Heartworm Disease

Heartworm Disease

Heartworm disease can cause serious problems for both dogs and cats. Although the same organism infects both dogs and cats, their clinical symptoms are very different.  Heartworm disease can be easily prevented in both dogs and cats.  Dogs must have a negative test for heartworm disease before they can start taking heartworm preventative medication.  In Missouri, we recommend year round treatment with heartworm preventatives due to the mild climate.  As an added benefit, our preferred heartworm preventative also controls fleas and ticks. The American Heartworm Society recommends annual heartworm testing for all canine patients.  Even one missed dose or a dose that is more than a couple days late, can allow an infection to develop, which necessitates annual testing.

What is heartworm disease?

Heartworm disease is caused by a parasitic worm that is transferred between mammals by mosquitoes. Although several mammals can become infected with  the parasitic worm, dogs, wolves, coyotes and foxes are ideal hosts for the disease since the worm can mature into adulthood and reproduce inside their bodies. Ferrets and cats are also susceptible to heartworm disease. In rare cases even humans can contract heartworm disease, but most of the worms die before reaching adulthood making them less of a threat to the heart, lungs and other internal organs.  It is important to note that since heartworm disease is transmitted by mosquitoes, it is not necessary for animals to go outside to become infected.  The mosquitoes are more than happy to come into the house and spread the infection there!

How does heartworm disease spread?

Mosquitos are the only known transferring hosts of heartworms. While a mosquito is feeding on the blood of an infected animal with advanced stages of the disease, it ingests baby worms, or microfilaria living in the infected animal’s blood stream. After about two weeks, these baby worms have reached a level of maturity where they are infectious. Once a mosquito bites a susceptible host; it transmits the infectious larvae onto the skin of the animal.  The larvae then makes its way into the puncture wound left from the mosquito's bite and infects the animal. If your dog is on heartworm preventative medication, the larva typically dies quickly without causing harm. If your pet is not on heartworm preventative medication, the heartworm can live years without being detected. Then, as the worm grows and reaches adulthood, it starts causing serious complications

What are the symptoms of heartworm disease in dogs?

Because a heartworm can live up to 7 years, the symptoms are dependent on the stage of infection and the number of worms an animal has.

    Stage 1 – No symptoms present other than the occasional cough

    Stage 2 – Occasional cough and exercise intolerance

    Stage 3 – A cough, exercise intolerance, and labored breathing. Signs of heart and or lung damage are usually seen on a chest x-ray at this stage.

    Stage 4 – Heartworms have caused severe damage to the animal's heart, lungs, liver, and kidneys. In this stage, heart failure with or without Caval syndrome is likely. Caval syndrome is when there is a large burden of worms blocking blood flow in the right side of  the heart. This creates a sudden life-threatening problem in need of quick surgical intervention. At this point, most dogs do not live long enough to receive the life-saving surgery.

What are the symptoms of heartworm disease in cats?

Heartworm disease in cats is usually manifested by respiratory symptoms.  Initially, symptoms may appear similar to asthma or bronchitis.  Quite frequently, intermittent vomiting is present as well.  Heartworm infections in cats can be very difficult to diagnose, often requiring many expensive tests that must be periodically repeated to establish the presence of heartworm disease.  Heartworm infection in some feline patients will progress to heart failure and caval syndrome, but this is much less common than in dogs.

How is heartworm disease treated?

Heartworm treatment for dogs is possible, but treatment is costly and hard on your pet’s body. It typically requires multiple veterinary visits, blood tests, x-rays, and painful injections. Many patients that are treated for heartworm disease suffer damage to their lungs as the heartworms die.  This makes extended confinement and exercise restriction necessary for an uncomplicated recovery. There is no drug approved to treat heartworm disease in cats. Cats that are suffering from heartworm disease require symptomatic treatment until the worms die naturally and are removed from the lungs by the immune system. Oftentimes, this results in severe damage to the lungs which results in lifelong symptoms. The best thing for you to do for your pet is to prevent heartworm disease from ever developing.

How is heartworm disease prevented?

Dogs and cats should be started on heartworm prevention medication  as young as possible. Dogs that are under 6 months of age do not require a test before starting preventative medication.  As long as they weigh at least two pounds, most puppies and kittens can receive heartworm preventives as early as their first check-up. Every US state has reported heartworm infections, so every dog and cat in the US is at risk of coming into contact with the disease. There are three types of preventive options, but a veterinarian must prescribe them all. 

1.    Monthly heartworm prevention pill

2.    Monthly topical solution

3.    Bi-annual preventative injection  (Not currently available at our clinic) 


Next steps for a healthier pet:

If your pet is not currently on a heartworm prevention regimen or you are more than two weeks late on administering your monthly heartworm preventative, than consult with your veterinarian or make an appointment today.  


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