Heartworms do not lay eggs like other worm parasites; instead they give live birth and the baby worms are called Microfilariae. Microfilariae are released into the circulatory system in hope that they will be slurped up by a mosquito taking a blood meal and carried to a new host.
Heartworm disease, as the name suggests, are worms that infect the heart. In Australia, the prevalence of heartworm disease is highest in subtropical/tropical areas (Darwin up to 100%, Brisbane 34%) and northern temperate areas (Sydney 25%). Dogs are the most important hosts, but cats may also be susceptible.
Heartworm prevalence in Australia
Queensland and New South Wales are considered to be areas of higher risk whereas rates in South Australia are low (4,5). Dogs and cats living in rural areas are more at risk of infection than their urban counterparts (6).
Cats, however, are not ideal hosts, even though they can develop a heartworm infection. That means your dog and wild canines act as the greatest reservoir of adult heartworms. Inside your pet's pulmonary artery, the female heartworms give birth to tiny larvae (L1) called microfilariae.
The infective larvae enter the dog's body when the mosquito bites the dog. These infective larvae migrate into the bloodstream and move to the heart and adjacent blood vessels, maturing to adults, mating, and reproducing microfilariae within 6 to 7 months.
Heartworm is spread by mosquitoes that bite an infected host and then pass the parasite to another host during a blood meal. Aedes, Anopheles, and Mansonia species of mosquito are all capable of transmitting heartworm.
After treatment, the adult worms die and are carried by the blood to the lungs where they lodge in small blood vessels. There they decompose and are absorbed by the body over a period of several months.
When the infected mosquito bites another dog, the mosquito spreads the infective larvae to the dog through the bite wound. In the newly infected dog, it takes about 6 to 7 months for the infective larvae to mature into adult heartworms.
While the risk of heartworm is more prevalent in spring and summer when there are more mosquitos, a pet can get heartworm any time of year.
In 6 to 7 months, these infective larvae migrate inside the dog, eventually reaching the heart and vessels of the lungs, where they continue to grow to full maturity. The mature adult worms produce microfilariae of their own, which are available in the dog's blood to infect other mosquitoes.
How do dogs get heartworm? Just like malaria, heartworm is spread by mosquitoes. When an infected mosquito bites your pet, it injects baby heartworm larvae (known as microfilaria) into the blood.
Urban areas such as Seattle Boise, Idaho in the Northwest; Bismarck, North Dakota in the Upper Midwest; and Tucson, Arizona, in the Southwest also saw significant increases in heartworm rates.
The most convenient and effective heartworm prevention is a yearly injection administered by your veterinarian. This can be given from 12 weeks of age, then boosted at 6 months.
Heartworms can infect your pet year-round and preventing them is much easier, and healthier, for your pet than getting heartworms in the first place or treating heartworm disease afterward. Year-round prevention is key to keeping your pet heartworm free.
If you forget a month of preventative, make sure to give the next 2 doses on time to "catch up" for that missed month. If you miss several months in a row, you should give your vet a call for advice on when to plan for the next heartworm test and how to get started back on preventatives.
Symptoms of heartworm in cats
Heartworm was once considered endemic to South Australia; it's now believed that this is not the case. In fact, heartworm is known to be more prevalent in tropical states with warmer climates.
Signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen.
If given the right care and medication, the majority of dogs can recover from heartworm disease and enjoy a healthy, long life. However, the survival rate for dogs with heartworm infection is incredibly poor in the absence of therapy or treatment.
Rarely, cases may be so advanced that it is safer to treat organ damage and keep the dog comfortable than it is to risk negative effects associated with killing the heartworms. Dogs in this condition are not likely to live more than a few weeks or months.
You should know that although heartworm can be fatal, it is very treatable if caught early. And even more importantly, your veterinarian can prescribe preventatives that are highly effective in stopping your dog from getting heartworm in the first place.
Dogs are typically heartworm tested prior to transport, but due to the life cycle of the heartworm, there is a window where dogs may test negative but still have heartworms. Heartworm in these dogs may go undetected for several months or more, allowing them to serve as a source of infection for other dogs.
Is heartworm detection/diagnosis immediate? No, heartworms are undetectable until they have finished maturing. Because of this, it can take 6-7 months before modern vet tests detect that your pet has been infected. However, they can live for years and reproduce inside the animal.
Prognosis: heartworm treatment success rates
With the three-dose adulticide protocol described above, in conjunction with doxycycline and macrocyclic lactones as recommended by the American Heartworm Society, 98% of dogs will be cleared of heartworm infection.
Myth #3: If my pet has heartworms, I will see them in her feces. Although many worm types, such as roundworms and tiny hookworms, are shed in your pet's feces, heartworms do not live in the gastrointestinal (GI) tract, and are not found in feces.
No one wants to hear that their dog has heartworm, but the good news is that most infected dogs can be successfully treated. The goal is to first stabilize your dog if he is showing signs of disease, then kill all adult and immature worms while keeping the side effects of treatment to a minimum.