Canine heartworm (Dirofilaria immitis) is a parasitic disease that was first discovered in Florida by R F Jackson, DVM in the 1950 era. Dogs die from heartworm disease. For years it was a disease of down south. The next hot spot area seen was in urban northern areas, such as Minneapolis. It was brought back by "snowbirds" that transported their dogs south for a winter vacation. What has been learned since is amazing.
The current understanding shows:
This parasite syndrome consists of female worms that may live up to 5 years in the dog, they are 4-6 inches long; reside in the chambers of the heart and in the lung (pulmonary arteries). Male heartworms are shorter and may not live as long as females. The mature adult worms combine and reproduce the microfilaria (offspring) that are picked up by mosquitoes. Mosquitoes incubate and develop through two larval stages. When a female mosquito bites / feeds on a host the third stage larvae are secreted back into a dog. After being injected back into the dog there are two more molts before maturity. This process takes 5-7 months to go from microfilaria to adult heartworm.
Diagnosis is primarily in clinic; based on the presence of microfilaria in whole blood (Modified Knott's test) or a direct blood smear, Heartworm antigen in blood, serum or plasma, ultrasound evaluation, thoracic x-rays. Heartworm antibody titers are usually submitted to a reference laboratory that has sophisticated testing capabilities.
Advanced cases show severe debilitation of a dog. Fluid (ascites) in abdomen, liver disorders, extreme breathing difficulty in the lungs, with no exercise tolerance are clinical signs seen in chronic heartworm or caval syndrome.
Treatment:
The only FDA approved treatment for dogs is an organic arsenic compound. This is approximately 90% effective in curing / eliminating adult heartworms. Condition of an individual case, time of year treatment, and multiple infection stages will modify if a second treatment will be needed. While this treatment is effective it may cause the death of the dog. There may be an immediate anaphylactic reaction or in 5-7 days when the arsenic starts to kill the adult worms an antigenic cascade causes respiratory problems. The treatment staging, blood testing, radiographs, and supportive medications are expensive. The risk for complications with treatment can be significant.
Prevention of canine heartworm:
My goal as a veterinarian is to teach prevention. There are lots of preventatives for canine heartworms: daily and monthly preventatives, tablets, chewable tablets, some topical monthly formulations are available. There are pluses for some drawbacks for others. The best preventative is convenient for the owner, easily administered, given on the correct schedule and well tolerated by the pet. These choices are best tailored to owner preference during annual exam. There is the option of using a preventative which includes an antiparasitic (intestinal dewormers) for routine monthly control.
Other animals known to get heartworm include raccoons, wolves, foxes, coyotes and cats.
Cats are not a normal species for heartworm. There is no FDA approved treatment for feline heartworm. The medication used in dogs, Immiticide (melarsomine), is very toxic in cats. Cats get heartworm at the rate of 1 positive cat for approximately every 10 positive dogs. Heartworm in cats will cause sudden unexpected death. There are FDA / EPA approved heartworm preventative medications available for felines.
Dr Mike
6/1/2008