180509P - EPIDEMIOLOGY OF COMMUNICABLE DISEASES

Epidemiology lecture for prosthetics diploma students at King Fahad Medical City on May 9, 2018 by Professor Omar Hasan Kasule Sr MB ChB (MUK), MPH (Harvard), DrPH (Harvard)

DEFINITION OF TERMS:

• A communicable or infectious disease is a disease transferred from person to person by micro-organisms. 
• Infection is the lodging, growing, and multiplication of micro-organisms in the host’s body. 
• Invasion is the mere presence of micro-organisms without necessarily multiplying. 
• Infectivity is the ability of the agent to lodge and grow in the host. 
• Pathogenicity is the ability of the agent to cause disease. 
• Virulence is the ability to cause severe disease. 
• The term epidemic is used to refer to a widespread disease whereas the term outbreak is used to refer to localized disease. 

MECHANISMS OF PATHOGENICITY:

• Pathogenicity is affected by invasiveness, toxigenicity, and hypersensitivity. 
• Invasiveness: Agents may invade and destroy tissues (eg shigella)
• Toxigenicity: Agents may cause disease by producing toxins (eg botulinum), 
• Hypersensitivity: Agents may cause disease by allergic reactions (like Mycobacterium tuberculosis). 

CONCEPT OF THE EPIDEMIOLOGICAL TRIANGLE:

• The communicable disease model comprises the agent, the host, and the environment. Disease transmission occurs when a susceptible host and a pathogenic agent exist in an environment conducive to disease transmission. 
• The concept of the communicable disease triangle simplifies the discussion of communicable disease. The triangle consists of the agent, the host, and the disease. They interact among one another. 
• Elements of the environment that affect disease transmission are: climate (temperature, rain, and wind patterns), vegetation (swamps, forest, and desert), water sanitation, air pollution, excreta disposal, housing, occupation (farm, factory). 
• Poor sanitation and crowding increase the transmission of microorganisms. Breeding places near homes, forest reservoirs, and soil help the survival of organisms and their vectors.

AGENT: CLASSIFICATION OF MICRO-ORGANISMS - 1:

• Microorganisms can be classified by structure as prokaryotes (bacteria, rickettsiae, and Chlamydia), eukaryotes (fungi, protozoa, and helminths), and viruses; 
• Microorganisms can be classified by transmission: feco-oral (ameba, giardia, shigella, vibrio cholera, salmonella spp, hepatitis virus A&E, poliovirus, and tapeworms), soil contact (trichuriasis, Ascaris, hookworms, Strongyloides, and tetanus), water contact (schistosomiasis and guinea worm), skin contact (chickenpox and smallpox), air transmission (measles, pertussis, diphtheria, and tuberculosis), contact with body fluids (trachoma, syphilis, gonorrhea, and HIV), and arthropod-borne (dengue virus and the malarial protozoan).

AGENT: CLASSIFICATION OF MICRO-ORGANISMS - 2:

• Microorganisms can be classified by infectivity, virulence, pathogenicity, toxigenicity, etc 
• Microorganisms can be classified as saprophytic (live on dead organic matter), parasitic (depend on and harm the human host), commensals (depend on but do not harm the human host), and symbiotic (mutual benefit). 
• Obligate parasites (e.g. viruses) cannot exist outside their host. 
• Normal bacterial flora is opportunist pathogens. 

AGENT: MICROORGANISMS:

• Viral infections are recognized by detection of viral antigens or antibodies to viral antigens using serological technics (neutralization, hemagglutination inhibition, complement fixation, fluorescent antibody, radioimmunoassay, and ELISA) and cell culture. 
• Diagnosis of bacterial infections is based on cultivation & identification, gram staining & microscopy, biochemical reactions, use of specific antibodies, and DNA hybridization. 
• Diagnosis of fungal infection is difficult and is based on clinical or histological examination. 

AGENT: PROTOZOA:

• Protozoa are single-celled eukaryotic organisms widely distributed in the world in both parasitic and free-living forms. The species that are epidemiologically important are plasmodium, toxoplasma, and pneumocytes. 
• Plasmodia cause malaria that is the most important protozoal disease in terms of mortality and morbidity. P. falciparum causes more mortality and morbidity than P.ovale, P. vivax, and P. malariae. 
• Toxoplasma Gondi has a high prevalence with little clinical disease. 
• Pneumocytes carinii is an opportunistic infection in HIV/AIDS. 
• Helminths are flatworms (cestodes), flukes (trematodes), and roundworms (nematodes). The most important helminths epidemiologically are schistosomiasis, hookworm, Strongyloides, echinococcus, tenia, and Toxocara. 

