By: Christen Woodley, DVM
What’s not to like about living in the sunshine state? Waves, sunshine, parks, beaches…mosquitoes?? We are blessed in many ways, but thanks to our long-lasting seasons of heat and humidity Florida is also host to many vector-borne diseases, the most famous of which in pets is heartworm disease. Heartworm disease has been diagnosed in all fifty states but is considered an endemic here. And though canines are represented in heavier numbers than felines, the rate of incidence in cats is often 5-15% of the dog rate in the area.
Mosquitoes are the vector for the transmission of heartworm disease in dogs and cats. It begins when a mosquito takes a blood meal from an infected host. The young worms pass through larval stages within the mosquito and then are transmitted to the dog and cat when the infected mosquito bites them. Once in the host, they molt through the last larval stage and into adults. Pets can test positive as early as six months after infection. For this reason, we place dogs and cats on heartworm prevention prior to six months of age, ideally as early as 8-12 weeks of age. Specifically for canines, we advise a heartworm antigen test annually, and sometimes even every 6 months for brief periods if switching preventative medication or if the dog has missed doses of prevention. Though temperatures can get a little cooler in the winter it is never cold enough here to eradicate the risk of mosquito infection, and therefore we do advise pets in Florida stay on year-round prevention. In an endemic area such as this, it is possible for an animal to become infected due to even one delayed or skipped a dose.
For dogs that do become infected with heartworms, immature and mature forms of the worm can subside in the heart and in the pulmonary arteries, vessels that connect the heart and lungs. This can lead to overload and inflammatory stress on the heart and lungs. If caught early your dog may have no outward symptoms, but as things progress we can see coughing, exercise intolerance, trouble breathing, a distended abdomen, lethargy, poor appetite, etc. Worm burden numbers can be small but in dogs are more often large. If caught before things have progressed too far heartworm disease is treatable in dogs with an injectable medication called Melarsomine. However, treatment is still not without risk and to be done properly is done over the course of months. It also requires exercise restriction for months, which can be difficult mentally on the dog and you as the owner. Not to mention the cost of treatment costs more than many people are likely to pay for heartworm prevention over the course of their dog’s entire lifetime. It is way better on all accounts to do the monthly preventative!
Yes cats can get heartworm disease!!!!! Even indoor cats!! Can anyone ever say that they have never seen a mosquito indoors? I think not! These ladies are NOT shy. Statistics say 25-30% of heartworm-infected cats are indeed indoor cats. Felines are a more inhospitable host and therefore most infections often only consist of one or a few adult worms if they survive to adulthood. For this reason reproduction of the heartworm is difficult if not impossible. Also, the worms most often don’t make it to the heart in cats so most of the sequela to feline heartworm infection involves the massive amount of inflammation that is set up in the lung tissue. This causes Heartworm Associated Respiratory Disease “HARD” which can often be difficult to distinguish from bronchitis/asthma. Interestingly enough clinical signs in cats can vary from coughing/ wheezing/ breathing difficulty, vomiting, neurological signs, weight loss, lethargy, and sudden death. Heartworm testing is the main way to distinguish these conditions. In cats, the most common heartworm testing is an antibody test, in contrast to dogs in which we use an antigen test. Antigen tests look specifically for a protein only found within an adult female worm, which is likely to be found in the larger worm burdens dogs can harvest. In felines with such small worm populations, there may not be an adult female and therefore an antigen test can come up falsely negative. An antibody test is more sensitive in that if it is negative that is reliable.
However, a positive antibody test can indicate a mature worm, an immature worm, or a past infection. The antibody test can remain positive for a long time after the exposure. In cats where heartworm disease is suspected, it is advised to run a combo antigen/antibody test. Treatment in felines is not the same as in dogs because the injectable medication Melarsomine is toxic to cats. Though there is no “treatment”, management is aimed at controlling the clinical signs if present, usually using a steroid such as prednisolone to help control the inflammatory response. Sadly some cats cannot survive the severe inflammation the worms set up in their system and do not survive.
As you can now see the effect of even one heartworm can be tragic, even in indoor only pets such as felines. Remember that one in four cases of feline heartworm disease occurs in cats that live indoors exclusively! Therefore on the advice of the American Heartworm Society, we do advise year-round heartworm prevention in our endemic area for canines and felines. There are many available products for dogs and cats that prevent heartworms and even combo product for heartworm and flea control. All heartworm prevention is prescription and available through your veterinarian. Please contact us to discuss which prevention may be best for your pet!