Hookworms, although not the most common parasite in dogs, have a tendency to cause severe symptoms in their host.  Like roundworms, adult hookworms lay microscopic eggs in the intestine that are passed out of the body with the feces.  Adult worms are not likely to appear since they stay attached to the intestine for most of their life.  In order to properly diagnose an animal for hookworms, a veterinarian must look at the feces under a microscope in order to find the eggs.

Hookworms can cause a distended abdomen, but are much more likely to cause diarrhea first.  In fact, hookworms can cause lethargy (tiredness and inactivity), lack of appetite, and diarrhea, many of the same symptoms as parvovirus.  Large infestations of hookworms have the ability to cause the same type of bloody diarrhea associated with parvo.  These types of infestations are sometimes mistaken for parvo which is why its essential to have an animal tested for both parvo and hookworms if he or she shows any of the above symptoms. 

Hookworms are rather simple to treat (unlike parvo) and animals diagnosed with these worms usually receive a series of doses of deworming medication in order to rid them of the parasite. 

Hookworms can kill puppies if they are left untreated and can be transmitted to humans through the environment.  Pick up feces regularly and keep areas clean.  Since hookworms can infect their host via the mouth as well as the skin, any areas that have been used by an infected animal should be thoroughly cleaned and treated.