There are three important components that play an important role in successful transmission of microbial diseases. These are (a) reservoir, (b) mode of transmission, and (c) susceptible host.
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Reservoir
Reservoirs of microbial infections are human, animal, plant, soil, or inanimate matter in which organisms usually live, mul- tiply, and cause the infections with or without overt clinical manifestations. Humans are usually the common reservoirs of many of the microbial infections. Animals are reservoirs of zoonotic infections, such as plague (e.g., rats), rabies (e.g., dogs), cysticercosis (e.g., pigs), etc.
Sources of infections: The sources of infections may be endogenous and exogenous:
■ Endogenous sources: The source of infection is the normal bacterial flora present in the human body. These bacteria are usually nonpathogenic but in certain situations become pathogenic and cause infections at different sites in the same host. For example, Escherichia coli present as normal flora of the intestine may cause urinary tract infection in the same host. Similarly, viridans streptococci present as a part of the normal flora of the mouth may cause infective endocarditis.
Virulence factors Bacteria
Capsule
Polysaccharide capsule
Polypeptide capsule
Streptococcus pneumoniae Klebsiella pneumoniae Haemophilus influenzae Salmonella Typhi Neisseria meningitidis Bacillus anthracis
Pili protein Escherichia coli
Protein A Staphylococcus aureus
M protein Streptococcus pyogenes
V and W proteins Yersinia pestis
TABLE 10-1 Important bacterial surface virulence
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■ Exogenous sources: The source of infection is from outside the host’s body. Most of the microbial infections are exoge- nous in nature. The exogenous sources include the following:
1. Humans: Humans are the most common sources of infections caused by the microorganisms. They may be either patients or carriers. The patient suffering from an active infec- tion is an important source of infection to others.
A carrier is a person who harbors pathogenic microorgan- isms without showing any signs and symptoms of disease.
Carriers are also important sources of infections. A carrier may be (a) healthy carrier, (b) convalescent carrier, (c) temporary carrier, and (d) chronic carrier.
■ Healthy carrier is the host who harbors the microorgan- ism without ever suffering from the disease caused by that microorganism.
■ Convalescent carrier is the host who continues to harbor the microorganism even after recovering from the clinical disease caused by the same pathogen.
■ Temporary carrier is the host who harbors the microorgan- ism up to 6 months after recovering from the disease caused by the same pathogen.
■ Chronic carrier is the host who harbors the microorgan- ism for many years after recovering from the clinical disease caused by the same pathogen.
2. Animals: Animals are also important sources of infection for humans. The symptomatic as well as asymptomatic animals can transmit infections to humans. Asymptomatic animals act as a reservoir of human infections. These are called as reservoir hosts. Infections transmitted from animals to humans are called zoonotic infections.
The examples of zoonotic infections include bacterial (e.g., plague, anthrax, bovine tuberculosis, etc.), viral (e.g., rabies, Japanese encephalitis, etc.), fungal (e.g., dermatophytic infec- tions), and parasitic (e.g., toxoplasmosis, cysticercosis, hydatid disease, etc.).
3. Insects: Insects, such as mosquitoes, ticks, mites, flies, fleas, and lice may transmit a wide variety of microorganisms to the humans (Table 10-2). The diseases transmitted by the insects are collectively referred to as arthropod-borne diseases and the insects transmitting these pathogens are called vec- tors. Insect vectors may transmit the infection in two ways:
mechanical transmission and biological transmission.
■ Mechanical vectors: Insects (e.g., domestic flies) carry enteric bacteria (Salmonella typhi, Shigella spp., etc.) mechani- cally on their legs, wings, and surface of the body and trans- fer them to food.
■ Biological vectors: These are the vectors in which the microorganisms multiply or undergo a part of their life
Organism Disease Reservoir Insect
Bacteria
Yersinia pestis Plague Rodents, especially rats Rat fleas
Francisella tularensis Tularemia Rabbits and other rodents Ticks
Borrelia recurrentis Relapsing fever Humans Lice
Borrelia burgdorferi Lyme disease Rodents and deer Ixodes ticks
Rickettsia rickettsii Rocky mountain spotted fever Dogs, rodents, and ticks Ticks
Rickettsia prowazekii Epidemic typhus Humans Lice
Ehrlichia chaffeensis Ehrlichiosis Deer and rodents Ticks
Parasites
Leishmania spp. Leishmaniasis Humans, dogs, and wild canine Sandfly (Phlebotomus)
Trypanosoma spp. Trypanosomiasis Humans, armadillos, antelope,
and cattle
Reduviid bug, tsetse fly
Plasmodium spp. Malaria Humans Anopheles mosquitoes
Wuchereria bancrofti and Brugia malayi
Filaria Humans Mosquitoes (Culex, Aedes,
Mansonia spp.) Viruses
Japanese encephalitis virus Japanese encephalitis Birds Culex tritaeniorhynchus mosquitoes
West Nile virus West Nile fever Birds Culex mosquitoes
Chikungunya virus Chikungunya Humans and monkeys Aedes aegypti mosquitoes
Dengue virus Dengue Humans Aedes aegypti mosquitoes
Kyasanur forest disease virus Kyasanur forest disease Forest birds and small mammals Tick
Yellow fever virus Yellow fever Monkeys and humans Aedes aegypti mosquitoes
TABLE 10-2 Microbial diseases transmitted by insects
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cycle before being transmitted to humans. Rat fl ea and female Anopheles mosquitoes are the examples of biological vectors that transmit plague and malaria, respectively, to humans by biting.
