Why is it important?
Dogs are naturally host to a range of unpleasant internal parasites. The most important of these are roundworms and tapeworms, but hookworms and pinworms are also a problem for many dogs. Some roundworms pose a very real threat to human health as well - their larvae (present in and around dog droppings) can invade the gut if poor hygiene is followed, and may even crawl in to the liver, brain or eyes (we call this visceral larval migrans, and it's most significantly a problem in children). The best way to control these parasites is with an effective worming programme for your dog.
How do I choose?
It is important to make sure that whatever wormer you're using covers all the major groups of worms - tapeworms, for example, aren't sensitive to most conventional wormers, so modern products usually contain an ingredient such as praziquantel which will put an end to them. However, with so many options out there, it's important that you understand what is available:
(1) Herbal and homeopathic remedies
The use of garlic and wormwood to control intestinal worms is an ancient practice; however, it is not ideal in dogs. These products may have some effect on roundworms, but they are also both toxic to dogs, so only very low doses can safely be used (this is probably why the effect on the worms seems fairly limited). Homeopathic remedies, meanwhile, have been extensively studied and the current evidence suggests they should be used as a complementary, not an alternative, medication.
(2) Pet shop or supermarket worm tablets
There are a wide range of different active ingredients available in these products, but the vast majority contain piperazine or nitroscanate. These products are effective against some kind of worms, but nitroscanate in particular is notoriously toxic in high doses, so you have to make sure the dose is very accurate! In addition, these products are usually only effective against a narrow range of worm species.
(3) Vet or Pharmacist over-the-counter worming liquid
These liquids (suitable for nursing mothers and young puppies) contain fenbendazole, a very safe, gentle wormer that is effective against roundworms and some protozoal parasites.
(4) Vet or Pharmacist over-the-counter worming tablets
These are combination tablets, with several different active ingredients. By combining them (typically, pyrantel, oxantel and praziquantel), the tablets are able to combat all the common worm species.
(5) Prescription worming tablets
Again, these are combination drugs, but containing more potent active ingredients (such as milbemycin), meaning they are often effective against an even wider range of parasites. However, they can only legally be purchased from your vet, or with a veterinary prescription.
(6) Prescription worming spot-ons
There are an increasing number of spot-on products that kill fleas and also some worms (often based on selamectin or moxidectin). In general, these are effective against roundworms, sometimes hookworms or whipworms, but not tapeworms. Again, these can only legally be purchased from your vet, or with a veterinary prescription. There is also an increased risk of harm (at overdose) in Collies and other herding-type dogs. Normal doses are safe, but these drugs should NEVER be given at more than the recommended dose; or in combination with other treatments such as milbemycin, as they can cause fits, or even death, in susceptible dogs.
When should I worm?
Regarding WHEN, it is important that most dogs are wormed regularly, to prevent the parasites "building up" in their system, and to stop them from completing their life-cycles. In particular, it is vital to worm bitches when they're in pup (some worms can invade the puppy even before birth, and also in the bitch's milk). Similarly, young puppies should be frequently wormed, even before they get their vaccinations - talk to your vet for details.
How often should I worm?
HOW OFTEN is a slightly more complex issue. Worms may be transmitted by eating live prey or fresh meat; by sniffing around in the park; or even by fleas. As a result, all dogs are at some risk of worm infestation. In general, we'd recommend worming every 3 months; but if your dog has a habit of eating roadkill, or catching rabbits, for example, more frequent worming (every 6 or even every 4 weeks) would be advisable.
Our dogs will always be at risk from worm infestations, but with a regular worming programme, they can be controlled. Talk to your vet for more details.