Routes of transmission

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Infection control and prevention depends on disrupting the transmission of pathogens from their source (the infected animal or human) to new hosts (animal or human) or locations.7,12 Understanding routes of disease transmission and how it contributes to the spread of organisms allows for the identification of effective prevention and control measures not only for specific diseases, but also other pathogens transmitted by a similar route, including unanticipated infectious diseases.12 The transmission of microorganisms can be divided into the following five main routes: direct contact, fomites, aerosol (airborne), oral (ingestion), and vectorborne. Some microorganisms can be transmitted by more than one route.

Direct Contact Transmission
Direct contact transmission occurs through direct body contact with the tissues or fluids of an infected individual. Physical transfer and entry of microorganisms occurs through mucous membranes (e.g., eyes, mouth), open wounds, or abraded skin. Direct inoculation can occur from bites or scratches. Examples include organisms such as rabies, Microsporum, Leptospira spp., and staphylococci, including multidrug-resistant (MDR) species methicillin-resistant Staphylococcus aureus and Staphylococcus pseudintermedius (MRSP). This is probably the most common and highest-risk route of pathogen transmission to patients and personnel.

Fomite Transmission
Fomite transmission involves inanimate objects contaminated by an infected individual that then come in contact with a susceptible animal or human. Fomites can include a wide variety of objects such as exam tables, cages, kennels, medical equipment, environmental surfaces, and clothing. Disease examples include canine parvovirus and feline calicivirus infections.

Aerosol (Airborne) Transmission
Aerosol transmission encompasses the transfer of pathogens via very small particles or droplet nuclei. Aerosol particles may be inhaled by a susceptible host or deposited onto mucous membranes or environmental surfaces. This can occur from breathing, coughing, sneezing, or vocalization of an infected individual, but also during certain medical procedures (e.g., suctioning, bronchoscopy, dentistry, inhalation anesthesia). Very small particles may remain suspended in the air for extended periods and be disseminated by air currents in a room or through a facility. However, most pathogens pertinent to companion animal veterinary medicine do not survive in the environment for extended periods or do not travel great distances due to size and as a result require close proximity or contact for disease transmission. Examples of common aerosolized pathogens include Bordetella bronchiseptica, canine influenza, and canine distemper virus.

Oral (Ingestion) Transmission
The ingestion of pathogenic organisms can occur from contaminated food or water as well as by licking or chewing on contaminated objects or surfaces. Environmental contamination is most commonly due to exudates, feces, urine, or saliva. Examples of diseases acquired via oral transmission include feline panleukopenia and infections caused by Campylobacter, Salmonella, Escherichia coli, and Leptospira.

Vector-Borne Transmission
Vectors are living organisms that can transfer pathogenic microorganisms to other animals or locations and include arthropod vectors (e.g., mosquitoes, fleas, ticks) and rodents or other vermin. Vector-borne transmission can be an important route of transmission in climates where these pests exist year-round and may be brought into the practice by an infested patient. Examples of vector-borne diseases include heartworm disease, Bartonella infection, Lyme disease (borreliosis), and plague.

Zoonotic Transmission
It is important to remember many animal diseases are zoonotic and therefore pose a risk for the healthcare team as well as clients. The transfer of these agents can occur by the same five routes of transmission described above. Examples of zoonotic pathogens include Microsporum, Leptospira, Campylobacter, and Bartonella.

AHS (alcohol-based hand sanitizer); HAI (hospital-acquired infections); ICP (infection control practitioner); ICPB (infection control, prevention, and biosecurity); MDR (multidrug resistant); MRSP (methicillin-resistant Staphylococcus pseudintermedius); PPE (personal protective equipment); SOP (standard operating procedure); SSI (surgical site infection)