Within minutes of walking outside last week to assess how my fall garden was doing, I (Perry Jameson) was harassed by mosquitoes. There were so many and they were making me so uncomfortable I had to rush indoors to get away from them. All of the rain from Irma, along with our warm late-summer heat, has produced the perfect breeding grounds for them. And they are out in full force.
As a veterinarian, the first thing I thought about was how all of these mosquitoes would be transmitting heartworms from infected pets to dogs and cats. The more mosquitoes the more likely your pet (and mine) will be exposed to an infected one.
When a mosquito bites an infected dog, it ingests heartworm larvae circulating in the dog’s blood. When this mosquito goes to feed on another dog or cat, it will inject these larvae under the skin. The larvae grow in here for one to two months before entering the blood stream. Over the next four to five months, they continue to mature and travel usually to the blood vessels in the lungs.
Once established they remain here for the rest of their lives (about 6 years). At around 6 months the adult worms begin releasing larvae into the blood stream. When a mosquito bites and feeds it becomes infected and can now infect another cat or dog.
The blood vessels in the lungs are the main location where the adults live. The infected animal’s immune system tries to fight off the parasite. In dogs they are usually unsuccessful but this immune response is what produces the clinical symptoms. Most commonly, coughing and labored breathing like you would see with asthma.
If the worms remain present long enough the chronic inflammation will begin to cause the blood vessel walls to thicken. This can produce a condition called pulmonary hypertension. Here the pet has high blood pressure in the arteries in the lungs even though the systemic blood pressure is normal. Over time the heart muscle will become stressed having to force blood into this high-pressure system resulting in heart failure.
Heartworms in this location are best treated with a medication called Immiticide. This drug is very effective at killing the adult worms. The biggest concern is, as the worms die, there is the potential for them to lodge in the lung’s vessels, obstructing them. This can result in labored breathing, fevers and in the worst cases death. Strict exercise restriction is recommended for the month following therapy to reduce the likelihood of this happening.
An alternate therapy is the “slow kill” method. This is where ivermectin is given and the adult worms are slowly killed over time (months to years). While this method is much less expensive than Immiticide, I do not recommend it. The dog is chronically infected, allowing for the inflammation to persist, which can permanently damage the heart and lungs.
The lungs are the primary location for infections; however, worms can end up in other vessels. Almost 20 years ago, Dr. Bianucci had a patient in which the worms had lodged in the arteries supplying blood to the dog’s back paws. The blood supply was obstructed, causing the tissue to die. This dog had learned to shift all of its weight forward and could walk with just his front two legs. Dr. Bianucci was able to amputate the dead rear paws and this dog was, fortunately, able to continue having a good quality of life.
The heart is another location where the adult worms may lodge themselves. When this occurs they are living in the large veins that carry blood to the right side of the heart. With all these worms in this location most dogs will develop symptoms of right heart failure, with the most common being accumulation of fluid in the abdominal cavity. These dogs are often anemic as a result of the worms in the heart and large veins damaging red blood cells as they pass through.
This type of infection occurs when dogs have a large number of worms and often have been continually infected over a long period of time. At this stage, medications to kill the worms cannot be given. The only treatment is to surgically extract the worms from the heart.
The dog is placed under general anesthesia and an incision is made into the jugular vein of the neck. With the aid of fluoroscopy (moving radiographs) a device is passed into the jugular vein down to the heart and the worms are extracted one to two at time. It is impossible to get them all, as most dogs also have worms in the lung vessels, but just removing some will allow the anemia and heart failure to resolve. Once recovered from surgery they then have to be treated medically to kill the remaining worms.
Of course, these patients are not the ideal candidates to be going under anesthesia. In my experience about half survive worm retrieval and half do not.
Cats are less commonly infected and the worms only live about one to two years. However, they can develop similar symptoms as dogs. The frustrating problem is they do not tolerate the Immiticide. Controlling their symptoms until the worms die is the only therapy.
Like most illnesses, prevention is the best medicine. There are many options available for prevention, from every six-months injections, monthly oral pills to topical drops. It is much less expensive (both financially and more important emotionally) to prevent heartworm disease than to treat once infected.
Dr. Henri Bianucci and Dr. Perry Jameson are with Veterinary Specialty Care LLC. Send questions to firstname.lastname@example.org.