Pathogenicity and Transmission of Infection PDF

# Pathogenicity The capacity to initiate disease. It requires the following attributes: - Transmissibility or communicability from one host or reservoir to a fresh host. - Survival in the new host. - Infectivity or the ability to breach the new host’s defenses. - Virulence: a variable that is multi...

# Pathogenicity The capacity to initiate disease. It requires the following attributes: - Transmissibility or communicability from one host or reservoir to a fresh host. - Survival in the new host. - Infectivity or the ability to breach the new host’s defenses. - Virulence: a variable that is multifactorial and denotes the capacity of a pathogen to harm the host. Virulence in the clinical sense is a manifestation of a complex bacterial-host relationship in which the capacity of the organism to cause disease is considered in relation to the resistance of the host. ### The pathogenesis of bacterial infection includes: - Initiation of the infectious process. - Mechanisms that lead to the development of signs and symptoms of disease. ## Types of Bacterial Pathogens Bacterial pathogens can be classified into two broad groups: - **Primary pathogens** - **Opportunistic pathogens** ### Primary Pathogens Primary pathogens are capable of establishing infection and causing disease in previously healthy individuals with intact immunological defenses. However, these bacteria may more readily cause disease in individuals with impaired defenses. ### Opportunistic Pathogens Opportunistic pathogens rarely cause disease in individuals with intact immunological and anatomical defenses. Only when such defenses are impaired or compromised, as a result of congenital or acquired disease or by the use of immunosuppressive therapy or surgical techniques, are these bacteria able to cause disease. Many opportunistic pathogens, e.g., coagulase negative staphylococci and Escherichia coli, are part of the normal human flora and are carried on the skin or mucosal surfaces where they cause no harm and may actually have beneficial effects, by preventing colonization by other potential pathogens. However, introduction of these organisms into anatomical sites in which they are not normally found, or removal of competing bacteria by the use of broad-spectrum antibiotics, may allow their localized multiplication and subsequent development of disease. ## Terms in the Bacterial Pathogens - **Infection:** Multiplication of an infectious agent within the body. - **Multiplication of the bacteria that are part of the normal flora is generally not considered an infection.** - **Invasion:** The process whereby bacteria, animal parasites, fungi, and viruses enter host cells or tissues and spread in the body. - **Microbiota:** Microbial flora harbored by normal, healthy individuals. - **Nonpathogen:** A microorganism that does not cause disease; May be part of the normal microbiota. - **Pathogen:** A microorganism capable of causing disease. # Transmission of Infection ## 1. Source of infection Bacteria (and other microorganisms) can adapt to a variety of environments which can act as a source for infection that include: - Soil - Water - Food ## 2. External sources such as: - **Insect vectors** - **Animals (reservoir or zoonosis)** - **Humans (either as patients or as healthy carriers)** Some bacteria that commonly cause disease in humans exist primarily in animals and incidentally infect humans. For example, Salmonella and Campylobacter species typically infect animals and are transmitted in food products to humans. The Clostridium species are ubiquitous in many environmental sources and are transmitted to humans by ingestion (eg, C perfringens gastroenteritis and C. botulinum [botulism]) or when wounds are contaminated by soil (eg, C perfringens [gas gangrene] and C tetani [tetanus]). ## 2. Modes of transmission of infection Pathogenic organisms can spread from one host to another by a variety of mechanisms. These include: ### 1. Contact: - **Direct contact** include sexually transmitted diseases such as syphilis, gonorrhea. - **Indirect contact** (via fomites such as clothes, pencils and drinking cups). ### 2. Inhalation - **Droplet nuclei (such as meningococci and TB)** - **Dust (such as streptococcal and staphylococcal infection, pneumonia and tuberculosis)** ### 3. Ingestion of food or drink contaminated by pathogens: - **Foodborne** such as (dysentery) - **Waterborne** such as (cholera) ### 4. Inoculation The pathogens can be inculcated directly into human skin or mucosa via different ways such as (wounds, burns, dog bite, using contaminated needles and syringes). ### 5. Insects (Vector borne) Diseases can be transmitted via insects include dysentery, gastroenteritis and diarrhea. ### 6. Congenital: Infection can happen when pathogens cross the placental barrier and reach the fetus in utero (such as in congenital syphilis). Sometimes the infant acquired the pathogen during passage through birth canal such as in gonorrhea. ### 7. Iatrogenic and laboratory infections - Pathogens transmission can happen for example during administration of injections, lumber puncture, exchange transfusion, dialysis, catheterization and other. - Laboratory personnel handling infectious material and doing mouth-pipetting are particularly at risk. Many opportunistic pathogens that cause nosocomial infections are transmitted from one patient to another on the hands of hospital personnel. - The clinical manifestations of diseases (eg, diarrhea, cough, genital discharge) produced by microorganisms often promote transmission of the pathogens. - Examples: Vibrio cholera can cause voluminous diarrhea, which may contaminate fresh water; drinking water or seafood may be contaminated; ingestion of contaminated water or seafood can produce infection and disease.