Prevalence of Giardia infection in households of Giardia cases and risk factors for household transmission.

Background: Giardia is a leading but neglected cause of infectious gastroenteritis worldwide and is treatable. There is a substantial burden of undetected Giardia in the UK and for every one case of Giardia reported to national surveillance there are 14 cases in the community. We aimed to ascertain the prevalence of, and risk factors associated with secondary household Giardia infections to assess the burden of infection and inform control measures.

Methods: We identified all giardiasis cases notified in nine local authorities in Lancashire between June 2014 and June 2015, and invited their household contacts to submit faecal specimens for Giardia testing and complete a risk factor questionnaire. We estimated the proportion of households with additional Giardia infection. We compared household risk factors between households with and without additional Giardia using Fisher's exact test. We used multivariable logistic regression to identify independent risk factors for additional Giardia infections.

Results: We identified additional Giardia infections in 30% (27/91) of included households. A total of 41 infections were found from 212 household members, of which 37 were asymptomatic. The majority of infections were assemblage B (57%) but there were also a high number of mixed infections (20%). Risk factors significantly associated with additional household infections were; having children under 5 years in the household (odds ratio 42; 95% confidence intervals 10-178) and the presence of gastrointestinal illness in the household before the onset of the index case (odds ratio 9; 95% confidence intervals 1.5-48).

Conclusions: Our finding of a high household prevalence of asymptomatic infection has raised the public health question of whether treatment of asymptomatic household contacts may be justified in preventing Giardia re-infection of the index case or in preventing secondary cases and household clusters. We recommend the communication of this risk in household contacts of Giardia and reinforcement of standard hygiene controls.