Giardia

Quick revision

Giardia is a protozoan parasite that causes giardiasis, a common infectious gastroenteritis worldwide.
It exists in two forms: motile trophozoites and resilient cysts, with cysts responsible for transmission.
Transmission occurs via contaminated water, food, or direct contact, with cysts surviving in cold, damp environments.
In the UK, giardiasis rates have increased due to improved diagnostics, with outbreaks linked to swimming pools and poor hygiene settings.
Humans are the primary hosts, though zoonotic transmission from animals (e.g., dogs and livestock) is possible.
Symptoms include watery diarrhea, bloating, flatulence, weight loss, and malabsorption; asymptomatic carriage is common.
Diagnosis relies on stool microscopy, antigen detection tests (e.g., ELISA), and PCR for molecular typing.
Treatment includes nitroimidazoles like metronidazole or tinidazole; nitazoxanide is an alternative for refractory cases.
Infection control emphasizes water safety, hand hygiene, and exclusion policies for symptomatic individuals in high-risk settings.
Special populations include children, immunocompromised individuals, and travelers to endemic regions, who require tailored prevention strategies.

Giardiasis, caused by the protozoan parasite Giardia duodenalis (syn. G. lamblia, G. intestinalis), is a leading cause of infectious gastroenteritis worldwide.

Structure

G. duodenalis exists in two morphologically distinct forms:

Trophozoites: Pear-shaped, motile cells (10–20 μm long) with two nuclei, four pairs of flagella, and a ventral adhesive disc for epithelial attachment. They lack mitochondria and Golgi apparatus but possess mitosomes, reduced organelles involved in iron-sulfur cluster biosynthesis.
Cysts: Oval, dormant forms (8–12 μm) with a protective two-layered wall, four nuclei, and fragments of the ventral disc1. Cysts survive for months in cool, damp environments and resist chlorination.

The parasite’s life cycle involves ingestion of cysts, excystation in the duodenum, trophozoite replication, and subsequent encystation in the large intestine.

Epidemiology

In England and Wales, laboratory reports of giardiasis increased from 3,342 cases in 2008 to 4,793 in 2017, possibly due to improved molecular diagnostics. Regional disparities exist, with the South West of England reporting the highest rate (20.1 cases/100,000 population in 2017). Asymptomatic infections are common, with an estimated 14 undetected cases for every reported case.

Key UK Trends (2017):

Demographics: Highest incidence in males aged 45–49.
Seasonality: Peak reporting in September.
Outbreaks: Two non-foodborne outbreaks linked to swimming pools.

Certain groups are considered to be at high risk for Giardia infection

Children (especially infants and those in daycare settings),
travelers to areas with poor sanitation,
immunocompromised patients
Cystic fibrosis

Reservoir and Host Specificity

G. duodenalis comprises eight host-adapted genetic assemblages:

Assemblage	Primary Host	Zoonotic Potential
A, B	Humans, animals	High (zoonotic)
C, D	Dogs	Low
E	Livestock	Low
F	Cats	Low
G, H	Rodents, seals	Rare

Assemblages A and B are responsible for most human infections, with potential transmission from domestic animals and contaminated water.

Transmission

Fecal-oral route: Ingestion of cysts via contaminated water (e.g., inadequate filtration), food, or direct contact.
- Environmental resilience: Cysts survive in cold, damp conditions and resist standard chlorination.
- High-risk settings: Swimming pools, childcare facilities, and households with poor hygiene.

Lifecycle

Cysts (resilient, dormant forms & Infective stage) are immediately infectious when excreted in feces and can survive for months in cool, damp environments.
Cysts are ingested via contaminated water, food, or through fecal-oral contact. In the duodenum, acidic stomach conditions and bile trigger cyst breakdown, releasing two trophozoites per cyst.
Trophozoite activity: Motile, replicative forms attach to the small intestine’s mucosal lining via a ventral disc, absorbing nutrients and multiplying via binary fission. As trophozoites migrate to the colon, neutral pH and bile salts induce transformation into cysts, which are excreted in feces.
No intermediate hosts are required; direct fecal-oral cycle

Incubation Period

Symptoms typically develop 1–3 weeks after cyst ingestion. If the symptom develops within a week, it may not be Giardia.

Clinical Features

Symptomatic infection: foul smelling large-volume watery diarrhea, bloating, flatulence, weight loss, and malabsorption. It may last up to 4 weeks.
Asymptomatic carriage: Up to 50% of cases, particularly in endemic regions.
Chronic infection: Prolonged symptoms in immunocompromised individuals and in some cases where treatment was not offered. The patient may continue to have loose stool, steatorrhea, malabsorption (fat, fat soluble vitamins), weight loss, and failure to thrive.

Humoral immunity (especially IgA) plays a significant role in protection against Giardia. Patients with a deficiency in this form of immunity (agammaglobulinaemia, common variable immunodeficiency, CF) tend to have severe disease. Cell-mediated immunity is also important (IL-17) but less compared to humaral form.

Complications

Hypersensitivity, failure to thrive, Chronic irritable bowel syndrome, biliary infection.

Investigation

Stool microscopy: Direct visualization of cysts/trophozoites (low sensitivity due to intermittent shedding). Often needs more than one sample to test (we recommend 3 samples)
Antigen detection: ELISA or immunofluorescence assays (higher sensitivity).
Molecular methods: PCR for assemblage typing in outbreak settings.

Management

First-line therapy: Nitroimidazoles (e.g., metronidazole for 5–7 days).
Alternatives: Tinidazole (single dose) or nitazoxanide for refractory cases.
Asymptomatic cases: Treatment may be warranted in households to prevent reinfection.

Even after treatment, the shedding of trophozoits and cysts may continue for up to 6 months.

Infection Control

Water safety: Boiling or filtering water in high-risk areas.
Hand hygiene: Emphasis on handwashing after toileting and before food handling.
Outbreak management: Exclusion of symptomatic individuals from pools and childcare settings until 48 hours post-diarrhea resolution.

Special Populations

Children under 5: Higher risk of household transmission (odds ratio 42 vs. older groups).
Immunocompromised: Prolonged symptoms and risk of chronic malabsorption.
Travelers: Advise avoidance of untreated water in endemic regions.