“Do I need to give heartworm prevention in the winter?”- Owner of “Lucy” in Pataskala, Ohio
The short answer is, “Yes.” But this is a blog post because I am rarely satisfied with a short answer.
Heartworms are icky…. and deadly. So two strikes right there. You may have seen a model of rubbery plastic heartworms lodged in a plastic dog heart in your vet’s office. Or, if you’re old enough, a real dog heart, worms plugging the great vessels, floating in a murky jar of formaldehyde. They are meant to educate, they are part of the long answer.
Difilaria immitis– the lyrical (and technical) name of these parasites that can be a foot long and live for years in the heart, cause damage to that vital bit and the other vital bits around it like the lungs. Heartworms have two hangouts. The “dog house” (yes, they can infect cats too, we can cover that another time) and the “mosquito daycare bus”. The adult heartworms live in the large vessels that are meant to carry blood to the heart of the “dog house”. When a male heartworm and female heartworm meet and fall in love, they share a special adult heartworm hug and over the course of their romance, make millions of little heartworm babies. With the consistency of George Forman, the babies are all called “microfilaria”.
These microfilariae(more than one microfilaria) are independent types and wander off with the blood to all parts of the “dog house”. This is how the “mosquito daycare bus” picks them up; a mosquito comes along for a snack, bites the dog and collects microfilariae with lunch. The microfilariae hang out in the “mosquito daycare bus” for a few weeks. They grow from babies to toddlers, to kids, and into sassy ‘tween microfilariae that don’t want to be in daycare anymore. So, they migrate to the mouthparts of the mosquito and wait for the next bus stop. When the mosquito gets hungry and stops the bus, the ‘tween microfilaria hop off, infecting a new dog, and are ready to become adults. Let’s make this new house a home! It takes months for the microfilaria to become adult worms. They spend this time migrating through the tissues of the new “dog house” until they find their happy place-arteries of the lungs and the heart where they can grow into full-size heartworms.
They start a profile on Heartworms Only, swipe right and, well, you know.
As adult worms accumulate and damage the heart and lungs, the dog may start showing outward signs of these parasitic tenants. The hair coat can become dull and clumpy, the dog may get tired more easily, sleep more, and may begin to cough. By the time the dog is showing signs of heartworm infection, the damage can be pretty severe.
Mosquitos like warm weather, picnics by stagnant water and Bloody Marys. Well, any blood drink will do. They live in all regions of the U. S., but are particularly successful in the warm, swampy areas of the Southeast. You know what else is successful there? Hurricanes and flooding, which bring more water and leave more puddles (happy mosquitoes) as well as also force humans and their pets to relocate. The animals that are rescued and not reclaimed in these natural disasters are often adopted into other areas of the country. With these animals can come the diseases of those regions.
Consider Hurricane Katrina, which ushered in a new level of natural disaster for humans and their pets. It was a colossal disaster. An article in DVM360 estimates more than 600,000 animals were affected in this tragedy. Many of these died, but the volunteer rescue groups tried to save as many as possible. Many of those animals were shipped out of the Southeast to rescue groups all over the country. The American Heartworm Society estimates over 250,000 pets were rescued and adopted in this tragedy. A study done by Dr. Julie Levy at the University of Florida found that nearly 50% of rescued dogs tested positive for heartworm disease. (They brought a lot of other stuff too…Toxoplasma gondii, anyone?)
I don’t think there has been a definitive correlation between natural disaster pet displacement and the increase in the number of heartworm cases in the US, but the incidence continues to rise and has risen 22% from 2013 to 2016 according to the American Heartworm Society.
In 2016, reporting clinics in our area of Ohio were in the 6-25 cases per clinic or the 25-50 cases per clinic zones. Not far to our southwest are clinics reporting 50-99 cases per clinic. That is to say, heartworm is here. Heartworm is in Gahanna, the Hocking Hills, at the metro parks and even in your house (ever tried to fall asleep at night and hear that high pitched whine?).
What about cold, snowy Maine? Check out the incidence maps made by the American Heartworm Society from 2001-2016. *** White colored areas indicate low numbers of infection, and shades of red from light (fewest cases) to dark (most cases) show our country looking from mildly embarrassed to sunburnt. Even Maine looks like her Mom started singing “Isn’t She Lovely” at middle school pick-up.
But it’s cold! Mosquitoes don’t like cold. (the Finnish Anopheles mosquitos do-heat kills them off!) That’s true. The magic degree for heartworm is 57degrees F. This is the temperature at which heartworm babies can start the journey to becoming ‘tweens, and mosquitos are buzzing around. So, as long as the temperature stays below 57…consistently.
When I moved to Ohio, I heard “If you don’t like the weather, wait an hour.”
Things to consider about mosquitoes and winter. Some mosquito species (ex. Aedes albapictus- primary transmitter of heartworm in the Southeast) lay eggs and then the adult dies, and they “overwinter” in egg stage. This would prevent transmission. Unless it stays above 51 F, then the adult just carries on! Other species (Culex spp.) can “overwinter” by hibernation, in the adult stage in warmer areas like damp basements and sewers. These mosquitoes, more prevalent in the North and Midwest, can come out of hibernation as a potentially infective adult. Otherwise, I do love a mild winter day in Ohio!
So let’s talk “dollars and sense”.
“ Buckeye” is a 51 lb adorable lab mix.
To keep him on Heartguard heartworm preventative will cost $9.85 per month, or $118.20 a year. Heartguard is a once a month ivermectin-based product that kills the heartworm microfilaria. It does not prevent a mosquito from stopping the bus and letting the ‘tweens off into your dog, but it kills those ‘tweens and teens before they can become adult heartworms.
If Buckeye is diagnosed with heartworm disease (a simple blood test done at minimum every two years by your vet that identifies pregnant adult heartworms) then Buckeye can be treated for heartworm. If damage has already occurred to his heart and lungs, that is not repairable with treatment. Treatment is meant to kill the adult heartworms, and also the microfilaria. Treatment takes several months, careful monitoring of Buckeye and costs around $1000.
Strike three! Icky, deadly, expensive.
Are you absolutely determined to not give heartworm preventative in the winter? You can make sure you keep your environment free of standing water and damp areas. Well, the environment of everyone in a three-mile radius, because this is the upper limit of what most mosquitoes can travel.
There are many products out there that are labeled for pets and claim to deter mosquitoes. Be cautious about their use, as there is no oversight on this category of product, and some- even “natural” products -can be dangerous to your pet. I am a big fan of essential oils- but you have to use them properly! Get educated before using any of these products. **** Try this website for a resource.
Keeping your animal inside can help when mosquitoes are most active, any time of year. There are mosquitoes that are dusk prevalent, but the Aedes albapictus favors afternoon in the sunshine.
If a day reaches 57 degrees, you are in potential heartworm transmission warmth!
Do you need to give heartworm prevention over the winter?
The short and long answer is, to keep your dog safe, “Yes.”
Have a question about your pet? Post your question below and it may be chosen as the next topic of discussion for ‘Ask a Gahanna Vet’.