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Worming working dogs – here’s what you need to know

Is it safe to worm young puppies? Vet Neil McIntosh tells you what you need to look out for

black Labrador puppy

Puppies can be born infected if the bitch carries dormant larvae

Responsible gundog owners know that worming working dogs regularly is part of the routine. But what about puppies? I recently received this query from a gundog puppy owner. (Read our tips here on choosing a gundog puppy from a litter.)

Q: I have heard of young puppies dying after they have been wormed. Why does this happen?”

A: Over the years I have heard of post-worming deaths but they always seem to have happened to a ‘friend of a friend’. Nevertheless, it is possible that puppies with very heavy worm burdens might have suffered intestinal blockage (and subsequently death) if large numbers were killed at once, as could occur with effective wormers. I am assuming that proper treatments were used at the correct dose and that these ‘deaths’ were not caused by toxicity. Let’s look at the background of why worming working dogs is important. (Read here for information on lungworm in dogs.)

worming working dogs


Roundworms in dogs are the most common nematode parasite seen in small animal practice. They are troublesome wee creatures, with a complicated life cycle. To use their scientific name, Toxocara canis eggs are passed in the faeces of infected dogs but have to undergo a maturation process before they become infective.

This is important because it explains why fresh faeces are relatively safe and should be collected well before the two-week period that embryonation of the egg requires. Once ingested by a dog, the mature eggs hatch in the duodenum (upper part of the small intestine) and larvae then take one of two intriguing routes.


In puppies, the larvae penetrate the intestinal wall and travel via the lymphatic vessels, portal vein of the liver and pulmonary artery to reach the lungs. They then leave the bloodstream and climb up the respiratory tract from the oxygen exchange area, the alveoli, through the bronchioles and onwards to the trachea (windpipe). They are then coughed up and swallowed, allowing them to mature in the small intestine, where the whole process starts all over again.


In older dogs, the larvae that hatch from the eggs in the duodenum penetrate the bowel wall and migrate to various tissues in the body, in particular the muscle. Once they have arrived at their destination, they become dormant, rarely completing their lifecycle. Unless…

puppies with mother

Pregnant bitch

In the pregnant bitch (and in a false pregnancy), some of the dormant larvae are stirred from their slumber.

After about the sixth week of gestation, they re-enter the bloodstream, cross the placenta and develop in the liver of the foetus. Puppies are, therefore, born infected and, within a week or so, the larvae will have travelled from the liver to the lungs, from where they are coughed up and swallowed. As a result, eggs can appear in the pup’s faeces from around three weeks. Also, once the bitch has given birth, dormant larvae migrate to the mammary glands, infecting the pups directly when they feed. The larvae that are consumed with milk develop into adults in the intestine without having to travel to the lungs. Cunning.

Mice and birds

Occasionally, mice and birds can ingest embryonated eggs (that’s why you need to poop scoop!) If these paratenic hosts are eaten by a dog, the larvae develop directly into adult worms in the intestine. (Read here why dogs shouldn’t drink from puddles.)

Transmission to humans

Toxocara canis is a zoonotic parasite. When humans (especially the young) ingest Toxocara eggs (from soil, contaminated objects, fruit, vegetables and the coats of dogs), larvae can migrate to the lungs, liver, nervous system and eyes.

Symptoms include lethargy, seizures, itching and progressive loss of vision. While antibody levels show that 2% to 31% of people have encountered the parasite, actual recorded cases are around two per million people. No matter the numbers, proper worming of dogs is safe and effective, despite the urban myths.

Worming working dogs

Setting aside the possibility of puppy death when large numbers of worms are present, Toxocara canis infections cause poor growth, loss of condition (classic pot-bellied look), pneumonia, diarrhoea, vomiting, fatty liver and kidney granulomas. All can be avoided.

Pregnant bitches should be wormed daily from the 40th day of pregnancy until at least the second day after whelping with Fenbendazole at a dosage of 25mg/kg. (Practically speaking, this is ¼ml Panacur 10% per kg body weight.) Puppies should be wormed every two weeks from the second week of age up to 12 weeks with 50mg/kg Fenbendazole daily for three days. (So ½ml per kg bodyweight daily for three days every two weeks.)

Virtually all bitches will have dormant larvae, no matter what you think. Research into treatment showed it reduces infections in puppies by nearly 90%. Carrying out one-off worming working dog puppies as they depart for their new homes is simply not good enough.