Host-parasite Interactions
Infectious Diseases. These are caused by infectious agents which are bacteria, viruses, fungi, protozoa and helminths.
Parasite is a living organism (bacteria, viruses, fungi, protozoa, helminths) that lives in another organism, and receives shelter and nourishment. However, in medical science parasitology traditionally deals with animal parasites- protozoa and helminths.
Strict or obligate parasite is an organism that cannot live without a host. That is they have no free-living existence. Examples: Treponema pallidum, viruses, malarial parasite.
Facultative parasite is an organism that has both a free-living and a parasitic existence, e.g. Clostridium species, Pseudomonas species.
Pathogen. Pathogen is an organism which can cause disease.
Nonpathogen. An organism that does not cause disease. It may be a member of normal flora.
Opportunistic pathogen is an organism (nonpathogen, commensal or saprophyte) that can cause disease only in immunocompromised individuals that is having impaired
resistance. Example : Cytomegalovirus, Pneumocystis carinii, Atypical mycobacteria, opportunist fungi.
Saprophyte is an organism that lives on dead organic matter.
NORMAL FLORA (Commensals)
Microorganisms that is present on the skin and mucous membrane of normal (healthy) persons. One particular microorganism of the normal flora may be a nonpathogen, or opportunistic pathogen. Normal flora is of two types: (1) Resident flora. These are microorganisms regularly present in the region at a given age, e.g. Viridans streptococci in mouth and throat, Escherichia coli in intestine, (2) Transient flora. The microorganism is present only for hours to weeks, e.g. Streptococcus pyogenes in throat. Microorganisms of transient flora play no role when the normal resident flora remains intact. But if the resident flora is disturbed than the microorganisms of transient flora may colonize and produce disease. See chapter 62.
Beneficial Functions of Normal Flora. Examples: (1) In mouth and lower b jvel an invading pathogen may fail to compete for nutrients and receptor sites with normal flora, (2) Some bacteria of the bowel produce antimicrobial substances, (3) In new born, bacteria acts as a powerful stimulus for the development of immune system, (4) Bacteria of the gut can produce
vitamin K.
Harmful Effects of Normal Flora. Clinical diseases by opportunist pathogen of normal flora arise under : (1) when the organism leaves the normal site and localizes to another site, e.g. Escherichia coli in the urinary tract from gut, (2) competitive advantage due to antibiotic therapy, e.g. colitis by Clostridium difficile, (3) In immunocompromised individuals.
Parasite is a living organism (bacteria, viruses, fungi, protozoa, helminths) that lives in another organism, and receives shelter and nourishment. However, in medical science parasitology traditionally deals with animal parasites- protozoa and helminths.
Strict or obligate parasite is an organism that cannot live without a host. That is they have no free-living existence. Examples: Treponema pallidum, viruses, malarial parasite.
Facultative parasite is an organism that has both a free-living and a parasitic existence, e.g. Clostridium species, Pseudomonas species.
Pathogen. Pathogen is an organism which can cause disease.
Nonpathogen. An organism that does not cause disease. It may be a member of normal flora.
Opportunistic pathogen is an organism (nonpathogen, commensal or saprophyte) that can cause disease only in immunocompromised individuals that is having impaired
resistance. Example : Cytomegalovirus, Pneumocystis carinii, Atypical mycobacteria, opportunist fungi.
Saprophyte is an organism that lives on dead organic matter.
NORMAL FLORA (Commensals)
Microorganisms that is present on the skin and mucous membrane of normal (healthy) persons. One particular microorganism of the normal flora may be a nonpathogen, or opportunistic pathogen. Normal flora is of two types: (1) Resident flora. These are microorganisms regularly present in the region at a given age, e.g. Viridans streptococci in mouth and throat, Escherichia coli in intestine, (2) Transient flora. The microorganism is present only for hours to weeks, e.g. Streptococcus pyogenes in throat. Microorganisms of transient flora play no role when the normal resident flora remains intact. But if the resident flora is disturbed than the microorganisms of transient flora may colonize and produce disease. See chapter 62.
Beneficial Functions of Normal Flora. Examples: (1) In mouth and lower b jvel an invading pathogen may fail to compete for nutrients and receptor sites with normal flora, (2) Some bacteria of the bowel produce antimicrobial substances, (3) In new born, bacteria acts as a powerful stimulus for the development of immune system, (4) Bacteria of the gut can produce
vitamin K.
