Heartworm disease in dogs and cats

Heartworm disease is transmitted to pets by mosquitoes

Does my dog need heartworm preventative medication year round even if he/she is mostly a house dog? This is one of the most common questions we get in everyday veterinary practice. Heartworm disease is a parasitic disease transmitted by mosquitoes. After being bitten by a heartworm infected mosquito, immature stages of heartworms called heartworm larvae gradually mature over many months and eventually end up at their final place of maturation in the pulmonary arteries of a dog or cat and/or to a lesser extent the heart.

Depending upon the individual susceptibility of a dog or cat, the average ambient temperature in the part of the country a guardian lives, and the number of heartworm carrying mosquitoes in an area, all of these factors affect the risk of infection in a given area. Heartworm positive animals have been documented in all 50 states, and the rate of infection can vary from 2 to 4 percent depending upon the factors mentioned above.

In cats, the risk is somewhat lower; however, heartworm infection in cats and subsequent pathology of the lungs and heart are being increasingly recognized and appreciated by the veterinary community. Symptoms of heartworm infection can range from sub clinical (meaning no symptoms at all), varying degrees of exercise intolerance, coughing and in severe cases abdominal swelling and right sided heart failure.

On occasion, a clot can form as the body attempts to rid itself of the heartworms, which can get into the circulation and potentially lodge in critical organs causing kidney, liver damage, or even sudden death. And while cats are often considered a “dead end” host for heartworms with most cats showing no symptoms, others cats are more sensitive to the presence of even one or two adult heartworms and the larvae, showing varying degrees of respiratory symptoms that are often mistaken as feline asthma, as well as chronic or intermittent digestive symptoms such as vomiting.

Heartworm transmission in the northern two thirds of the country lasts only several months a year, but in the south, transmission can occur year round. However with the increasing recent global warming, as well as the interstate travel that many people are now doing around the country at various points throughout the year, the American Veterinary Heartworm Society is now recommending year- round prevention. Their recommendations also come at a time when there is increasing concern of what are called zoonotic diseases, which are diseases potentially transmitted from animal to human. Most of the current monthly heartworm preventative medications have additional ingredients that also protect against zoonotic diseases, such as roundworms or hookworms.

There are plenty of safe monthly heartworm preventative medications on the market, with a long track record of reliability such as Heartgard Plus and Sentinel. Some of the newer topical flea/tick products also protect against heartworms; however, I remain a proponent of the tried and tested monthly preventatives that have been around for years, rather than the topical medications for heartworm prevention.

Many clients will often ask why their vets require heartworm testing every year. And while medically speaking, giving even a heartworm positive dog any of the newer monthly preventatives is not a problem in most cases, it is still a good idea to have your dog heartworm tested once a year. Although these products are close to 100 percent effective in heartworm prevention, there are still potential individual dogs that are uniquely susceptible to heartworm infection, even with diligent client monthly compliance in administration. If your pet is on heartworm prevention year-round, some veterinarians will allow every other year blood testing.  However, if an animal guardian misses a few months  of their pet’s heartworm preventative, it is NOT critical that they have them immediately tested before restarting the pills, which many veterinary offices still erroneously tell their clients.  This immediate testing was only important when using the old daily heartworm prevention medications that were on the market years ago, because in those cases if one gave a heartworm positive dog one of the old daily pills such as Filaribits, there could be an increased risk of an allergic or life threatening anaphylactic reaction. While reactions to heartworm medications can occasionally occur, most pets tolerate them just fine and enjoy the chewable flavors that they often are packaged with.

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  1. Don’t lie to me to sell your product. Dogs can’t get heartworm in Alaska during the winter.Shame on you I will take my business elsewhere

  2. I’d like to meet that mosquito that can make it through a Wisconsin winter!

  3. Hey guys just dont buy the meds if you dont want them!

    Pet Meds offers a good service!

  4. I currently live in Washington State and every vet I have spoke to about heartworm has told me my dog does not need this medication in this area of the country. Believe me, if anyone was to tell me to protect my dog from this disease I would. Your information is wrong when you write that heartworm is a problem in all 50 states. Please correct your information.

  5. As a Ph.D. entomologist and expert in mosquito-borne diseases, I can say that this article appears more designed to sell product than correctly inform the public. In months in which there is virtually no chance of your dog being bitten by a mosquito, there is no chance of it becoming infected. Here in New York State, that mosquito-free period is basically November 1 thru May 1st. So it is sufficient to give my dog heartworm preventative medication solely in the months of April (a month in advance of mosquitoes) thru October.

