Heartworm is a severe and potentially fatal disease that affects pets (and in rare cases humans), in many parts of the world including Australia. It is caused by blood-borne parasites that infect the heart, lungs and associated blood vessels, causing organ damage and failure. Heartworm disease affects dogs, cats and other mammal species, such as ferrets, foxes, and dingoes.


Heartworm in dogs

Heartworms that live inside dogs grow into adults, mate and reproduce. Dogs are ideal host for heartworm, so when untreated heartworm numbers increase, and it is possible for dogs to harbour large numbers of heartworms which live in the major blood vessels from the heart to the lungs. The damage caused by heartworms is long lasting and can have a significant impact on a dog’s long-term health and quality of life. Prevention is therefore of utmost importance, and if heartworm disease is detected then prompt treatment is essential.


Signs of heartworm in dogs

The severity of heartworm disease is determined by the number of worms present in the dog, how long the dog has been infected with heartworm, and how the dog has responded to the infestation.  Heartworm disease is categorised as either asymptomatic, which means that dogs either do not exhibit visible symptoms or the symptoms are minimal, moderate, where dogs usually become intolerant to exercise and develop a cough - and may have sustained some organ damage which is diagnosed with blood tests - and severe, where in addition to exercise intolerance, dogs can faint and suffer chronic heart, kidney, or liver failure. When severe, heartworm can also cause laboured breathing, hypertension (high blood pressure), and tachycardia (fast heartbeat).


Heartworm in cats

Heartworm disease is quite different in cats than in dogs. While dogs are the ideal host for heartworm, cats aren’t, and in cats heartworms usually do not survive long enough to become adults. Also, the numbers of heartworms that infect cats are generally fewer, and may be limited to one or a few, compared with dogs who can potentially harbour a hundred. Many cats carrying heartworm may not harbour any adults, but immature heartworms can wreak havoc nonetheless and cause serious respiratory disease. The medication that is used to treat dogs is not suitable for cats, and as there is no approved drug therapy, heartworm prevention is imperative, as it is the only way to protect cats from the disease.


Signs of heartworm in cats

Signs of the disease in cats can also range from mild to severe and include loss of appetite and weight loss, coughing, and vomiting. Fainting, seizures, or difficulty walking may also be symptoms, as well as fluid accumulating in the abdomen. The disease is often not detected in cats until the animal is seriously ill and the first signs may be collapsing, or death.


Some dogs and cats develop a reaction similar to an allergy, which causes a variety of symptoms similar to asthma. It occurs more often it cats and can cause death, even when the infection is from only a few heartworms.

How is heartworm disease contracted?

Heartworm's lifecycle provides an insight into why prevention of the disease is so important. It starts with the disease, microfilaria, circulating in the blood of the host animal. A mosquito when it bites the infected animal, consumes the circulating microfilaria in the blood. Once inside the mosquito’s body, the microfilaria starts to develop into larva which ultimately migrate to the mosquito’s salivary glands. When a mosquito bites an animal the larvae burrows into the host animal via a small bite wound. It’s virtually impossible to keep mosquitoes out altogether, therefore both indoor and outdoor pets are at risk.


Once inside an animal it continues to develop, and over the next six to seven months becomes sexually mature. It’s not until this point that the infection can be detected by a heartworm test. The larva works its way through the host’s tissues all the way to the heart from approximately two and a half months after initially entering the animal. Most heartworm larvae get to the host’s heart within three months. This is where they remain, and grow quickly in size and length. The heartworm continues to live in the heart until it dies, usually between five and seven years.


As well as being long living, heartworms can grow to surprisingly long lengths

Heartworms continue to grow in size after becoming sexually mature adults at which point the females start passing microfilaria into the blood of the host animal. This is the reason some animals become infected with numerous heartworms. They can become twisted masses of spaghetti-like infections and consequently block the normal flow of blood, causing pressure to build. Adult females can grow up to 35cm, whereas males are usually shorter, and it takes fewer heartworms to cause problems in smaller animals.


Diagnosis of heartworm disease and prognosis for infected animals

Being a serious and progressive disease, the earlier heartworm is detected, the better the chances of recovery for an infected animal. As there are few early signs of disease when a dog or cat is infected – and in some cases no signs at all – blood tests conducted by a vet are necessary to in order to detect the presence of heartworm. If tests are positive, further tests may be ordered, to identify signs of damage caused by the disease.


Heartworm infection is harder to detect in cats than in dogs, because they are less likely to carry adult heartworms. Screening of cats involves tests to detect exposure to heartworm larvae. X-rays or ultrasound may also be used to identify heartworm infection.


For those dogs with mild to moderate cases of heartworm infection, the prognosis is good. However, dogs with more severe cases can experience complications from the treatment, which involves strong medication to kill serious infestations.


Just one worm can make a cat very sick. Despite there being no approved drug treatment, cats with heartworm disease can often be assisted with veterinary care and the goal is usually to stabilise the animal implement a long-term management plan. Surgical removal of heartworms may be possible in some cases.


The good news is that heartworm is preventable

A prophylaxis (preventative) medication recommended by a vet can prevent heartworm. There are a variety of medications which include pills, spot-on topical medications, and injections. Oral and topical products must be used monthly, and the injectable versions last for six months or a year depending on the age of the patient when given. They work by eliminating the larval (immature) stages of the heartworm parasite.


In as little as 51 days, heartworm larvae can mature into adulthood, at which point they cannot be eliminated by preventives. This is why it is critically important that heartworm preventives are administered strictly on schedule. Late medication can allow immature larvae to mature, or “molt”, and when lapses in medication occur, heartworm starts to spread. Some pet owners prefer annual injections to avoid the risk posed by missing a scheduled monthly treatment.


Prevention is always better than cure

People sometimes think that if their pet doesn’t mix with other animals, then they can’t catch heartworm disease. This of course isn’t true, because the disease isn’t spread directly from animal to animal, but rather by mosquitoes, and it’s impossible to tell whether a mosquito is infected. Numerous local cases of dogs who have tested positive to heartworm suggests that complacency is dangerous.


Heartworm is easily prevented but expensive and complicated to treat, and treatment for heartworm is not always successful. Don’t put your pet’s health at risk; Make sure they have appropriate and consistently administered heartworm preventative medication. Any of the team at Vets4Pets can help you chose one that is suitable for your pet. We also carry a range of approved medications in our online store. If you have any questions about prevention, testing, or treatment, don’t hesitate to get in touch with any of our hospitals.