Harmful Effects of Normal Flora. Clinical diseases by opportunist pathogen of normal flora arise under : (1) when the organism leaves the normal site and localizes to another site, e.g. Escherichia coli in the urinary tract from gut, (2) competitive advantage due to antibiotic therapy, e.g. colitis by Clostridium difficile, (3) In immunocompromised individuals.
HOST
Host is the harbouring organism of a parasite. They fall into
three groups or classes:
1. Definitive host. In this host the adult stage of animal parasites lives and sexual reproduction takes place. Man is the definitive host for all animal parasites except malarial parasites and hydatid tapeworm.
2. Intermediate host. In this host the asexual reproduction takes place, or the larval stages of animal parasites develops. When development of larval stage takes place in two different hosts they are called 'first' and 'second' intermediate hosts. Man is the intermediate host for malarial parasites and hydatid tapeworm. Man is both definitive and intermediate host for Taenia solium and Trichinella spiralis.
3. Paratenic host. A carrier or transport host.
Note : The term infestation is used when the parasite lives on the outer surface. They are arthropodes- mites, ticks and insects.
INFECTION
Infection is the multiplication of an infectious agent within the body. Multiplication of pathogenic bacteria (e.g. Salmonella typhi) even if the person is asymptomatic is taken as an infection. Multiplication of bacteria of normal flora at its normal site is not an infection. However, if they multiply and cause disease it is an infection, e.g. Escherichia coli when causes diarrhoea.
An infection involves the following : (1) Source or reservoir of infectious agent, (2) Transmission of the infectious agent from the source to the host (3) Susceptible host- Portal or route of entry of the agent in the host, its localization, multiplication and finally host-parasite interactions which result in either (a) Destruction of the agent or, (b) Infectious disease.
Source (Reservoir) :
1. Human Source.
(a) Exogenous source : Patient or carrier,
(b) Endogenous source : The individual himself.
Carrier. A person with asymptomatic infection which can be transmitted to another susceptible person. An individual may be a carrier : (a) in the incubation period (Incubatory carrier), e.g. in measles, (b) during convalescence (convalescent carrier), e.g. typhoid fever, or (c) for a prolonged period, e.g. typhoid fever.
2. Foods and Drinks
(a) Food :Any contaminated food, (b) Water contaminated with bacteria of typhoid fever, cholera, diarrhoea and dysentery,
(c) Milk contaminated with salmonella, M. bovis.
3. Animals : Zoonoses are diseases which are transmitted from infected animals to humans.
(a) Cow : Bovine tuberculosis, brucellosis by Brucella abortus, Salmonella food poisoning, anthrax, Taenia saginata, (b) Fowl. Salmonella food poisoning by eggs and meat, (c) Dog. Rabies, hydatid disease, Weil's disease, (d) Horse. Tetanus and glanders, (e) Goat. Anthrax and brucellosis (by B. melitensis ), (f) Sheep. Anthrax and tetanus, (g) Cat. Cat scratch disease, (h) Rat. Plaque, Weil's disease, (i) Parrot and pigeons. Psittacosis.
4. Soil. Tetanus, gas gangrene.
1. Definitive host. In this host the adult stage of animal parasites lives and sexual reproduction takes place. Man is the definitive host for all animal parasites except malarial parasites and hydatid tapeworm.
2. Intermediate host. In this host the asexual reproduction takes place, or the larval stages of animal parasites develops. When development of larval stage takes place in two different hosts they are called 'first' and 'second' intermediate hosts. Man is the intermediate host for malarial parasites and hydatid tapeworm. Man is both definitive and intermediate host for Taenia solium and Trichinella spiralis.
3. Paratenic host. A carrier or transport host.
Note : The term infestation is used when the parasite lives on the outer surface. They are arthropodes- mites, ticks and insects.
INFECTION
Infection is the multiplication of an infectious agent within the body. Multiplication of pathogenic bacteria (e.g. Salmonella typhi) even if the person is asymptomatic is taken as an infection. Multiplication of bacteria of normal flora at its normal site is not an infection. However, if they multiply and cause disease it is an infection, e.g. Escherichia coli when causes diarrhoea.
An infection involves the following : (1) Source or reservoir of infectious agent, (2) Transmission of the infectious agent from the source to the host (3) Susceptible host- Portal or route of entry of the agent in the host, its localization, multiplication and finally host-parasite interactions which result in either (a) Destruction of the agent or, (b) Infectious disease.
Source (Reservoir) :
1. Human Source.
(a) Exogenous source : Patient or carrier,
(b) Endogenous source : The individual himself.
