Pathways of Infection

PATHWAYS OF INFECTION

The various routes by which organisms gain entrance to the body are protected by defense mechanisms. If these mechanisms are breached in some way, infec-tion can occur. Once inside the body, the infection may spread directly via the bloodstream or lymphat-ics. One method of containing and preventing infec-tions is to protect these pathways and maintain the barriers. Think of the various ways by which this pro-tection can be achieved as the different pathways of infection are addressed.

Skin

Keratinized epithelium, sweat, and sebum are some protective barriers present in skin. When the conti-nuity of skin is breached by laceration, burns, or re-duced blood supply, the barrier is no longer effective, and the risk of infection increases.

Infection may be acquired through the skin by di-rect physical contact (e.g., herpes simplex, ringworm,impetigo), by infection when the skin is disrupted (e.g., tetanus), orby injection into the skin by vectors carrying infectious agents (e.g., malarial parasites in-jected by mosquitoes). Infections may also be ac-quired by injection by humans,such as transfusion of infected blood and blood products and contaminated needles. Certain infections are spread by direct pene-tration of the skin by the infectious agent(e.g., hook-worm larvae).

Infection may also be transmitted by indirect con-tact with infected body fluids via towels, shared uten-sils, or bedding.

Respiratory Tract

Entry of pathogens through the respiratory tract is prevented by the presence of mucus and cilia that move the mucus toward the mouth. Defense cells, an-tibody secretions, and lymphoid tissue (tonsils) in the mouth and pharynx also protect the respiratory tract. Depression of the cough reflex by drugs; interference with ciliary transport, as in alcoholism, cold, and loss of ciliated cells as a result of smoking; and bronchial obstruction as a result of various causes can all con-tribute to the weakening of the barrier and an in-creased risk of infection. Inhalation of droplets car-rying infectious agents is the usual mechanism of transmission.

Gastrointestinal Tract

This tract is protected by lysosomes, antibodies pre-sent in various secretions and the pH of secretions that are not conducive to growth and multiplication of mi-croorganisms. The mucosal lining and the growth of natural intestinal flora in the colon also serve as a pro-tective barrier. Entry of infectious agents through the gastrointestinal tract is via infected food and drink, in-cluding fecal contamination.

Genitourinary Tract

This tract is normally sterile. But risk of infection is increased by obstruction of urinary flow, catheteriza-tion, and alteration in normal flora by prolonged use of broad-spectrum antibiotics and others. Entry of infection through this tract is more common in women because of the shortness of the urethra.