Invasion

Invasion (from Latin invasio — penetration, incursion) is a medical term denoting the process of penetration, establishment, and subsequent parasitism of protozoa or helminths in the human body.

The term differs from the concept of “infection,” which is usually applied to bacteria, viruses, and fungi.

Etiology and pathophysiology

The mechanisms of invasion are diverse and depend on the parasite species. Infestation can occur via two main routes:

• active (percutaneous, i.e., transcutaneous) — larvae physically burrow through intact skin upon contact with contaminated soil or water (as in hookworm infection or schistosomiasis);
• passive (alimentary) — occurs through the ingestion of protozoan cysts (Giardia), helminth eggs (for example, Ascaris), or their larvae (for example, Trichinella or pork tapeworm) with unwashed vegetables, untreated water, or inadequately cooked meat.

After entering the body, the parasite uses an array of mechanisms to evade the host immune response. The infective forms of helminths are covered by a dense protective cuticle resistant to digestive enzymes and antibodies.

Some helminths are capable of antigenic mimicry: they alter their antigenic structure, acquiring receptors similar to human receptors, and become “invisible” to leukocytes.

The pathophysiology of invasion comprises several factors:

• mechanical tissue damage — attachment to the intestinal wall and penetration into target organs (muscles, liver, eye, brain);
• absorption of host nutrients;
• toxic-allergic reaction — to parasite waste products.

Clinical Significance

Parasitic invasions are often asymptomatic or mimic various diseases depending on the affected organ. They more commonly manifest as gastrointestinal or respiratory symptoms or as an allergic reaction such as urticaria.

A common laboratory marker of helminthic tissue invasion is persistent eosinophilia (an elevated blood eosinophil count), and less commonly, anemia (a decreased hemoglobin level).

The following methods are used to diagnose invasions:

• specific serological tests (ELISA) — performed to detect antibodies to parasites (Echinococcus, Toxocara, Trichinella); tissue biopsy is generally of limited diagnostic value in these cases;
• stool microscopy using concentration methods — used to diagnose Giardia trophozoites and cysts.

Parasitic invasions are treated with antiparasitic agents. Their action may be directed at:

• disruption of neuromuscular transmission in helminths — causes paralysis;
• inhibition of synthesis and damage to the DNA structure of pathogens;
• destruction of parasite cell membranes.