METHODS OF TRANSMISSION - 1:

• Common vehicle spread is by water, air, or food. 
• Serial transmission is human to human, human to animal to human, or human to the environment to human. 
• Infection can be exogenous or endogenous. Auto-infection by Strongyloides and E.coli. 
• Horizontal transmission (human to another) is more common than vertical disease transmission (intra-uterine from the mother to fetus ?CMV, ?toxoplasma, ?rubella, ?HSV, ?syphilis, ?TB, ?VZ.). 
• The natural portals of entry into humans are the respiratory tract (common cold, influenza, measles, TB, whooping cough), the urogenital tract (gonorrhea, syphilis, herpes, HIV), the alimentary tract (amebic dysentery, shigellosis, polio, and cholera), the mucous membranes, the skin, the placenta (rubella, syphilis, and HBV), and the parenteral portal (intravenous and sexual). 

METHODS OF TRANSMISSION - 2:

• Air-borne infections are TB, influenza, histoplasmosis, and legionellosis. 
• Vehicle-borne transmission is by contaminated materials or objects (fomites). 
• Vectors may be arthropods (such as mosquitoes, fleas, flies, lice, and ticks), zoonoses, plants, or other vehicles. They transmit organisms mechanically (eg flies and cockroaches) or hematophagous (eg ticks, lice, fleas, mites), or biologically. 
• Zoonoses are diseases whose reservoirs are vertebrate animals and are transmitted to humans by accident for example plague, rabies, Rocky Mountain spotted fever. 
• Anthroponooses are diseases whose reservoirs are human for example measles. 
• Plants can be vectors of disease when they are contaminated by micro-organisms and are eaten raw. 

THE HOST:

• Humans can be an intermediate, definitive, reservoir, and accidental hosts. Humans can also be in a carrier status. 
• Humans can be healthy, incubational, convalescing, or chronic carriers. 
• Susceptibility to infection is determined by age, heredity, gender, pregnancy, nutritional status, lifestyle and behavior, personal hygiene, and immune resistance (natural or acquired; passive or active). 
• Immune resistance is the main barrier to infection by micro-organisms. 

THE DISEASE:

• Clinical severity can be described as mild, moderate, severe, and fatal. 
• Clinical manifestations are: asymptomatic, latent, sub-clinical, and clinical. 
• The 4 stages of natural history are pre-pathogenesis, pre-clinical, clinical, and chronic. 

• An epidemic occurs due to changes in the agent, host susceptibility, and effective transmission. The agent may be new, may increase in number, or may change in virulence. 
• An adequate susceptible population is required to sustain and propagate the epidemic. 

CONTROL and PREVENTION OF COMMUNICABLE DISEASE:

• Control is by identification of cause, notification, treatment of cases using drugs, prevention, and surveillance. The strategy is attacking the agent at the source, interrupting transmission, and reducing the susceptible population. 
• Measures for the healthy host are active immunization, passive immunization, chemoprophylaxis, behavioral change (sexual, dietary), physical isolation, and increase of host resistance by better nutrition and health care. 
• Measures for the diseased host are chemotherapy, isolation, quarantine, restriction of activity, and behavioral change. 
• Vector control is by chemicals, environmental and biological control. 
• Control of animal reservoirs is by active immunization, restriction of movement or reduction in number, chemoprophylaxis and chemotherapy. 
• Environmental control is by water sanitation, safe drinking water, excreta disposal, and food sanitation. 
• Control of the agent is by cleanliness, refrigeration, disinfection, and sterilization. 

DISEASE NOTIFICATION:

• Disease notification plays a central role in disease control. 
• The notifiable diseases are: AIDS, anthrax, botulism, brucellosis, cholera, congenital rubella syndrome, diphtheria, encephalitis, gonorrhea, H. influenzae, Hansen’s disease, leptospirosis, lyme disease, measles, plague, paralytic polio, psittacosis, rabies, syphilis, tetanus, trichinosis, tularemia, typhoid, and typhus. 

PREVENTION:

• Primary prevention is the prevention of initial contact and/or infection. 
• Secondary prevention consists of diagnosing and treating cases. 
• Tertiary prevention limits chronic disability by physiotherapy, supportive care, and surgical correction of deformities.