MODES OF TRANSMISSION OF PATHOGENS In order for an infection to spread, there must be three things: 1. A source of infection (bacteria, virus, fungi): could be sick person, carrier (asymptomatic), a person’s own microflora, contaminated objects 2. A susceptible host: resistance to infection can vary greatly, depending on the host’s health status and on the virulence factors of the infecting agent 3. A route of transmission from person or site to another: There are five main routes of transmission – some organisms can infect through more than one route! ROUTE #1: CONTACT TRANSMISSION There are two types of contact transmission – direct and indirect. DIRECT CONTACT: • Direct physical transfer of a microorganism from person to person such as sexually transmitted diseases, or bowel flora contaminating urinary tract. • Diseases spread exclusively by direct contact are unable to survive for significant periods of time away from a host. Sexually transmitted diseases are almost always spread through direct contact, as they are extremely sensitive to drying. • Examples: HIV/AIDS, Hep B/C, Chlamydia, HPV, Gonorrhea, Syphilis, Herpes INDIRECT CONTACT: • Contact with contaminated objects such as bed linen, surgical instruments, needles, door knobs and handles, washroom objects, supermarket cart handles, cups, dishes, computer keyboards, toys, pens, pencils, money, etc. • Some organisms (such as Norwalk virus) are capable of surviving on surfaces for an extended period of time. • Examples: Athlete’s foot, flu virus, cold virus, HPV, chicken pox, Hep A, mono, mumps, scarlet fever, etc. Organisms transmitted through contact must be able to get past body’s barriers – through mucous membranes (including urinary and vaginal tracts), eyes, mouth or through cuts in skin – very few organisms can overcome the skin barrier! ROUTE #2: AIRBORNE TRANSMISSION • Some organisms can survive in the air for periods of time before landing on a host, a surface that might be touched by a host or being inhaled by a host. • They can be spread through ventilation systems and can be widely dispersed on air currents. • Examples: certain fungal infections, chicken pox, anthrax, tuberculosis, measles • Fortunately, only a limited number of diseases are capable of airborne transmission. DROPLET TRANSMISSION: • Many infectious agents cannot survive exposure to the drying air for a long period of time – but can be transmitted rather quickly through droplets spread by sneezing, coughing or talking. • These organisms can survive long enough to be inhaled directly or to land on a surface and be picked up by a host. • Examples: Cold, flu, diphtheria, meningitis, plague, whooping cough, Strep infections, etc. ROUTE #3: FOOD AND WATER TRANSMISSION • This includes diseases spread through a fecal/oral route. • Infection is caused through ingestion of infectious organisms • Food and water can become contaminated in a number of ways: - Organisms naturally present in food – need to be cooked properly - Infection from sewage - Improper handling of raw foods If proper hygienic and sanitation practices are not in place, the microorganisms in the feces may contaminate the water supply through inadequate sewage treatment and water filtration. Fish and shellfish that swim in contaminated water may be used as food sources. If the infected individual is a waiter, cook, or food handler, then inadequate hand washing may result in food being contaminated with microorganisms. Examples: Cholera, Hep A, Polio, Salmonella, Typhoid, Listeria, E. coli ROUTE #4: VECTOR BOURNE TRANSMISSION • Vectors are animals that are capable of transmitting diseases without contracting the disease itself. • Examples include: flies, mites, fleas, ticks, rats, and dogs and even farm animals • Since vectors are mobile, they increase the transmission range of a disease • Vectors can spread disease through biting, through their feces and through carrying the infectious agent on their bodies. • Organisms that are infected by the disease, from which vectors can obtain the infectious agent, are called resevoirs. • Examples: malaria (mosquitos), Lyme disease (ticks), rabies, West Nile Virus (mosquitos), Japanese encephalitis (mosquitos), dengue fever (mosquitos) From WHO website ROUTE #5: TRANSPLACENTAL TRANSMISSION (VERTICAL TRANSMISSION) • Passed from mother to fetus through the placenta. • Examples: Toxoplasmosis, chicken pox, Hep B, Epstein-Barr, Syphilis, rubella, herpes • Infants can also be infected as they pass through the birth canal. • Examples: Gonorrhea, chlamydia, HPV, Streptococcal diseases NOTE: HIV can be transmitted to the fetus during pregnancy, but it is rare. Most transmission takes place during childbirth. Transmission can also occur from breastfeeding if the mother breastfeeds for more than 24 months – breastfeeding exclusively for the first 6 months is recommended.