Intestinal Flagellates and Ciliates

Giardia is the most common pathogenic intestinal protozoan worldwide and a leading cause of waterborne diarrhoeal disease. It exists as several morphologically identical but genetically distinct assemblages, with Assemblages A and B causing the great majority of human disease.

The Giardia trophozoite is pear-shaped (piriform) in outline, measuring roughly 10 to 20 micrometres in length. It has a distinctive bilateral symmetry. Two nuclei, each with a prominent central karyosome, sit side by side in the anterior half of the cell. This gives a characteristic "face-like" appearance on wet mount microscopy.

Four pairs of flagella provide motility, classically described as a "falling leaf" motion when viewed live.

On the ventral surface lies a large concave adhesive disc (ventral sucking disc). This specialised cytoskeletal structure attaches the organism firmly to the intestinal epithelium. Suction-based attachment, rather than tissue invasion, is central to how Giardia produces disease.

The trophozoite is the metabolically active, replicating, and colonising form. It is fragile outside the host and is not the transmissible stage.

The cyst is oval, thick-walled, and measures roughly 8 to 12 micrometres. The mature form contains up to four nuclei. Visible internal structures, such as axonemes (flagellar remnants) and curved median bodies, are useful identifying features on stool microscopy.

The resistant cyst wall allows Giardia to survive for extended periods in cold water, including chlorinated water at standard treatment concentrations. Giardia therefore remains a well-recognised cause of outbreaks linked to municipal and recreational water supplies despite conventional chlorination.

The cyst is both the infective and the diagnostic stage most commonly identified in formed stool.

Infection follows ingestion of as few as 10 to 25 cysts. This unusually low infective dose contributes to Giardia's high transmissibility.

Sources include:

• Contaminated drinking water, including apparently clear mountain streams contaminated by animal or human faeces (a classic scenario in hikers and campers).
• Recreational water (swimming pools, lakes).
• Contaminated food.
• Direct person-to-person transmission via the faecal-oral route, particularly in childcare settings, institutions, and among men who have sex with men through anal-oral contact.

Zoonotic transmission from domestic and wild animals (dogs, cats, beavers, livestock) is recognised. The relative contribution of zoonotic versus anthroponotic (human-to-human) transmission varies by Giardia assemblage and setting.

After ingestion, excystation occurs in the duodenum. It is triggered by gastric acid exposure, followed by the alkaline, bile-rich environment of the proximal small intestine. Each cyst releases two trophozoites.

Trophozoites colonise the duodenum and proximal jejunum in large numbers. They attach to the brush border of enterocytes via the ventral adhesive disc.

Giardia does not invade the mucosa or the bloodstream. It does not typically produce a marked inflammatory or destructive histological lesion. Disease results predominantly from mechanical and functional disruption of the epithelial surface rather than tissue invasion. This distinguishes its pathogenesis fundamentally from Entamoeba histolytica.

Heavy trophozoite colonisation produces a carpet-like coating of the duodenal and jejunal mucosa, mechanically interfering with nutrient absorption at the brush border.

Contributing mechanisms:

• Disruption of brush border enzyme activity, notably disaccharidases. This produces secondary lactose intolerance that can persist for weeks after clearance of infection.
• Shortening of intestinal microvilli.
• Increased intestinal permeability.
• Mild inflammatory change in some cases.

The combined effect is malabsorption of fats, sugars, and fat-soluble vitamins. This produces steatorrhoea, bloating, flatulence, abdominal cramping, and weight loss. The steatorrhoeic stool is bulky, greasy and foul-smelling, without blood or mucus, which distinguishes it from the dysenteric pattern of invasive amoebiasis.

Chronic or recurrent giardiasis in children is an important and under-recognised cause of failure to thrive and growth faltering in endemic settings, due to cumulative malabsorption.

Infection can be asymptomatic, acute and self-limited, or chronic and relapsing. Chronic infection is more likely in individuals with IgA deficiency or other humoral immune impairment. This reflects the predominantly antibody-mediated (rather than cell-mediated) nature of protective immunity against Giardia.