  6. Dr. Michael Dym, VMD veterinarianNovember 2, 2009 at 3:59 pm

    While you are most likely correct Dan in the transmission of heartworm disease in more northern colder areas of the country being restricted to the warmer months you cited, with recent global warming it is sometimes hard to know when to start and when to begin monthly heartworm prevention medications, especially in those seasonal transition months. There are also other reasons that the American Heartworm Society and other parasitologists recommend year round monthly heartworm prevention, especially with the increasing mobile human population who often travel back and forth from northern climates to the southern climates, as well as the fact that the monthly heartworm preventatives also prevent intestinal parasites that are potential zoonotic risks to particularly younger human infants and toddlers. There is also a very prestigious parasitologist on staff at Cornell University veterinary school named Dwight Bowman, who has recently published newer research on the biology of the mosquito in terms of heartworm transmission. Given your similar background and credentials as Dr Bowman, you may want to contact him and discuss the issue further.

  7. Dr. Michael Dym, VMD veterinarianNovember 2, 2009 at 4:52 pm

    According to the veterinary experts on cardiology and parsitology, heartworm positive animals have been documented to occur in all 50 states. This likely also has to do with some of the reasons I posted under other blog comments above, including a much more mobile human population who travels back and forth from colder climates to the deep south.

  8. Dr. Michael Dym, VMD veterinarianNovember 2, 2009 at 4:54 pm

    @Clay & Hugh:

    It has become the policy of the American Heartworm Society, which was established in 1974, for vets to now recommend year round heartworm prevention in all areas. The reasons why stem from the fact of a now more mobile human population that travels often to the southern warmer states with their pets, where transmission rates are higher, as well as the fact that the heartworm prevention medications most commonly used also protect against other intestinal parasites, some of which can be zoonotic or risk to humans, particularly infants. Also many people often forget to administer the preventatives during the warmer months, and so if a person simply gets into the habit of giving their pet the heartworm prevention medication once monthly, than there is less liklihood of missing doses during the higher risk times of year in today’s busy life styles. But your observations are indeed correct. That heartworm transmission by itself usually only occurs 4-5 months a year in the Northern third of the country, and for those clients who prefer to use less drugs or medications on their pets, as long as they remember to have pet tested each year, and restart during the warmer months, than stopping in the Winter time in the areas asked about is not going to place most pets at risk for heartworm disease.

  9. I had a dog as a teen and my parents did not give her the heartworm prevention medications doing the slow months. She died! Am still hurt and now I have another dog 🙂
    I would never not give her the medication and I live in the Wisconsin Winter. Am not sure how all this works but I love my dog so much that I will not take the risk!

  10. I have had 3 dogs in my life time. Unfortunately 2 have passed from cancer and liver disease. When it comes to giving medicine I consider my pets part of the family. I would never not take care of my pets as I would my children. Thankfully my children are healthy, but loosing two pets to disease I have cried a million tears with a broken heart. Please think of your pets as part of the family and take good care of them. It’s only money, I’d rather have my animals!!

  11. Dr. Michael Dym, VMD veterinarianNovember 22, 2009 at 7:57 pm

    Hi Robyn. Thanks for your wonderful supportive words. I do wholeheartedly agree with you.

  12. Dr. Michael Dym, VMD veterinarianNovember 22, 2009 at 8:02 pm

    Thanks for sharing. In most cases there is certainly no problem in keeping dogs on year round heartworm prevention.

  13. Pingback: PetMeds®: Chronic Coughing or Gagging in Dogs and Cats | 1800PetMeds Blog

  14. In about spring of 2005 some dropped a large white dog off at our house, a week later another white and also a large black with white feet and tail tip. About 4-6 months later, a chocolate lab showed up. They all seemed to know each other. We had four other (small) dogs to support, but we had the 4 new ones neutered and got shots for them. We were paying for heartworm meds for the 4 little ones and really couldn’t afford for 4 big ones. Since neighbors had old dogs who hadn’t been given meds we thought it to not be much of a chance to take. We lost the first dog about a year ago – didn’t know how old he was. Then the first week this Dec. we suddenly lost the black one. We took the other 2 to the vet, one suddenly very sick – with heartworms. We took the lab in and she had heartworms, was started on antibiotic and aspirin and died today – 3rd day of treatment. The one at the vet is barely hanging on. Why all 3 at almost the same time?

  15. Dr. Michael Dym, VMD veterinarianDecember 19, 2009 at 11:11 pm

    I am sorry to hear of your experience with your first two dogs. Most likely the latter two dogs acquired heartworms during their stressful prior times before you acquiring them. Sounds like you did everything you could. Heartworm treatment can have some risk of side effects in some animals on rare occasions. That is why prevention is best option.

  16. Thanks for responding. The vet put the two remaining dogs on two weeks of aspirin and antibiotic preparatory to treatment, but they both died on the third day of aspirin/antibiotic.

