CHAPTER 7 PRINCIPLES OF DISEASE © Andy Crump / Science Photo Library.

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1 CHAPTER 7 PRINCIPLES OF DISEASE © Andy Crump / Science Photo Library

2 WHY IS THIS IMPORTANT? Infectious diseases can be understood through their cause and development (etiology) and characterization. Understanding how infectious diseases develop will help you understand they spread and can be controlled. It is important to understand the differences between normal microbial flora and abnormal or infectious microbial organisms.

3 OVERVIEW

4 USEFUL DEFINITIONS A disease is any negative change in a person’s health. Etiology is the cause and development of a disease. Normal / indigenous microbial flora are the useful microorganisms normally found in the body.

5 THE ETIOLOGY OF DISEASE Proof of etiology can be found using Koch’s postulates.

6 THE ETIOLOGY OF DISEASE In some cases, Koch’s postulates cannot be used because some organisms cannot be grown in pure culture: –Treponema pallidum (syphilis) –Mycobacterium leprae (leprosy) –Viruses and rickettsial organisms

7 FIVE STAGES OF DISEASE Incubation period – the time between the initial infection and the first symptoms. The more virulent the pathogen, the shorter the incubation time. Prodromal period – when the first mild symptoms appear.

8 Period of illness – when the majority of symptoms manifest and when the immune response is at its highest level. Period of decline – when symptoms subside. During the period, secondary nosocomial infections can occur of a nature more serious than the original infection. Period of convalescence – when the patient actively regains strength and returns to health.

9 DURATION OF DISEASE Disease duration can vary, affected by health of host. There are four categories of disease duration: –Acute diseases develop quickly and last only a short time e.g. measles. –Chronic diseases develop slowly but last for a long time e.g. tuberculosis. –Latent diseases remain in the host after the symptoms disappear and can become reactivated years later e.g. chicken pox/shingles. -Sub-acute diseases have gradual onset (usually 6 to 12 months) and are almost always fatal e.g. sclerosing pan-encephalitis

10 PERSISTENT BACTERIAL INFECTIONS Examples of persistent bacterial infections include:

11 THE SCOPE OF INFECTIONS Infections can be localized. –A local infection is contained (walled off) such as a boil or an abscess. –Local infections are the easiest to deal with medically. Infections can be systemic. –Systemic infections occur when pathogens move away from the initial infection location (also known as the focus of infection). –This movement is usually associated with the blood or the lymphatic system.

12 TERMS RELATED TO SYSTEMIC INFECTIONS Bacteremia – bacteria in the blood Septicemia – Organisms growing in the blood Toxemia – toxins in the blood Viremia – viruses in the blood

13 TYPES OF INFECTION Primary – the initial infection which has acute onset of symptoms. Secondary – seen in people that are already weakened from a primary infection and can be more dangerous. Subclinical – no symptoms are visible even though the individual is infected. These people are carriers of the disease and can infect others. Pneumonia secondary to influenza Secondary vulvovaginal candidiasis

14 TOXIC SHOCK AND SEPSIS Toxic shock and sepsis are two different clinical situations that can result from infection.

15 Staphyloccocal toxic shock syndrome (TSS) Staphyloccocus aureus Toxic shock syndrome toxin (TSST-1) Super Antigen – cytokine storm Streptococcal toxic shock like syndrome (TSLS) Streptococcus pyogenes M protein mediated activation of neutrophils Scarlet Fever Toxic shock is a massive leakage of plasma from the circulatory system. Causes a dramatic drop in blood pressure (hypotension) Fatal for 30–70% of patients TOXIC SHOCK

16 SEPSIS Sepsis is a general term referring to the presence of the pathogen or toxin in the blood. There are two forms of sepsis: –Severe sepsis –Acute septic shock

17 SEPSIS Severe sepsis is characterized by systemic inflammation and organ dysfunction. –It is accompanied by abnormal temperature, heart rate, respiratory rate, and white blood cell count. –It induces elevated numbers of liver enzymes and altered cerebral function. –Severe sepsis kills slowly over a period of weeks with minimal tissue inflammation or damage.

18 Acute septic shock has a sudden onset and death occurs in 24 to 48 hours. –It causes widespread tissue inflammation and cell damage. –Hypotension despite aggressive fluid resuscitation (>6 liters). SEPSIS