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. 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.

  2. 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.

  3. 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.

  4. 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

  5. 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.

  6. 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.

  7. 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.

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

    Thanks for clarifying my point.

  9. 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

  10. 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.

  11. 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!

  12. 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.

  13. 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.

  14. 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.

  15. 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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