The parasite known as heartworm is spread through the bite of a mosquito that has recently fed on an infected animal. The tiny creatures migrate from the mosquito to the new victim, where they circulate for some time in the bloodstream, too microscopic to be harmful. It takes nearly 3 months for the larval heartworm to develop into a larger worm that takes up residence in the host's heart, and even longer before the worms' offspring, microfilariae, are detectable on certain heartworm tests.

The presence of a few worms in a dog's heart may never cause visible symptoms. A large infestation manifests as coughing, lethargy, convulsions, vomiting, blindness, fainting, and weight loss. The 5-14 inch worms can clog blood flow through the heart, in some cases with fatal complications. The inflammation sparked by the immune system response can cause joint pain. Getting rid of the infestation requires expensive treatment and weeks of bedrest. A significant portion of dogs die during treatment from pieces of dead worm lodging in vital organs.

Reducing mosquito population

Mosquitoes are the only way that heartworm can be spread between dogs or wild canines like coyotes. Be proactive in keeping down the population of these pests. Mosquitoes can breed in flowerpot saucers, old tires, knot-holes or crotches in trees, landscape fountains and stagnant swimming pools. If you live on property, call your friendly local vector control officer to consult on non-toxic options, many provided free of charge.  Your neighbors will be happy to participate in getting rid of as many of the pests as possible. While you're at it, make sure your neighbors are giving their own dogs heartworm preventative-- many people are unaware that their healthy-looking dog could be a "Typhoid Mary", harboring and spreading heartworms every time a mosquito bites them.

Reducing mosquito bites

Keeping your dog indoors during peak mosquito-feeding times may help but is not a surety-- mosquitoes still get inside. A study in LA found that, among the few dogs which tested positive (67 out of 4,350), 50% were described by their owners as “always indoors”. It's NOT a good idea to use mosquito repellent made for humans on your dog. Ingredients in repellents can be TOXIC to canines, either absorbed through the skin or licked off. Use only dog-safe repellents made from essential oils.

Treating for larval-stage heartworm

Fending off mosquitoes is a bit like herding cats and eventually an infected mosquito may get through to your dog. There are a number of mild oral insecticides that will kill any developing larvae before they become dangerous adult worms and take up residence in the heart.  The top three are ivermectin (brand-names Heartgard® , Merial, Inverhart™ and Virbac); milbemycin oxime (Interceptor®, Sentinel, and Novartis); and selamectin (Revolution® and Pfizer). No doubt about it, these are toxic bug-killing chemicals. Selamectin flushes out in a week or so but ivermectin persists in fat tissue for much longer (toxicology).

Is a year-around, monthly dose necessary?

To reach a growth stage where transmission to dogs is possible, the larvae in the mosquitoes must enjoy sustained temperature of at least 57*F, day and night, for 10 to 14 day (see study). Around human habitations, a mosquito might conceivably find a warm spot that meets this minimum temperature even when “actual” temperatures are much lower. But the critical time to treat your dog with larvae-killing pesticides is during the months when night temperatures are over 57*F, which varies by region.

The manufacturers of these drugs recommend monthly doses. However, since heartworm larvae take 70-90 days to develop into adults (life cycle), many dog owners choose to dose on a 45 or even 60 day interval to reduce the amount of toxins in their dogs' system. In fact, even four-month intervals of ivermectin have been shown to be 95% effective.

Given the lack of heartworm transmission during low-temperature months, it is your choice whether to treat year around. "Bonus" pesticides in commercial heartworm preventatives also suppress the population of other, less dangerous types of worms that typically inhabit canine GI tracts. In healthy adult animals, these populations are not usually a problem. Granted, parasites in any shape or number are creepy, but administering any type of medication in the absence of an actual threat to the animal's quality of life raises questions. If you don't treat year around, a yearly heartworm test is advised.

Money-saving options

Name-brand heartworm preventatives are available by prescription only. Buying from your prescribing vet's pharmacy is a nice way to support your care-provider and for medium and small dogs, name-brand preventatives are not terribly expensive. If you are pinching pennies, you can ask to take the vet's prescription elsewhere: "people" drugstores and online pet-med sites may be less expensive. PetTrust is much cheaper than brands such as Heartgard.

Some dog owners buy generic ivermectin without a prescription. It is available in .08% concentration, marketed as "sheep drench", at feed stores (about $30 for several lifetimes' supply) and from other suppliers at other concentrations. These products are not approved by the FDA for oral use in dogs, so your vet cannot administer or recommend and manufacturers cannot advertise it for the purpose. Shasta Dogs cannot recommend it either. Anyone considering the use of raw ivermectin needs to read this.

Certain collie-type dogs with the MDR1 mutation are very sensitive to high levels of ivermectin, though a normal heartworm dose is usually tolerated fine. It's easy to have a dog tested for the mutation.

What if my Dog TESTs POSITIVE for heartworm?

Many dogs recover well from adult heartworm infestations. They may even live long and happy lives with a minor number remaining in their systems. Treating a heartworm case can be expensive ($700 is a rough number, depending on size), so check with the local Humane Society and vets to find out if financial help is available. See national foundations that aid with vet bills on the Need Help With Pet Costs page.

For heartworm-positive dogs that appear to be in good health, without coughing, lethargy, or other signs of a heavy heartworm load, some people opt not to intervene to kill the worms. Preventing the establishment of new worms is still important. There are also some home remedies. See this article for info.

Some myths about heartworm

Can heartworm be caught from drinking water with mosquito larvae or passed to pups through a mother's milk?

No. Because of the heartworm's complex 5-stage lifecycle, there's only one transmission method: the bite of an infected mosquito.

My grandparents' dogs didn't get heartworm meds and they lived to a ripe old age. Isn't this modern heartworm hysteria a myth to sell a toxic and expensive drug?

There has been a huge increase in the heartworm population in the last forty years, for a number of reasons. For instance, back when DDT was popular, there were far fewer mosquitoes to spread disease. Now in wetter and warmer areas of the country, the risk of contracting heartworm can be more than 50-50. Fortunately, drought decreases the number of mosquitoes and new heartworm infections temporarily fall. But during the wet period in Northern California, 1994-96 , infection rates amongst adult wild coyotes were found to be 90%.

I think my dog is pretty healthy and has a good immune system. Why isn't that enough?

Unfortunately, healthy heartworms are well-equipped to withstand attacks from their host's immune system. A 1994-96 study of wild coyotes (presumably exercising well and eating a natural, organic diet), found that 9 out of 10 adult coyotes were infected with heartworms. Will your dog be one of the lucky few?

If heartworm disease isn't fatal, why should I worry about it?

True, heartworm disease is usually not fatal. The heartworms need their host alive, after all! But it can be painful and affect the dog's health. The body's immune system is at constant war with the foreign body. High levels of antibodies circulating in the blood cause inflammation, tissue damage and pain in areas such as the eye, kidney, and joints. Imagine having a large splinter in your hand, and the accompanying inflammation, oozing, and swelling. A 6-14" heartworm attached to your dog's artery wall triggers the same kind of immune response to the foreign body. Granted, a few heartworms in a large dog are probably not going to affect quality of life, but the more worms that are introduced by later mosquito bites, the more "splinters" in the dog's artery walls and stress on its system. It is much easier and safer to prevent heartworm disease than to treat it.

CDC Heartworm page
Petmed Heartworm page-- great description!

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