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Posted on 02-03-2015

If you could do one simple thing to prevent your pet from getting a serious and possibly fatal disease, would you do it? Most pet lovers would say yes. Unfortunately, every year dogs, cats, and ferrets are diagnosed with heartworm disease, which could have been easily avoided if only they were placed on a monthly heartworm preventative.

What is it?

Heartworm disease is a parasite infection caused by the long, thin, threadlike worm Dinofilaria immitis. These worms can grow to a foot in length and reside in the pulmonary arteries of infected pets. These are the arteries that take blood from the heart to the lungs. The worms affect the flow of blood and oxygen to the rest of the body.

Over time, infected animals can develop heart failure and lung disease, resulting in symptoms such as lethargy, coughing, difficulty breathing, loss of appetite, fluid retention, difficulty exercising, coughing up blood, and loss of weight. This results in a poor quality of life for your pet and can possibly lead to their death.

How does your pet get it?

Heartworm disease is present in all 50 states and in many other countries around the world. Domestic pets and some wild animals such as wolves, coyotes, and foxes can be infected. The disease is transmitted when a mosquito bites and takes blood from an animal with microfilaria (baby worms) in their bloodstream. These microfilaria continue to develop in the mosquito till they reach the infective larva stage. When the mosquito bites another animal for a blood meal, the larva enter the bloodstream of the animal, and it becomes infected. The larva grow into adult worms in approximately 6 months.

What happens in dogs?

In dogs, an adult worm can live for 5-7 years. Dogs usually have multiple heartworms present at one time. Frequently, there will be more than 20 heartworms, but the worms can number in the hundreds. The severity of the heartworm disease depends upon the number of heartworms present, how the dog's body reacts to the heartworms, how long the dog has been infected, the age of the dog, and whether the dog has any other medical problems. Early in the disease and with low numbers of heartworms, the dog may have little to no symptoms. However, as the length of time and heartworms increase, the dog will show symptoms such as coughing, difficulty breathing, tiring easily with minimal exercise, loss of appetite, fatigue, fluid in the abdomen, and weight loss.

What happens in cats?

Heartworms have a more difficult time surviving and reproducing in cats. An adult worm usually lives only 2 years. Cats usually have between 1 and 3 heartworms when they are diagnosed. However, because of their smaller body size even the presence of one heartworm can result in serious illness. Symptoms of heartworm disease in cats include vomiting, coughing, weight loss, difficulty breathing, and loss of appetite. Other symptoms may include seizures, fainting, difficulty walking, or fluid in the abdomen. Unfortunately, in some cats their first symptoms are sudden collapse and death.

How can it be prevented?

The consequences of heartworm disease can be serious for your pet. The best way to avoid infection and the dangers of heartworm disease is through monthly preventive medication. The American Heartworm Society recommends the medication be given all year round to give maximum protection for your pet . Medications and dosages for dogs and cats are different, so please don't give your medication for one pet to another. Also, the most effective dosage of medication depends on your pet's weight, so it is important that the medication be adjusted as your pet's weight changes. In most cases, a prescription tablet given once a month will be all that is needed to protect your pet from heartworm disease. In some cases, your veterinarian will prescribe a medication that is applied to the skin monthly for heartworm prevention. Some heartworm medications also control other internal and external parasites. It is important to remember that preventative medications kill the microscopic larva that may be present in your dog's blood. It does not kill adult heartworms. Please consult your veterinarian for the right medication and dosage for your pet.

What is heartworm testing all about?

There are several tests for detecting the presence of heartworms in your pet. To perform the tests, a small blood sample must be obtained from your pet. One test is called a microfilaria test. This test detects microscopic baby worms in your pet's blood. A second test is a heartworm serology or antigen test. This test detects proteins in the blood, which come from adult heartworms. A third test sometimes performed in cats is a heartworm antibody test, which checks for the presence of antibodies in the blood and indicates that a heartworm infection has occurred in your cat.

When do dogs need to be tested?

