Dirofilariose em cães: sinais, tratamento e prevenção

How heartworm spreads

An infected dog carries tiny immature worms, called microfilariae, in its blood. When a mosquito bites that dog, it takes some of them in. In warm weather they develop into infective larvae inside the mosquito in about two weeks. When the mosquito bites another dog, the larvae enter through the bite wound. They travel through the body to the lung arteries and grow into adult worms. This takes about six months. Female worms can reach about 12 inches (30 cm) in length, and adult worms can live for five to seven years in a dog.

Dogs cannot pass heartworm directly to each other. A mosquito is always needed. Still, an infected dog with microfilariae in its blood is a source of infection for local mosquitoes, so treating infected dogs also helps protect other dogs in the area.

Cats and people

Cats can be infected by the same mosquitoes, though far less often than dogs. In cats, even one or two worms can cause serious breathing problems and sometimes sudden death. There is no approved treatment to kill adult worms in cats, so prevention matters even more for them.

People cannot catch heartworm directly from a dog. In rare cases an infected mosquito passes larvae to a person. The larvae usually die before they mature, but they can leave a small lung nodule that may look like a tumor on an X-ray.

Where heartworm is common

The larvae need warm weather to develop inside mosquitoes, so heartworm is most common in warmer regions. In Europe it is well established across the south, including Greece, and in Bulgaria, Romania and Slovenia. A recent survey also found it in dogs in Serbia and North Macedonia. In a study of more than 1,700 pet and stray dogs from ten Bulgarian cities, about 13 percent tested positive. The parasite is now spreading north, helped by a warmer climate and by dogs that travel or are adopted across borders.

A related species, Dirofilaria repens, is found in many of the same areas. It lives in the tissue under the skin. In dogs it often causes no signs, or only small, painless lumps. It matters mainly because it is the most common cause of Dirofilaria infection in people in Europe. In people it can cause a lump under the skin or a worm in the eye. The monthly preventives used against heartworm also help protect dogs against this worm.

Sinais a observar

Many infected dogs show no signs for months, especially when only a few worms are present. Signs usually develop slowly as damage builds up in the lungs and heart. Very active dogs tend to be affected more severely. Watch for these changes.

• A mild cough that does not go away
• Tiring quickly or not wanting to exercise
• Weakness or fainting after exercise or excitement
• Faster or harder breathing
• Reduced appetite and weight loss
• A swollen belly from fluid buildup, in advanced disease

Any of these signs is a good reason to see a veterinarian. This is even more true for a dog that has lived in or traveled to an area where heartworm is common.

Quando é uma emergência

Heartworm disease can suddenly become severe. When a large number of worms move into the right side of the heart and block blood flow, a condition called caval syndrome develops. It is more common in small dogs and is life-threatening. Vets may try to remove the worms surgically through a large vein in the neck. Dead worms can also block blood vessels in the lungs, either when they die naturally or during treatment. Contact a veterinarian or an emergency clinic right away if you see any of these signs.

• Sudden labored breathing or gasping
• Pale gums
• Urine that looks dark red, brown or like coffee
• Coughing up blood
• Collapse

How vets diagnose it

Vets check for heartworm with blood tests. The most common test detects a protein released by adult female worms, and many clinics can run it in minutes. A second test looks for microfilariae in a blood sample. Not every infected dog has microfilariae, so a negative result on that test alone does not rule out heartworm.

Both tests miss very recent infections. They usually cannot detect an infection that is less than about five to seven months old, because the worms are not yet mature. A dog that tests negative soon after being exposed may still be infected.

When a test is positive, vets usually confirm it with a different test. Chest X-rays, a heart ultrasound and routine blood work then show how much damage the worms have caused. This helps the vet decide how to treat the dog safely.

How treatment works

Treatment is done under close veterinary care and usually takes several months. The plan depends on how sick the dog is, so the details vary. Exercise is restricted from the day of diagnosis, because physical activity speeds up the damage the worms cause. Dogs with severe disease may first need medicines to stabilize the heart and lungs.

A common approach begins with a heartworm preventive to kill young larvae, together with a month-long course of an antibiotic. The antibiotic targets bacteria that live inside the worms. This weakens the worms and reduces inflammation when they die. About a month after the antibiotic course ends, the vet starts a series of injections into the muscles of the lower back to kill the adult worms. The injections are usually spread over one to two months. An anti-inflammatory medicine is usually given as well.

The weeks after the injections carry the highest risk. As the worms die, fragments can lodge in the lung vessels. Strict rest lowers this risk. That usually means a crate or small room and short leash walks for bathroom breaks only, for about six to eight weeks after the last injection. Some months after treatment ends, the vet repeats the tests to confirm the worms are gone.

Some owners ask about using monthly preventives alone to let the adult worms die slowly. Heartworm specialists do not recommend this as a first choice. It can take a year or longer, and the lungs and blood vessels keep getting damaged in the meantime. Your veterinarian can explain the options for your dog.

How to prevent it

Heartworm is far easier and cheaper to prevent than to treat. Prescription preventives kill the larvae a dog picked up in the previous month, before they can grow into adults. They come as monthly tablets or chews and monthly spot-on products. In some countries there is also a long-acting injection, given by the vet to dogs older than six months. Many of these products also control some intestinal worms. There is no vaccine against heartworm.

In most parts of Europe where heartworm occurs, mosquitoes spread it from about spring to late autumn. European guidelines advise starting prevention before the mosquito season and continuing until late autumn. For southern Europe they suggest protection from May until the end of November, and all year in areas where heartworm is very common. Some vets advise giving it all year anywhere, because the mosquito season changes from year to year and a routine is easier to keep. Ask your vet what fits the area where you live.

Preventives do not kill adult worms, so dogs older than about seven months are usually tested before they start. Giving a preventive to a dog with microfilariae in its blood can also cause a reaction. Puppies under seven months can usually start without a test and are tested later. A yearly test is recommended even for dogs on prevention. Mosquito repellent products may add some protection, but they do not replace a heartworm preventive.

Advice for adopters

Dogs adopted from southeastern Europe may have been bitten by infected mosquitoes before adoption. At Paw Squadron, the first stops after every rescue are the vet and a bath, with a checkup, deworming and treatment against fleas and other parasites. Before adoption, every animal gets a checkup, treatment, vaccinations and a microchip. Routine deworming is aimed at worms in the gut and is not a test for heartworm. A recent heartworm infection also does not show on a blood test for several months, so follow-up testing after arrival is still recommended.

European guidelines suggest that dogs with an unknown history that leave an area where heartworm is common receive two months of heartworm prevention, then have blood tests six and twelve months later. Mention where your dog came from at the first vet visit, and ask which tests and which prevention plan your vet recommends. If you live in or travel to an area where heartworm occurs, keep prevention going through the mosquito season or all year, as your vet advises.

If you have questions about a dog you would like to adopt, send us an adoption inquiry. You can find more pet care guides in PawPedia.