Principles of Disease and Epidemiology Chapter 14 Principles of Disease and Epidemiology

Principles of Disease and Epidemiology Pathology Study of disease Etiology Study of the cause of a disease Pathogenesis Development of disease Infection Colonization of the body by pathogens Disease An abnormal state in which the body is not functionally normally

Normal Microbiota and the Host Transient microbiota may be present for days, weeks, or months Normal microbiota permanently colonize the host Symbiosis (تعايش) is the relationship between normal microbiota and the host

Normal Microbiota and the Host: In commensalism, one organism is benefited and the other is unaffected. In mutualism, both organisms benefit. In parasitism, one organism is benefited at the expense of the other. Some normal microbiota are opportunistic pathogens.

Normal Microbiota and the Host: Locations of normal microbiota on and in the human body Figure 14.2

Normal Microbiota and the Host: Microbial antagonism is competition between microbes. Normal microbiota protect the host by: occupying niches that pathogens might occupy producing acids producing bacteriocins Probiotics are live microbes applied to or ingested into the body, intended to exert a beneficial effect.

Koch’s Postulates Koch's Postulates are used to prove the cause of an infectious disease. Figure 14.3.1

Koch’s Postulates Koch's Postulates are used to prove the cause of an infectious disease. Figure 14.3.2

They are summarized as follows: 1. The same pathogen must be present in every case of the disease. 2. The pathogen must be isolated from the diseased host and grown in pure culture. 3. The pathogen from the pure culture must cause the disease when it is inoculated into a healthy, susceptible laboratory animal. 4. The pathogen must be isolated from the inoculated animal and must be shown to be the original organism.

Exceptions to Koch's Postulates Some microbes have unique culture requirements: Treponema pallidum is known to cause syphilis (الزهري), but virulent strains have never been cultured on artificial media. The causative agent of leprosy (الجذام), Mycobacterium leprae, has also never been grown on artificial media. Many rickettsial and viral pathogens cannot be cultured on artificial media because they multiply only within cells.

Classifying Infectious Diseases Symptom A change in body function that is felt by a patient as a result of disease Sign A change in a body that can be measured or observed as a result of disease. Syndrome A specific group of signs and symptoms that accompany a disease.

Classifying Infectious Diseases Communicable disease A disease that is easily spread from one host to another (either directly or indirectly) Chickenpox, measles, genital herpes, typhoid fever Contagious disease A disease that is easily spread from one host to another.(directly) Chickenpox, measles Noncommunicable disease A disease that is not transmitted from one host to another.(tetanus: Clostridium tetani)

Occurrence of Disease Incidence Fraction of a population that contracts a disease during a specific time. Prevalence Fraction of a population having a specific disease at a given time. Sporadic disease Disease that occurs occasionally in a population. Endemic disease Disease constantly present in a population.(common cold) Epidemic disease Disease acquired by many hosts in a given area in a short time.(influenza) Pandemic disease Worldwide epidemic.(influenza, AIDS) Herd immunity Immunity in most of a population.

Severity or Duration of a Disease Acute disease Symptoms develop rapidly but lasts only a short time; influenza Chronic disease Disease develops slowly (tuberculosis, Hepatitis B) Subacute disease Symptoms between acute and chronic (panencephalitis) Latent disease Disease with a period of no symptoms when the patient is inactive(shingles)

Extent of Host Involvement Local infection Pathogens limited to a small area of the body (boils and abscesses) Systemic infection An infection throughout the body (measles) Focal infection Systemic infection that began as a local infection Bacteremia Bacteria in the blood Septicemia Growth of bacteria in the blood

Extent of Host Involvement Toxemia Toxins in the blood Viremia Viruses in the blood Primary infection Acute infection that causes the initial illness Secondary infection Opportunistic infection after a primary (predisposing) infection (streptococcal bronchopneumonia following influenza) Subclinical disease No noticeable signs or symptoms (inapparent infection).. Poliovirus and hepatitis A virus

Predisposing Factors Make the body more susceptible to disease Short urethra in females Inherited traits such as the sickle-cell gene (Malaria) Climate and weather Fatigue Age Lifestyle Chemotherapy

The Stages of a Disease

Reservoirs of Infection Reservoirs of infection are continual sources of infection. Human — AIDS, gonorrhea Carriers may have inapparent infections or latent diseases Animal — Rabies, Lyme disease Some zoonoses may be transmitted to humans Nonliving — Botulism, tetanus Soil

Transmission of Disease Contact Direct Requires close association between infected and susceptible host Indirect Spread by fomites Droplet Transmission via airborne droplets Bacterial :Tuberculosis, Pneumococcal infection, Meningococcal infection, Staphylococcus aureus infection. Viral: Chickenpox, Measles, Influenza, Fungal: Aspergillus, Cryptococcus neoformance

Transmission of Disease Figure 14.6a & 8

Transmission of Disease Vehicle Transmission by an inanimate reservoir (food, water) Vectors Arthropods, especially fleas, ticks, and mosquitoes Mechanical Arthropod carries pathogen on feet Biological Pathogen reproduces in vector

Transmission of Disease Figure 14.6b, c

Nosocomial (Hospital-Acquired) Infections Are acquired as a result of a hospital stay 5-15% of all hospital patients acquire nosocomial infections Figure 14.7, 9

Relative frequency of nosocomial infections Figure 14.10

Common Causes of Nosocomial Infections Percentage of nosocomial infections Percentage resistant to antibiotics Gram + cocci 34% 28%-87% Gram – rods 32% 3-34% Clostridium difficile 17% Fungi 10%

Infection by hospital equipment Infection from surgical instruments is now rare. But instruments can convey infection from one patient to another. Following infections have been reported to be transmitted from instruments: Rhizopus Mucor Clostridium Salmonellosis (through rectal thermometers ) Hepatitis Mycobacterium tuberculosis

Infection by inoculation The infections through inoculation can be transmitted in the following ways: Through blood and blood products. Accidental injury from sharp contaminated insruments. Through unsterile syringes and needles.

Infection by inoculation Various organisms and diseases which have been reported to be transmitted through this mode include: Hepatitis B,C,D. Brucellosis Salmoellosis Treponemes Toxoplasmosis

Emerging Infectious Diseases Diseases that are new, increasing in incidence, or showing a potential to increase in the near future. Contributing factors: Evolution of new strains V. cholerae O139 Inappropriate use of antibiotics and pesticides Antibiotic resistant strains Changes in weather patterns Hantavirus

Emerging Infectious Diseases Contributing factors: Modern transportation West Nile virus Ecological disaster, war, expanding human settlement Coccidioidomycosis Animal control measures Lyme disease Public Health failure Diphtheria