Control Measures for Infectious Diseases

• Personal behavior

• Vaccination

• Vector control

• Disinfection– Removal– Inactivation

Prevention or Cure

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 SystemBone Marrow Stem Cells

Blood lineage

Red Blood Cells

Platelets

GranulocytesEosinophils, Neutrophils, Basophils

Monocytes

Macrophages

Lymphoid lineage (lymphocytes)

NK Cells

Pre-B Plasma cells

Memory B-cells

Pre-T(thymus)

T-helper cellsT-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, side-effects, 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 infectionExample: West Nile

Flavivirus

Disease agentTarget organisms Reservoirs

?Vector

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

Ultra-violet light• UVA, UVB, UVC

– 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 nmUVAUVBUVC

Factors Influencing DisinfectionEfficacy and Microbial Inactivation

• Microbe type: Resistance to chemical disinfectants:• Vegetative bacteria: Salmonella, coliforms, etc.

• Enteric viruses: coliphages, HAV, SRSVs, etc.

• Protozoan (oo)cysts, spores, helminth ova, etc.– Cryptosporidium parvum oocysts– Giardia lamblia cysts– Clostridium perfringens spores– Ascaris lumbricoides ova

• Acid-fast bacteria: Mycobacterium spp.

Least

Most

Factors Influencing Disinfection Efficacy

and Microbial InactivationType 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.