Ok, an attempt at debunking…
This starts off the main problem with most of the claims. They have some kernel of truth, but it is mixed with blatant misinformation.
Yes, there are areas of the country where heartworm is rare. There are plenty others where it is incredibly common. I don’t understand what the author means by “pretty rare in much of the country”. I mean, the completely white areas are less than half the mainland US area, and many of the areas with high incidence of heartworm are also densely populated areas in the US, meaning that finding a dog with heartworm there will be more common than in the deserted areas.
And of course, this goes without mentioning, but most likely, the incidence is what it is because people use preventive treatment with their dogs. The survey is done on reporting clinics, and the clinics report about the patient they get. If the clinic makes a good job in having a good part of their patients taking preventive medicine, their incidence is going to be lower than what it would otherwise be. So the map is the incidence (which is alarming in many places), even with the use of prevention by part of the population. It would be worse if no dog was taking preventive medicine.
OK, this is true, in and of itself. For one single microfilaria to reach full adulthood in a non-medicated dog, it will take 5-6 months before it can cause clinical signs. There are many holes, though, in this part:
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Heartworm is not just a one-time thing. Mosquitoes transmit heartworm, and they inject many stage L3 larvae each time they bite a dog. So your dog is bombarded with potential L3 larvae each time a mosquito bites. And mosquitoes come unannounced! Therefore, you dose every month so that the protection against the mosquito bites for the next month remains.
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While most preventive drugs do and can last a bit longer than one month recommended daily dose, the long-term accumulative effects take time. In other words, in order to get months of protection from a single dose of preventive, the dog must have been on continuous, on schedule treatment for a while.
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It used to be a bigger problem with older drugs, but if you wait a while, in a zone known for having a high incidence, you carry the risk of, when skipping months and giving the pill just once in a while, cause a massive die-off of larvae. Unlike zombie threads (heh), zombie heartworm parasites can and do kill, by causing anaphylactic (hypersensitivity) reactions.
OK, first of all, the life cycle is not broken, it is just extended. The larvae cannot develop to the next stage, but they don’t necessarily die, and they’ll reenter the cycle once the conditions are met. How else would it make it through winter, otherwise? And yet each year we get new ones!
Also, the data is from 1998. That’s 15 years ago. Do YOU accept all medical recommendations that were present 15 years ago? Do YOU trust an incidence map from 15 years ago to be actual now?
Seasonal treatment can be a sensible idea in many parts, but this is something best discussed with the dog’s regular veterinarian, not assuming that an article off the internet citing epidemiology and date from more than a decade ago is still valid in their region and for this year.
And personally, I’ve lived in a region where it gets way warmer than the minimum temperatures way before June.
Bwahahahahaa… OK, they all work by killing the microfilaria through L3. Hint: microfilaria is the L1. The baby heartworm are microfilaria, the L1. What your dog gets when it is bitten by a mosquito is the L3.
The idea of prevention comes because they kill the parasite before it can cause disease. The disease is really caused by L4 and L5, so killing the parasite before it can cause disease is a preventive measure. Similar as to how people that are going to malaria-infested regions take anti-malarial drugs before the travel.
Well… d’uh! Most medicine are toxic poisons to the organisms they are attacking. In the case of most heartworm preventive medicines, they have been found to be effective at killing the pre-L4 parasite without affecting the animal (with rare occasions, see below). They are macrocyclic lactones and macrolides, part of a range of drugs.
Unless the animal has continuously been taking them, unfortunately, no, these preventives are not effective against killing the adult parasite. Continuous (as in, monthly) use of the preventive can, though, render the adult female sterile, which can decrease the burden and eventually the worm will die (because they are not immortal). But this can take years.
Also, I’m getting tired of the author calling all life stages microfilarial. They are not! Stop it!
I don’t deal with human medicine, but I seem to remember rare cases of human infections with Dirofilaria immitis (the nematode). Since humans are not the right host, the diseases (while very very rare) where also very very severe.
OTOH, do you know what other species can also rarely get them, but then it is lehtal? CATS!!! Cats can and do get heartworm, and their clinical sign is “found dead”. Unlike dogs, which can handle quite a load of parasites before dying, kittycats keel over with as little as 3-4 adult worms found at necropy.
This is true. But not all collies are affected, and there are several other drugs on the market that have been found to be collie-safe.
Perhaps in the rare clinic, although I see more plastic models now. Easier to handle, slightly less gross. But I can tell you, I’ve seen enough of them throughout my training and career, which spans… Oh, about 15 years since I started working at vet clinics until my current position. I’ve also seen them in cats. I’ve also seen them in cases my coworkers have had. It is not uncommon for me, although granted, I’ve always lived in high-incidence area. The idea that they need to go to Mexico for that is… absurd.
One benefit of going to the veterinarian is that, in case there is a problem with dosage, as can happen with the compounding doses mentioned there, or if there is a product recall or a bad lot or a breakthrough infection… If you have the paper trail through your vet that any of that happened to you, the drug company WILL pay for most if not all the “real” heartworm treatment (the adulticides are expensive). I’ve seen this happen a couple of times.
Microfilaria (the L1) can be produced by the adults (L5) living in the dog when they have not yet caused clinical signs. As mentioned earlier, a massive die-off of these baby heartworm can cause a hypersensitivity reaction and kill the dog. This is why they do the testing. The other one is to check for breathrough infection. No drug is ever 100% safe, and in the case of heartworm, better to treat the infection early, before clinical signs have had a chance to really develop, than to wait for coughing and respiratory problems to occur.
The author has not kept up with the literature either, and that is 3 years old, anyways…
Here are the latest canine treatment guidelines, as of 2012. The drug melorsamine is the expensive drug.
I’ll repeat again… Macrocyclic lactones ARE NOT considered to be effective at killing adult parasites, which are the main cause of disease.
I think that is it for now. Any questions?