Canine Heartworm

Heartworms are a common parasite that can be easily transmitted to dogs and cats via mosquitoes. If a heartworm infection is left untreated it can result in death. The remainder of this article will explore the spread, signs, diagnosis, treatment, and prevention of heartworms in dogs. 

Spread. Immature heartworms are picked up by mosquitoes when they bite an infected dog. The immature worms develop slightly within the mosquito and are passed to an uninfected dog when the mosquito bites for its next meal. The heartworms migrate to the bloodstream and find their way to the pulmonary artery, near the heart. The immature worms will then develop into adult worms. In turn, they will produce more immature heartworms. These young heartworms will then circulate in the dog's bloodstream and can be picked up by a mosquito and spread to another dog. Thus, the cycle continues.

Signs. A dog with heartworms may show a wide variety of signs, depending on the severity and duration of the infection. Signs will vary from none to coughing, exercise intolerance, lethargy, weakness, collapse, heart failure, and death. 

Diagnosis. Due to the development of "in-house" heartworm tests, a dog can be diagnosed quickly and accurately with a very small blood sample. Bloodwork, including a CBC and chemistry panel, and chest x-rays may be necessary to help determine the health status and extent of infection.

Treatment. Currently, there is not a single medication that kills immature and adult heartworms. Thus, if a dog is diagnosed as heartworm positive, there are two "phases" of treatment--one to kill the adult heartworms and one to kill the young heartworms. There are only two medications approved for the treatment of adult heartworms.

The older medication, Caparsolate, is given in the vein whereas the newer medication, Immiticide, is given in the muscle. While both medications can have severe side effects, Immiticide is generally the safer of the two medications. Once the adults are dead, oral medication will be given to kill any young heartworms that are still present in the bloodstream of the dog. 

Dogs being treated for heartworms may be put on additional supportive medications like antibiotics, aspirin, or steroids. The decision to use these and other medications are often based on the severity of the infection, the likelihood of complications, and/or the occurrence of complications during treatment. 

In addition to the medication given to destroy heartworms, strict confinement and exercise limitation, periodic physical examinations, and a repeat heartworm test 2 months after the initial treatment are all necessary to ensure the success of the treatment.

Once a dog has been treated and tested negative, it is critical to put them on a monthly heartworm preventative to ensure that they do not get another heartworm infection.

Preventative. Heartworm treatment is long, risky, and expensive. Thus, it is ideal for you to prevent your dog from ever becoming heartworm positive. Fortunately, there are several medications available that will kill young heartworms before they become adults and cause problems. The most common monthly medications that are available are Interceptor, Heartgard, Revolution, and Sentinel. (Sentinel contains Interceptor and thus will also kill young heartworms.) Daily heartworm preventative pills are available but are generally less popular than monthly pills.

There is also a six-month injectable product called Proheart, available. Currently, it is recommended that all dogs over the age of 8 weeks be given heartworm medication all year, even in the winter months. Mosquitoes only need an average temperature of approx. 45 degrees to be present. It is not uncommon for dogs on preventative 8-9 months a year to become positive over the winter, thus it is safer to keep your dog on the preventative all year. Dogs who are not on heartworm preventative or who are not on in the winter should be tested each spring. It is acceptable for a dog that is on year-round preventative to 

be tested every other year. 

For our current clients, heartworm preventatives are available online at our webstore. A current heartworm test is required for purchase. 

The information provided is for educational purposes only and is not intended to take the place of your regular veterinarian. Please do not hesitate to contact your regular veterinarian if you have questions regarding your pet.