Eventually the snow will melt and spring will bring rain. Warmer weather and excessive ground water brings the perfect conditions for mosquito proliferation. There are approximately 50 species of mosquitoes native to Missouri according to the Department of Conservation. Even-though warm weather is usually associated with those pesky insects there are species active during every season of the year. The only reprieve is during freezing temperatures but warmer environments such as in the house or barn may allow mosquitoes to overwinter.
Mosquito’s can serve as vectors to transmit diseases to birds, animals, and humans. Most commonly we think of St. Louis encephalitis, Eastern and Western Equine encephalitis, and West Nile virus. However, I want to focus on a parasite that utilizes the mosquito for its life cycle. The filarial worm (Dirofilaria immitis) causes Heartworm disease in both our canine and feline pets.
This parasite has a complex life cycle requiring the mosquito for development and propagation. Lets start with a mosquito that takes a blood meal from an infected animal (dog, cat, coyote, etc). Microfilaria (L1 larvae) are ingested by the mosquito and develop through three stages of molting. At this point the larva migrate and erupt from the mosquito’s labium as it finds a new host to feed on. The larvae molt two more times within the tissues of the new mammal host. They migrate to the heart as immature worms hitching a ride within the bloodstream. Blood pressure then pushes the young adult worms into the small arteries within the lungs. As they mature and grow lengths of 10 inches can be reached. Eventually they fill up the right side of the heart and start producing new microfilaria (L1 larvae). This completes the life cycle of the heartworm.
Every patient is different in regards to severity of the disease process occurring at the time of diagnosis. Often, smaller patient size relates to more severe disease. The adult worms cause irritation and inflammation of the blood vessels within the lungs leading to longterm damage. Due to the “clogging effect” within the blood vessels and heart, excessive pressure is created which can lead to right sided congestive heart failure. Other common symptoms include vomiting, coughing, lethargy, weight loss, and damage to other organs due to restricted blood flow.
Treatment of this condition can be as complex as its development. There is only one product approved for the destruction of adult canine heartworms currently available. This product is given in a painful series of injections followed by confinement to reduce the chances of a thromboembolism occurring. Other medications may be added to the regime including blood thinners, vasodilators, diuretics, and anti-inflammatory drugs. Some medications might need to be continued for the remainder of the pet’s life to compensate for the damages done. Now, larvae infections can be treated and prevented by macrolide therapy given once a month. However, once L4-L5 larval stages exist then the prior discussed treatment will be required to clear the infection. Regardless of the options available every doctor has preferences, and treatment should be tailored to each patient.
Why treat when we can prevent? Both having a heartworm infection and treatment of that infection has life threatening potential. Plus, it’s expensive! There are several different products for administering prophylactic care for your pets. The three basic forms are monthly topical products, oral tablets, and subcutaneous injections that provide 6 months coverage. Many of these products contain multiple drugs that also protect against intestinal parasites, mites, and fleas. Make sure you use the appropriate medication for your pet’s species, and make sure you give the correct dosage every month. This is easy if you have established a relationship with a veterinarian, as it is their job to inform you of safe options. For those that have managed to slip through the cracks and purchase these products from sources other than veterinarians, you are the only quality control for the pet!
From atop my soapbox I ask you to: never discontinue these products over winter, never stretch 6 plus weeks between administration, never split these products between pets, and stop ordering medications from online pet pharmacies! Veterinarians are not making their sole living dispensing these products; there is no sham to gouge the public. We are following the recommendations of the American Heartworm Society, and our medical training. Splitting medications has the potential to under-dose one pet while overdosing another. Furthermore we want to build a relationship with you and your pet to provide better care and service. No matter how professional online pharmacies may seem, they will not be able to assist you when Lucky gets sick. Get to know a medical professional prior to that possible emergency!
New puppies and kittens should receive several rounds of vaccines starting as young as 6 weeks of age. Some heartworm preventatives can also be started as early as 6 weeks of age without heartworm testing. By giving preventative products not only are you protecting that pet, but stopping the spread of the disease to any other animal by disrupting the life cycle. I encourage you to visit the American Heartworm Society’s webpage at www.heartwormsociety.org for more information.


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