  17. Dr. Michael Dym, VMD veterinarianDecember 25, 2009 at 9:44 pm

    I am sorry to hear of your loss. It certainly sounded like your vet and you did everything possible.

  18. i have a cat that drinks ALOT of water but when i look up it is very unclear what the problem is. He shows no other signs in any search i do about it. He is normal except for the water drinking. I live in puerto rico and he is most times very active Maybe he show a little sign of being nervous but most time he seems normal. Please let me know asap as i am giving him to a good family because have to relocate back to states

  19. Hi Everyone

    I must say my doggie of 10 years past away from Heartworms last april and I live in Wisconsin! My vet told me he didnt need it but 8 months out of the year and in the 4 months off he got heartworms. He had no symptoms and the vet didnt test but every other year so by the time we found out it was too late I still have my little maltese and he is doing well but I still miss my big teddy bear. Now I give my Maltese interceptor heartworm meds all year. It is a shame for the little bit i saved in the 4 months I didnt give Teddy his interceptor and it took his life. And the vet cant take away the pain I felt when my baby died : ( My baby died for me being a cheap skate. Dont make the same mistake!

  20. Dr. Michael Dym, VMD veterinarianJanuary 2, 2010 at 11:50 pm

    Thanks for sharing with us. I have seen on rare occasion dogs become positive for heartworms, even when on year round prevention, as there is no such thing as 100% protection. Also some pets will be susceptible no matter what one does. It sounds like you love your dogs very much.

  21. Dr. Michael Dym, VMD veterinarianJanuary 2, 2010 at 11:53 pm

    Many possibilities in cats of drinking ALOT of water. Should have vet exam including blood testing and urine analysis to check for diabetes, kidney disease, liver disease and thyroid problem. If all negative, then start to look at diet. Cats on processed dry commercial pet foods can sometimes drink LOTS of water. I would definitely make change over to mostly natural wet food or meat based home made diet. Pet guard and wysong are my favorite commercial pet foods. Many cheaper over the counter pet foods often use alot of salt and preservatives that increase thirst. Also remember cats are true carnivores and did NOT evolve to eat carbohydrate based dry kibble diets many people erroneously feed.

  22. Hi
    I must say my doggie of 10 years past away from Heartworms last april and I live in Wisconsin! My vet told me he didnt need it but 8 months out of the year and in the 4 months off he got heartworms. He had no symptoms and the vet didnt test but every other year so by the time we found out it was too late I still have my little maltese and he is doing well but I still miss my big teddy bear. Now I give my Maltese interceptor heartworm meds all year. It is a shame for the little bit i saved in the 4 months I didnt give Teddy his interceptor and it took his life. And the vet cant take away the pain I felt when my baby died : ( My baby died for me being a cheap skate. Dont make the same mistake!

  23. Cate,
    I live in California and was told the same thing by my vets about this area of the country, but I googled the actual statistics and there absolutely are cases of heartworm in California, as there are in Washington state as well. Addressing the issue with my vet again, she admitted that even she uses heartworm meds for her pets despite saying I didn’t need to. The risk may be small, but as someone who knows what heartworms can do, it’s not a risk I want to take.

  24. Dr. Michael Dym, VMD veterinarianJanuary 10, 2010 at 9:07 pm

    It certainly makes sense what you report here. Thanks for sharing and informing others.

  25. Dr. Michael Dym, VMD veterinarianJanuary 10, 2010 at 9:10 pm

    As long as you dont see rare reactions then of course can give it safely year round. However your dog likely acquired the heartworm during warmer months, as this parasite is not transmitted from mosquitoes unless average ambient temperature is 65 degrees or higher for 30 days, which does not occur in colder months where you are. !!!! No preventative is 100% , including vaccines, etc But good advice to those who live in your state.

  26. Dr. Michael Dym, VMD veterinarianJanuary 14, 2010 at 1:09 am

    HI Charles: Sorry for the delay in response. Cats who drink alot of water like this can have many underlying conditions including diabetes, liver disease, kidney disease or infection as well as an overactive thyroid to name the most common conditions. A thorough workup at vet would be needed to work your cat up and do blood work and urine analysis for proper diagnosis and treatment. Sometimes salty diets can be involved so I would try one of healthier long term natural diets with proven track records such as wysong or pet guard which you can find on line. ALso cats who are fed dry food only will drink more, so best to incorporate mostly wet if not all wet food or meat based diets with our feline friends.

  27. I have had three dogs in my lifetime one passed away at 16, one at 15, and one still living 12 ears old. I have NEVER used heartworm medication and live in Houston Texas.