Carrier. A person with asymptomatic infection which can be transmitted to another susceptible person. An individual may be a carrier : (a) in the incubation period (Incubatory carrier), e.g. in measles, (b) during convalescence (convalescent carrier), e.g. typhoid fever, or (c) for a prolonged period, e.g. typhoid fever.
2. Foods and Drinks
(a) Food :Any contaminated food, (b) Water contaminated with bacteria of typhoid fever, cholera, diarrhoea and dysentery,
(c) Milk contaminated with salmonella, M. bovis.
3. Animals : Zoonoses are diseases which are transmitted from infected animals to humans.
(a) Cow : Bovine tuberculosis, brucellosis by Brucella abortus, Salmonella food poisoning, anthrax, Taenia saginata, (b) Fowl. Salmonella food poisoning by eggs and meat, (c) Dog. Rabies, hydatid disease, Weil's disease, (d) Horse. Tetanus and glanders, (e) Goat. Anthrax and brucellosis (by B. melitensis ), (f) Sheep. Anthrax and tetanus, (g) Cat. Cat scratch disease, (h) Rat. Plaque, Weil's disease, (i) Parrot and pigeons. Psittacosis.
4. Soil. Tetanus, gas gangrene.
Modes of Transmission
1. Water-borne, e.g. cholera and other diarrhoea)
diseases, enteric fever from contaminated
water.
2. Food-borne (contaminated food) e.g. enteric fever, salmonella food poisoning (e.g. eggs of duck, foul). Milk and milk products- enteric fever, bovine tuberculosis.
3. Air-borne e.g. Droplets. During coughing, sneezing and talking: Diphtheria, tuberculosis, measles, chickenpox, mumps, influenza,
4. Dust-borne. e.g. tuberculosis,
5. Soil- Tetanus, gas gangrene.
6. Contaminated fomites like beddings, clothings, utensils: Diphtheria, enteric fever, food poisoning.
7. Direct contact- STD like gonorrhoea, syphilis, AIDS.
8. Transplacental: Rubella, toxoplasma, congenital syphilis.
9. Insect borne. (a) Actively, e.g. malaria, filaria, kala azar, dengue, plague, yellow fever, (b) Passively, e.g. diarrhoea) diseases, enteric fever, salmonella food poisoning.
Portal or Route of Entry
(1) Alimentary system by ingestion, (2) Respiratory system by inhalation, (3) Genitourinary system, (4) Skin due to trauma, bite of insects, (5) Placenta.
Self-infection and cross-infection
Self-infection is the infection that occurs from the patient's own flora. Examples : (i) infection of an wound of a patient by staphylococci carried by the individual in his nose, (2) coliforms and anaerobes released from his bowel during surgery.
Cross infection is the infection derived from other patients or healthy carriers by direct spread as droplets during talking, coughing, sneezing, by air-born dust, food, fluids etc. Cross-infection is common in hospitals.
2. Food-borne (contaminated food) e.g. enteric fever, salmonella food poisoning (e.g. eggs of duck, foul). Milk and milk products- enteric fever, bovine tuberculosis.
3. Air-borne e.g. Droplets. During coughing, sneezing and talking: Diphtheria, tuberculosis, measles, chickenpox, mumps, influenza,
4. Dust-borne. e.g. tuberculosis,
5. Soil- Tetanus, gas gangrene.
6. Contaminated fomites like beddings, clothings, utensils: Diphtheria, enteric fever, food poisoning.
7. Direct contact- STD like gonorrhoea, syphilis, AIDS.
8. Transplacental: Rubella, toxoplasma, congenital syphilis.
9. Insect borne. (a) Actively, e.g. malaria, filaria, kala azar, dengue, plague, yellow fever, (b) Passively, e.g. diarrhoea) diseases, enteric fever, salmonella food poisoning.
Portal or Route of Entry
(1) Alimentary system by ingestion, (2) Respiratory system by inhalation, (3) Genitourinary system, (4) Skin due to trauma, bite of insects, (5) Placenta.
Self-infection and cross-infection
Self-infection is the infection that occurs from the patient's own flora. Examples : (i) infection of an wound of a patient by staphylococci carried by the individual in his nose, (2) coliforms and anaerobes released from his bowel during surgery.
Cross infection is the infection derived from other patients or healthy carriers by direct spread as droplets during talking, coughing, sneezing, by air-born dust, food, fluids etc. Cross-infection is common in hospitals.
