Transmission routes

Definition

Epidemiology, the study of disease transmission, identifies the organisms responsible for disease, the hosts that transmit the disease, and the possible routes of disease transmission. Infections and pathogens (disease-causing agents such as bacteria, viruses, and protozoa) may be spread from animal to animal, animal to human (zoonotic transmission), or human to human through several means. Pathogens are not limited to one transmission route; some have many. Properly classifying how pathogens are transmitted is critical to the effective treatment of an infected person and to controlling the spread of disease.

Types of Transmission

Vertical. Vertical transmission is the passing of a disease-causing agent from one generation to another, as when a parent transmits a pathogen to potential offspring at conception, during the perinatal period (pregnancy), at labor and delivery, or shortly after birth. The pathogen may be transferred either in sperm, by crossing the placenta, by fetal or newborn exposure to secretions or blood, or through breastfeeding.

Common examples of vertically transmitted diseases include human immunodeficiency virus (HIV) infection, hepatitis, cytomegalovirus, and toxoplasmosis. Pregnant women may be asymptomatic, but affected fetuses can have severe disease presentation after birth depending on the timing of exposure. Infections can lead to multiple medical and developmental concerns or to fetal or neonatal death.

Women (both pregnant and those planning to become pregnant) are screened on routine blood work for many of the diseases that show vertical transmission. Treatments, although specifically tailored to the individual disease, include maternal or neonatal medication (or both), changing delivery route from a vaginal delivery to a cesarean section to avoid maternal-fetal contact in the birth canal, and avoiding breastfeeding. Vaccines for certain diseases, such as hepatitis B, are administered in the newborn period to prevent disease development and transmission to future generations.

Horizontal. Horizontal transmission refers to the passing of a disease or pathogen from one person to another in the same generation. The disease is not passed from woman to fetus in the perinatal period. Symptoms depend on the specific type of acquired infection. Horizontal disease transmission occurs by direct or indirect pathogenic contact.

Direct contact occurs when a susceptible person touches or otherwise physically comes into contact with the pathogenic source or an infected person. Therefore, for transmission to occur, the source of disease and the potential recipient must be close to one another. This transmission can occur when touching an open wound, a mucous membrane, blood, or saliva. Sexually transmitted diseases occur by reproductive transmission, a form of direct contact. The respiratory route is also considered a pathway for direct transmission of disease.

Indirect transmission occurs when a person has contact with an object or host that carries the pathogen, allowing transmission of disease without physical contact between persons. This may occur through vector transmission (through an insect or animal) or fomite transmission (through contaminated, inanimate objects).

Sexual and reproductive. Sexually transmitted diseases (STDs) are transmitted by direct contact through touching or sharing of saliva or body secretions. One sexual partner’s infection may be transmitted to the other partner through genital, anal, or oral contact. Risk factors include unprotected sexual intercourse.

More than twenty-five STDs exist, including HIV infection, chlamydia, trichomoniasis, gonorrhea, and herpes. STDs are very common, with around 20 percent of the population contracting an STD at some time during their life. Laboratory blood work and a physical exam will diagnose STDs. Antibiotics, antiviral medications, and vaccinations are available for some STDs. Abstinence or condom use are recommended measures to prevent future transmission of disease.

Respiratory. The respiratory system is a common pathway for direct contact, whereby an ill person coughs, spits, or sneezes contaminated respiratory droplets of saliva onto a susceptible person. The pathogen enters the body through the nose, mouth, or eye and causes infections such as influenza, whooping cough (Pertussis), respiratory syncytial virus (RSV), chickenpox, the common cold (Rhinovirus, Adenovirus), and strep throat. Airborne transmission of pathogens occurs when droplets become smaller in size and when the liquid in those droplets evaporates to a significant degree.

Precautions to avoid contracting an illness by droplet contact include good hygiene practices, such as handwashing and avoiding sneezing directly into the hand. Infected persons should minimize direct contact with others. Antibiotic and antiviral medications are prescribed to treat certain infections.

Airborne. An airborne (aerosol) route differs from the respiratory route when infective droplets become smaller in size and when they become more evaporative, making it possible for the pathogen to travel farther distances. Most pathogenic droplets cannot survive when airborne, and their survival is affected by factors such as temperature and the general environment. A pathogen that replicates in the respiratory tract has the potential to be transmitted through airborne contact. Additionally, pathogens that replicate in water may also become airborne by means of shower heads or water fountains.

Airborne transmission is divided into two categories: long-range and short-range. Short-range transmission involves pathogenic droplets traveling less than three feet (one meter) from the person who sneezes or coughs. Therefore, for infection to be transmitted by air-flow exchange, the infected person and a susceptible person must be close to one another.

