Heartworm disease

Jennifer Ryan

Heartworm disease poses a major health threat to both dogs and cats, and it’s on the rise. Present throughout the United States and Canada, the disease strikes pets, if unprotected by preventive heartworm medicines, who have been bitten by a mosquito carrying contagious microfilariae.

A mosquito becomes a carrier of heartworm disease after it bites an infected animal. With the blood of the animal, the mosquito also takes in microfilariae, which circulate in the animal’s blood and are the young offspring of adult heartworms living in the animal’s heart.

Once in the mosquito, the microfilariae take 10 to 14 days to mature into infective larvae, after which they are ready to infect animals the mosquito goes on to bite. Only certain animals are commonly susceptible to contracting heartworms, and cats and dogs both make the list, though the disease remains more prominent in dogs.

While more prominent in dogs, it is more devastating to cats. Dogs often survive the disease with the help of skilled veterinary care and timely treatments, but cats are often not so fortunate. A principle reason for this difference is that no drug is currently approved to treat heartworms in cats specifically. While a drug used to treat dogs can be applied to cats, significant side effects often result, not the least of which is that, as the heartworms begin to die as a result of the medicine, the worms travel from the chambers of the cat’s heart into the pulmonary arteries, which can cause sudden death.

The treatment of heartworm disease in dogs is often met with more positive results, but it, too, is a burdensome undertaking that carries its own share of risk. Canine-specific drugs do exist, and are the most common method of treatment, but, for advanced cases, surgery may be needed.

Before patients can be treated though, veterinary practices must first diagnose pets with the disease. Pet owners can aid in this endeavor by knowing the symptoms to look for. While dogs may not show signs immediately, symptoms include difficulty breathing, coughing, lethargy, apathy, weight loss, and a rough hair coat.

Cats’ symptoms, on the other hand, are both more difficult to isolate to heartworm disease and are less consistent between patients. Approximate symptoms, though, often include coughing, rapid or labored breathing, weight loss, and vomiting—symptoms that are also common to many other diseases. Laboratory tests are needed for both cats and dogs to know definitively whether they have contracted the disease.

While the symptoms may be confusing and difficult to detect, prevention of heartworm infection is straightforward and is recommended for dogs and cats throughout the United States and Canada. Several drugs exist to protect pets against the infection and, with proper administration, are nearly 99 percent effective. Consult with your veterinarian to have your pet tested for heartworms and to obtain preventive medications.

As with many health considerations, the importance of prevention cannot be overstressed, and heartworm disease proves no exception. The overarching need for preventive care to fortify pets against the contraction of heartworms is undeniable, especially as the incidence of the disease increases. Meet with your veterinarian today to ensure that you do all you can to protect your furry loved ones from an avoidable disease and the trauma it can incur.

Based in Denver, Colo., where she lives with her Labrador-mix, Jennifer Ryan works for the American Animal Hospital Association.


Photo credit: iStock images


Comments (1) -

Jennifer Skiba
Jennifer SkibaUnited States
3/30/2014 10:55:36 PM #

Heartworms Development Requires Sustained Day & Night Weather Above 57˚F

The University of Pennsylvania vet school (in a study funded by Merial) found: “Development in the mosquito is temperature dependent, requiring approximately two weeks of temperature at or above 27C (80F). Below a threshold temperature of 14C (57F), development cannot occur, and the cycle will be halted. As a result, transmission is limited to warm months, and duration of the transmission season varies geographically.”

Knight and Lok agree: “In regions where average daily temperatures remain at or below about 62˚F (17˚ C) from late fall to early spring, insufficient heat accumulates to allow maturation of infective larvae in the intermediate host [the mosquito], precluding transmission of the parasite.”

The Washington State University vet school reports that laboratory studies show that maturation of the worms requires “the equivalent of a steady 24-hour daily temperature in excess of 64°F (18°C) for approximately one month.”  In other words, it has to be warm day AND night or development is retarded even if the average temperature is sufficiently warm. They add, that at 80° F, “10 to 14 days are required for development of microfilariae to the infective stage.”

Jerold Theis, DVM, PhD, says, “If the mean monthly temperature is only a few degrees above 14 degrees centigrade [57 degrees F] it can take so many days for infective larvae to develop that the likelihood of the female mosquito living that long is remote.”

I have never found this temperature-dependent information on a website promoting “preventatives,” but only in more scholarly works not easily accessed by the public. There is, as far as I can find, only one mention of temperature on the Heartworm Society (on the canine heartworm page) and none in the Merck/Merial Veterinary Manual site or Merial’s heartworm video — even though Merial funded the UPenn study.

Quoted from this website.

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