MIC 252 Features of Surface and Systemic Infections

1. MIC 252 Features of Surface and Systemic Infections • Learning Outcomes • State Koch’s postulates • Describe general characteristics of local and systemic infections • Explain the factors which prevent surface infections from spreading further • Explain why some microbes risk spreading through blood and lymph

2. Normal Flora?, Soil, Water, Foul, Rodents, etc Contact, fomites, droplet, vehicle, airborne, arthropods Portals of entry Traverse epithelium, basement membrane, junctions, invade cells, spreading factors Hydrophobicity, surface charge, pili, fimbriae, capsules, LPS, LTA

3. Definitions • Disease is an abnormal state in which part or all of the body is not properly adjusted or is incapable of performing normal functions • Infection: is the invasion and growth of pathogens in the body with or without disease • Infectious disease: A clinically manifesting disease of humans or animals resulting from an infection

4. Causes of Infectious Disease • Quite difficult to show that a specific bacterial species is the cause of a particular disease • More than 100 microbes that commonly cause infection  some remaining in the body for many years afterwards • Several hundreds cause less common infections • In 1884 Robert Koch proposed a series of postulates to link a microorganism to be accepted as the cause of a given disease

5. Koch’s Postulates

6. Types of Infections • A local infection affects a small area of the body • A systemic infection is spread throughout the body via the circulatory system • A secondary infection can occur after the host is weakened from a primary infection • A subclinical (inapparent) infection does not cause any signs of disease in the host • A continual source of infection is called a reservoir of infection. • People who have a disease or are carriers of pathogenic microorganisms are human reservoirs of infection • Zoonoses are diseases in wild and domestic animals that can be transmitted to humans.

7. Measurement of Infection • Infectious dose (ID50)- number of cells required to cause infection in 50% of the subjects. • Lethal dose (LD50)- number of cells required to cause death in 50% of the subjects.

8. Surface Infections • Many microorganisms multiply in epithelial cells at site of entry on the body surface • Local spread takes place on fluid-covered mucosal surfaces  large scale movement of fluid spread the infection to distant areas on the surface e.g., in the GIT • In the URT high winds (coughing and sneezing) splatters microbes onto new areas or into opening of sinuses in the middle ear • Gentler downward trickle of mucus during sleep may spread microbe into the LRT •  Large areas of the body surface can be involved within a few days with shedding to the exterior

9. General Characteristics of Surface Infections • Short incubationperiod(length of time between infection with the agents and onset of symptoms of disease) <1 week • Microbes are replicating very rapidly • Nonspecific defenses are key to control - the early nonspecific defense mechanisms like phagocytosis, complement activation, interferon, and NK cells are key to controlling the infection - no time to wait for specific defenses to activate • E.g., Common cold, Gastroenteritis, Gonorrhea

10. Systemic Infections • Microorganisms spread systemically through the body via lymph and blood • Undergo a series of stepwise invasions of various tissues before reaching the final site of replication • Can take weeks to develop and shed to the exterior

11. General Characteristics of Systemic Infections • Longer incubation times, >1 week  Ebola virus, 12-21 days incubation; HIV, several years • Microbes replicate more slowly • Specific defenses are key to control - because of longer incubation times, specific defenses can activate and play a role in controlling infection • Measles, Typhoid fever

13. Factors Which Prevent Surface Infections From Spreading • Temperature • e.g., Rhinovirus- upper respiratory tract is 33°C, lower respiratory tract is 37°C. Rhinovirus can't replicate at 37ºC • e.g., Mycobacterium leprae is also temperature sensitive, which accounts for its replication being more or less limited to nasal mucosa, skin and superficial nerves • Site of budding • e.g., Influenza and parainfluenza viruses invade surface epithelial cells of the lung, but are liberated by budding from the free (external) surface of the epithelial cell, not from the basal layer from where they could spread to deeper tissues

15. Why Risk Spread –especially through Blood and Lymph • Many microbes are obliged to spread systemically because they fail to spread and multiply at the site of initial infection • e.g., measles and typhoid fever  almost no replication at initial respiratory or intestinal infection  spread systemically and then delivered back to the same surface where they then multiply and shed • Other microbes have committed themselves to infection by one route and multiplication and shedding at another • E.g., mumps and hepatitis A virus  infect via respiratory and alimentary routes but must spread through body to invade and multiply in the salivary glands and liver • It can only get to these distant sites if it spreads through the blood and lymph