Heartworm (Dirofilaria Immitis)
 
     Heartworms are large worms that lodge in the right side of the heart and in the pulmonary vessels of the lung.  Due to consequent inflammation of the pulmonary vessels as well as the mass of the parasites, the heart has to work much harder to pump blood to the lungs. The heart must work harder and harder to accomplish its output.  As the heart tries to compensate for the added amount of work, it becomes larger.  As the heart becomes larger it looses its strength.  Thus the hearts of dogs with heartworm, therefore, become old before their time.  As the heart becomes weak, almost every organ in the body becomes affected because of a lack of the proper blood flow to these organs. The immune system is also stimulated so that autoimmune reactions such as arthritis can occur.
     Not all heartworm infestations are as meaningful to the body as that described above.  The dog can tolerate light infestations and the dog can very successfully out live the 5-year life expectancy of the worm.
     Heartworms and most other nematodes can be prevented by using Heartguard, which can be purchased from a veterinarian.  Dogs become initially infected with microfilariae which then become adult heartworms 6 months after they are bitten by a mosquito which is carrying microfilaria (baby worms) of an adult heartworm from another dog.  After the mosquito bite, the microfilariae travel through the connective tissues of the body and 5 or 6 months later become adult worms in the heart and pulmonary vessels.  Preventive medication must be administered regularly.  All dogs can be injected with microfilaria from the bite of a mosquito, but only those who are on preventive medication will kill these microfilaria, up to 2 months after infection. It is necessary, therefore, to administer medication from June 1 until November 1 of each year,
     If a dog has heartworm it can be treated by using an arsenical compound which will kill the adult worms and is much less toxic to the tissues of the body than the earlier treatment.  Since dogs with heartworm may have marginal function of some of the organs of the body, administration of this medicine is performed with great precaution by the veterinarian after much testing of the dog.  This can allow concurrent therapeutic regimens to create fewer hazards from the injection of the arsenical.
     Testing a dog between the months of April and June will provide a check on possible infestation throughout the previous summer.  If the testing is performed later than June, it is wise to test the dog again the following April to check for possible infestation prior to administration of preventive medication.  If administration of preventive medication is not done in a very consistent manner it has been recommended to test dogs each April before the new preventive medicine administration season.
     If dogs can be kept on heartworm prevention and carriers treated there may soon be a day in New England when heartworm disease will be very rare.