Long-range transmission occurs when droplets travel more than one meter, generally because of pressure differences in air flow from ventilation systems, open windows, a person’s movement, or temperature. Dust particles containing bacteria or viruses containing the infectious agent may be carried by air currents before being inhaled or before landing on other surfaces. In general, when an infectious agent has the capacity for long-range transmission, it is also pathogenic in the short-distance range.

The symptoms of disease from airborne transmission ultimately depend on the type of pathogen, the inhaled dose, and the immune response. Common diseases include tuberculosis, measles, the common cold, chickenpox (Varicella), and SARS/Covid-19 (SARS-Cov-1 and SARS-CoV-2). Influenza is spread by direct contact or by airborne transmission. The international 2003 outbreak of severe acute respiratory syndrome (SARS), thought to originate through water aerosols, demonstrates the long-range capacity of airborne illness and the possibility that it may cause a global pandemic if not adequately controlled. Although most airborne transmission occurs through human illness, pathogens can be disseminated through biological warfare and bioterrorism.

Inadequate ventilation is a significant risk factor for airborne transmission. Precautionary measures include using proper ventilation systems when working with known pathogenic agents, isolating or quarantining infected persons, and improving door and window seals.

Fecal-oral. The fecal-oral route of transmission typically involves food, water, or objects contaminated with either animal or human feces or urine that are ingested, leading to the oral transmission of disease. Transmission of disease by contaminated food or water is extremely common and remains a global public health burden. Populations at greatest risk include young children, older adults, pregnant women, and immunocompromised persons, such as those with HIV or acquired immunodeficiency syndrome (AIDS).

Food-borne transmission, commonly known as food poisoning, occurs as part of the fecal-oral pathway. Food-borne illness is defined as a disease resulting from the consumption of food or beverages contaminated either by a microbial pathogen or by a toxic substance. Infections typically have an incubation period before they lead to illness. A common area affected is the gastrointestinal tract, with diarrhea, nausea, and vomiting. Exact symptoms pertain to the type of substance and dose ingested.

There are more than 250 food-borne illnesses and diseases that infect over 48 million individuals annually, such as those caused by the bacterium Escherichia coli and the bacterium Salmonella. Laboratory analysis of stool samples can be performed to identify some of the causative agents. Because symptomatic persons are often dehydrated, rehydration with oral fluids containing electrolytes is often recommended. Some bacteria respond to antibiotics, but normally, the clinical course self-resolves within a few days.

Causes of foodborne illness include improper handling of food items; improper cooking, so that food remains raw or undercooked; drinking unpasteurized milk; eating contaminated produce; and having direct contact with animals, such as at petting farms or zoos.

When commercially preparing food, one should use proper handwashing techniques and wear gloves. Other food preparation recommendations include thoroughly cooking food, separating food to prevent cross-contamination, refrigerating food when not in use, and washing all produce. Any symptomatic person who prepares food for consumption is advised to discontinue work and remain home to prevent the spread of infection.

A specific category of food-borne illness is waterborne disease. Water provides an excellent breeding ground for many infectious agents. Pathogens that are excreted in fecal matter may contaminate water, leading to indirect fetal-oral waterborne transmission. Contaminated water is often used for drinking, bathing, or swimming. Measures to prevent waterborne illness include chlorinating swimming pools, avoiding water with hazardous wastes, regularly sterilizing bathtubs and sinks, and drinking treated water only. Drinking contaminated water is the most direct transmission of waterborne disease, but contaminated water can also cause illness indirectly—eating shellfish that lived in contaminated water, eating or drinking from an object washed in contaminated water, eating produce washed in contaminated water, or brushing one's teeth with contaminated water can all lead to illness.

Vector. Vector-borne transmission requires the use of a “vehicle” (or vector), such as an insect or arachnid, to disseminate the infection to humans. By definition, a vector can carry a disease agent but will not develop symptoms.

Vector-borne transmission occurs either mechanically or biologically. With mechanical transmission, the pathogen “uses” the vector only as a means to deposit itself, involving no replication or change in the pathogen. The infectivity of the vector is greatest within the first day of pathogen exposure, but even if the mechanical vector were to be eliminated, the pathogen would remain because it would be able to find another route for infection.

With biological transmission, also known as cyclical transmission, the pathogen uses the vector as a host to replicate and develop. For example, the pathogen “uses” a mosquito’s bite, which is infected with the pathogen, to cause malaria. After the initial vector infection, the vector (the mosquito) will remain infected forever, and the host (the person bitten) will remain infectious for some time only. Unlike with mechanical transmission, eliminating biological transmission reduces, or eliminates, the disease incidence.

