Heartworm Prevention and Testing
Over the last few years, we have seen a steady increase in the number of heartworm cases in our canine patients. Previously, they were all pets that had been adopted from southern states, and not been on monthly prevention prior to their rescue. However, in the last two years, we have started seeing dogs with heartworm disease that have never left the New England area. We suspect part of the reason for local dogs becoming infected is that we have a larger fox and coyote population. They are wild canines, that all carry heartworms in their blood, and are seeding the mosquitos, that then infect local canine pets. Another reason is likely that Spring weather is arriving sooner, and Winter frosts later, allowing for a longer mosquito season. The last reason, and the only one we can do anything about, is that because traditionally, New England has not been an epidemic are for heartworm disease, many caretakers are not vigilant about making sure their pets get their prevention every month. All it takes is for your pet to miss two doses in a row, and any larval worms that have been injected into their blood stream by mosquitos, will become adult worms living in the dog’s heart. Since it takes 6-8 months for the larval worm to grow into an adult, a dog will not test positive until 6-8 months after his or her initial infection, or just under a year from when the caretaker missed the dose.
In the past, we have been flexible with authorizing heartworm prevention to animals that may have missed a month here or there, because the risk of actual infection was very low. However, with recent issues with on line pharmacies allowing refills that have not been authorized and recent upticks in the number of cases we are seeing, we are being much more careful about prescribing heartworm prevention only when we are sure your pet is not at risk for having the infection.
Monthly heartworm only kills off the larval worms for the first 4 -6 weeks after initial infection and it has NO activity against larval worms older than 6 weeks. It takes about 8 months for those larvae to grow into adult worms living in the heart. The heartworm test ONLY detects adult worms, it does not detect the larvae. When you give heartworm prevention, the process of the larval worms dying stimulates an immune response in your dog that can lead to life threatening allergic reactions. The risk of reaction is greater, the more larval worms circulating in your dog’s blood stream. If you miss more than 2 doses of heartworm prevention, it is possible that enough larval worms will survive to become adult worms, making more larvae to circulate in your dog’s blood stream, increasing the risk for an allergic rection if you start prevention without testing.
For example, in December 2019 your Dog tests negative for heartworm, but you do not start prevention until April because it was a cold winter. However, there were a few warm days in February where the mosquitos came out, and your dog became infected. In November of 2020, your dog is tested again, and is still negative because the larvae haven’t matured enough to be detected by the test. At this stage, your dog is infected with maturing larvae. These larvae are too young to give a positive on the test, but they are also too young to be making baby worms, so it is still SAFE to give the prevention at that time. By December, they are full fledged adults and they are now making larvae that are starting to build up in your dog’s blood stream. IF you started the prevention in November and continued it monthly from there on, the preventative would kill off the larva as fast as the adults make them, and your pet would be unlikely to have a reaction, but would test positive the following year and require treatment to kill off the adults.
However, if you chose again not to start the prevention until the following Spring, the adults would have had 4-5 months to make babies that build up in your pet’s blood stream. Giving heartworm prevention at that point would involve a risk for an allergic reaction as those baby worms die off. Admittedly, in the Northeast that risk is still relatively low, and significantly lower than it is in Southern states. However, the incidence is higher than it was just 5 years ago, and significantly higher than it was 10 years ago, which is why testing policies are changing. We do not want to do anything that puts your pet at risk.
So, once your dog tests negative for heartworm, start the prevention right away, and keep him or her on it year-round. That way, you will only need one test a year. We will still ask that you test yearly for two reasons. First and most importantly, the 4dx test we use to screen for heartworm not only tests for adult heartworm infection, it tests for 3 different types of tick born disease, which are epidemic in this area and not completely preventable even with tick prevention and Lyme vaccination. Secondly, no one is perfect, and it is pretty easy to miss a couple of months without realizing it. I do it myself, often.
IF we can not see a record of either selling or authorizing a prescription for heartworm prevention in a way that shows that your pet has likely been on prevention close to year-round, we will ask that your pet be tested prior to approving a prescription request. This is for your pet’s safety. We also will not approve any refills on medications through the pharmacy. Too often we are finding that the pharmacy ‘s put pets on auto delivery and send requests either when you have not asked for them, or have refilled requests from you, when we have not authorized those requests. We also will only authorize request where there is either a fax or prescription paper trail, unless you are using our VETSOURCE on line pharmacy. The reason we make an exception for our Vetsource account, is that they sed a digital document directly to your pet’s medical history when you make a prescription request, when we authorize it, and once it is actually delivered. We do not have the software that allows a paper trail for the digital prescriptions with other on-line pharmacies.
Please keep in mind, every single day, we have over 100 prescription requests. Each has to be approved by a doctor to make sure the medication, the dose and the dosage are correct and appropriate for your pet’s treatment. So please, call a couple of days before you need your medications to give us time to address your request in a timely manner, and PLEASE, just keep your dog on year-round heartworm prevention. It is cheaper to do that, than it is for an emergency visit because your dog has a reaction, and FAR and AWAY cheaper than treating your dog for heartworm. I am not saying you need to buy it from us, I am just asking you to give it every month.
