There are two main Classes of worms affecting dogs and cats: The nematodes or roundworms, and the cestodes or tapeworms.
Roundworms
There are many types of roundworm, but the ones which affect our pets, and sometimes us, most often are;
Ascarids:Toxocara canis, Toxascaris leonina, Toxocara cati.
These are large (up to 10cms) white worms and are found worldwide. These worms have a complex life cycle. Toxocara canis and Toxascaris leonina are found in dogs, and Toxascaris leonina and Toxocara cati in the cat. Puppies and kittens are infected by their mother, and adults are infected by swallowing the almost invisible eggs, generally by swallowing contaminated grass. The eggs hatch in the animals' intestine into larvae which can be carried to the lungs in the bloodstream, often then making their way up the windpipe causing gagging and coughing. Most larvae return to the intestine, grow into adults several inches long, and produce up to 200,000 eggs per day. These are passed in the faeces. This disintegrates and the eggs can last on the ground for years after the faeces have disappeared. Sometimes an adult worm is vomited up or is passed in faeces. They can look like rubber bands, bits of string, or spaghetti. Since puppies often eat bits of string and rubber bands, and then vomit it, it can be difficult to know what has been produced.
Some larvae encyst in the body and in pregnancy these larvae mobilise about three weeks before birth and infect the foetus. Even if she never becomes infected again, the bitch/queen has enough dormant eggs to infect every subsequent litter. Pups heavily infected in the womb may die a few days after birth. Most fatalities occur when the larvae in the lungs cause a pneumonia.
We have to assume that every pup and kitten is infected. They should be wormed every two weeks from two weeks to twelve weeks of age and every three months thereafter.
Children do not get worms from dogs or cats, they get them from other children. However, Ascarid eggs, if swallowed by a human can cause a condition known as Visceral Larva Migrans. The egg forms a cyst randomly in the body. This can result in occasionally serious complications such as a granuloma in an eye, or a cyst in the brain, but mostly the effects are minor. Incidence is commonest in children in close contact with domestic pets, and or who frequent public parks where there is contamination of the ground with dog faeces. To protect human health, good personal hygiene, poop scooping, and regular worming are essential. It is important to use effective worming treatments. Garlic may work for vampires, it doesn't work for worms.
Strongylids (Lungworms): Angiostrongylus vasorum.
Dogs (mostly) are infested by eating infected slugs and snails, or (it is suggested) their mucous trails on grass etc. Puppies under 6 months are particularly at risk. The larvae migrate from the stomach to the lungs (causing a soft cough), but can also migrate anywhere in the body, causing serious problems including seizures, bleeding, heart failure and death. They are particularly found in the right ventricle of the heart and the blood-vessels of the lungs. This may account for it sometimes mistakenly and confusingly called French Heartworm.
Ancylostomids (Hookworm): Ancylostoma caninum, Uncinaria stenocephala.
These are the most common hookworms in dogs, Ancylostoma tubeforme being the commonest in cats. Infection is most commonly contracted in kennels, especially greyhound kennels. The symptoms include anaemia and even death due to blood loss. Especially in puppies a blood transfusion may be necessary.
Filarids (Heartworm) Dirofilaria immitis
These affect the dog, occasionally cat, and rarely man. The mosquito is the intermediate host. So far, we do not have heartworm in the UK, although dogs travelling to the Mediterranean can be infected. We advise preventative measures beginning 4 weeks before travel to heartworm areas. In severe cases (these worms live in the heart!) heart failure, collapse and death are possible. Milder cases show signs of weakness, listlessness, but fatal consequences (heart failure) are common.
Tapeworms:
Again, there are also many types of tapeworm;
Dipylids: Dipylidium caninum.
Affects both dogs and cats. They are normally carried by the flea bite, and these are the ones that look like grains of rice. They are common, and are not known to cause serious problems. (The flea larva feeds on a tapeworm segment as it breaks down, and the adult flea develops with the tapeworm egg already inside. When the flea is swallowed by the dog or cat – which is how most fleas die – the egg develops into a new tapeworm in the dog or cat, and this tapeworm puts out segments in the faeces, or just per rectum, onto the carpet. The segment breaks down. Flea larvae devour the segment and the cycle continues).
Taenids: Echinoccus granulosus
Echinococcus granuosus granulosa affects dogs and wild canids, but not foxes
Echinococcus granulosus equinus affects the dog and the fox
Echinococcus multilocularis affects the dog, cat, and wild dogs.
Tapeworms have intermediate hosts, like the flea in Dipylidium. The Echinococcids form hydatid cysts in their intermediate hosts. For E.g.granulosa the intermediate hosts are domestic and wild ruminants, man, and the pig. For E.g.equinus they are equines. For E.multilocularis they are rodents and similar.
If the cyst of E.g.granulosa forms in man (by swallowing a larvum) the cyst can form in the liver, chest, or abdomen, may be as large as a football, and will require surgical removal. With E.multilocularis (The Pet Passport Tapeworm) if the cyst forms in man the cyst can destroy the liver and other organs and is not particularly susceptible to treatment. As this is a life-threatening condition, and this tapeworm does not occur in the UK, the tapeworm treatment for cats and dogs (potential carriers) before returning to the UK is very important.
This is a whistle-stop look at the common worms which our cats and dogs encounter. It is very easy to under-estimate the risks of human infection, and yet I see people plucking a blade of grass on Clapham Common (one of the most worm-egg contaminated spots in London) and putting it in their mouths! I hear tales of vets abroad merely signing the forms but not administering the tapeworm treatment. It has gone out of fashion to wash one's hands after handling animals or before eating, but the risks are real and serious. Regular, effective worming is only one part of the picture.