Toxocara canis
Toxocara canis is a roundworm harboured in almost all puppies. The adult worm can reach up to 18cm in length and live in the small intestine of the dog. Puppies with a small number of worms often show no symptoms, however large numbers can be fatal, causing a pot belly, diarrhoea, vomiting, weight loss and intestinal obstruction.
The worm infects all members of the ‘canid’ family including foxes and wolves.
It has a complicated life cycle that varies as the dog ages. In puppies up to 2 months of age, the infective egg is ingested and passes through the stomach in to the small intestine. Throughout this route the egg’s shell is digested and a larva released. This larva then burrows through the intestinal wall and through the hepatic portal system, reaching the liver. From here it enters the vena cava leading it to the heart where it then travels to the lungs and finally reaches the trachea. It then migrates up to the pharynx where it is swallowed back in to the intestines. Throughout this journey the larva undergoes a series of moults to reach its mature, adult form. This migratory route is typical of the ‘ascarid’ family of worms to which Toxocara canis belongs.
The adult then resides in the small intestine, producing eggs. The time frame from infection to egg production is 4-5 weeks. These eggs are tough and can survive in the environment for years. When the puppies are around 6 weeks of age, there tends to be a spontaneous expulsion of worms which can be a disgusting sight in their faeces!
In dogs older than 2 months, the worms take a different route. The eggs make their way through to the small intestine as with puppies and the larvae then penetrate the intestinal mucosa and enter the venous portal system to the liver, continuing their route to the heart. From here, they migrate to the lungs and then back to the heart. They then enter multiple tissues of the dog such as the liver, kidney and muscles where they reside in a ‘waiting state’.
Waiting for what? Well. In female dogs, the larvae are waiting for pregnancy. When the hormonal changes that come with pregnancy are detected, the larvae awaken and migrate to the placenta and mammary glands. Here they infect the puppies before they are born, and enter the milk ready to further infect the puppies at birth. Not all larvae are activated during a bitch’s first pregnancy, so infections of every litter are likely.
The ‘waiting’ larvae are around 0.5mm in length. If a male dogs becomes infected, he is a dead end host and no further infection occurs.
The worm’s life cycle is described as ‘direct’, as it only needs one host to complete its cycle. It can however have ‘paratenic’ hosts. This means a host, non-essential to the worm’s life cycle, in which no development happens. These include, rodents and birds. Eggs infect these hosts and develop in to ‘waiting’ larvae which reside in the tissue of the animal until it is eaten by a member of the canid family. Humans can also be paratenic hosts. The ‘waiting’ larvae in humans can cause serious pathogenic effects. The worm is associated with three disease syndromes in humans:
- Visceral larval migrans – this is where the larvae migrate though tissues and can cause inflammatory reactions, fever and asthma
- Ocular larval migrans – here the larvae migrate to the retina of the eye causing impaired vision
- Covert toxocarosis – this produces non-specific clinical signs
Around 55 cases of Toxocara canis infections in humans are diagnosed every year. Around 2.5% of British people are confirmed as seropositive for the infection, showing most cases are asymptomatic. Sadly it is mostly children who pick up the infection through playing in mud and sandpits infected with eggs from canine faeces.
Puppies, along with some adult dogs (and wildlife such as foxes), shed eggs in their faeces, constantly infecting the environment and making infection hard to control. Treatment tends to be regular worming of puppies from 2 weeks of age. Most treatments only to kill the adult worms, however this prevents the eggs from entering the environment, breaking the life cycle where possible. The ‘waiting’ larvae are very hard to kill and treatment involves dosing a pregnant bitch every day from day 40 of pregnancy until 2 days post whelping. This would costs the breeder £70-£75 per bitch, so it is clear why most breeders don’t opt in to this treatment.
In general, our aim as veterinarians is to control the disease as much as possible and provide clients and the public with knowledge of the worm for their own safety as well as their pet’s.