Heartworm Prevention
Heartworm Treatment

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WARNING: ProHeart 6 is back on the market

Important New Information Regarding Heartworm Treatment and Doxycycline

Any dog that is currently infected with heartworms should be treated with doxycycline. If the infected dog will be treated with Immiticide (fast-kill method), it is best to give the doxycycline prior to beginning Immiticide treatment, as this should make the treatment much safer, by greatly reducing the potential for embolism and allergic reaction to the death of the worms. There may also be benefit in continuing to give doxycycline intermittently during treatment. If anyone has dogs currently undergoing treatment with Immiticide, I would start doxycycline immediately, as it may be beneficial even if prior treatment was not done.

The ideal treatment regimen is not known, as studies have not yet been done in dogs. Merial (the makers of Immiticide) recommends giving doxycycline at normal doses for 30 days before starting treatment with Immiticide. However, a laboratory study done on wolbachia showed that intermittent treatment with antibiotics was more effective in depleting Wolbachia than continuous treatment. The study showed that stopping the doxycyline for 7 days actually kills more wolbachia than continuous treatment. The study was short term, and showed only that more wolbachia were killed after 14 days when doxy was given for 7 days and then stopped for 7 days than when it was given for the full 14 days, and that more wolbachia were destroyed after 21 days when doxy was given for 14 days and then stopped for 7 days than when it was given for the full 21 days. I would conclude, therefore, that you should stop the doxycycline for one week prior to beginning treatment with Immiticide, but I don't know whether it would be best to give doxy for 30 days followed by one week off, or for 21 days followed by one week off, or for 14 days followed by one week off, or possibly even to give it for two weeks, stop for one week, give it for another two weeks, and stop for another week before starting treatment with Immiticide.

Doxycycline should also be given to dogs that are being treated with monthly Heartgard (slow kill method) or any type of alternative heartworm treatment method, as it will weaken the heartworms, prevent them from reproducing, and reduce the chance of adverse effects caused by the heartworm infection itself, and by the worms dying. Wolbachia will repopulate over time, so the treatment with doxycycline should be repeated intermittently. A study on cattle infected with onchocerca volvulus (a filarial parasite similar to heartworms that cause a disease called River Blindness) showed that the Wolbachia repopulated within six months following short-term (two week) daily treatment with oxytetracycline. A combination of this short-term treatment with long-term intermittent treatment (double the dose, or 20 mg/kg, injected once a month for six months), eliminated 80% of the adult female worms as well as sustaining the depletion of Wolbachia. See this abstract for more information. In dogs, this might translate to giving doxy for two or three weeks at normal doses to start with, then repeating the treatment at twice the normal dose for one week out of each month as long as adult heartworms are present. The double dosage (10 mg/kg twice a day) is used to treat tick disease, so it is safe.

Veterinarians may contact Merial, the manufacturer of Immiticide, for more information on this topic, if needed.

Here is some additional info on the topic that I was able to find, though most of it is highly technical and still in preliminary stages of research:

Are Heartworms getting worse? A Seminar: Part 6 of 6 Doxycycline’s Role in the Treatment Process (May 2009). The article discusses doxyclycine's effect on heartworms and wolbachia, along with treatment and dosage guidelines.

Combined ivermectin and doxycycline treatment has microfilaricidal and adulticidal activity against Dirofilaria immitis in experimentally infected dogs. (October 2008)
"Treatment with ivermectin (IVM; 6 microg/kg per os weekly [that's a regular monthly Heartgard dosage given every week]) combined with doxycycline (DOXY; 10 mg/kg/day orally from Weeks 0-6, 10-12, 16-18, 22-26 and 28-34  resulted in a significantly faster decrease of circulating microfilariae and higher adulticidal [killed more adult heartworms] activity compared with either IVM or DOXY alone. . . . Results indicate that the combination of these two drugs causes adult worm death."