Puppies should be started on preventative medication as soon as possible, preferably before 8 weeks of age. Under 6 months of age, a puppy can be started on medication without heartworm testing. After a dog becomes infected, it takes at least 6 months for a dog to test positive, so testing before that is not helpful. The dog will need to be tested 6 months after starting medication, then annually thereafter. Dogs older than 6 months will need heartworm testing before starting preventative medication.  It is important to know the heartworm status of the dog before giving preventatives because heartworm positive dogs can have a severe reaction to the medication. Once the preventative is started, the dog will need to be tested at 6 months, then annually thereafter. If there is a lapse in preventative medication, the dog will need to be tested before restarting the medication, then at 6 months, then annually after that. Also, testing is recommended when switching to a different preventative.    

When do cats need to be tested?

Heartworm infections are more difficult to detect in cats than dogs. Cats should be tested before starting preventative medication. Retesting should occur as recommended by your veterinarian. The frequency will depend upon your cat's risks for heartworm disease and their exposure to other heartworm infected pets.

How is heartworm infection treated in dogs?

If testing shows that your dog is infected with heartworms, a second confirmatory test is performed. If this is positive, additional studies or tests will be required. A complete physical exam will be performed to assess your pet. Also, additional blood work will be needed to gauge your pet's general health, blood count, liver and kidney function, and electrolytes. Chest x-rays may be needed to determine the severity of your pet's infection. In some cases, an ECG or echocardiogram of your pet's heart may be necessary. Once these studies are completed, a treatment plan can be created. This plan will vary depending upon the severity of your dog's infection, the symptoms your dog is experiencing, and your dog's medical history and condition.

Immiticide is the FDA approved drug of choice for killing adult heartworms in dogs. This medication is injected into the back muscles of the dog. This is given in one of two ways. The two-day series involves one injection a day on two consecutive days. The split protocol uses three doses. The first injection is given, then one month later two additional injections are given on two consecutive days. The split protocol is usually recommended due to its increased safety and effectiveness.

During the treatment period and for at least a month afterward, your pet's exercise must be restricted. The degree of restriction depends upon the severity of the infection. As the worms die and breakdown, there is an increased risk of fragments blocking blood flow to the heart and lungs. This can cause a blood clot to the lungs (pulmonary thromboembolism) and result in lung damage or the death of your pet. Increased activity or exercise increases the chances of damage to the heart and lungs. This is one of the most important things that you can do for your pet.

Other medications will also be needed. Doxycycline is used to kill bacteria (Wolbachia) associated with the adult heartworms. It is given twice a day for 4 weeks. With this bacteria eliminated, heartworm survival is decreased. The chance of breathing complications after heartworm treatment is also reduced.

The microfilaria, or baby worms, are killed with another medication. This treatment may need to be repeated in a month if repeat testing shows that the microfilaria have not been totally eliminated.

Steroids, such as prednisone, may also be given with decreasing doses over several weeks. They reduce inflammation and the chances of lung complications during and after heartworm treatment.

Treatment can be very successful for dogs with little to no symptoms. This is why early detection and treatment is important. Also, early treatment makes it less likely that the dog will suffer permanent lung or heart damage, which could result in heart failure. As the severity of the infection and symptoms increase, the chances of complications and damage rise. However, even in many of these cases, heartworms can be eliminated successfully leading to a better quality of life for the pet.

How is heartworm infection treated in cats?

To aid in diagnosis, your pet will undergo a complete physical examination as well as additional testing such as blood work and chest x-rays. Other studies may include an echocardiogram of the heart or an angiogram (injection of dye with x-rays to view the pulmonary arteries). Currently, there is no FDA approved treatment for heartworms in cats. This is why prevention is so important. However, your veterinarian can treat the symptoms from heartworm infection. Coughing, wheezing, and breathing difficulties  can be improved by steroid treatment. Supportive care such as oxygen, intravenous fluids, activity restriction, drugs to improve heart function, drugs to open the airways, and antibiotics may also be used. Unlike dogs, cats can sometimes clear the heartworm infection if given the supportive care they need, since the life span of the heartworm is only about two years in cats.

Heartworm disease is an infection that can have serious and sometimes fatal consequences for your pet. It can be prevented in nearly 100% of cases by just giving the appropriate monthly preventative medication in a timely manner.  Don't let your pet's health be at risk. Talk to your veterinarian today about your options for heartworm prevention to keep your pet active and healthy for years to come.

Sources:

AVMA, PetPlace, and American Heartworm Society.

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