronic.
- Temporary carrier – Carriage lasts for less than six months.
- Chronic carrier – Carriage may last for several years and sometimes even the rest of one’s life.
- Contact carrier – A person who acquires the infection from a patient.
- Paradoxical carrier – A carrier who acquires it from another carrier.
Animals:-
- Act as sources of human infection.
- Reservoir hosts – The infection in animals may be symptomatic. These animals serve to maintain the parasite in nature and act as the reservoir of human infections.
- Zoonoses – Infectious diseases transmitted from animals to humans.
- Zoonotic diseases may be;
- Bacterial – Plague from rats, caused by Yersinia pestis which is an enterobacteriaceae.
- Viral – Rabies from dogs, Lyssani viruses, Rabies virus, Australian Bat Lyssani.
- Protozoal – Toxoplasmosis from cats, Toxoplasma gondii.
- Helminthic – Hydatid disease from dogs, Echino coccosis – tapeworms (Echino coccus)
- Fungal – Zoophilic dermatophytes from cats and dogs.
Insects:-
- Arthropod-borne disease – The diseases caused by transmission of pathogens by blood sucking insects.
- Vectors – Insects such as mosquitos, ticks, mites, flies, fleas and lice that transmit infections.
- Mechanical vectors – When the transmission of pathogens by vectors is mechanical. For e.g. Transmission of dysentery or typhoid bacilli by the domestic fly.
- Typhoid bacilli – Salmonella typhi
- Dysentery bacilli – Shigella sonnei, Shigella dysenteriae, Shigella flexneri
- Biological vectors – When the pathogen multiplies or undergoes part of its developmental cycle in the body of the vector.
- For e.g. Aedes aegypti mosquito – Yellow fever
- Anopheles mosquito – Malaria
- These vectors transmit the infection only when the pathogen has multiplied in them sufficiently or has undergone a developmental cycle.
- The interval between the time of entry of the pathogen into the vector and the vector becoming infective is called the extrinsic incubation period.
- Insects also act as reservoir hosts, e.g. Ticks in relapsing fever and spotted fever.