Around 80 percent of the world’s population is at risk of contracting a vector-borne disease. At greatest risk are persons living or traveling in tropical and subtropical climates. These climates are hot and humid, making conditions ideal for disease transmission. Many vector-borne illnesses were once thought to be controlled, but because of vector drug resistance, airline travel, and mass migrations, for example, these diseases began rising. Between 2001 and 2023, vector-borne diseases doubled in the United States.

Preventive measures against infections from vector transmission include pest control in the home, in food stores, in food preparation areas, and in healthcare settings. Healthcare settings and homes are especially at risk for diseases related to mice, rats, and cockroaches.

Nosocomial. A nosocomial infection, sometimes called healthcare-associated infections (HAIs), is an infection that is acquired in a medical setting, typically a hospital, during a person’s care. Organisms of greatest concern for nosocomial infection include Enterococci (a common cause of nosocomial bloodstream infections), Escherichia coli (gastrointestinal illness), Vancomycin-resistant Staphylococcus aureus (VRSA; causes a rare antibiotic-resistant staph infection), and Pseudomonas aeruginosa (common in intensive care units). It is estimated that annually, over one million persons acquire these infections in the United States, and thousands die from resulting complications. However, estimating the rate of nosocomial infection is difficult.

Nosocomial infections are caused by direct or indirect contact with endogenous or exogenous agents, which may be bacteria, viruses, or fungi. An endogenous agent is a pathogen that comes from a previously infected or colonized site in the patient’s own body. An exogenous agent is a pathogen that comes from outside the patient’s body. Many of the exogenous pathogens are found on inanimate objects that carry disease, such as medical equipment, supplies, and clothing. The pathogens may also come from staff members, other infected patients, and visitors. Transmission caused by a medical procedure is termed “iatrogenic” and may confer a risk for infections such as methicillin-resistant Staphylococcus aureus (MRSA) or Creutzfeldt-Jakob disease.

Impact

Understanding the route of disease transmission is of utmost importance for treating an infected person and for ensuring public health. However, because significant overlap exists among the transmission routes for some illnesses, it is a challenge to determine the actual route of transmission that led to illness and, therefore, to treat and control disease.

While many forms of disease transmission can be controlled, data show that on an individual level, care is not always taken to prevent illness. Also, threats of global pandemic still exist. Continued efforts from governmental and public health agencies are needed to educate the public on disease transmission so that illness and its associated healthcare costs can be reduced or eliminated.

Bibliography

Aitken, C., and D. J. Jeffries. “Nosocomial Spread of Viral Disease.” Clinical Microbiology Reviews, vol. 14, no. 3, 2001, pp. 528–46.

Brower, Vicki. “Vector-Borne Diseases and Global Warming: Are Both on an Upward Swing?” EMBO Reports, vol. 2, no. 9, 2001, pp. 755–57, doi.org/10.1093/embo-reports/kve193. Accessed 23 Dec. 2024.

"Infectious Diseases: Modes of Transmission." Memorial Sloan Kettering Cancer Center Library, 7 Mar. 2024, libguides.mskcc.org/infectiousdiseases/Transmission. Accessed 23 Dec. 2024.

Martinson, Francis E., et al. “Risk Factors for Horizontal Transmission of Hepatitis B Virus in a Rural District in Ghana.” American Journal of Epidemiology, vol. 147, no. 5, 1998, pp. 478–87.

Morrison, Leanne G., and Lucy Yardley. “What Infection Control Measures Will People Carry out to Reduce Transmission of Pandemic Influenza? A Focus Group Study.” BMC Public Health, vol. 9, no. 1, 2009, pp. 1–11, doi.org/10.1186/1471-2458-9-258. Accessed 23 Dec. 2024.

Rajendram, Rajkumar, et al. Features, Transmission, Detection, and Case Studies in COVID-19. Academic Press, 2024.

Stein, R. A., and M. Chirilã. “Routes of Transmission in the Food Chain.” Foodborne Diseases, 2017, pp. 65–103, doi.org/10.1016/B978-0-12-385007-2.00003-6. Accessed 23 Dec. 2024.

Tang, J. W., et al. “Factors Involved in the Aerosol Transmission of Infection and Control of Ventilation in Healthcare Premises.” Journal of Hospital Infection, vol. 64, no. 2, 2006, pp. 100–14, doi.org/10.1016/j.jhin.2006.05.022. Accessed 23 Dec. 2024.