Heartworms in dogs and cats

Mosquitos are the main culprit behind the spread of the heartworm. When a mosquito bites an infected animal, it carries the undeveloped baby worms called microfilaria in it, which develop into infective larvae over a period of 10-14 days. 

If the same mosquito bites another animal, the infected larvae are deposited onto the surface of the skin.

Once the larvae seep inside a new body, they take approximately six months to develop into sexually mature adult heartworms. After these worms mature, they can live for about 5-7 years in dogs, and up to 2-3 years in cats. 

Within three months, the larvae migrate through the animal’s bloodstream and reach the blood vessels of the heart and lungs. These worms then damage the blood vessels, thus reducing the heart’s pumping ability, resulting in damage to both lungs and the heart.

Dogs are more prone to get heartworm disease. The worms easily mature into adults, mate and produce offspring in a dog’s body. If not treated, the disease can be fatal for dogs as the worms can rapidly increase from a single worm to a hundred.

Dogs with heavy worm burdens - especially those who have been infected for a long time or are very active often - are more likely to show symptoms of heartworm disease. Depending on the severity, the symptoms of heartworm disease in dogs could be divided into four classes:

• Class 1: The dog does not show any symptoms 
• Class 2: The dog may show mild to moderate symptoms such as an occasional cough and tiredness after mild activity. 
• Class 3: The dog may show severe symptoms like persistent cough and lethargy, breathing difficulty, reduced appetite, weight loss, frequent fainting.
• Class 4: At this stage, the worm burden is so high that blood flowing back to the heart is physically blocked by a large mass of worms. This stage is known as caval syndrome and it is life-threatening. It is marked with a sudden onset of laboured breathing, pale gums, a swollen belly due to excess fluid in the abdomen and dark bloody or coffee-coloured urine.

For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest X-rays.

You must take your dog to a vet as soon as you observe any of the above symptoms.

• Blood tests could be performed by your veterinarian to detect the presence of adult heartworm infection in your dog. 
• Antigen test is most commonly performed to detect the presence of adult female heartworms in dogs.
• The antibody test is also done to determine the exposure to heartworms.
• Further tests, such as chest X-rays and an echocardiogram (an ultrasound of the heart), could be done to prove the diagnosis, to evaluate the severity of the disease and to discover the best treatment plan for your dog.

It is much better to prevent heartworm infection than to treat it as there are substantial risks involved in treating a dog for heartworms.

• Your vet may ask you to hospitalize your dog in order to provide therapy, such as intravenous fluids, drugs to treat lung and heart symptoms, antibiotics, and general nursing care.
• The goal of your vet while treating a heartworm disease would be to kill the adult worms and microfilariae present in your dog as safely as possible. 
• Your dog may require additional medications to help control the body’s inflammatory reaction against the debris of dead worms that gets collected in the lungs.
• If your dog reaches to the stage of caval syndrome, then he/she would require quick surgical removal of heartworms. The surgery is risky, and even with surgery, most dogs with caval syndrome do not survive.

Heartworm infection is preventable. Here are a few things you can do to prevent heartworms in your dog:

• Several medications that are both safe and effective at preventing infection are available from your veterinarian. Heartworm medications are highly effective, but dogs can still become infected. Missing an administration date may have serious consequences. If you miss a dose, you should contact your veterinarian regarding administration of the medication.
• Heartworm prevention can be started for puppies under seven months of age even without an antigen test. But they should initially be tested six months after the first visit and yearly after that, to ensure that the puppies are not prone to heartworms.
• Adult dogs over seven months of age and who have not been on a preventive course need to go through antigen testing before starting heartworm prevention. They should also be tested six months after the prevention regime starts and yearly after that.
• Annual testing is necessary - even when dogs are on heartworm prevention year-round - to ensure that the prevention program is working.

Heartworm disease in cats is very different from heartworm disease in dogs. The cat is an atypical host for heartworms, and most worms in cats do not survive to the adult stage. Cats with adult heartworms typically have just one to three baby worms which don’t even mature into adulthood in most cases. 

This means that heartworm disease often goes undiagnosed in cats. Also, immature worms cause a great deal of damage in the form of a condition known as Heartworm Associated Respiratory Disease (HARD), which is a type of inflammatory lung disease (ILD). Moreover, medications used to treat heartworm infections in dogs cannot be used in cats, so prevention is the only means of protecting cats from the effects of heartworm disease.

Signs of heartworm disease in cats can be very subtle or very dramatic. 

• The symptoms may appear as coughing, asthma-like attacks, periodic vomiting, lack of appetite or weight loss. 
• Occasionally, an affected cat may have difficulty walking, experience fainting or seizures, or suffer from fluid accumulation in the abdomen. 
• Any of these symptoms could mean that your cat requires urgent medical care.

Diagnosing heartworm infection in cats is harder than in dogs because cats are much less likely to have adult heartworms than dogs.

• A series of different tests may be needed to help determine the likelihood of heartworm infection as the cause of your cat's illness and, even then, the results may not be conclusive. 
• In general, both antigen and antibody tests are recommended for cats to give the best chance of detecting the presence of heartworms. 
• Your veterinarian may also use X-rays or ultrasound to look for heartworm infection.

Unfortunately, there is no approved drug therapy for heartworm infection in cats, and the drug used to treat infections in dogs is not safe for cats. 

• If your cat is diagnosed with heartworms, your veterinarian may advice medications to give symptomatic relief or surgery to remove the heartworms.
• Most of the time, you may come across spontaneous clearing of heartworms in your cat but the damage they cause could be permanent. 
• If your cat has been detected with worms in the lungs, but does not present any signs of respiratory distress, then your vet may recommend chest X-rays every six to 12 months to ensure early detection.
• In the presence of mild symptoms, small doses of prednisolone may be administered to the cat to help reduce inflammation.
• Your veterinarian may recommend hospitalization in severe cases to provide therapy, such as intravenous fluids, drugs to treat lung and heart symptoms, antibiotics and general nursing care. In some cases, surgical removal of heartworms may be possible.

Prevention is critical as there is no approved treatment for heartworm infection in cats.

• Cats that live in colder areas, where mosquitoes are seasonal, should be on monthly preventative for at least six months of the year. 
• It is important to give your cat monthly heartworm preventatives throughout the year, which are available in both spot-on and pill form. 
• Preventatives keep new infections from developing if an infected mosquito bites your cat.

Ferrets, even those kept indoors, are also at risk of heartworm infection. The rate of development is rapid in the case of ferrets. Even a single worm can cause serious disease in a ferret which could be fatal.

Signs and symptoms:

Ferrets with heartworm disease have decreased activity level, coughing, trouble breathing, and overall weakness. Heart failure can occur in severe cases.  

Diagnosis:

A veterinarian may perform chest X-rays and an ultrasound of the heart to determine if your ferret has heartworm disease.

Treatment:

Unfortunately, there are no specific drugs to cure heartworm disease in ferrets. Medications could be given to treat the symptoms.

Prevention:

A medication containing imidacloprid and moxidectin as active ingredients could be given by your vet to prevent heartworms in ferrets. It is a topical solution that is applied monthly, throughout the year, onto the skin of ferrets.