All Pets Veterinary Clinic


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 itís 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 dogís 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 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, an
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 is
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

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 the 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 (see
link below). A currrent 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.

Karen Blakeley, DVM, MPH
27 September 2009