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 breast-feeding.
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
breast-feeding. 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 (that is, through an insect or animal) or through fomite transmission (that is, 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,
gonorrhea, and herpes. STDs are very common; one study
estimated 333 million new cases in one year of syphilis, gonorrhea,
trichomoniasis, and chlamydia.
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, chickenpox, the common cold,
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 the avoidance of 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, and
chickenpox. It remains debatable if influenza is spread mostly 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 public health burden worldwide. The Centers for Disease Control and
Prevention estimates that, annually, 325,000 hospitalizations
and 5,000 deaths are related to this disease route. Populations at greatest risk
include young children, the elderly, pregnant women, and immunocompromised
persons, such as those with human immunodeficiency virus infection or acquired
immunodeficiency syndrome.
Food-borne transmission, commonly known as food
poisoning, occurs as part of the fecal-oral pathway.
Food-borne illness is defined as 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, such
as those caused by the bacterium Escherichia coli and the
bacterium Salmonella. Laboratory analysis on 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 food-borne 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.
One should use proper handwashing technique and should use gloves when commercially preparing food. Other food preparation recommendations include thoroughly cooking all 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.
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.
Nearly one-half of the world’s population has 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 are on the rise.
Preventive measures against infections from vector transmission include pest control in the home, in food stores, in food preparation areas, and in health care settings. Health care settings and homes are especially at risk for diseases related to mice, rats, and cockroaches.
Nosocomial. A nosocomial infection 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 Escherichia coli, Enterococcus, Staphylococcus aureus, and Pseudomonas. It is estimated that annually, two million persons acquire these infections in the United States, and approximately twenty thousand persons die from resulting complications.
Nosocomial infections are caused by either direct or indirect contact with
endogenous or exogenous agents. 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 or Creutzfeldt-Jakob
disease.
Aitken, Celia, and Donald J. Jeffries. “Nosocomial Spread of Viral Disease.” Clinical Microbiology Reviews 14 (2001): 528-546. Review of literature detailing the risks of specific viruses for transmission in the medical setting. Includes discussion of respective hygienic precautions.
Brower, Vicki. “Vector-Borne Diseases and Global Warming: Are Both on an Upward Swing?” EMBO Reports 2 (2001): 755-757. Examines the medical and political controversy about whether or not warmer global temperatures are increasing the incidence of vector-borne illnesses.
Martinson, Francis E., et al. “Risk Factors for Horizontal Transmission of Hepatitis B Virus in a Rural District in Ghana.” American Journal of Epidemiology 147 (1997): 478-487. A case study discussing risk factors and suggestions for reducing horizontal transmission of disease.
Morrison, Leanne G., and Lucy Yardley. “What Infection Control Measures Will People Carry Out to Reduce Transmission of Pandemic Influenza?” BMC Public Health 9 (2009): 1-11. A pilot study assessing public knowledge and willingness to comply with infection control recommendations to reduce disease transmission.
Tang, J. W., et al. “Factors Involved in the Aerosol Transmission of Infection and Control of Ventilation in Healthcare Premises.” Journal of Hospital Infection 64 (2006): 100-114. Discusses common and less recognized diseases that may be a risk for airborne transmission. Also illustrates common mechanisms for spreading airborne illness.
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