Some parasites are human specific, but a lot of them will manage just fine in many mammals. Health and hygiene is likely a major factor, but managing human waste in a sanitary way is one of the big jobs of community health and hygiene efforts; that and access to clean water. Proper health protocols wouldn't allow for untreated night soil to be used as fertilizer, but maybe they don't have access to treatment or other fertilizer, so there you go.