Dr. Farrington
Over the last few years, we have seen a steady increase in the number of heartworm cases in our canine patients. Previously, they were all pets that had been adopted from southern states, and not been on monthly prevention prior to their rescue. However, in the last two years, we have started seeing dogs with heartworm disease that have never left the New England area. We suspect part of the reason for local dogs becoming infected is that we have a larger fox and coyote population. They are wild canines, that all carry heartworms in their blood, and are seeding the mosquitos, that then infect local canine pets. Another reason is likely that Spring weather is arriving sooner, and Winter frosts later, allowing for a longer mosquito season. The last reason, and the only one we can do anything about, is that because traditionally, New England has not been an epidemic are for heartworm disease, many caretakers are not vigilant about making sure their pets get their prevention every month. All it takes is for your pet to miss two doses in a row, and any larval worms that have been injected into their blood stream by mosquitos, will become adult worms living in the dog’s heart. Since it takes 6-8 months for the larval worm to grow into an adult, a dog will not test positive until 6-8 months after his or her initial infection, or just under a year from when the caretaker missed the dose.
In the past, we have been flexible with authorizing heartworm prevention to animals that may have missed a month here or there, because the risk of actual infection was very low. However, with recent issues with on line pharmacies allowing refills that have not been authorized and recent upticks in the number of cases we are seeing, we are being much more careful about prescribing heartworm prevention only when we are sure your pet is not at risk for having the infection.
Monthly heartworm only kills off the larval worms for the first 4 -6 weeks after initial infection and it has NO activity against larval worms older than 6 weeks. It takes about 8 months for those larvae to grow into adult worms living in the heart. The heartworm test ONLY detects adult worms, it does not detect the larvae. When you give heartworm prevention, the process of the larval worms dying stimulates an immune response in your dog that can lead to life threatening allergic reactions. The risk of reaction is greater, the more larval worms circulating in your dog’s blood stream. If you miss more than 2 doses of heartworm prevention, it is possible that enough larval worms will survive to become adult worms, making more larvae to circulate in your dog’s blood stream, increasing the risk for an allergic rection if you start prevention without testing.
For example, in December 2019 your Dog tests negative for heartworm, but you do not start prevention until April because it was a cold winter. However, there were a few warm days in February where the mosquitos came out, and your dog became infected. In November of 2020, your dog is tested again, and is still negative because the larvae haven’t matured enough to be detected by the test. At this stage, your dog is infected with maturing larvae. These larvae are too young to give a positive on the test, but they are also too young to be making baby worms, so it is still SAFE to give the prevention at that time. By December, they are full fledged adults and they are now making larvae that are starting to build up in your dog’s blood stream. IF you started the prevention in November and continued it monthly from there on, the preventative would kill off the larva as fast as the adults make them, and your pet would be unlikely to have a reaction, but would test positive the following year and require treatment to kill off the adults.
However, if you chose again not to start the prevention until the following Spring, the adults would have had 4-5 months to make babies that build up in your pet’s blood stream. Giving heartworm prevention at that point would involve a risk for an allergic reaction as those baby worms die off. Admittedly, in the Northeast that risk is still relatively low, and significantly lower than it is in Southern states. However, the incidence is higher than it was just 5 years ago, and significantly higher than it was 10 years ago, which is why testing policies are changing. We do not want to do anything that puts your pet at risk.
So, once your dog tests negative for heartworm, start the prevention right away, and keep him or her on it year-round. That way, you will only need one test a year. We will still ask that you test yearly for two reasons. First and most importantly, the 4dx test we use to screen for heartworm not only tests for adult heartworm infection, it tests for 3 different types of tick born disease, which are epidemic in this area and not completely preventable even with tick prevention and Lyme vaccination. Secondly, no one is perfect, and it is pretty easy to miss a couple of months without realizing it. I do it myself, often.
IF we can not see a record of either selling or authorizing a prescription for heartworm prevention in a way that shows that your pet has likely been on prevention close to year-round, we will ask that your pet be tested prior to approving a prescription request. This is for your pet’s safety. We also will not approve any refills on medications through the pharmacy. Too often we are finding that the pharmacy ‘s put pets on auto delivery and send requests either when you have not asked for them, or have refilled requests from you, when we have not authorized those requests. We also will only authorize request where there is either a fax or prescription paper trail, unless you are using our VETSOURCE on line pharmacy. The reason we make an exception for our Vetsource account, is that they sed a digital document directly to your pet’s medical history when you make a prescription request, when we authorize it, and once it is actually delivered. We do not have the software that allows a paper trail for the digital prescriptions with other on-line pharmacies.
Please keep in mind, every single day, we have over 100 prescription requests. Each has to be approved by a doctor to make sure the medication, the dose and the dosage are correct and appropriate for your pet’s treatment. So please, call a couple of days before you need your medications to give us time to address your request in a timely manner, and PLEASE, just keep your dog on year-round heartworm prevention. It is cheaper to do that, than it is for an emergency visit because your dog has a reaction, and FAR and AWAY cheaper than treating your dog for heartworm. I am not saying you need to buy it from us, I am just asking you to give it every month.
Dr. Farrington