  28. Dr. Michael Dym, VMD veterinarianMarch 14, 2010 at 9:57 pm

    One of my holistic vet colleagues lives in Texas and he does not even use heartworm medicine in his practice. Heartworm disease is sporadic in animals with incidence being 2 to 3.5 percent depending upon where you live in country. That is not a high percentage, which is why your dogs likely did not come down with it, as they were not likely susceptible. However given heartworm is preventable, I would at least recommend it in your younger dogs during transmission season. Many people use it year round in south.

  29. i rescued a dog she is 12 and in the last 3 months she has lost 15lbs i took her to her vet and i was rtold that she had heartworms he gave me Smz Tmp Tab 960mg and panacur and iverhart plus. i do”not understand should i get her weight back up before i start her on the iverhart plus or should i start giving her them now. i do not want to lose her she is very playfull and i think that i can hold on to her for 2 to 3 more years. please help me how i go about treating her. thank you kay

  30. Dr. Michael Dym, VMD veterinarianApril 11, 2010 at 11:09 pm

    HI Kay: I would FIRST have your dog retested to confirm if true heartworm positive, as I occasionally see false positive tests on in house heartworm tests done at vet office. So when you have test redone, have it sent to outside lab like Idex labs or Antech diagnostics, which most vets use as their outside lab. As for decision about using iverhart, I suppose your vet feels that given age that treatment with immiticide injections(the approved treatment for heartworm infected dogs) would be too risky but I would ask your vet about that. IF pet not candidate for immiticide treatment, then certainly monthly iverhart treatment is good second option as with continuous use after 1-2 years most of not all adult heartworms die off or become inactivated. I am a bit perplexed about choice of antibiotic SMZ. Given that many heartworm positive dogs have parasite called Wolbachia infecting them at same time, antibiotic of choice is usually doxycycline in that case. But I would ask your vet to confirm test result first and about immiticide issue.

  31. Pet meds seem to be the only mean to get rid of heartworms. If you find another way to ease the worm then you are welcome to share with people.

  32. Yeah, as Doc Dym mentioned, I too have seen a false positive, which two of my pooches actually. They certainly happen, so I would definitely take her back in, Kay.

  33. Dr. Michael Dym, VMD veterinarianSeptember 29, 2010 at 10:54 pm

    Thanks for clarifying my point.

  34. I have 3 three year old dog, Boston terrier. She is very healthy. My vet has put her on revolution and nemex Tabs. The tabs are for roundworm and hookworm. My question is, does she need both to keep her safe? I thought the revolution does it all. She is a little over 20lbs. We live around Boston Ma. I have asked the vet about this, but he gives me the run around. I’m confused, can you help me with this question? Thank you Christen

  35. Dr. Michael Dym, VMD veterinarianJanuary 17, 2011 at 8:51 pm

    Usually vets will use nemex to get rid of active infections and then revolution as preventative but you can try the revolution by itself and then recheck the stool in 3-4 weeks to see if still have the worms. Either way is fine.

  36. I have 5 rescued dogs, all behaving normally, maintaining healthy weight, and doing great!

    However, the 2 much older ones, 1 large retriever mix, 1 small chi mix, tested heartworm positive about 3 months ago, and the vet chose to switch from Revolution to Iverhart Plus instead of harsher treatments, saying sometimes it will clear up on a future test.

    Do dogs ever become ‘heartworm free’ just on Iverhart alone? I am supposed to give it religiously all year to all 5 dogs, and all are thriving.

    The two that tested ‘positive’ are doing great in every other way, especially given their ages!

    So they may actually turn up ‘heart-worm free’ if the meds are continuous? I ask because in years past, I had heard that only harsh, sometimes dangerous treatments could cure heartworms. Please help with my confusion!!

    Thank you so much!

  37. Dr. Michael Dym, VMD veterinarianOctober 13, 2011 at 12:30 pm

    Hi Betty:
    Yes year round continuous iverhart may clear your dog in 1-2 years. Not 100 percent though. Other holistic options do exist like black walnut and other herbs, but you would need to consult with a holistic vet. See websites http://www.AHVMA.org http://www.AVH.org http://www.drpitcairn.com to learn about vets who offer other treatment options. Many offer phone consults.

  38. Dr. Michael Dym, VMD veterinarianOctober 13, 2011 at 12:30 pm

    I am one of those vets who offers phone consults on this as well.

  39. I have a seventy pound boxer not really overweight just a big dog. She is 4yr 2mo old. She has not been screened since 09′ and has not been on preventative for maybe 2yrs. She lives in Texas in and outside the home. Question, Can I start her on preventative until I can afford the test at the Vet clinic. I have heard yes and no plus I remember hearing it could kill them when I was a child. Thanks J.B.

  40. Dr. Michael Dym, VMD veterinarianMay 1, 2012 at 5:50 pm

    It is usually best to have a heartworm blood test first at the veterinary office before starting heartworm preventative medication.

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