**Author details**

**5. Giardiasis epidemiology**

18 Current Topics in Giardiasis

which include:

• institutionalized individuals [53]; • men who have sex with men [54];

The *Giardia* cysts are overall highly infectious, and as few as 10 cysts can cause an infection in an individual. Giardiasis prevalence rates have been reported consistently as high among young children from developing countries, with high rates of repeated infection even within the first year of life. However, many developed countries have many regions with endemic giardiasis or regular outbreaks. At these countries giardiasis outbreaks are particularly common during the summer months (likely due to recreational swimming exposure) or throughout the year around day-cares and nurseries, infecting children under 5 years old—and their caregivers—the most [45]. In fact, an investigation of 242 outbreaks, affecting 41,000 persons, reported that most outbreaks resulted from waterborne (74.8%), foodborne (15.7%), personto-person (2.5%), and animal contact (1.2%) transmission, with waterborne outbreaks been

Surveillance data cases have shown that giardiasis infects populations with a bimodal age distribution, peaking at ages 0–9 years and 45–49 years, without gender preferences [35], and within areas that are endemic, giardiasis commonly shows a seasonal pattern, with most cases occurring in the summer months due to a recent history of drinking untreated surface water and a history of swimming in a lake or pond or swimming in any natural body of fresh water [47]. Other risk factors that have been reported as associated to giardiasis in endemic areas include living in areas that use at-risk tap water (i.e., filtered or unfiltered surface water [48,

One of the most common mechanisms of transmission of *Giardia* infections is a waterborne transmission but also can be transmitted by fecal-oral transmission with contaminated food or direct fecal-oral contact among family members, person-to-person contact, and sexual transmission (oral-anal contact). Although it is unclear which ones are clinically the most important, there is a common understanding about the populations at high risk of giardiasis,

• immunocompromised individuals (chronic variable immunodeficiency, hypogammaglobulinemia, HIV, immunosuppressed individuals, cystic fibrosis, and others) [55, 56]; and

• international travelers or any subject (hikers, campers, sportsman's adventures, and others) exposed to drinking untreated water from lakes, streams, and swimming pools [57].

Waterborne transmission is recognized as the most common transmission, with numerously documented outbreaks throughout the world [46, 58]. This includes the consumption of contaminated water from pools, rivers, or lakes, as well as from contaminated drinking water, either unpurified or inadequately purified. There have been multiple documented cases of

that largest ones in terms of number of cases per outbreak [46].

49] or unfiltered shallow well water [48]) or in rural areas [49].

• diaper-age children who attend day-care centers [50, 51];

• adults that work in child-care organizations or day-care centers [52];

Antonio Marty Quispe Gutiérrez1,2

Address all correspondence to: drantonioquispe@gmail.com

1 Johns Hopkins Bloomberg School of Public Health, International Health Department, Baltimore, Maryland, USA

2 Instituto de Evaluación de Tecnologías en Salud e Investigación (IETSI), EsSalud, Lima, Peru
