Epidemiology and management of nitroimidazole-refractory giardiasis

Giardia is one of the most prevalent gastrointestinal protozoan pathogens globally. Nitroimidazole drugs are usually the first line of treatment, but nitroimidazole-refractory giardiasis (NRG), where infections fail to respond to these agents, is increasingly reported. At our centre, for laboratory-proven refractory giardiasis, a stepwise treatment ladder is used with albendazole plus tinidazole as second-line and mepacrine as third-line therapy.

We conducted a retrospective review between 2020 and 2024 at the Hospital for Tropical Diseases, London. Laboratory records identified adults and children with Giardia detected by stool microscopy or molecular methods. Treatment details and clinical information were extracted from electronic patient records. NRG and clearance failure were defined as a positive molecular test at least two weeks after completion of treatment.

A total of 531 cases were identified. Of these, 59.0% (N = 311) were diagnosed through screening of high-risk individuals attending clinics for people seeking asylum and refugees (PSAR; Cohort A). The remaining 41.0% (N = 220) were symptomatic patients (Cohort B).In Cohort A, the prevalence of NRG was 36.3% (49/135), with children showing significantly higher odds of clearance failure compared to adults (adjusted OR, 4.81; 95% CI, 1.09-21.40; P = 0.0367). In Cohort B, 34.1% of treatment-naïve travellers had NRG. Travel to India was strongly associated with NRG (adjusted OR, 4.19; 95% CI, 1.72-11.00; P = 0.002). Second-line treatment regimens with albendazole plus tinidazole were effective, and third-line mepacrine therapy was well tolerated with no adverse reactions and successful in nearly all cases.

NRG was common in both asymptomatic and symptomatic individuals, particularly among children seeking asylum and travellers who had visited India. NRG likely results from multiple factors, including protozoal drug resistance and host-related factors. While our current treatment ladder appears effective, further research is needed to understand the mechanisms underlying treatment failure.