Updated November 29, 2020
We have a problem. It’s a good problem. Young vets graduate and work without ever seeing a case of heartworm in dogs. If they do, it’s a rare dog from a very disadvantaged background.
Compare this with the 1980s and 1990s. It’s hard to find a dog owner from that time who didn’t know a dog that died from heartworm in Adelaide.
Like most vets of my age I’ve been unlucky to diagnose and treat a lot of cases of heartworm. I hope it will never happen again, but I’ve got good reasons to be worried. This story is typical:
- Several years ago, before we settled on the DV foster care program, I was looking for a charitable outlet for Walkerville Vet. I contacted a well-known dog rescue organisation, and offered to help by providing free desexing, vaccines, heartworm prevention and microchips.
- We arranged a meeting and it all went well until the subject of heartworm came up. I was told that they didn’t believe heartworm was a problem, and they didn’t use prevention. In fact, they wouldn’t use or recommend prevention even though I was offering it for free!
- In the end, I had to withdraw my offer; they weren’t going to budge and I wasn’t going to do a half-baked job.
Wow do we have short memories; the folk memory of heartworm is already disappearing, even though the disease hasn’t. This happens every time we get success in preventing a disease. Take these three for example:
- Polio: my father was expected to never walk again
- Diphtheria: my grandmother carried a tracheostomy scar for life
- Distemper: my parents as young vets saw this kill more dogs than anything else (read more about distemper here)
Is Heartworm In Adelaide?
Yes, but it wasn’t always the case. When I was a child visiting my NSW relatives, I remember them giving heartworm prevention even though we didn’t in SA. It seems that sometime in the 1970s South Australian dogs began to get infected.
We suspect it started when people started moving around more for work. In moving to SA they would of course bring the family dog. An infected dog would then act as a reservoir for infecting other dogs.
Another theory is that it spread via the Riverland. Regardless, by the 1970s, Adelaide researchers using greyhounds were finding it in some dogs at post mortem, and by the 1980s vets were seeing disease in pet dogs. By the 1990s it was a big problem.
Prevention and treatment of heartworm have since been a big success. Follow the link to learn more about heartworm and see a creepy sight we regularly saw down our microscope.
Is Heartworm Prevention Necessary?
So if heartworm disease is now rare, why do you still need to use prevention? There are two good reasons:
- Dogs with heartworm are still found throughout Adelaide. If one lives within the range of a mosquito, your dog will become infected if not protected.
- The concept of ‘herd immunity’
What is Herd Immunity?
My best analogy is a bushfire. For a spot fire to spread there needs to be enough dry fuel around to keep the fire moving.
If only some fuel is dry, sparks will sometimes land and catch fire, and sometimes not, and the fire will gradually die out.
For heartworm in one dog to spread, there need to be enough dogs not on prevention within mosquito range to keep the disease spreading. If the mosquitoes bite mostly protected dogs, like sparks landing on damp grass, the disease will peter out.
Herd immunity for heartworm depends on:
- The population density of dogs
- The percent of dogs protected
That’s why the occasional dog not on heartworm prevention (or the occasional unvaccinated dog) stays well; herd immunity is how these at-risk dogs still remain protected.
“Herd immunity is vulnerable to the free rider problem. Individuals who lack immunity, primarily those who choose not to vaccinate, free ride off the herd immunity created by those who are immune. As the number of free riders in a population increases, outbreaks of preventable diseases become more common and more severe. … If large numbers of people in a community free ride, herd immunity in that community is lost.” https://en.wikipedia.org/wiki/Herd_immunity
What happens next all depends on dog owners. I’m pleased to say that our clients almost 100% follow our recommendations. It’s up to us to keep levels of protection high enough to never let heartworm spread again.
What about cats? It’s complicated. Read about the heartworm in cats here.
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By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. These articles are from a series regularly posted on email and Twitter. Subscribe via email here to never miss a story!