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Heartgard during winter months?


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I tried to search for an answer to this question in the older threads of the forums, but no one seemed to have mentioned it.


When our vet first started Ouzo on heartworm medicine, it was at the end of May. Ouzo was 6 months old by then. I "think" :rolleyes: I remember him saying that in Colorado, due to the colder weather, there is no need for the heartworm preventative medicine during winter months. Is this true, or am I imagining it?! I don't want to risk anything, but I'm wondering if it's true. I will call them next week, since I gave him the last doze of the 6 months package at the end of October, so his next doze would be due at the end of November.


I checked on the Heartgard site and it says that "Heartgard should be given at monthly intervals during the period of the year when mosquitoes (vectors) potentially carrying infective heartworm larvae, are active". This kindda supports what the vet had said.


When I asked him about Frontline or something against parasites, he again said that in Colorado there's no real danger for fleas or ticks, so conclusion, no Frontline for Ouzo.

No parasites ever either.


Is that true?

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I don't know about the parasites, but Bo just had a checkup with the vet - she said the same thing: to start him on the heartworm med in the spring. Mosquitos aren't an issue due to the cold weather. So they will send me a reminder card, probably in April or May, draw blood for the heartworm test, then start him on the med.

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There's no time of year around here when mosquitoes are not active. There's cold weather, sure, but we have warm spells when the mosquitos emerge.


Be sure this is not true of your area if you plan to go off HW meds.

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Back in the dark ages when I worked for a vet (in Virginia), it was common practice to take dogs off HW meds for the winter months. Generally if the dog wasn't kept on preventive year-round, the owner was told to test in the spring before restarting prevention. Now it seems that many vets suggest keeping the dogs on preventive year-round AND testing yearly. Did your vet suggest testing before restarting the preventive in the spring? That said, I live in the hot, humid south where we rarely get a break from the mosquitoes, so my dogs stay on year round. Some board members (in Washington State, for example) are in areas where the mosquito vector for heartworms doesn't exist and so they don't need to use preventive at all. All this is to say that to be absolutely sure it's safe to take Ouzo off preventive for the winter months, you would need to know if mosquitoes remain active through the winter (or any part of winter) in your area. I would bet that either Cooperative Extension or a local university (Colorado State) would have an answer to that question. And of course if you have an unusually warm winter or early warm spring, you'd have to adjust your preventive schedule to take that into account.


As for fleas and ticks, again I think the low temps you experience in Colorado would mean you don't have to worry about fleas or ticks in the winter months. Again I would consult with a unversity or extension service to get the skinny on that (that's not to day your vet isn't correct, but from experience I can say that vets don't always know, especially when it comes to the prevalence of ticks and the resulting tick-borne diseases they may carry). Sadly, we in the south must battle those critters year-round as well. Sigh.



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Does Denver ever warm spells in the winter, warm enough for the bugs to come out?


Will you be traveling to warmer parts of the country during the winter?



This is from the American Heartworm Society; while MN is not CO it gives you an idea what is being recommended for colder climates.


I live in Minnesota. How long should my dog be on heartworm prevention?


Dogs have been diagnosed with heartworms in almost every county in Minnesota, and there are differences in the duration of the mosquito season from the north of the state and the south of the state. While your veterinarian can best advise you, dogs in Minnesota should generally be on preventive from June through November. Remember, if the dog travels out of state, a longer duration of administration (or year-round administration) may be advised.


American Heartworm Society

On the other hand there are species of mosquito that overwinter as bred females (possibly infected); one can imagine them feeding occuring during warm spells in the winter.



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Originally posted by juliepoudrier:

As for fleas and ticks, again I think the low temps you experience in Colorado would mean you don't have to worry about fleas or ticks in the winter months.

One thing to consider is whether your dog is around other dogs. The first time I had a flea infestation was in the dead of a Chicago Winter. My dog must have picked fleas up at obedience class. Since I take my dogs to shows and my club, they're on flea preventative all year.
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Thank you guys, for all your answers! I will discuss it with my vet (I don't have absolute trust in him...), the entire clinic seems more ol' school, so your comments, Julie, might be right on the money.


