Richard Gerhold, veterinary parasitologist, sets the record straight about monthly preventive medications and the possibility of pets still picking up parasites. He discusses a case where a dog picked up hookworms, even though the client was believed to be compliant on a monthly preventive regimen.
Source: DVM360, January 3, 2017.
The preventive effectiveness and the continued contamination of the premises of hookworm larvae are two separate issues.
One final consideration should include determining if the dog has been ingesting wild or domestic ungulate feces. The eggs of the GI nematodes of ungulates may be confused with hookworm eggs. If these are GI nematode eggs, they are just passing through the dog and are not infecting the dog.