Oral transmission

Definition

Oral transmission is the acquisition of bacteria, viruses, fungi, and parasites through the mouth, either by ingestion or by absorption through the oral mucosa.

Fecal-Oral Transmission

Escherichia coli and other enteric bacteria, viruses, and parasites are transmitted when the feces of a person or animal are inadvertently swallowed. This may occur when hands are not washed after using a toilet, after changing a diaper, after working in dirt or soil, after petting animals, and after cleaning up after animals. Surfaces in daycare centers and in public restrooms may be invisibly covered with such microbes unless they are frequently disinfected.

Fecal-oral transmission may also occur when raw fruits and vegetables that are grown in or are otherwise in contact with soil fertilized with manure are not thoroughly washed before they are eaten. Similar foods may also become contaminated when harvesters or food preparers handle them with unwashed, stool-contaminated hands. Food handlers should keep their hands clean by washing with soap and warm water. Cooking food at a high temperature for a sufficient length of time kills these bacteria and parasites.

Fecal-oral transmission may also result from swallowing swimming pool water that has not been sufficiently chlorinated. Similarly, lake or river water may be contaminated with animal feces and should not be ingested. To ensure safe drinking water when camping or in other outdoor situations, water may be boiled, filtered, or chemically treated.

Foodborne Transmission

Uncooked meat typically contains bacteria; poultry is known to harbor Salmonella. Fish and shellfish that were caught from contaminated water sources may transmit disease. Handling raw meat and neglecting to wash one’s hands and the food preparation surface afterward may lead to the contamination of other foods and subsequent bacterial ingestion. Meat should always be stored at the proper temperature before cooking, and frozen meat should be thawed in the refrigerator rather than on the kitchen counter to discourage the multiplication of bacteria. Surfaces that come in contact with raw meat juices should be thoroughly disinfected.

A dental cavity is an infectious disease, and studies have shown that parents and caregivers inadvertently infect infants and toddlers with cavity-causing bacteria when they sample the child’s food to check the food’s temperature. An Indigenous Alaskan cultural practice is to chew solid foods before feeding these foods to infants, incidentally transmitting cavity-causing bacteria and other oral pathogens. Persons who engage in this practice should use a chlorhexidine mouthwash before each feeding.

Sharing beverages is another means of oral transmission. Bacteria, viruses, and fungi that live in the mucous lining of the mouth, tongue, and throat may be shed in saliva that is washed back into a beverage after drinking, thus contaminating the beverage for the next drinker. For this reason, beverages that come in containers should be poured into individual cups for serving more than one person.

Objects and Surfaces

Meningococcal disease caused by Neisseria meningitidis may be transmitted by sharing contaminated objects (fomites), such as eating utensils, drinking glasses, drinking straws, and water bottles. These bacteria live in the mucous lining of the throat and are shed in liquids and on surfaces where they may be immediately picked up by other people. Similarly, viruses such as the influenza virus may be transmitted by sharing toothbrushes and drinking glasses. Dental caries may be transmitted from an adult to a child when the adult puts a pacifier in his or her mouth to clean or moisten it before giving it to an infant or toddler. Thus, personal items should not be shared.

Person-to-Person Transmission

Herpes simplex virus types 1 and 2 may be transmitted through the oral mucous membranes by kissing. Infectious mononucleosis and meningococcal disease may also be passed by kissing. The human immunodeficiency virus (HIV) may be transmitted through the oral mucosa by oral sexual acts involving infected semen or blood.

Dental Procedures

Infective endocarditis develops in some people following dental procedures. Oral surgery, such as tooth extraction and root canal therapy, creates access by which bacteria that typically live in the mouth get into the bloodstream. Nonsurgical procedures such as dental prophylaxis, with or without periodontal therapy, may disturb areas of inflammation, increase blood flow, and increase the amount of bacteria; when sharp instruments remove epithelium and the calculus, the oral mucosa barrier is compromised and bacteria enter the circulatory system. Persons who have had heart surgery, particularly valve replacement, or who have abnormal heart valves are most at risk of infective endocarditis and must take a prophylactic antibiotic one hour before undergoing dental treatment.

Other Modes of Transmission

Hepatitis B, C, D, and G may be transmitted through piercing of the tongue, lip, or cheek, which compromises the oral mucosa barrier.

Impact

Foodborne, orally transmitted, disease accounts for 48 million illnesses and 3,000 deaths annually in the United States. One negligent food handler may be responsible for a disease outbreak requiring costly public health intervention. Outbreaks have occurred in college dormitories and other close living quarters. To minimize oral transmission, hands should be washed thoroughly with an antibacterial soap not only when visibly soiled but also and especially when preparing and eating food.

Bibliography

Harrison, Lee H., et al. “Invasive Meningococcal Disease in Adolescents and Young Adults.” Journal of the American Medical Association 286 (2001): 694-699.

Mandell, Gerald L., John E. Bennett, and Raphael Dolin, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. New York: Churchill Livingstone/Elsevier, 2010.

"Foodborne Illnesses and Diseases." Food Safety and Inspection Service, US Department of Agriculture, 10 Dec. 2024, www.fsis.usda.gov/food-safety/foodborne-illness-and-disease. Accessed 4 Feb. 2025.

Tanzer, Jason M. “Dental Caries Is a Transmissible Infectious Disease: The Keyes and Fitzgerald Revolution.” Journal of Dental Research 74 (1995): 1536-1542.

Younai, Fariba S. “Oral HIV Transmission.” Journal of the California Dental Association, February 2001.