Most pet owners are aware that worms can be a serious problem for pets. They also know that worms carry with them the potential to produce serious illness, or even kill pets. This is particularly true in young puppies and kittens but also holds true for adult animals. Since most pets are treated for intestinal parasites early on in their lives, owners assume that worms no longer pose a threat thereafter. Unfortunately, nothing could be further from the truth.

The most common type of worm is the roundworm family. Their significance lies not only in their prevalence but also in their ability to cause serious illness such as weight loss, vomiting, diarrhea, coughing and death. Roundworms also pose a public health hazard, especially when children accidentally ingest eggs.

In people, blindness may result or internal organs may be affected. If dog or cat stools are not picked up promptly from parks, eggs in these feces remain in the environment, active in soil or sand boxes for long periods of time. Children should always be coached to routinely wash their hands with soap and water after playing outside in soil, sandy or grassy areas.

Second on the list of common worms are hookworms. These worms parasitize the digestive tract by ingesting blood. They damage the lining of the intestines, leading to bloody diarrhea and, in some cases, serious blood loss and death. These also pose a public health risk in that the larval forms can cause a skin rash or creeping eruption in people. Ensuring that the mother cat and dog are clear of tapeworms and hookworms before pregnancy is very important parasite prevention strategy.

Tapeworms are also quite common in both dogs and cats. They occur when a pet swallows infected fleas. Few pets actually suffer serious ill effects from a tapeworm infection except those with heavy burdens, and owner disgust is usually the prime motivator for treatment. Owners tend to diagnose tapeworms when they notice worm segments ("rice particles") around their pet's anus in the fur, or segments moving across the carpet or floor. Helpful preventive measures include prevention of hunting and flea control for your pets..

A less common type of worm, called whipworm, can infect dogs. They often cause bloody diarrhea, leading to dehydration, weight loss and anaemia.

Toxoplasma and Giardia are single-cell parasites, not true “worms”, but are also internal parasites.

Cats can shed toxoplasma cysts in the stool, and contact with cysts by pregnant women can lead to serious problems. Toxoplasma only has a full life cycle in felines, completing the life cycle in the intestines and shed out in stool. Other common sources of Toxoplasma include handling uncooked or undercooked meats during meal preparation, and gardening in flowerbeds contaminated with infected cat feces. It is important that pregnant women do not clean cat litter boxes as a preventive measure, and wear gloves for handling uncooked meat, and during gardening. An adjunct for prevention of these parasites can be accomplished by keeping your cat as an indoor cat.

Giardia is another single celled parasite that has human health significance, causing so-called Beaver Fever in people. Usually, water is the source but evidence points to transmission between pets and humans. Diarrhea, bloating and mucous malodorous stool are typical signs in all animals. A vaccine is available for prevention in dogs, and avoiding exposure to water contaminated with feces is another helpful prevention strategy.

Treatment of intestinal parasites is straightforward. These days, prevention therapy is a standard part of puppy and kitten care. Medications currently available for both prevention and treatment are safe and some have quite a wide spectrum, often killing several types of parasites with one single treatment. An important step in the fight against intestinal parasites is having your pet's stools tested at least once a year by your veterinarian. For pets that are at high risk, such as pets that spend a lot of time outdoors, routine treatment with appropriate preventive medication may be recommended by your veterinarian. Some dewormers require more than one treatment to clear the parasite, so follow prescription instructions closely.