Giardia

Morphology and life cycle forms

The causative agent can exist in two radically different morphological forms, which ensures its survival both within the host and in the external environment.

• Vegetative form (trophozoite). It has a flattened pear shape measuring 12–15 by 5–9 µm. There are two nuclei inside the cell. The parasite uses four pairs of flagella for movement, which produce a specific rotational motion. Attachment to the intestinal mucosa occurs via a specialized organelle — the adhesive disc.
• Cyst. Formed to withstand unfavorable conditions. It is oval in shape (8–12 by 7–10 µm) and securely protected by a thick double-contoured wall. A feature of the mature cyst is the presence of four nuclei.

Epidemiology and life cycle

The main source of infection is humans (a patient with pronounced clinical symptoms or an asymptomatic cyst carrier). Animals less commonly act as a reservoir: dogs, cattle, as well as wild species (deer, beavers, muskrats). The transmission mechanism is fecal-oral.

Transmission factors include contaminated water, food, as well as dirty hands and household items. Cases of massive waterborne outbreaks of infection are known. A special risk group is gay men, among whom infection with giardia and/or amebae can reach 40%.

Life cycle features:

1. Ingested cysts reach the upper parts of the intestine, where they transform into trophozoites.
2. Using the adhesive disc, the parasites tightly attach to epithelial cells (adhesion of microvilli of the plasmalemma occurs).
3. Reproduction occurs via longitudinal binary fission into two.
4. Reaching the lower, less favorable parts of the intestine, the trophozoites encyst (become covered with a protective shell) and are excreted with feces.

Cysts demonstrate high resistance: they remain viable in cold water and soil for more than two months, tolerate low temperatures, and do not die during standard water chlorination.

Pathogenesis and clinical presentation

Massive reproduction of vegetative forms in the duodenum and jejunum leads to mechanical blockade of the mucosal membrane. This entails a disruption of parietal digestion processes and normal intestinal motility disorders. An immune component is also involved in the pathogenesis: T lymphocytes can damage the jejunal mucosa. The severity of the disease directly correlates with the host's overall resistance level.

Clinically, giardiasis manifests as diarrhea, signs of enterocolitis, weight loss, decreased appetite, and systemic metabolic disorders.

Physicians distinguish three leading syndromes in the manifest form:

• Gastroenterocolitic;
• Cholecystopancreatic;
• Asthenic.

Diagnostics, treatment, and prevention

In response to the invasion, both cellular and humoral immunity are formed in the body. However, laboratory confirmation is required for diagnosis.

Diagnostic methods:

• Microscopy. This is the gold standard. Cysts are sought in formed stool using Lugol's iodine staining. In case of diarrhea or in duodenal aspirate material, living vegetative forms can be detected in native preparations (a differential feature is the trophozoite's movement like a "falling leaf").
• Serology. ELISA and indirect immunofluorescence assay (IFA) methods are used, evaluating the increase in the titer of specific antibodies.

Treatment and prevention: To eradicate the pathogen, nitroimidazole drugs (metronidazole, tinidazole, ornidazole) and nitrofuran drugs (furazolidone) are used. Preventive measures are based on strict adherence to sanitary and hygienic rules, similar to the prevention of amebiasis.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of the immune response: the role of T lymphocytes in mucosal damage
• Zoonotic reservoir of infection and its epidemiological significance
• Pathogenesis of cholecystopancreatic and asthenic syndromes
• Pharmacodynamics of nitroimidazoles and nitrofurans
• Features of serological diagnostics (interpretation of ELISA and IFA)

Other topics in "Flagellates (Leishmania, Trypanosoma, Giardia, Trichomonas)"

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