Insects, besides acting as vectors, also act as reservoir hosts (e.g., ticks in relapsing fever).
4. Food: Food items contaminated with pathogens also act as source of infection and cause diarrhea, dysentery, food poison- ing, and gastroenteritis.
5. Water: Water contaminated with microorganisms also acts as a source of infection and transmits water-borne diseases, such as leptospirosis, cholera, dysentery, hepatitis A infection, etc.
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Modes of transmission
Microbial pathogens causing various infectious diseases are transmitted from one host to another by many ways: (a) con- tact, (b) inoculation, (c) ingestion, (d) inhalation, and (e) vectors (Table 10-3).
1. Contact: Transmission of microorganisms from person to person occurs by direct or indirect contact. Transmission by direct contact occurs through the acts of touching, kissing, sex, etc. Hence, this mode of transmission is also known as person- to-person transmission. The diseases transmitted by direct contact include common cold, staphylococcal infections, and sexually transmitted infections (e.g., gonorrhea, syphilis, and AIDS, etc.). The term contagious disease was used earlier for the disease acquired by direct contact.
Microorganisms can also be transmitted by indirect con- tact through inanimate objects, such as clothings, handker- chief, toys, etc., called fomites. The fomites, contaminated by microbial pathogens, act as a vehicle for their transmission.
Influenza, tuberculosis, and certain superficial fungal infec- tions are examples of diseases transmitted by fomites.
2. Inoculation: Infections can be transmitted by inoculation of microorganisms directly into tissues of the host. For example, tetanus is transmitted by direct inoculation of Clostridium tetani spores present in soil to the injured tissues in the host. These spores then germinate to vegetative forms of bacteria and migrate along the neural tissues to cause teta- nus. Similarly, rabid dogs through their act of biting inoc- ulate rabies virus directly to host tissue and cause rabies in humans.
Iatrogenic infection occurs following the use of unsterile syringes and equipment in a hospital. Hepatitis B and C and HIV infections are the examples of iatrogenic infections caused by use of contaminated syringes and that of contaminated blood and blood products.
3. Ingestion: Ingestion of water and food contaminated with microorganisms can transmit a wide variety of micro- bial infections. For example, food poisoning caused by Bacillus cereus is transmitted by rice contaminated with bac- terial spores that survive boiling. If the rice is cooled and reheated, the spores of the bacteria may germinate to the bacteria that produce a heat-stable toxin that induces vomit- ing. Cholera, typhoid, food poisoning, hepatitis A, poliomy- elitis, and many parasitic infections are the other examples of diseases transmitted by ingestion of contaminated food and water.
4. Inhalation: Infections are transmitted by inhalation of droplet nuclei that are discharged into the air by coughing, sneezing, or talking. Respiratory pathogens are shed into the environment by patients in secretions from the nose or throat during coughing, sneezing, or talking. Small droplets (less than 0.1 mm in diameter) become airborne as minute particles or droplet nuclei (1–10 m in diameter), while large droplets fall down to the ground. Infections are transmitted by inhalation of these droplet nuclei containing respiratory pathogen, which remain suspended in air for a long period of time.
Measles, influenza, whooping cough, tuberculosis, aspergil- losis, etc. are few examples of infectious diseases acquired by inhalation.
5. Vectors: Mosquitoes, flies, fleas, ticks, mite, and lice are the vectors that transmit many diseases as mentioned earlier.
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Susceptible host
The infective agent enters the body by four main routes:
(a) genital tract, (b) respiratory tract, (c) gastrointestinal tract, and (d) skin. The pathogens can be transmitted either as vertical or horizontal transmission.
Vertical transmission: Certain bacteria (Treponema palli- dum), viruses (rubella and cytomegalovirus), and parasites (Toxoplasma gondii) can be transferred from mother to fetus by a process called vertical transmission (Table 10-4). The organ- isms can be transmitted vertically by three ways:
(a) Across the placenta,
(b) Within birth canal during birth, and (c) Through breast milk.
Modes of transmission
Disease Causative agents
Human to human
Direct contact Gonorrhea Neisseria gonorrhoeae Indirect contact Dysentery Shigella dysenteriae
Blood-borne Syphilis Treponema pallidum
Transplacental Congenital syphilis Treponema pallidum Nonhuman to human (animal origin)
Direct contact Cat-scratch disease Bartonella henselae Lyme disease Borrelia burgdorferi Through animal
excreta
Hemolytic uremic syndrome
Enterohemorrhagic Escherichia coli (EHEC) Handling of fomites Skin infection Staphylococcus aureus
Soil borne Tetanus Clostridium tetani
Water borne Legionnaire’s disease Legionella pneumophila TABLE 10-3 Different modes of transmission
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Table 10-4 summarizes a list of diseases transmitted vertically.
Horizontal transmission: Unlike vertical transmission, hori- zontal transmission occurs from person to person and is not from mother to offspring (Fig. 10-1).