Last week at our hospital we treated two dogs for heartworm disease. It seems like this year we treat a dog every other week for heartworms. This is a big jump—we used to treat one or two cases a year. So, I took a look back in our records to see what the actual numbers are for heartworm treatment at our practice.
We heartworm test about 3,000 dogs a year with a blood sample for the adult worms. Here are the results, for positive tests: In 2016, 4 dogs; in 2017, 8 dogs; in 2018, 7 dogs; in 2019, 15 dogs; so far in 2020, 23 dogs. That is a significant upward trend in our area. What accounts for the tripling of cases in just a few years?
A few basics about heartworm disease. Mosquitoes transmit the disease from dog to dog (also fox and coyote). The mosquito bites the dog with the infective larvae (microfilaria) and takes in these baby worms with the blood meal. The microfilaria then develop into the infective stage within the mosquito. When the infected mosquito bites another dog, the microfilaria are injected into this dog. In about six months the microfilariae will develop into adult worms and live in the heart. They can grow to be a foot long and live for five to seven years. Because these worms live so long there can be many different ages and stages living in one dog. This is especially the case in the southern states where mosquitoes are very active all year long.
Severity of the illness depends on how many worms are in the heart and associated blood vessels. Dogs can have mild infections if there are just a few worms. Heartworm positive dogs can also have severe respiratory distress, lethargy and a chronic cough. It is recommended that all dogs have a blood test for adult heartworms annually and that owners give a monthly prescription medication to prevent the disease.
It is estimated by the American Heartworm Society that there are about 1 million dogs that are positive for heartworm in the U.S. The hotspots are in the south, especially Mississippi where 10 percent of the dogs test positive. After hurricane Katrina, 250,000 dogs were adopted and shipped all over the United States. A large number of them were infected with heartworms. This was the beginning of the migration north of heartworm disease. Climate change is also contributing to the increased incidence of heartworm in our area. Mosquitoes can be blown great distances from the high winds of tropical storms. The mosquitos themselves are also adapting to colder climates in the north and can survive indoors.
Most of the cases we diagnose in Granby are from newly adopted dogs from the south. How are these puppies and dogs sneaking in here with heartworms? It appears that most rescue groups blood test for heartworm before shipping them north. The tricky part is that it is important to test twice. Once in the shelter and again six months later to be sure the pet is truly negative. This is because it takes six months for the worms to mature into adults. We have diagnosed many heartworm positive dogs from the south because the second test never happened. It makes sense that heartworm disease is more endemic in Granby with warmer winters, more foxes and coyotes among us and lots of rescue dogs.
Treatment for heartworm is a big deal. First, it is important to confirm the disease and have a medical workup to decide if the disease process is advanced. Then there is a protocol to follow that includes antibiotics, steroids, and strong, sometimes painful, intramuscular injections. Exercise restriction is very important because the dying parasites shower into the lungs and cause further pathology. It takes nine months to complete the treatment when a final test is done to confirm that the worms have been eliminated. Then it is back to administering the preventatives each month.
Some may ask why test annually if you give the heartworm medication each month. These medications are highly effective but nothing is 100 percent. You and I can easily skip a dose or give it late. The dog may vomit up the medication outside. It is very important to test every year to be sure not to miss a positive dog and have the disease progress. Most veterinarians will not prescribe the monthly preventatives without a negative test.
The most important things to remember is to test your rescue dog from the south by following your veterinarian’s recommendation. Also, heartworm test your dog each year and give the preventive medication all year long. Just because we are coming into winter is no reason to discontinue the medication..