Visiting Vet: Heartworm sneaks through

There are not tests available for every stage of the heartworm.

Courtesy American Heartworm Soci

Sunflower is a typical rescue dog, transported here from Texas as a 10-week-old puppy by a pet adoption group. She arrived with six littermates, and all the paperwork showing puppy shots, fecal tests, health certificates, and doses of heartworm preventative. At 14 weeks old, she was adopted, and her new mother brought her in for an introductory visit. Sunny seemed healthy enough. We continued her puppy shots, adding Lyme and rabies vaccination. A fecal sample determined that those intermittent soft stools were the result of infection with a protozoal parasite called giardia. We treated that, arranged for her to be spayed when she reached the appropriate age, and sent her on her way with a 12-dose pack of monthly Heartgard pills to prevent heartworm infection and a cheerful, “See you next year.”

Here it is … next year. Sunflower has doubled in size since last fall. She’s not quite as happy to see me as I am to see her, but I’m used to that. “Relax,” I told her. “This is just a routine visit.” I didn’t tell her she was due for four vaccines, and the annual in-house blood test called a 4DX we recommend to screen for Lyme, anaplasmosis, ehrlichiosis, and heartworm. Sunny tolerated the blood draw, but she’s a scrappy little flower, and took some convincing to settle down for the other injections. Rabies first. Then distemper-parvo. Then kennel cough. That one’s an easy oral vaccine. Finally Lyme. I always do Lyme last, waiting until the 4DX is done to be sure my patient does not have Lyme disease before giving the vaccine. Sunny didn’t have Lyme. She had heartworm.

Heartworm. We don’t see a lot of it here in the Northeast. It’s a parasite carried by mosquitoes. Here’s the life cycle. It starts with a “host” dog or coyote who is already infected. Mr. Host has adult heartworms residing in his heart and/or pulmonary arteries. We’re not talking little worms here. Adult female worms can be 14 inches long. An infected dog can have 25, 50, even 100 adult worms inside. If both male and females are present, they reproduce. Heartworms do not lay eggs. They have live young, dumping hundreds of tiny baby worms called microfilaria, or L1 larvae, into the blood. These L1 larvae cannot infect another dog directly. They have to go through Mr. Mosquito.

So Mr. Mosquito bites Mr. Host dog, and ingests L1 larvae in the blood meal. Inside the mosquito over several weeks, L1 larvae molt to L2, then L3. L3 larvae are the ones that can infect the next dog. Now here’s one reason we don’t see as much heartworm up north. If the temperature drops below 57°, mosquitos die. If Mr. Mosquito dies, that breaks the chain of transmission of heartworm. In sunny Texas, however, Mr. Mosquito never dies. He bites the next dog, say a puppy like Sunflower, who then gets infected. Have I lost you yet? Hang on, we’re almost done. The L3 larvae hang out in Sunny’s skin for a while, molting to L4, then L5. At the L5 stage, the larvae enter the bloodstream and eventually go to the heart and lungs and mature into adults. (If you’re not squeamish, there’s a cool animated version of this on Youtube: 

How is it possible that Sunflower, whose Massachusetts mom has been giving monthly heartworm pills all year, has heartworm? Because preventive medications work on a very limited part of the worm’s life cycle, specifically killing only L3 and L4 larvae in the skin. Once those little critters get to the bloodstream and molt to L5, the preventive medications no longer affect them. I’ve seen this happen before. A very young puppy down South gets infected. By the time that pup is started on preventive medication, the parasite has already gotten to the L5 stage. Too late to stop the infection.

Why didn’t I test Sunflower when she first arrived from Texas? Because we have two types of heartworm tests for dogs, and both require the presence of adult female worms. The 4DX test looks for antigen from the surface of the adult female worm, and it takes six to nine months from the time the mosquito infects a dog with L3 larvae until adult female worms are present. There is no way to test for L3, L4, L5, or immature worms, or even male adult worms. When Sunflower tested positive on the 4DX test, I was shocked, but also knew we occasionally get false positive results. I needed to confirm with a second test. I started with the simplest. I put a drop of her blood on a slide, added a cover slip, and examined it under the microscope. At the very first glance, I had confirmation: a microscopic microfilaria wiggling away in that drop of blood. Creepy.

The American Heartworm Society still recommends a specific treatment protocol. Radiographs to assess the patient’s overall condition. A month of doxycycline (because of a bacteria called Wolbachia; just take my word for it, it’s important), then a series of two to three painful injections in the back muscles of an arsenical medication. The dog absolutely must have severely restricted exercise throughout and after treatment to try to avoid life-threatening complications like a pulmonary embolism. 

While reviewing the current literature, I came across a treatment protocol that was new to me, called Mox-Dox, which uses a combination of oral doxycycline and topically applied monthly parasiticide poured on the back and absorbed through the skin. Mox-Dox is not the gold standard at this time, but the research seems promising, and several reliable veterinary references suggested it as a reasonable alternative. I had not been looking forward to giving Sunflower those painful injections … and Mox-Dox is considerably more affordable. We’ve decided to try this method on Sunflower. She’ll still have to stay on a leash for many months, but I hope she will tolerate it well, and eventually come to be happy to see me.