Worms or helminths are internal parasites that drain pets of blood and nutrients. Dogs and cats get worms that can be classified by shape into round or flat. Three types of round-shaped worms (nematodes) live in pet intestines: roundworms, hookworms, and whipworms. Two types of flat-shaped tapeworms (cestodes) live in pet intestines and other organs, including the liver: echinococcus and taenia. Dogs commonly get hookworms, whipworms, roundworms, and tapeworms. Cats commonly get roundworms, hookworms, and tapeworms, but not whipworms.
Pets can also get lungworms, esophageal worms, bloodworms, kidney worms, liver flukes, and intestinal protozoa. The most common pet worms are roundworms, and hookworms. Heartworms (Dirofilaria immitis) are not intestinal parasites but are parasites that live in the heart. Heartworms are not spread like regular intestinal worms, but are spread by mosquitoes. They infect both cats and dogs and can cause death.
Almost universally, puppies and kittens have intestinal worms. Puppies and kittens whelped in large kennels or in unhygienic conditions have the greatest worm burdens. Pets using pet parks, doggy day cares with communal potty areas, farmyards, and city streets where feces is on the sidewalk are prone to having worms. Pregnant and lactating females have worms. Pets kept in humane shelters generally have intestinal worms.
Puppies and kittens are born with intestinal worms. This occurs because worms resting within the mother's body are stimulated to grow by hormones secreted during pregnancy. The newly developed worms within the mother pass through the placenta (transplacental) and into the young before they are born. In addition, worms are passed through the mother's milk (transmammary) to the young. Puppies, kittens, and lactating bitches and queens should be wormed routinely.
Adult pets pick up worms from eggs that stay in the soil after pets have defecated. Some worm eggs survive for years in soil. Pets also pick up worms from: