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 *** Please consult your personal veterinarian when asking about your own pet. We are giving this information as a reference only.


Why are all of our adult dogs tested for heartworm prior to being adopted out?



The map speaks for itself! (the map shows heartworm infections in dogs in 2001) No dog is safe from heartworm. In fact, we rescued some local dogs recently that were Heartworm Positive, so even local dogs are not immune! (They were treated and are doing fine)

Please put your dogs on a monthly heartworm preventative! A $6 a month pill will save you a lot of money in the long run and will save your furry friend a lot of pain and suffering! The treatments are rough!

*It takes 6-7 months for the heartworms to develop enough to actually be seen/found on a heartworm test. The reason we test only dogs/puppies over six months of age is because testing a dog under 6 months of age would render itself useless, even if the pup was bitten by an infected mosquito the day it was born, the tests would still come up as being negative.


Here are some heartworm facts:

  • The mosquito is the only known vector for transmitting heartworm.
  • The average lifespan of heartworms in untreated pets is 5-7 years in dogs and 2-3 years in cats.
  • Virtually 100% of dogs exposed to infective heartworm larvae become infected; the percentage drops to 61% to 90% of cats.
  • Microfilariae are found in the blood of 80%-90% of dogs, while only 20% of cats have these in their blood.
  • Heartworm infection in cats exists everywhere heartworm in dogs exists.
  • The American Heartworm Society (AHS) estimates that only 55% of dogs in the U.S. are currently on a heartworm preventive, leaving 27 million dogs at risk of acquiring heartworm disease.
  • A study performed at North Carolina State University indicated that 27% of cats infected with heartworms were solely indoor cats.
  • Heartworms may infect more than 30 species of animals including coyotes, foxes, wolves and other wild canids, domestic cats and wild felids, ferrets, sea lions, and humans.
  • Prevention is far more effective and less costly than treatment.
  • Research suggests that heartworm disease could be virtually eradicated using available preventives.
  • Only 55% of the dogs in the United States get some kind of heartworm preventive, leaving 27 million dogs at risk.


Heartworm transmission

Adult  heartworms, about six inches long, live in the heart and large blood vessels.  These adult male and female worms  produce thousands of microscopic baby worms. 

Baby heartworms do not grow up in the dog where they were born.  If they did, the dog would quickly die and so would the heartworms.

Next, a mosquito comes along and bites the infected dog, sucking up baby heartworms. It probably isn't too good for the mosquito, but this is what the worms have been waiting for. During the next month, inside the mosquito, the heartworm babies grow into heartworm teenagers, a stage partway between baby and adult. 

Now, the mosquito bites another dog, infecting it with teenage heartworms,  which are now ready to develop into adults.  After six or seven more months, the life cycle is complete. The new adult male and female heartworms are busily producing thousands of babies.

The heartworm has 5 separate larval stages referred to simply as L1, L2, L3, and so on. 
It has two separate cycles which combined makeup the lifecycle of the heartworm. 
One cycle takes place in a mosquito and the other inside a dog or cat.

The mosquito is infested when it bites a dog which is harboring L1 (MICROFILARIAE). This can only happen if the dog is also harboring the L5, or adult, male and female heartworm as the Microfilariae are their offspring. These Microfilariae can live for up two years in the dogs blood but must be taken up by a mosquito in order to develop any further. If they are not they will simply die of old age.

Once the mosquito is infested, the larva must go through to stages of development or molts L2, and L3, before they can infect another dog. This, mosquito, stage takes anywhere from two weeks to several months depending on the weather. The warmer the weather the faster the development.

The importance of temperature:
While the larva are developing in the mosquito, the temperature MUST remain above 
57 degrees Fahrenheit at all times, day and night. If at any time during the mosquito stage the temperature drops below 57 F, the development is halted and must start over. 
It is only the L3 larva which are capable of infesting your dog.

Now lets say that a mosquito has bitten an infested dog and the temperature has remained above 57°for a minimum of 14 days since that bite and the mosquito bites your dog. Still your dog is not infested because the L3 larva are deposited in a tiny droplet of mosquito saliva adjacent to the bite not injected into your dog as many would have us believe. Providing the humidity and temperature are such that the droplet does not evaporate, the L3 larva must swim through the saliva and into the mosquito bite, thereby entering your dogs system.
Once inside your dog the L3 larva must spend the next two weeks or so developing into the L4 larva. During this period of time the larva is living in the skin, not the blood of the newly infested dog. The L4 will continue to live and develop in the skin for the next three or four months where it develops into the L5 stage.

Once it makes this development into the F5 it then leaves the skin and enters the circulatory system. The L5 or young adult then migrates to the heart and pulmonary arteries. Once there it will mate approximately 6 to 7 months after entering the dog's body. That is of course assuming that the dog has been infested with both male and female larva. This mating produces the Microfilariae.

A word on "preventatives"

The most popular heartworm "preventives" Heart guard and Interceptor are not really preventives at all, rather they act by killing the microfilariae, L3, and/or L4 larva in an infested dog. Interceptor kills the L3s, and L4s while Heartguard will kill the L4s and some of the youngest L5s. In other words they're poisons. Neither of them kills the fully adult heartworms.

There are basically two standard tests for heartworm. One is called the antigen or occult test which tests for the antigens produced by the adult female heartworm. This test does not show the presence of microfilaria. The other is the microfilaria test. This test, of course, tests for microfilariae. Both Heartguard and Interceptor kill microfilariae. Therefore if ones dogs have been on either of these products they will test negative for heartworm when given the microfilaria test even though they may be infested with adult heartworms. It is not common, but it does happen. 

Resources for you:

Dog Owner's Guide to Heartworm

Heartworm Disease in Dogs and Cats

Heartworm Information Center

Heartworm Treatment


*Some dogs in our rescue that we have treated for heartworm:

sanya Sanya came to us Dec. 2007 with 5 tiny pups, then tested positive for heartworm! We treated her, now she is the spokesdog for a wildlife rehab program!

emmitt Emmitt was an owner turn. He was treated and is living with a wonderful family.

skeeter Skeeter wasn't supposed to be in our rescue at all! He came with a transport of dogs and accidentally made it on board. We decided to accept him, rather than send him back. He came from Tennessee and was heartworm positive. Midway through his treatments, he had complications. He was rushed to the ER but we couldn't save him. His blood platelets dropped and his body was hemorrhaging. R.I.P. Skeeter! If only your pet parents had paid that $6 a month for your heartworm pills!!!!

 gigi Gigi was another dog that tested positive for heartworm!

mj This is MJ and her new family who saw her through her extensive treatment. Her family decided to adopt her, despite all her health issues. What a trooper both MJ is AND her new family!!!


Please, put your dog on heartworm preventative and save them from needless suffering and/or death!!!!





PO Box 626 •  East Amherst, NY 14051  •   •  info [ at ]