Heartworm – the silent killer of dogs

Back in the 1970’s to 2000 the incident of heartworm in the dogs of Queensland was very high – to the stage that if an owner did not have their dog on heartworm prevention then within 12 months I would have guaranteed that dog would have an adult worm growing in its heart.

As more and more owners got their dog on heartworm prevention and as society become more and more aware of the dangers of mosquitoes (they spread heartworm) and started to minimise the breeding grounds that heartworm needs to reproduce (such as stagnant water) then the incidence of heartworm started to decrease.

The number of dogs (and cats) that we now see with heartworm is quite low BUT it is still important that you, the pet owner continue to give your dog’s heartworm prevention. If you stop giving the preventative medicine then the incidence of heartworm will increase until it leads to massive death numbers and debilitation numbers in our canine and feline society. At the moment there has been an outbreak of the condition in the Mackay area and literally dozens and dozens of dogs have had to be treated or sadly have died from the condition.’

I do not want to see the same happen here in Bundaberg.

So what is heartworm?

It literally is a worm that lives in the heart and lungs of dogs (and some cats). It attaches to the wall of the heart and blood vessels of the lungs and feeds off the passing blood cells. It wafts around in the blood and can lead to the formation of clots (which can cause strokes or loss of blood supply to areas of the heart and lungs) or can grow in such numbers that the colony of the heartworm literally blocks the blood vessel.

I have post mortemed dogs with literally hundreds and hundreds of heartworm living in the right side of the heart and have caused heart blockages where the animal either has died or has become so debilitated that it could not live with any quality of life.

How do they breed?

There are female and male heartworm and when they reproduce they produce small microscopic larvae called Microfilaria and they travel around the blood vessels until a mosquito sucks them up when the “mozzy” is feeding. Inside the mosquito, these larvae grow until they are ready to infect another animal.

When the mosquito is feeding, these microscopic larvae are injected into the new dog and again travel around the body until they are ready to become adults (this takes around 6 months).

The life inside the mosquito is paramount – without that phase the heartworm cannot develop to become an adult. The mosquito is called The Intermediate Host.

The life of an adult heartworm is measured in years (not days) and once they become established inside the animal they continue to do damage for the whole of the heartworm’s life.

How do you prevent your dog getting heartworm?

There are tablets that can be given daily or monthly that prevent the infective larvae from developing any further.

1. Daily tablet MUST be given every day to be effective – if you miss a day then your dog is at risk. These tablets can also be a little irritant to the mouth of the dog so it can become a chore each day to get the dog to take the tablet.

The daily tablets can be deadly if given to a dog that already has heartworm. If you’re starting your dog on daily tablets then please get the animal checked for heartworm by a veterinarian BEFORE you start.

The reaction to the tablet is fast and deadly within a very short time, and if we are lucky enough to get the animal to our animal hospital while it is still alive the treatment can be very expensive and does not always work.

1. Monthly tablets are safer and can be given to a dog with heartworm but they are a prevention medication NOT a treatment. We would always recommend that an owner gets their animal tested before treatment – if the dog already has the heartworm adults then trying to prevent an already existing problem is pretty much a waste of time and money.
2. Yearly injection of a subcutaneous medicine deposit is the way I would recommend all owners do. You do not have to remember to give the medicine, you do not have to depend on the dog swallowing the tablet and not vomiting afterwards, you do not have to depend on the intestine absorbing the medication. This is the method I use for my own dogs and is the method I would recommend for all dogs. Again – these injections are a preventer not a treatment so if your dog has not been on heartworm prevention for over 6 months (it takes 6 months after the mosquito bites the animal until we can detect the heartworm adult) the I would recommend a blood test is done before the injection is given to makes sure the animal is clear of the disease.

Do I recommend all dogs should be on some form of Heartworm prevention?

YES, I DO!

I have seen lots and lots of dogs die from heartworm (especially in my early years as a vet) and it is a horrible condition to watch. Treatment is expensive and complicated and animals being treated need to be totally confined for a number of weeks.

I know that heartworm still exists in Australia and I know if owners become complacent then the chance of heartworm coming to our area is high. I would rather prevent a problem as serious as Heartworm rather than have my animal permanently debilitated or dead because I became “slack”

Can cats get heartworm?

Yes they can but the incidence is low and the cat’s immune system seems to be a lot more effective to stop the heartworm growing to a stage where it becomes life-threatening.

Most cats with heartworm show more signs of lung problems with wheezing and breathing difficulties. They respond (usually) to treatment of cortisones and antibiotics and the death rate of heartworm infestation is very low.

Cats can be very difficult to give oral medication and because the incidence of clinical problem is so low – most vets (myself included) do not recommend preventative medication.

But each owner has to make a decision – you cannot use daily tablets and there is no yearly injection that can be given to cats. There are some monthly “spot-ons” that can be applied easily but each owner must weigh up the cost compared to the benefits.