Heartworms were first diagnosed in 1856 and the disease is seen across the country. "The American Heartworm Society and Merial partnered to survey 12,000 veterinary clinics across the United States. The survey found mor that more than 250,000 casees of heartworm were reported in 2004." - www.heartwormsociety.org How does my dog or cat get it? It is transmitted through a single mosquito bite. If a mosquito is infected with heartworms, then it can transmit the disease when it bites your pet. Depeneding upon susceptibility, some species can contract heartworm disease easier than others. For example, dogs are more susceptible than cats to heartworm disease. Am I at risk? Yes and not really. Humans can get heartworm disease from a mosquito bite however they have natural resistance against this disease. An infected dog CANNOT transmit the disease directly to another dog nor human. If youare are concerned about yourself or other human members of your family, please contact your physician at once. Who else gets heartworm? Canines (dogs, wolves, coyotes, fox, etc...) are the heartworms host which are needed for reproduction.Other animals that have been shown to get heartworm are: domestic cats, bobcat, jaguar, tiger, muskrat, raccoon, ferret, otter, bear, horse, orangutan, gibbon, sea lion and man. In these other animals, the heartworm infection may not necessarily show up in the pulmonary arteries like dogs. The worms can get 'lost' and end up having aberrant migration to other organs. How is it diagnosed? Blood tests looking for the disease are highly accurate. Unfortunately the hearworms must be at least 7 months old in order to show up on the standard screening test. If a heartworm screening test is positive, additional testing to evaluate the disease state will likely be necessary. These may include chest radiographs (X-Rays), ECG and cardiac ultrasound. What is the heartworm cycle? Heartworms have different live stages. While the worms that live in heart can be a foot long, another stage is called the microfilariae which is microscopic and can be present in both the blood stream and within the mosquito. Microfilariae mature into infective juveniles inside the mosquito. The infected juvenile is transmitted into the dog (or cat or human) when the mosquito bites the dog. The juveniles migrate tot he right side of the heart and mature into adult worms that live primarily in the pulmonary arteries. The adult worms sizes are follows: females are often 6 - 14 inches long and the males are about half the size of the female. The adult worms reproduce (as early as 2-3 months of infection) and the females produce microfilariae which can then be ingested by more mosquitos to continue the life cycle. What kind of mosquito poses the problem? Unfortunately as many as 30 different species of mosquitos can carry heartworms. There may be up to 300 adult heartworms in a single dog and the worms can live as long as 7 years and in ther time produce literally millions of microfilariae.
Heartworm Symptoms & Treatment
What are the symptoms? The symptoms are created from heart valve interference, the clogging of the pulmonary arteries, blood clot formation and reduction of blood supply to other organs. The organs affected include: the heart, the lungs, the liver and the kidneys. The lung an liver damage is often a result from the microfilariae obstructing small blood vessels and capillaries. The most common signs include:
a soft, dry chronic cough
shortness of breath
listlessness, depression, nervousness
loss of stamina, weakness
heart murmur and abnormal breath sounds
heart attach and spontaneous death have also been seen
What are the treatments? The most common medication is Melarsomine dihydrochloride which is injected as recommended by the American Heartworm Association on a three injection schedule. Some cases are treated with long term use of a heartworm preventative. What is Caval Syndrome? It is a severe life threatening syndrome which occurs in severely infested patients. When the large number of worms prevents proper closure of the tricuspid valve, blood flow becomes congested within the liver. It is noticed with an acute onset of severe lethargy and weakness. Diagnosis is made by echocardiographic (ultrasound) evaluation of the heart. If surgical extraction of the worms is not done immediately, the patients usually die within days of caval syndrome.
Wolbachia and Heartworms
Wolbachia picture from microbiology.ucsc.edu
What is Wolbachia? Dirofilarial Wolbachia is a type of rickettsial bacteria that has developed as an endosymbiont (living within) to heartworms. The presence of this bacteria within the heartworms is also associated with further disease of the canine patient. The wolbachia surface proteins (WSP) appear to be able to have an inflammatory response within the patient. It is hypothesized that some of the pulmonary vasculopathy of the canine heartworm patient may be due to WSP response. What is the treatment? While treatment of the hearworm disease with melarsomine will also kill the wolbachia, some clinicians will place the patient on a course of doxycyline prior to heartworm therapy. Doxycyline therapy may reduce the incidence of hypersensitivity reactions. It may also reduce pulmonary inflammation.
What is the prevention? There are a number of excellent heartworm preventatives available. Which one is the best? It really depends upon the pet (age, breed), what other parasites you need to control, and which mode of delivery that you feel comfortable with. Oral Preventatives (listed below each describes other parasites covered by the medication)
American Dog Ticks
What if I miss a dose? There is a wide overlap from many of the preventative medications. If the lapse is over 3 months in duration, then immediately re-institute prevention therapy and have your pet tested for heartworms in 7 months following the re-institution of the preventative medication. If the gap is less then 2 months or less, then re-institute therapy and have your pet tested on his or her regular schedule. Should I get my pet test yearly even if I never miss a dose? The American Heartworm Society recommend yearly testing despite the high efficacy of the preventative medications on the market. Many medications, like birth control, may have excellent efficacy however the reality of potential problems still exists. The test is usually easy to perform and relatively inexpensive.