Giardia lamblia cases of infections
If you have or can get access to it, you might try looking in the Incidence and Prevalence database: http://thomsonreuters.com/incidence-and-prevalence-database/

Another possibility is the GIDEON database: http://www.gideononline.com/. It is possible to sign up for a 15-day trial.

For Europe, statistics are available from the WHO CISID at http://data.euro.who.int/cisid/ (select "all infectious diseases", then "Giardiasis"). 

The WHO does not have any global statistics available on their website, so otherwise you might have to piece together data from individual publications. Some examples:

Thailand, 2005: http://www.ncbi.nlm.nih.gov/pubmed/16438174 

Germany, 2006: http://www.ncbi.nlm.nih.gov/pubmed?term=19404678

United States, 2006-2008: http://www.cdc.gov/mmwr/preview/mmwrhtml/ss5906a2.htm

Portugal, 2002-2008: http://www.parasitesandvectors.com/content/5/1/22

Quatar, 2008: http://www.parasitesandvectors.com/content/4/1/211

Tajikistan, 2009: http://www.parasitesandvectors.com/content/4/1/195

Ivory Coast, 2009: http://www.parasitesandvectors.com/content/4/1/96

Tanzania, 2011: http://www.parasitesandvectors.com/content/6/1/3

Ghana, 2006-2009: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3170632/?report=classic

Canada, 1999/2002: http://www.biomedcentral.com/1471-2334/5/72

Canada, 1990-1998: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC194172/

China, 2012-2013: http://www.biomedcentral.com/1471-2334/14/25

Ethiopia, 2009: http://www.biomedcentral.com/1471-2334/13/419

Kenya, 2010-2011: http://www.biomedcentral.com/1471-2334/13/243