Up to 40% of adult dogs and 70% of puppies may be infected with Toxocara canis or Toxascaris leonina. In adult dogs they are often carried in the absence of any clinical signs, although they may cause problems in puppies. They are large, buff coloured worms about 10-18 cm in length which live in the dog's intestine as adults.
THE LIFE CYCLE
The life-cycle of the Toxocara roundworm is interesting and quite complex.
Toxocara roundworms have two main ways of infecting dogs:
1: Adult worms living in the intestine produce eggs which are shed in the faeces of infected dogs. Once shed the eggs take about 2 weeks to develop into the infective stage if the weather is warm or much longer in the cold, (the process is temperature dependent). These infective eggs are then eaten by an animal.
2: Toxocara may also form dormant cysts in the muscle of infected animals. Here it waits until the animal is killed or dies and the infected muscle tissue is eaten by a carnivore.
Once the worm eggs or dormant cysts are ingested by an animal the eggs will hatch or the larvae of the cysts will reactivate inside the intestines. The resulting first stage larvae will then penetrate the small intestine and enter the blood stream in which they are carried to various organs.
What happens next depends upon what type of animal has been infected:
In adult dogs the larvae migrate in the blood to organs such as the muscles where they form cysts in the tissues. Larvae in the body tissues remain dormant unless a bitch becomes pregnant in which case they will reactivate to cross the placenta or go into the milk to infect the pups. Reactivation of dormant larvae occurs about 6 weeks into pregnancy.
In puppies the main route for the larvae is to go from the intestine to the liver then on to the lungs. From here they work their way up the bronchi to the pharynx and are then swallowed by the dog to enter the intestines again, where they develop into adult worms. The life-cycle takes around 4 weeks to complete in puppies.
If eggs are swallowed by animals other than dogs the larvae migrate to the muscles of that animal where they form dormant cysts and wait for the muscle tissue to be eaten by a dog so they can complete their life-cycle.
The process of the worm migrating through the body tissues is known as visceral larval migrans and this is a natural part of the life-cycle of this worm. Visceral larval migrans may occur in species other than dogs if they happen to ingest Toxocara eggs. The larvae need to get back to a dog to complete their life-cycle. They attempt to do this by migrating to the muscles and residing there as a dormant form. Here they are likely to be eaten with the muscle tissue by a dog when the animal dies.
TOXOCARIASIS IN DOGS
Up to 40% of adult dogs may have toxocariasis. Many adult dogs will have been infected as puppies. Routes of infection also include the ingestion of transport hosts such as mice which carry the encysted larvae in their tissues. Adult dogs may have dormant larval cysts in their body which will reactivate if a bitch becomes pregnant. A bitch may also become re-infected by eating her pup's faeces.
Around 70% of puppies will be infected with T.canis. These puppies may become infected before birth from their mothers (as the worms may cross the placenta). They may also be infected via the mother's milk or from contact with the mother's faeces. In the puppy, the worm will begin to migrate through the body tissues via the blood. The lungs and the liver are included in the route of migration and the worm eventually finds its way back to the intestines to start shedding eggs and begin the life-cycle once more.
Puppies infected with Toxocara may show no signs of clinical disease. However some puppies will show clinical signs especially if there is a high worm burden. Infected puppies may fail to gain weight or become lethargic. They may develop a pot bellied appearance, so they look fat in the middle but skinny elsewhere. As the larvae migrate they may cause respiratory problems in the pup such as coughing or pneumonia. The puppies may show signs of gastrointestinal upset with vomiting or diarrhoea. (If Toxocara worms are vomited they may look a bit like elastic bands because they are often coiled and similar in colour.) If severely affected the puppies may become weak, emaciated and die. Adult dogs are generally more resilient to infection and show few clinical signs.
ZOONOTIC POTENTIAL
People may become infected with Toxocara if they somehow ingest the eggs which are shed in dog faeces. Children are particularly susceptible to toxocariasis because they are more likely than adults to ingest worm eggs while playing either with animals or on contaminated soil.
In humans the migration of Toxocara around the body (visceral larval migrans) may cause respiratory problems and fever. Problems also arise in cases where the worms do not migrate properly through the body to the muscles but "get lost" and end up in nervous tissue (such as the brain or spinal cord) or the eye. Toxocara canis has been implicated in problems such as epilepsy, eye tumours and blindness; it is presently unclear whether T.leonina is involved as well. In the eye the microscopic larvae can cause inflammation and a permanent scar may form. Around 100 people in the UK every year suffer permanent partial loss of vision due to this problem.
Puppies may be the main cause of human infection because adult dogs may develop some kind of immunity to these worms. Puppies start to shed worms at about 3 weeks of age with maximal shedding at around 6 to 12 weeks of age. Pregnant bitches are also a risk because the hormonal changes of pregnancy reactivate the worms so that they may infect the next generation of puppies.
TREATMENT
All dogs should be regularly treated for worms. Since dogs may carry Toxocara without showing any signs of infection it is best to assume your dog has them and treat accordingly. This is especially true of young puppies and breeding bitches who will very often be infected and shedding eggs. There are many products available which will treat Ascarids such as Toxocara in dogs. A drug called fenbendazole is the active ingredient in the preparations I usually recommend for this purpose. My favourite product for adult dogs is Drontal Plus which should be given at the correct dose every three months; this product treats all the worms normally found in dogs in the UK. For puppies I would recommend Drontal Puppy Suspension which should be given orally to the puppy on its own or mixed in the feed. Puppies should be wormed every two weeks until 12 weeks old, beginning at two weeks of age. For breeding bitches I like Panacur Favourites which should be given daily at the recommended dose for your dog from day 40 of pregnancy to 2 days post whelping. This helps prevent transplacental infection of the puppies. It is also recommended that bitches should be wormed about 25 days post whelping when she is likely to be shedding Toxocara eggs in her faeces.
PREVENTION
To prevent anyone in your family catching toxocariasis it is wise to treat your pet for worms regularly as discussed above. Many owners only worm their dog every 6 to 12 months but I would suggest an interval of around every three months. Worm puppies every two weeks until they are 12 weeks old then every month until they are 6 months old when they can begin the adult regime. Wash your hands well with soap and water after touching your dog or cleaning up faeces etc. Do not allow children to play in soil which may be contaminated; parks or other areas frequented by dogs are likely to have soil contaminated with Toxocara eggs.
There are approximately 6 million dogs in the UK producing around 1000 tons of faeces daily. A large proportion of this waste is likely to be contaminated with Toxocara. Since the eggs shed in dog faeces take around 2 or 3 weeks to mature to an infective stage, clearing away dog excreta will help to avoid the problem of toxocariasis. If eggs are allowed to get into the ground they are very difficult to destroy and may remain infective for up to three years. It is every dog owner's duty to clean up after their dog if it fouls in public areas.
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