Stool microscopy identifies cysts (in formed stool) or trophozoites (in loose or liquid stool, since trophozoites are rapidly destroyed once stool is fully formed and dehydrated).

Shedding is intermittent, and a single stool examination may miss infection. Three stool samples collected on separate days substantially improve sensitivity over a single sample.

Stool antigen detection (enzyme immunoassay) offers higher sensitivity than routine microscopy and does not depend on intermittent shedding to the same degree. Stool PCR provides high sensitivity and specificity and is increasingly used where available.

In selected cases where stool testing is repeatedly negative despite strong clinical suspicion, duodenal aspirate, the "string test" or duodenal biopsy may be used. The string test (Entero-Test) uses a gelatin capsule containing a coiled string, swallowed and later withdrawn to sample duodenal fluid and mucus. These tests are reserved for diagnostic difficulty rather than first-line use.

Balantidium coli is the largest protozoan parasite of humans and the only ciliate of established medical importance in humans.

The trophozoite measures roughly 50 to 100 micrometres (occasionally larger) and is ovoid. It is covered entirely by short cilia arranged in rows. These provide rapid, coordinated motility, distinctly faster and more directed than the pseudopodial or flagellar movement of other intestinal protozoa.

It possesses two nuclei of different types. A large, kidney-bean-shaped macronucleus is responsible for routine metabolic and vegetative function. A small, less conspicuous micronucleus is involved in genetic exchange. This nuclear dimorphism is unique among the medically important intestinal protozoa and is a distinguishing diagnostic feature.

A cytostome (primitive mouth structure) at the anterior end allows ingestion of bacteria, food particles, and host cells.

Pigs are the principal natural reservoir and are commonly colonised without disease, maintaining a large environmental burden of cysts in areas with pig farming. Balantidium coli is therefore a recognised zoonosis, and human infection correlates strongly with proximity to pigs and exposure to pig faecal contamination of water or food; other animals, including rodents and nonhuman primates, can also serve as reservoirs in some settings.

Transmission occurs via the faecal-oral route through ingestion of mature cysts, typically from water or food contaminated with pig faeces, or occasionally through direct contact in settings of close human-pig cohabitation. As with other cyst-forming intestinal protozoa, the cyst is the resistant, infective, and diagnostic stage, while the trophozoite is the invasive, disease-producing form.

After ingestion, excystation occurs in the small intestine and trophozoites colonise the large intestine, particularly the caecum and sigmoid colon.

Most human infection is asymptomatic or produces only mild, self-limited diarrhoea. In the majority of colonised individuals, the organism behaves as a luminal commensal.

In a smaller proportion of cases, trophozoites invade the colonic mucosa by mechanical boring aided by hyaluronidase-like enzyme activity. This produces ulcerative lesions that are histologically similar in gross appearance to the flask-shaped ulcers of amoebic colitis, though generally considered less deeply invasive on average.

Symptomatic disease presents as balantidiasis: diarrhoea or dysentery with blood and mucus, abdominal pain, tenesmus, and occasionally nausea and weight loss.

Severe disease with extensive colonic ulceration, haemorrhage, and perforation is described but uncommon. It occurs more frequently in malnourished or immunocompromised hosts.

Extraintestinal spread is rare but has been reported (appendix, mesenteric lymph nodes, rarely other organs).

Giardia and Balantidium coli both colonise different parts of the gastrointestinal tract by mechanisms that avoid deep tissue invasion in most infected individuals. They differ substantially in every other respect.

• Giardia is a flagellate confined to the small intestine. It produces disease through mechanical or functional disruption of absorption without significant mucosal ulceration. It presents with malabsorptive, non-bloody steatorrhoeic diarrhoea.
• Balantidium coli is a ciliate confined to the large intestine. It is capable of true mucosal invasion and ulceration in symptomatic cases. When symptomatic it presents in a dysenteric pattern with blood and mucus, resembling amoebic colitis more closely than giardiasis in its invasive form.
• Giardia is transmitted primarily via human and some animal reservoirs, with a very low infective dose.
• Balantidium coli is predominantly a pig-associated zoonosis. Human disease is comparatively uncommon worldwide, relative to its wide environmental distribution in pig-farming regions.