Dr Liesel van der Merwe is a small animal medicine specialist. Send her your questions: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Dr Liesel van der Merwe

I have to confess to being an avid reader of ‘You’ magazine. A recent edition ran an article regarding a boy who partially lost his vision due to immature worms infecting his eyes. Throughout the article the parents insisted that their animals were dewormed.

Three things struck me: What is the definition of adequate deworming? How should you establish a deworming regimen? What are the routes of infection? 

The two most common worms affecting dogs and cats are roundworm (Toxocara spp), easily visible to the naked eye and looks like spaghetti, and hookworm (Ancylostoma spp), very small and fine.

The lifecycles of both worms are similar and the dog or cat is the end host. Development to adults in the small intestine occurs in younger and immuno-incompetent animals, whereas older immuno-competent animals may develop somatic/tissue larvae in various tissues. Humans are not the true host, so the worm cannot reach maturity and migrates into the tissues and encysts.

Adult worms live in the small intestine where they feed on ingesta. The eggs, containing a second-stage larva (L2), are not infective when passed and develop to infectivity only from three weeks to months later. Roundworm eggs are very resistant and may remain infective for years and can survive sewage treatment and composting. Each female worm is responsible for 700 eggs per gram of faeces per day.

After the eggs are ingested they hatch in the small intestine and the L2 larvae migrate through the intestinal wall into the blood vessels, and make their way to the liver and from there, the lungs. The larvae mature to L3 in the lungs and migrate into the air passages and up the trachea, and from there they are coughed up to be swallowed and return to the intestine.  

Infected pregnant bitches are a major source of infection: the unborn foetus can be infected by dormant encysted L2 larvae, which become mobilised due to hormonal changes. These L2 larvae migrate to the lungs and mature to L3 just prior to birth.

In the newborn puppy the cycle completes and the worm moves to the gastro-intestinal tract. Some activated larvae will also penetrate the small intestine and mature into adult worms which will pass eggs.

Once infected, a bitch can infect all her subsequent litters with activation of encysted larvae. Puppies are the most important source of infestation of the environment, and in turn will re-infest the bitch.
The duration from ingestion of the egg to an adult passing eggs (pre-patent period) is about three weeks and in new-born puppies may only be 14 days.

Deworming is the key to managing infestation. If pets are frequently out of the property and walk in areas where other dogs walk, you’d need to deworm monthly.

Dogs which are confined to the property can be dewormed less frequently if an initial intensive course is applied. Bitches should be dewormed two weeks before giving birth to kill off the resting and mobilised larvae.

Symptoms are generally only significant in very young puppies. They may develop verminous pneumonia depending on the degree of infestation, and will cough, have an increased breathing rate and a nasal discharge. Common symptoms are ascites, vomiting, diarrhoea or constipation and on occasion bowel obstruction or perforation. This will all occur before the pre-patent period is over, which means that eggs will not be detected on a faecal flotation.

Eggs ingested by an intermediate, non-final host develop to L3 in the host and undergo a resting L3 stage in various tissues. This is what happens in people. The worms are not active and eating the tissue. The body’s reaction to the parasite contributes to the tissue damage and inflammation in the affected organ.

People get infected from the soil/lawn if environmental contamination has occurred; from sand (play pens); and eating unwashed contaminated fruit or vegetables. Patting pets is very unlikely to cause infection as contamination of the fur by eggs is very rare.

Both canine and feline roundworm can infect people and develop into a resting tissue stage. Migration into the eye is a preferred route in the canine roundworm. In tropical and subtropical regions another species of ringworm infects via the skin and causes cutaneous larval migrans.