**5. Giardiasis epidemiology**

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

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

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, which include:


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 cysts in the municipal water supply here in the United States, although such scenarios do not account for the vast majority of infections [35].

Foodborne transmission of *G. lamblia* is much less common than waterborne transmission, but there are many ways food can be fecally contaminated. For example, street food and any food prepared with the unclean hands of an infected subject could easily transmit giardiasis given a few cysts necessary to transmit the disease. Ingestion of 100 or more cysts is required to ensure infection in humans, but as few as 10 cysts have proven to be enough to infect a volunteer [59].

Fecal-oral transmission is also a significant mechanism of transmission and is the one responsible for the outbreaks in day-cares and nurseries. These outbreaks reflect the close contact between young children, who are significantly more likely to pass the parasite fecal-orally at day-cares than at home. For example, in the Netherlands, where around half of preschool children are cared for in day-care centers, a mean of 2.5 days a week, children at day-care centers are twice as likely to test positive to *G. lamblia* as their home-care counterparts [45], infecting around 4.2% of them [60].

Sexual transmission of *Giardia* is now a very well described form of oral-anal transmission and fecal-oral transmission among men who have sex with men. Currently, there exist a large body of publications that have led to improving our understanding of giardiasis as a sexually transmitted infection. According to these studies, prevalence rates of giardiasis among men who have sex with men range from 2 to 30% [61]. Although giardiasis is not a major cause of AIDS-associated diarrhea, the prevalence of giardiasis, as well as the chronicity of symptoms, is greater in patients with AIDS, especially in developing countries [62]. Overall, every immunocompromised group, like AIDS patients, is recognized as more susceptible to the development of chronic giardiasis [63].

To sum up, it is really important that healthcare providers consider *Giardia* as a differential diagnosis among high-risk populations that match giardiasis epidemiology, and if patients tested positive, it is really important that they provide patients with appropriate therapy and follow-up, as well as proper counseling to increase treatment compliance rate. And in the case of men who have sex with men, also encourage partner notification, and teach them strategies for preventing the transmission of this disease, including the discussion of the risk of enteric infections after oral-anal sexual contact.
