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War of the Worms


As veterinarian in private practice, there's one question I hear almost every day: "Why do I have to deworm my dog? I haven't seen any worms."

It's in the worms' best interest to stay hidden in order to survive. They are protected inside the warm--and for them, nutritious--canine intestine. But even though you don't see them, they disrupt the dog's health and pose a hazard to people in the dog's environment. Winning the war against these parasites means knowing the enemy--who they are, what they do, and how to kill them.

Which Worm's Which?

The roundworm (Toxocara) is the common intestinal worm of dogs. It looks like spaghetti-thin, round, and up to eight inches in, length. In adult dogs, round worms in the intestine absorb nutrients from the surrounding fluid; then they mate and lay microscopic eggs. Once the eggs pass out of the dog in his stool, they hatch into larvae ready to infest another dog. (The next dog becomes infested by eating the stool or grass contaminated with larvae.) The time from egg-laying to larvae depends on temperature. If it's hot, eggs can larvate in less than a day. If it's cold, they can stay dormant for months.

I find roundworms fascinating because of their migrations. These worms can actually tell the difference between a puppy and an adult dog, and change their travel routes accordingly. A larva eaten by a puppy less than 3 months old does not develop into an adult worm right away-it first has to go on a walkabout, The larva burrows through the pup's intestinal wall, tunnels through the liver, passes through the diaphragm, and finds its way into an airway in the lung. Once there, it's coughed up and swallowed, returning it to the intestine where it can mature and produce eggs. If enough worms migrate at one time, puppies can become quite ill. When infestations are severe, puppies may vomit up a pile of writhing worms or pass them in their stool.

In adult dogs, Toxocara larvae follow a very different path. They burrow through the intestinal wall as in puppies, but they end their journey in muscle tissue where they enclose themselves in cysts. These lay dormant forever, with one important exception: If a larva is lucky enough to encyst in a breeding female dog, it gets a reprieve. The hormonal changes of pregnancy trigger the larva to emerge. It then goes to one of two places-either into the uterus and directly into a puppy, or into the mammary glands where it ends up in the dam's milk. These two mechanisms explain how puppies may be infested at a very young age, or even be born with roundworms.

As you can see, roundworms have a remarkable ability to ensure their survival. That's why, in spite of modern treatments, we still see them in our dogs. Of course, there are other worms. For example, imagine a worm squirming our of your dog's anus and settling on your pillow. [Yuck! -Ed.] This is a common dog tapeworm (Dip ylidium caninum), another parasite with an amazing life cycle.

Tapeworm segments are parts of a long worm that lives in the dog's intestine. The segments look like rice grains, but they are actually egg packets, which must be eaten by an intermediate host for the tapeworm's life cycle to continue. The intermediate host for the common dog tapeworm is the flea.

Once inside the intermediate host, the tapeworm larvae hatch and migrate to the host's muscle tissue, encysting themselves to wait for the next stage of their lives. The next step is vital--the intermediate host must be eaten by a dog. This releases the larvae, which develop into adult tapeworms, shedding segments to continue the cycle. Since fleas cause skin irritation, dogs lick them off and swallow them. Dogs with heavy flea infestations can have significant tapeworm loads.

Though tapeworms are certainly an aesthetic problem, they are not really much of a health concern. Only with very heavy infestations do dogs drop weight, eat more, and lose coat quality. In such cases, the tapeworm is absorbing nutrients from the dog's food before the dog can benefit from them.

Another species of worm, the hookworm, is rare, but severe infestations can be deadly. Hookworms don't absorb nutrients from the intestine; instead, they suck blood. These worms can consume so much blood that dogs become severely anemic and die. Dogs pick up hookworms either by eating the worm eggs deposited in dog feces or when worms from the ground penetrate the skin. A dog can become infested simply by walking on a lawn.

Whipworm, a rarely seen parasite, also feeds on blood. It can trigger bloody diarrhea and weight loss, and is seen mainly in adult dogs rather than puppies.

Worms in dogs

The most common intestinal worm in dogs.
Only with heavy infestations do dogs drop weight and eat more.
Rare, but can be deadly.

Definitive Diagnosis

Now that you know the types of worms that can infest your dog, how do you tell if he has any? Checking the stool doesn't work unless you are looking for tapeworms. Tapeworm is diagnosed by seeing segments, either on your dog's bed or on the hair beside the anus.

If you have a puppy with a heavy infestation of roundworms, you might see poor growth, a pot-bellied appearance, diarrhea or vomiting, and a rough hair coat.

The other types of worms tend to be more of a problem in adult dogs and can cause weight loss, bloody diarrhea, and scruffy hair coats. Because most infestations are light and dogs generally do not show any outward evidence of worms, their presence can be confirmed only by finding eggs in the dog's feces. To do this, your veterinarian must analyze a fecal sample under the microscope.

Walloping Worms

The nice thing about worms is that they are easy to treat. Many deworming medications are available, from your veterinarian and at pet-supply stores. But because each product kills only certain species of worms, it is important to give the right one. If you are sure of which type of worms your dog has, pick a product developed specifically to kill them. If you don't know, ask your vet to check a fecal sample. Or you can use a broad-spectrum dewormer to treat all types of worms at once.

How often should you treat your dog for worms? Many veterinarians recommend deworming puppies every two to four weeks until they reach 16 weeks of age. Adult dogs should continue to be dewormed on a regular basis. If the risk of re infestation is high, or if there is a concern about transmission of worms to children, most dogs can be dewormed safely once a month.

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