How Does My Dog Get Roundworms?

Roundworms are one of the most common and important parasites of dogs. When present in large numbers they can be fatal for young puppies, and their immature form occasionally affects man.

How Do Dogs Get Roundworms?

The most common species of roundworm, Toxocara Canis, is found in dogs, foxes, and dingoes.

The smaller form, Toxascaris leonina, affects dogs and cats, whilst the common roundworm of cats, Toxocara cati, only rarely affects dogs. Roundworms are quite large, female worms can be up to 18cm long.

The lifecycle of Toxocara Canis, the largest and most common roundworm in dogs, is very complex. It has features that guarantee that infection is passed from generation to generation and a full understanding of the life cycle is required before roundworms can be adequately controlled.

The first method of infection is by a small puppy in an infected environment swallowing an infective egg. Eggs survive for a long time in the environment and many thousands are shed daily by an adult female worm.

The egg hatches in the puppy’s intestine and the larvae immediately burrow through the intestinal wall to reach the liver.

After a few days, it moves to the lungs, where it remains for a further few days before burrowing through the lung tissue to reach the esophagus and thence return to the stomach and small intestine. Here it completes its development into a mature egg-laying adult worm about 4 to 6 weeks after hatching from the egg.

It is important to realize that these immature worms cause considerable damage long before they can be detected by examining the feces for the presence of eggs.

As the puppy becomes older the eating of infected eggs will result in few adult worms finding their way to the small intestine, because in puppies older than 3 months immature worms hatched within the intestine become encysted in the tissues of the puppy.

In the female puppy, the roundworms do not begin to develop again until the dog becomes pregnant. This feature of the life cycle ensures that infection passes from the mother to the puppies developing within her uterus.

The larvae make their way from the cysts within the bitch through the placenta to the puppies during the last 14 days of pregnancy. Newly born puppies have immature worms within their lung tissue and during the first week of the puppy’s life migration to the stomach and intestine occurs.

These mature and can become egg-laying within 3 weeks of birth. This not only means that 3-week-old puppies have mature worms but they are passing thousands of infective eggs which add to the worm burden of their littermates through direct infection.

Within the bitch, larvae continue their migration from the cystic stage and complete their development within the intestine. These produce infective eggs, thus further contaminating the puppies’ surroundings.

Large numbers of immature roundworms can cause such severe damage to the liver and lungs that the puppy may die. Less severe infestations can produce intestinal disturbances, coughing and such severe changes to other organs that the proper development of the animal is interfered with.

Humans can be infected with the larval stages of roundworms by close association with badly infected puppies, or in the ease of young children, by putting objects into their mouth that are contaminated by infective eggs.

The ill-effects of the immature worms migrating in the human body are related to the organs involved. The most serious effects occur when the larval worms migrate to the central nervous system or tissues of the eyes.

Treatment of roundworm infestations using modern drugs is very successful.

When treating a nursing bitch and a litter of puppies, it should be remembered that the worms are not always at a stage of their life cycle when they are affected by worm treatments.

It is important to routinely treat puppies every 2 weeks, starting at 2 weeks of age. The bitch should be treated at the same time. Anthelmintics such as Canex, Lopatol and Ambex are all effective if used at the correct dosage and frequency.

Breeders are often lulled into a false sense of security when they worm their bitch before mating. As we have illustrated, at this stage the larval worms are in their cystic stage and are not affected even by massive doses of anthelmintics.

Bitches and puppies must be treated routinely after whelping. Young children should be discouraged from handling both bitch and newly born puppies. Every effort should be made to keep the bitch and puppies in surroundings that are easily cleaned.


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Hannah Elizabeth is an English animal behavior author, having written for several online publications. With a degree in Animal Behaviour and over a decade of practical animal husbandry experience, Hannah's articles cover everything from pet care to wildlife conservation. When she isn't creating content for blog posts, Hannah enjoys long walks with her Rottweiler cross Senna, reading fantasy novels and breeding aquarium shrimp.

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