Dr Liesel van der Merwe is a small animal medicine specialist. Send her your questions: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Dr Liesel van der Merwe

We often see dogs that are routinely dewormed or treated for ticks and fleas become infested. And this whole problem requires some unpacking. First of all, regularly or routinely does not imply frequently. Neither does it ensure that the product was used appropriately.

Often, as a rule of thumb, dogs are dewormed every three months. Where does this number come from? I don’t know. If one looks at the lifecycles of the worms most commonly found in dogs – hookworms and roundworms – their lifecycle from egg to egg-laying adult worm is only three weeks.

Newborn puppies are also at risk because they can be infected with larvae already hatched from the mother’s milk and through the placenta. They can thus also be infected with adult worms by three weeks of age. 

In a study with infected puppies, by 25 days of age, puppies were losing blood at the rate of 25% of their circulating volume. This is a massive loss and far exceeds their capacity to replace it, causing severe anaemia. These worms are obviously sucking blood as they are maturing, so the absence of evidence of worm eggs on a faecal examination does not exclude the possibility of verminosis in very young puppies.

Adult hookworms and roundworms lay thousands of eggs, which are passed in the faeces. These eggs are moderately to quite resistant to the environment, especially in milder climates where there is no snow and ice in winter. Thus the environment is contaminated and dogs will continually reinfect themselves.

The route of infection is oral, so any chewing, eating and nibbling of anything in the yard, of which the soil is contaminated, will infect the dog. And then within three weeks we start again contaminating the environment with worm eggs.  So the “deworm every three months” rule of thumb is useless. 

We should be treating our pets according to their lifestyle. Some dogs are taken to doggy parks every week or are regularly out and about running or walking with their owners. These dogs need deworming every three to four weeks to prevent any infection from maturing to fully fledged egg-laying adult worms.

The little lap dog that never leaves the property may need to be dewormed less frequently. If a stool sample is negative and the dog doesn’t leave the small property, it may not be needed at all.

However, some worms use intermediate hosts, which may ingest the eggs accidentally. The eggs hatch and the very immature larva lives in that host – a lizard or insect – until it is eaten by a dog and can then mature further. So, there is always a risk of some exposure to infection.

However, this is minimal compared to that of animals that are taken out and about and also that of very young puppies and their infected mothers. There is no one rule that fits all – treat your dog for its lifestyle.

This applies to tick and flea control as well. Where I live I never see ticks on my dogs, but if I go on holiday with them, I always treat them before we leave. However, if you are staying on a smallholding or a property on the edges of town, tick infestation is more common. Ticks are dropped off by other animals moving through the property and your pets are at risk even if they stay on the cut lawn.

It is too late to dose a tablet or use a topical treatment after you have noticed ticks on your dog. Once a tick has bitten and attached, it can immediately transmit infective parasites, which cause babesiosis (biliary) or ehrlichiosis.

Treatment must be applied according to directions. These medications are expensive, so make sure you are using them properly. Because our winters are so much milder these days, we are also seeing biliary in dogs throughout the year. There is no longer a safe winter period. In all these cases prevention is better than cure.