Source of Hospital acquired infections

The transmission of nosocomial infections requires three elements: source of infecting microorganisms, mode of transmission and susceptible host. The human source of nosocomial infection may be patients, hospital personnels or visitors. These people may have infectious diseases, be in the incubation period, or may be chronic carriers. Other sources of infecting microorganisms are inanimate objects that become contaminated e.g. examination tables or medical instruments and the environment, including air and water. Inanimate surfaces have been described as the source for outbreaks of nosocomial infections. The most common nosocomial pathogens may well survive on surfaces for months and can thereby be a continuous source of transmission if no regular preventive surface disinfection is performed. Most gram-positive bacteria, such as S. aureus (including MRSA), or Streptococcus pyogenes, survive for months on dry surfaces. Many gram-negative species, such as E.coli, Klebsiella spp., Pseudomonas aeruginosa, or Shigella spp., can also survive for months. Candida albicans as the most important nosocomial fungal pathogen can survive up to 4 months on surfaces.

Most viruses from the respiratory tract, such as corona, influenza or rhinovirus, can persist on surfaces for a few days. Viruses from the gastrointestinal tract, such as HAV, Polio- or Rotavirus, persist for approximately 2 months. Blood-borne viruses, such as HBV or HIV, can persist for more than one week. Herpes viruses, such as CMV or HSV persist from only a few hours up to 7 days.

The influence of humidity on persistence has been described inconsistently. For Enteroviruses and Rhinovirus, high humidity was associated with longer persistence. HSV and HAV can persist longer at low humidity. For most viruses, such as Astroviruses, Adenoviruses, Poliovirus, HSV, and HAV, low temperature is associated with a longer persistence. The type of material did not affect the persistence of Echo-, Adenovirus, Parainfluenza, Rotavirus, RSV, and Poliovirus. Other investigators found that persistence was favored on non-porous surfaces for Influenza virus, on formica and gloves for RSV. There are three main routes of infection transmission in hospitals including airborne, droplet and contact.  An infecting microorganism, however, can be transmitted by more than one route. For example, varicella (chicken pox) is transmitted both by the airborne and contact route at different stages of the disease.

Airborne transmission; transfer of particles 5 µm or less into the air either as airborne droplets or dust particles containing the infectious microorganism; can be produced by coughing, sneezing or procedures such as bronchoscopy can remain in the air for up to several hours; and can be spread widely within room or over longer distances. Droplet transmission; contact of the mucous membranes of the nose, mouth or conjunctivae of the eye with infectious particles larger than 5 µm that can be produced by coughing, sneezing or procedures such as bronchoscopy. It requires close contact between the source and the susceptible person because particles remain airborne briefly and travel only about 1 meter or less. Contact transmission; infectious agent transmitted directly or indirectly from one infected to a patient, often on the contaminated hands of a health worker.