what are the health issues related to untreated giardiasis, if not diagnosed and infected for several years?

Hi,
To begin with, here is the link to an article which gives an overview of this common intestinal infection:
https://www.empowher.com/giardiasis/content/giardiasis-common-intestinal-infection
Specific to your question, the symptoms of nausea, diarrhea and abdominal cramps can lead to weight loss and dehydration (loss of fluids). However, not all infected persons have symptoms. Symptoms usually appear 1 to 2 weeks after infection with the parasite. In otherwise healthy persons, symptoms can last 4 to 6 weeks. Occasionally, symptoms last longer.The parasite lodges in the upper intestine and can be hard to get rid of, so diarrhea can last for several weeks or months and cause weakness and weight loss. Dehydration can be a serious problem for children.
If left untreated, symptoms may persist for months. Malabsorption due to chronic Giardia infection has also been reported.
Giardiasis has been associated with several gastrointestinal disorders such as irritable bowel syndrome, inflammatory bowel disease and biliary tract dysfunction. Case reports indicate that giardiasis may also be involved with other health complications such as reactive arthritis and urticaria/pruritus or other dermatologic disorders.
All you wanted to know and more.
Maryann