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Control Measures for Infectious Diseases Prevention or Cure • • • • Personal behavior Vaccination Vector control Disinfection – Removal – Inactivation Personal behavior • • • • • Exposure avoidance Handwashing Skin protection Respiratory protection Prophylactic treatment The body’s defenses • Skin (passive) • Non-specific immune responses – Inflammation (cytokines, macrophages, activated lymphocytes), fever – Phagocytosis by macrophages – Antibody response: IgA, IgM • Specific immune responses – Antibody production: IgG specific to target – Memory cells (B-lymphocytes) Cells of the Immune System Bone Marrow Stem Cells Blood lineage Red Blood Cells Lymphoid lineage (lymphocytes) NK Cells Platelets Pre-B Granulocytes Eosinophils, Neutrophils, Basophils Monocytes Macrophages Plasma cells Memory B-cells Pre-T (thymus) T-helper cells T-suppressor cells Memory T cells Cytotoxic T cells Delayed hypersensitivity T cells Vaccination • Develop antibodies – attenuate disease • Personal or public health measure ? • Need to have “critical mass” vaccinated to achieve control of epidemic • Practical considerations: cost, sideeffects, duration of immunity Some examples • Smallpox • Flu • “Childhood diseases” – Measles, chickenpox • Rotavirus • Bacterial diseases ? – Tetanus – Anthrax Routes of Transmission • Person-to-person: Physical contact • Indirect person-to-person – Aerosol – Fomites • Vehicle-borne – Food, water • Vector-borne – Insects Vector-borne cycle of infection • Disease agent is a microorganism • Reproduces in a reservoir or host • Is transmitted by a vector Vector-borne cycle of infection Example: West Nile Target organisms Disease agent Flavivirus Vector Reservoirs ? Vector control • Vector-borne diseases – E.g. West Nile, malaria • Identify vectors, reservoirs – Information on vector life-cycles • Eradicate vectors, reservoirs – How ? Mosquitos • • • • • • • Pesticides Larvaecides Malathion Naled (an OP) Synthetic pyrethroids Mosquito traps Drain water pools Insecticides • Chlorinated hydrocarbons • Organophosphates • Carbamates Animal Reservoirs • Cryptosporidium parvum • Single host, eg Beef, calves Oocyst •Oocyst excysts, releases 4 sporozoites •Sporozoites invade intestinal epithlial cells •Sporozoites replicate asexually, differentiate into microgametes and macrogametes •Sexual replication •More oocysts Is vaccination an option ? • Vaccinate vectors ? • Reservoirs ? • Target species ? Attack disease agent directly • Inside host – antibiotics ? • In transmission media – Fumigation, sanitization, sterilization Disinfection • Physical – Heat, pasteurize, autoclave – Time/temperature dependence • Biological – Predation, competition • Chemical – Destroy versus prevent reproduction Water disinfectants • • • • • • Chlorine Chlorine dioxide Chloramines Ozone UV light Effectiveness differs with type of organism Chlorine • Strong oxidizing agent, relatively stable in water • Produced by chloralkali process, electrolysis of salt NaCl in water • Chlorine gas, dissolved in water > hypochlorous acid HOCl at low pH, most effective form • OCl- (hypochlorite ion) at higher pH – Cl2 + H2O <->HOCl + H+ + Cl– HOCl <-> H+ + OCl- • Maintains residual, (provides a disinfectant residual) • Formation of THMs • Offensive taste/odor Chlorine Dioxide • ClO2 • Strong oxidant, though weaker oxidizing agent than chlorine • More effective at higher pH • Gas, poorly soluble in water • Poor residual Chloramines • Monochloramine, NH2Cl • Need chlorine and ammonia gas, generated on-site • Weaker oxidizing agent than chlorine • Fewer THMs • Less offensive taste/odor • Poor but stable residual Ozone • • • • • • O3 Generated on-site Strong oxidizing agent Effective against Giardia Odor/taste not offensive Poorly water-soluble, no residual • UVA, UVB, UVC Ultra-violet light – low pressure mercury lamp: low intensity; monochromatic at 254 nm – medium pressure mercury lamp: higher intensity; polychromatic 220-280 nm • Less effective in opaque/colored waters • No residual • Attacks nucleic acids, forms pyrimidine dimers 100 290 320 400 nm UVC UVB UVA Factors Influencing Disinfection Efficacy and Microbial Inactivation • Microbe type: Resistance to chemical disinfectants: • Vegetative bacteria: Salmonella, coliforms, etc. • Enteric viruses: coliphages, HAV, SRSVs, etc. Least • Protozoan (oo)cysts, spores, helminth ova, etc. – Cryptosporidium parvum oocysts – Giardia lamblia cysts – Clostridium perfringens spores – Ascaris lumbricoides ova • Acid-fast bacteria: Mycobacterium spp. Most Efficacy and Microbial Inactivation Type of Disinfectant and Mode of Action: Free chlorine: strong oxidant; oxidizes various protein sulfhydryl groups; alters membrane permeability; oxidize/denature nucleic acid components, etc. Ozone: strong oxidant Chlorine dioxide: strong oxidant Combined chlorine/chloramines: weak oxidant; denatures sulfhydryl groups of proteins Ultraviolet radiation: nucleic acid damage; thymidine dimer formation, strand breaks, etc.
