By Dr. Becker
The only FDA-approved heartworm treatment drug, Immiticide, has been in 'temporary' short supply for about two years now.
The shortage, according to Merial, is due to "unforeseen technical difficulties during a planned manufacturing site transfer."
As almost all dog owners know, a heartworm infection is serious.
Adult heartworms can grow up to a foot in length and live as long as five years inside a dog.
They can clog pulmonary arteries, and if there is significant infestation, the worms back up into the heart itself and eventually fill it.
They cause blood clots, and the heart has to work abnormally hard to pump blood through plugged arteries.
Heartworms also cause serious inflammation in the arteries that can affect important organs like the liver and kidneys.
As is often the case when 'desperate times call for desperate measures,' in the face of the Immiticide shortage, the pet healthcare community has been faced with finding other means for treating active heartworm infections in dogs.
And as it turns out, one alternative therapy in particular is effective and much less costly and potentially deadly than the old standby treatment with Immiticide.
Why I Made the Switch from Immiticide
My own experience with Immiticide is that I've bought a small supply of it every year for several years, only to throw it away, unused, on its expiration date.
A few years ago it occurred to me this was a totally wasteful and expensive exercise, since I've never encountered a case of heartworm that couldn't wait 48 hours for me to order and receive a box of Immiticide.
Shortly after this revelation, the economic downturn descended upon us. The owners of the four heartworm-positive dogs I've seen since then have all opted for the less costly 'slow kill' treatment with a combination of ivermectin (heartworm preventive drug) and doxycycline.
As it turns out, low-dose ivermectin therapy in dogs with no sensitivity to the drug, in combination with the antibiotic doxycycline, can be an extremely effective, inexpensive option for treating heartworm infection.
The cost was about 50-75 percent cheaper than Immiticide and all four cases of infection cleared beautifully.
The only time I'll consider using Immiticide in the future (once it becomes available), is when I have an ivermectin-sensitive patient.
Heartworms are parasites, and it was recently discovered that another organism called Wolbachia lives inside heartworms.
"Wolbachia is a genus of rickettsial organisms (sort of like bacteria, but not exactly). They live inside the adult heartworm. These organisms seem to be protective or beneficial to heartworms and treating the dog with the antibiotic doxycycline seems to sterilize female heartworms (meaning they cannot reproduce). Wolbachia is also thought to be involved in the embolism and shock that result when heartworms die. The role of this organism is still being investigated."
Research indicates Wolbachia organisms worsen the effect of both the heartworms themselves and the adverse events associated with heartworm treatment, including allergic reactions, inflammation and embolism.
Doxycycline, a member of the tetracycline antibiotic group, kills Wolbachia. It also simultaneously weakens the heartworms and sterilizes them so they cannot reproduce, which lessens the damage they can do inside a dog's body. It also dramatically reduces the risk of adverse reactions to heartworm treatment.
A study published in 2008 demonstrated that treatment with a combination of ivermectin and doxycycline has the following effects on heartworm infections:
- Sterilizes female heartworms
- Prevents the infected dog from infecting other dogs via mosquitoes
- Hastens the death of heartworms
- Limits inflammation and other damage caused by the presence of heartworms in the body
- Reduces risk of serious adverse reactions to Immiticide
These effects are significantly improved when the two drugs are used together rather than one without the other.
The doses used for the study were ivermectin (Heartgard) given weekly at the usual monthly preventative dose for 33 weeks, and doxycycline given at 10 mg/kg daily during weeks one through six, weeks 10 and 11, 16 and 17, 22 through 25, and 28 through 33.
Questions about Dosing
Different theories and protocols exist for how much, how often and how long ivermectin and doxycycline, should be given in treating heartworm infections.
One recommendation is to give doxycycline at normal doses for 30 consecutive days before starting (in this case) Immiticide.
However, a laboratory study conducted in 2005 indicates intermittent treatment with doxycycline is more effective in killing Wolbachia than continuous treatment.
In another study published in 2010, 11 heartworm-infected dogs were given doxycycline daily for 30 days and ivermectin every 15 days for six months, with the following result:
One hundred percent of dogs became negative for circulating microfilariae by day 90, while 8/11 (72.7%) of dogs became antigen-negative by day 300. Of the 7 dogs that were positive for visualization of parasites at echocardiography, 6 (85.7%) became negative by day 300. Treatment was well-tolerated by all dogs. These results suggest that a combination of doxycycline and ivermectin is adulticide in dogs with D. immitis.
Using this therapy, the gradual death of adult heartworms dramatically reduces the risk of pulmonary thrombosis -- blood clots in the lungs that pose a serious adverse effect associated with other adulticides. This is the protocol I have used in my practice with 100 percent success.
The American Heartworm Society has its own set of dosing and care guidelines, which you can find here.
Surgery to Remove Heartworms
Also in response to the ongoing Immiticide shortage, veterinary cardiologists at the University of Florida are offering surgical extraction of heartworms from dogs with severe infections.
Using specialized instruments, these doctors can manually remove heartworms from the heart and pulmonary arteries by way of the jugular vein.
The UF DVMs consider the surgery a minimally invasive technique and an important option for dogs with severe symptoms and/or significant infestation with heartworms.
The procedure isn't without risk, of course. Dogs must be heavily sedated or undergo general anesthesia, and the surgery is costly. But according to Herb Maisenbacher, VMD, a clinical assistant professor in cardiology at UF:
"… for dogs with extensive disease, it could be another option to consider at a time when there are limited treatment options available."
If Your Dog Has a Heartworm Infection
The best advice I can offer you is to work with your veterinarian to determine the most appropriate course of action based on the level of infection and the health of your dog, as well as financial considerations.
Your vet may be able to treat the infection with ivermectin and doxycycline. Or Immiticide may be the better option and hopefully your vet knows how to source it given the shortage.
Some dogs also need a drug like Benadryl or a course of steroid therapy to mitigate the side effects of heartworm treatment.
And no matter whether you opt for a 'fast kill' or 'slow kill' approach, your dog will need to be kept inactive (which generally means confined to a crate or other small space) during treatment. So it's extremely important to understand what your pet can and can't do while undergoing heartworm drug therapy.
Surgery to remove the heartworms instead of waiting for them to die may or may not be an option for your pet as well.
The decision about the best way to treat a precious pet infected with heartworms is never easy, so I encourage you to work closely with your dog's health care providers to decide the best and safest treatment for your canine companion.
A Word about Natural/Holistic Heartworm Infection Treatments
At my practice I have had many owners request holistic options for treating heartworm. I have also had clients who decided to treat their pets with nutraceuticals purchased online.
Unfortunately, entirely drug-free heartworm treatment protocols have ranged from moderately successful to epic fails (dogs were still positive after treatment and their heart disease was worse).
Treatment for heartworm infection is one area where conventional veterinary medicine offers valuable options. Whether you elect to do the fast-kill method using Immiticide, or the slow-kill method using monthly Heartgard, either is preferable to leaving the dog untreated, or using unproven, alternative methods that may have no effect or even be harmful.
Because I have not found one specific entirely holistic protocol that is consistently successful at treating moderate heartworm infection, I recommend you discuss options for natural protocols with your holistic vet. There are some adjunctive protocols that can be beneficial, but they are tailor-made to each patient and must be designed and monitored by your vet.
Please keep in mind that heartworms are blood-borne parasites, not GI parasites. Natural remedies effective in treating GI parasites don't work on blood-borne parasites. A natural 'wormer' won't work for heartworms because heartworms live in your dog's bloodstream, not in the gastrointestinal tract.