Denver definetly gets warm spells (two days ago we broke the record with 80 degrees, now it's back in the 50's !), but I don't know enough about the insect population, if they are revived by such short and abrupt temperature spikes.


No traveling scheduled outside the state for Ouzo this winter.


He does get in contact with other dogs at the dog park, that is true (not that he especially seeks their company, all he wants is water and balls ), so I see your point about it.


However, he hasn't been on any Frontline or such (other than the flea collar we bought him when he was a pup and which he wore for max 2 months), and he hasn't had any fleas, ticks, etc.


And another argument for keeping him year arround on HG, a friend of mine who has a St. Bernard says her doc keeps her on HG 12 months a year.


But I think I better be safe than sorry.


Thanks again!

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I found this, not sure how accurate it is, but it does track with what I have heard from other people in my area. And my vets have never recommended HW preventives. Of course, my current vet recommended against lepto vaccination (which my dogs got anyway).


Is heartworm a problem in Whistler?


Heartworm is not a problem in Whistler due to our climate. The development of the microfilariae in the mosquito to the stage where they are infective is temperature dependent. The warmer the temperatures, the quicker they develop. For example, in a laboratory environment where the temperature is maintained at 18 degrees Celsius, it would take the microfilariae 30 days to develop to the infective stage. At a steady 27 degrees Celsius, it would take 10 to 14 days. There is another way that their development is temperature dependent. If the temperature drops below the developmental threshold of 14 degrees Celsius (for a couple of hours at night, for example,) that would be enough to retard maturation. Even if the average daily temperature supports continued development, our cool evenings prevent it. There have been two confirmed cases of heartworm in Whistler but both dogs were from Mexico. No Whistler dogs have contracted the disease.


18 degrees Celsius = 64.4 Fahrenheit, according to the conversion chart I found.




Allie + Tess & Kipp


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I found something, too, and it actualy mentiones Colorado:


"Colorado is fortunate to have one of the lowest incidences of heartworm infection in the country. Unfortunately, the prevalence of infection has been on the rise over the last decade. Some parts of Colorado, especially the lower river valleys, are reported to have an infection rate as high as 5%. Our warmer, moister seasons over the last few years may be partly responsible for the increase in heatworm disease we are seeing in Colorado ? the more mosquitos that are present, the greater the threat of infection. If you are seeing mosquitos in your area, then there is a chance for your dog to get heartworms. "


And, from the same site of an Animal Hospital in Fort Collins


"The season on the Front Range usually lasts from April through November, or until we have a hard freeze leaving no mosquitos. If you plan to travel outside the Front Range with you dog, be sure to call us to find out how serious the prevalence of heartworm may be in the area you will be visiting."




And one more from the Alameda East Animal Hospital site (the one featured in Emergency Vets on Animal Planet :rolleyes: )


"In our busy general practice, we diagnose one or two cases per year. For comparaison, when we took 25 Katrina evacuees last year, eight of them were heartworm positive"



I seem to agree with our vet now...

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The mosquito is an obligate intermediate host for the heartworm. The worm goes through 2 larval instars in the mosquito's mouth parts (from L3 to L5). The L3's are not infective - the L5 is the infective stage. When I was going to school, the short form was that if the temp drops below 70 degrees (Farenheit) at any time in the 2-week maturation stage, even for a few hours at night, it prevents the maturation of the worm. (This, BTW, is the reason we don't suffer severe heartworm issues in AK - we have ZILLIONS of mosquitos, but there are plenty of DAYS that don't make 70 degrees, let alone nights.) OTOH, if I went to a heartworm hotspot for even 10 minutes with my dog, I'd be doing preventatives. It's never been warm enough in the winter - since I've been here, anyway - to allow mosquito hatch. But Mark has a good point about winter hatch... the infectiveness (or lack thereof) of winter-hatch mosquitos would be an unknown for me, so I'd rather do the preventatives than take the chance on infestation.


If you're uncertain, you can call several local vets and get a consensus, or (if there is one) a local veterinary teaching hospital. Or you can err to the side of caution and treat if you're at all unsure of the risks. JMO, of course.

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