Does Your Heartworm Prevention Last ALL Month? SOME DO NOT!!!

I am reminded on a daily basis, that most clients do not have a good understanding of heartworm prevention.  How does it work? Why do we test every year, even if they give the medication every month? Why do we need to give it all year round?

I am going to try to simplify things a bit, but first off, how long does the medication live in the blood stream?  Your ORAL heartworm preventions only work for a few days (see references). Therefore, you have to think of them as a “dewormer” for heartworms.  If you give them once a month, then you are killing the larvae (L3 and L4), before they have a chance to mature to the next phase (where they will NOT respond to the medication).  The larvae (L3/L4) can mature in 45-65 days, so if you are 2-4 weeks late on your dose, there is a chance that you missed your window of prevention and some worms are in the next phase.

In the next phase, the larva is developing into an adult.  This phase can take 4-5 months until it matures into an adult.  During this time, your preventions can NOT kill them and we can NOT detect them with blood testing.  

The tests that we run at the office are ELISA tests, and they detect the protein secreted by the female ADULT heartworm (it takes 6 months from the mosquito bite to make an adult).  It takes 1-2 female heartworms to get a positive test. We now know there are some risks of false negative tests as well. “False-negative test results occur most commonly when infections are light, female worms are immature, only male worms are present, and/or the test kit instructions have not been followed. There are also suspected cases of antigen blocking from antigen–antibody complexes interfering with antigen testing, resulting in false-negative tests. Laboratory studies have shown that heating serum will release blocked antigen, and result in more positive test results (Velasquez et al, 2014).”

The American Heartworm Society recommends testing once a year.  If you missed a dose, there is always a chance for a few larvae to mature into the next phase.  There is always human error, but with the oral preventions, it is extremely important to give it every single month.

We also have to consider that parasites are very good at evading our chemicals and medications.  It is now generally accepted that isolated instances of resistant heartworms have been identified.   Efficacy of four commercially available heartworm preventive products against the JYD-34 laboratory strain of Dirofilaria immitis.  Blagburn BL, Arther RG, Dillon AR, Butler JM, Bowles JV, von Simson C, Zolynas R.  Parasit Vectors. 2016 Apr 5;9:191. doi: 10.1186/s13071-016-1476-7.

The take home point is this: talk to your veterinarian about the best means of prevention for your pet.  Every pet has different needs. There are high risk and low risk areas of the US. There are some pets with food allergies that do poorly on chewable tabs.  There are some dogs with seizure disorders that should not be on certain combination medications. There are some dogs that break out in a rash with topical products.  I do believe that multimodal prevention will be the most protective in those high risk areas. This includes a heartworm prevention, as well as a mosquito repellant/protection used together.

Dr. Leah Wulforst

Riverside Veterinary Clinic

Knoxville, TN


Ivermectin - elimination half life is 80.3 hrs +/- 29.8 hrs (Al-Azzam SI, Fleckenstein L, Cheng K-J, et al. Comparison of the pharmacokinetics of moxidectin and ivermectin after oral administration to beagle dogs. Biopharm Drug Dispos 2007;28:431-438. Available at:

Milbemycin - elimination half life is 11.6 hrs +/- 0.93 hrs (Xu Q, Li J, Shen Z, et al. An LC-MS method for determination of milbemycin oxime in dog plasma. J Chromatogr Sci 2014;52:999-1004. Available at:   There is an article in which the Milbemycin lasts longer in concentration when given in combination with spinosad, yet further research is required to determine whether the higher levels of MBO have any implications for improved effectiveness (Pharmacokinetics of spinosad and milbemycin oxime administered in combination and separately per os to dogs.  Holmstrom SD, Totten ML, Newhall KB, Qiao M, Riggs KL.  J Vet Pharmacol Ther. 2012 Aug;35(4):351-64. Doi: 0.1111/j.1365-2885.2011.01333.x. Epub 2011 Sep 6.


    • Topical (ie Advantage Multi) elimination half life is 35 days (Bayer HealthCare Animal Health. Advocate: the technical manual, ver 3.0.; 2005:16-20. Available at: Accessed October 5, 2015.)

    • Subcutaneous injection (ie Proheart) elimination half life is 35 days (Fort Dodge Animal Health. ProHeart6 (moxidectin) [FOIA summary]. Fort Dodge, IA, USA; 2005:100. Available at: Accessed October 1, 2015).  Proheart is given as an injection every 6 months for the prevention of heartworms.

Selamectin - elimination half life is 11 days as per Plumb’s Veterinary Drug Handbook, but I can not find a specific reference for this data.