What is it? Giardiasis (also known to backpackers and campers as “beaver fever”) is an infection of the small intestine caused by the flagellated parasitic protozoa Giardia intestinalis (also referred to as Giardia lamblia, or Giardia duodenalis). 
Occurring worldwide, infection of U.K. residents has traditionally been associated with travel to low income countries (with a reported prevalence of up to 30% compared to 2-7% in high income countries), but it is now thought that “Most patients with Giardia in the UK acquire their infection in the UK and not from overseas travel”. 
Giardia, which is typically found in lakes, streams, or ponds and private wells that have been contaminated by human, dog, deer, cattle or beaver faeces , lives outside mammalian hosts only by forming an oval protective cyst (8-19 µm (average 10-14 µm)) before excretion, which also allows it to survive in cold water for several weeks or months. The cyst has a degree of resistance to deactivation by chlorine, particularly at lower water temperatures.  The U.S. CDC states that “disinfection with iodine or chlorine has a low to moderate effectiveness in killing Giardia”, and further notes that “contact time, disinfectant concentration, water temperature, water turbidity (cloudiness), water pH, and many other factors can impact the effectiveness of chemical disinfection.” 
“Infection usually occurs through ingestion of G. intestinalis cysts in water (including both unfiltered drinking-water and recreational waters) or food contaminated by the faeces of infected humans or animals.”  It can also be passed on through the faecal-oral route (on hands or fomites).
Once infected, Giardia in the proximal small bowel (the first part of the small intestine, the duodenum) goes through a process of excystation and releases two trophozoites (10-20 µm in length) per cyst. These then reproduce by binary fission, attach themselves with a sucking disc to the mucosal epithelium (inner lining of the duodenum) and, using a method only recently identified,  break down cells to feed on nutrients.
Infection may be asymptomatic, but symptoms usually appear between one to three weeks after exposure (the most infectious phase in an individual is from when the symptoms start until two days after they’ve passed). These may include one or more the following intestinal indications:
• Diarrhoea (sometimes explosive and/or foul smelling)
• Greasy soft stool that can float
• Belching (with a bad taste)
• Stomach or abdominal cramps, and bloating
• Loss of appetite and weight loss caused by malnutrition
Less common symptoms are
• A mild fever with a temperature of 37-38°C (98.6-100.4°F)
Symptoms of Giardia infection may last from two to six weeks, but they can last longer or recur at intervals. There is no prophylaxis for Giardia. Treatment is by a course of antibiotics, and symptoms usually take a week to stop.
Where is it being contracted? Prior to a webpage content change in 2017, the U.K. NHS stated  that “There are more than 3,500 cases of giardiasis reported in England and Wales each year, although the true number is likely to be higher as many cases go undiagnosed. Around one quarter of these cases are thought to be contracted abroad, but many people don’t develop the symptoms until they return home.”
In 2016 that webpage listed the following regions and destinations as places where Giardia is widespread.
• sub-Saharan Africa – all the countries south of the Sahara desert, such as South Africa, Gambia and Kenya
• south and southeast Asia, particularly India and Nepal
• Central America
• South America
• the countries of the former Yugoslavia (Croatia, Serbia, Montenegro, Slovenia, Macedonia, and Bosnia and Herzegovina)
This information is still relevant to travellers.
In the most recently published U.K. government statistics on Giardia infection overseas, “Travel-associated Giardia infection in England, Wales and Northern Ireland: 2014” (released 2017) , it was noted that “a total of 84 different destinations were reported for Giardia infections in 2014.” The ‘top 10’ countries in which travellers from EWNI acquired Giardia in 2014 were listed as 1) India 2) Spain 3) Pakistan 4) Thailand 5) Turkey 6) Egypt 7) Greece 8) Cyprus 9) Morocco 10) Tunisia (with the total of “Other” countries reporting only a little over the total number of infections from India alone).
This ranking, of course, reflects the popularity of travel destination choices, rather than an absolute categorisation of risk, and it is likely that the significant subsequent decline in tourist numbers from the U.K. to Egypt and Tunisia (and to a lesser extent Turkey) might now alter the list. Conversely, there has been a significant increase in vacation and other travel to India and Spain (for India there was a rise between 2014 to 2017 from 884,000 to 1,006,000 trips, while for Spain over the same timeframe the rise was from 11.6m to 15.8m) that would suggest they might be keeping their ‘top’ positions.
However, at this point it should be noted that the U.K. Government figures show that over the period from 2005 to 2014 the number of cases contracted overseas compared to those of infections acquired at home fell from just 10% to a mere 7%, which is at significant variance with the NHS estimate of around 25%.
In an article published by the British Medical Journal (BMJ) in 2016, it was stated of Giardia that “Its incidence in the United Kingdom is underestimated because of the lack of diagnostic sensitivity of traditional faecal microscopy and the mistaken belief that it is mostly acquired abroad, so often only people reporting foreign travel are tested.” 
In an article published online in 2017 research was stated to show that “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.”  This latter report focused on transmission within households, but notwithstanding this factor, there is now growing recognition that the number of Giardia infections acquired in the U.K. are both on the rise, and ‘home grown’.
In 2016 the figure of reported  Giardia infections in England and Wales was 4,492 cases. At the 1:14 ratio described above, that would have equated to a total of 58,396 cases that were undiagnosed, and potentially untreated.
If you’d like to ensure that drinking water from unprotected sources both at home and abroad doesn’t add you to these statistics, and avoid finding out first hand about the joy of Giardia (not to mention the risk of passing it on to your family), the aquapure traveller™ has been independently tested  and proven to exceed international standards for the reduction of Giardia (and other parasitic protozoan pathogens, including Cryptosporidium and Cyclospora) in drinking water.
 See more on the naming of this parasite here.
 BMJ 2016;355:i5369.
 Note that “genotype […] assemblages A and B are capable of infecting several animal species, as well as humans.” See this information sheet for vets.
 See this research on the effect of chlorine on Giardia.
 U.S. CDC advice on backcountry water treatment.
 A statement by the World Health Organisation.
 BBC report from 2018 here.
 This information was located here, and was accessed 02 March 2016. That page has now been replaced with a default to this page.
 U.K. government report here.
 BMJ article here.
 Research article reported here.
 Public Health England Giardia spp data 2007 to 2016 (published May 2018) here.
 Our test results for performance against parasitic protozoans such as Giardia.