I’ve had a few questions about the risks posed by heartworm in imported dogs to local dogs. (This also speaks to just how often we import heartworm-infected dogs, but that’s another story…). Here’s a bit of a deeper dive into the topic based on a fictionalized (but very typical) scenario:

You live in Guelph, Ontario, Canada. Your neighbour imported a dog and just found out is has heartworm. Fortunately they detected the infection fairly early on and are going to treat it, but what’s the risk to your dog?

Here’s a quick summary of some important aspects of heartworm disease and its biology:

  • Heartworm is (most often) caused by the parasite Dirofilaria immitis.
  • Adult worms live inside the major blood vessels leaving the heart in dogs (with very severe infections the worms will be found in the heart as well, hence the name).
  • When at least one male and female adult heartworm are present, they produce microfilaria that are found in the dog’s blood.
  • Mosquitoes ingest microfilaria when they feed on an infected dog.
  • Microfilaria have to develop in the mosquito to the L3 larval stage to become infectious, and can then be passed on to another dog when the mosquito feeds again.
  • Parasite development in the mosquito is temperature dependent.

Larval maturation in mosquitoes stops below 14C, so we can consider “heartworm development units (HDUs)” which are calculated by looking at the mean daily temperature over time. If it’s over 14C, then heartworm development could occur; the amount of development is based on how far above 14C the temperature goes. For example, if the mean (not maximum) temperature was 20C yesterday, that’s 6 HDUs for that day. It takes a total of approximately 130 HDUs for the parasite to develop to the point where it can be transmitted by a mosquito bite to another dog.

Back to our scenario…

You find out that your neighbour imported the dog six weeks ago. They went to their veterinarian a couple of weeks ago and got confirmation that the dog has heartworm last week. He’s starting treatment this week.

Overall, the risk is low. A mosquito would have to bite the neighbour’s dog, get infected, the larvae would have to mature to an infectious state, then that mosquito would have to bite your dog. Unlikely, but not impossible.

If we really want to look into the risk more, we can calculate the HDUs, which can tell us whether there was even a chance for the recently-arrived dog to be a source of infection to others in the neighbourhood before treatment was started.

I can look up the local mean temperature data for Guelph for the last 6 weeks. When I sum up all the daily HDUs, there were only 74 HDUs during that period. That means that even if a mosquito bit the neighbour’s dog on the day of its arrival, there wasn’t enough heat/time combination for the parasite to become infectious in the mosquito. To reach 130 HDUs, the dog would have had to arrive a few weeks earlier. Any mosquitoes the dog may have infected in the meantime are not going to be infectious because we (likely) won’t have enough warm days this fall to continue that development. They are also not likely to overwinter.

If the neighbour’s dog wasn’t treated, the risk wouldn’t start until next summer. In 2024, Guelph hit 130 HDUs on approximately June 20. Our common heartworm preventive medications for dogs will kill L3 and early L4 larvae, so they are effective if started up to 4 weeks after exposure. For example, if a dog was infected on June 20, treatment by July 18 should still be effective at preventing adult heartworm infection (but I wouldn’t want to push that limit too much, earlier is better).

If the neighbour’s dog is treated successfully (as we would expect), the transmission risks are negligible, because the diagnosis was made – promptly – and based on the time of year heartworm development in mosquitoes was limited, and the dog will be treated before the risk starts again next year.

If the dog had been imported earlier in the year or the diagnosis was not made, then the risk increases. That’s why we want to make sure imported dogs are tested – ideally before they arrive, upon arrival and again about seven months later – to get them treated as soon as possible and to reduce the risks of local transmission.

Unfortunately, people don’t always test imported dogs, either before or after arrival. Some people don’t treat infected dogs (and strangely, some will even knowingly import an infected dog and then not treat it). These scenarios create risk of adding to our local heartworm burden, which is still pretty low in most parts of Canada compared to other parts of the world (but is increasing).

I do have some disclaimers. As with most things in infectious diseases, we have to realize that there can be exceptions, loopholes and data gaps.

  • The 130 HDU cutoff seems to be pretty reliable, but is based more on in-lab testing than complex field situations. However, the 74 HDUs accumulated during the time frame in my example is so far away from 130 HDUs that the risk is presumably negligible.
  • Looking at hourly versus daily mean temperatures is a bit more accurate, but hourly temperatures are not always available, and it’s a lot more work for fairly limited extra benefit. In this example once again, the 74HDUs is so far below the threshold that the extra detail wouldn’t make a difference here.
  • We’re also assuming that mosquitoes are not hanging out in warmer microclimates (e.g. inside a shelter or a house). If a mosquito was living somewhere warmer than the mean outdoor temperature, then larval maturation could be quicker.

Take home messages:

  • Test imported dogs for heartworm before arrival, ASAP upon arrival and 7 months later.
  • Start imported dogs on monthly heartworm preventive as soon as possible, and keep them on it at least until all this testing is complete.
  • Treat heartworm-infected dogs as soon as possible.
  • Use a routine heartworm preventive for all dogs during the at-risk (mosquito) season if heartworm is present in the area.
  • Realize that the risk of heartworm from imported dogs is case specific, and depends on dog, mosquito and climate factors.

Image from https://capcvet.org/guidelines/heartworm/