Giardia is named after the scientist Alfred Giard who described it in a report in 1895, but it wasn't until the 1970s that it was discovered that it wasn't a harmless gut parasite but that it caused an infectious intestinal disease - giardiasis.
Infection is by the fecal-oral route with the bug passing through two stages to achieve its lifecycle. It lives as a cyst in the environment (it can live in cold water for two to three months) and as the protozoa in the gut.
First noted in New Zealand in 1990 in river water south of Oamaru, subsequent testing found that giardia was widespread throughout the country. DOC as well as local authorities carried out studies as to its spread and the levels of occurrence.A 1992 Journal of Marine and Freshwater Research reported DOC's backcountry sampling as returning 19 positives from 90 samples taken. Wellington didn't fare so well; the Regional Council reported 63 per cent positive findings. In many developing countries giardia is endemic - you're going to find it in most water - and around 200 million infections occur each year in Asia, Africa and Latin America.
In 1996 giardiasis became a notifiable disease, meaning the Ministry of Health should be informed about outbreaks. Studies have shown that gross under-reporting is probable. Between 1997-2006, the average annual rate of notified giardiasis was 44.1 cases per 100,000 population or about 1650 reported cases each year. However true annual incidence because of non-reporting is likely to be over 160,000 people infected annually.
Outdoors there are two means of infection — poor hygiene or sanitation resulting in person-to-person transmission, or waterborne transmission. The first is pretty easy to deal with, wash your hands and take care disposing of toilet waste in the wilderness. The second is tougher, because giardia exists not only in humans but also in possums, deer, sheep, cattle and other mammals.
Self-reported exposure notes more than 35 per cent of infection as coming from untreated drinking water. Of these, 20 per cent reported overseas travel during the incubation period, so the chance of catching the disease from a back-country stream in New Zealand is still fairly low.
You can avoid infection by boiling, chemically treating or filtering water and using common sense. Remote streams above the bush line and where there are fewer animals will most likely have fewer cysts in the water. Walks where there are lots of visitors popping off-track for the call of nature are likely to have more polluted waterways. Ingesting as few as 10 of the cysts can cause the disease and if you swallow 25 it's guaranteed you'll be infected.
The symptoms of infection are diarrhea and abdominal pain. Most people don't display symptoms, hence hygiene is always important, because they are still carriers. Very few people require hospitalisation and death from infection is very rare.
The disease is self-limiting with the body expelling the parasite in the faeces, although re-infection from that source is possible if hygiene standards are poor. Untreated giardiasis can last for weeks. Treatment can be made with antibiotics, which shorten the course of the disease.