Heartworm and Wolbachia: therapeutic implications (December 2008)
"Preliminary observations suggest that administration of Doxyclcline plus Ivermectin for several months prior to (or without) melarsomine [Immiticide] will eliminate adult HW with less potential for severe thromboembolism than melarsomine alone." Doxycycline was given at the rate of 10 mg/kg/day for weeks 1-6, 10-11, 16-17, 22-25, and 28-33.

http://www.veterinarypartner.com/Content.plx?P=A&A=610
"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. If your veterinarian wants to pre-treat your heartworm positive dog with doxycycline, it may be because of concerns regarding this organism. As new information emerges, we will post here."

http://www.vetmed.auburn.edu/~blagbbl/Blagburnheskasymposium.pdf
"Wolbachia are intracellular bacteria that infect numerous species of filarial worms including heartworms. Many contend that these friendly inhabitants (endosymbionts) play a role in the pathogenesis of diseases caused by heartworms and other filarids. Contention is that host immune responses directed at Wolbachia can actually go awry and enhance the disease process in heartworm infections. Some also contend that elimination of Wolbachia spp. from heartworms may affect the survival of adult heartworms and may decrease the host’s errant immunologic responses when adult worms are killed or die."

http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2005&PID=10906&O=Generic
"Dirofilaria immitis [heartworms] the cause of heartworm disease in dogs and cats harbours an endosymbiont intracellular bacteria of the genus Wolbachia (a Rickettsia). Studies performed recently indicate that these bacteria may play an important role in the pathogenesis and immune response to filarial infection (Bandi et al, 2001)." This article goes on to say that Wolbachia may contribute to many of the side effects of heartworm disease, including inflammation, kidney disease, lung problems and allergic reactions.

http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2005&PID=10907&O=Generic
"Furthermore, preliminary data from our laboratory indicates that antibiotic treatment before adulticide therapy in dogs with heartworm disease leads to a decrease in pro-inflammatory cytokines, particularly IL-8."

http://lib.bioinfo.pl/meid:13044
This site has numerous other studies on human treatment, indicating that doxycycline, oxytetracycline (of which doxycycline is a derivative) and rifampicin (sometimes use in conjunction with doxycycline) are all effective against Wolbachia.

http://www.pnas.org/cgi/content/full/94/21/11154
"Bacterial relatives of Wolbachia include a number of agents that have arthropods as vectors and cause serious human diseases such as typhus, scrub typhus, erhlichiosis, and Rocky Mountain spotted fever. Studies of these bacteria require special containment facilities. In contrast, Wolbachia have so far been found only in invertebrates and are not known to cause mammalian disease."

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Where to find Diethylcarbamazine Citrate, the generic form of Filaribits (daily heartworm preventative):

    Interpet
    CanadaVet (ships from Australia)
    Pets Megastore
    Vet-Pet-Supplies-Online
    MavLab (manufacturer)

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Ivomec

I have also worked out the amount of ivermectin in Acarexx, used to treat ear mites in cats (and off-label in dogs), which might be useful for people who need to treat a large number of dogs. Contact me privately for more information (my contact information is at the bottom of the page).

Are Heartworms Getting Worse? Has a 2007 heartworm incidence map. Also talks about the possible emergence of heartworms that are resistant to current preventatives.

Heartworm The information here is slightly outdated, but it gives a very good overview of heartworm infection, prevention and testing, with some info on treatment.

U.S. Regional Map of Lyme Disease, Ehrlichiosis, Heartworm and Anaplasma Interactive map showing the incidence of hearworm and three types of tick disease in various states and counties.

Map of Quebec and Ontario showing incidence of heartworm and Lyme disease positive tests by IDEXX in 2007.

Information on timing heartworm preventatives:
    http://www.citadeltm.com/Heartworm.html (US)
    http://www.heartworm-hotline.org/  (California only)
    http://www.veterinarypartner.com/Content.plx?A=595 see When to Start Prevention Each Year

Emerging Issues in Heartworm Disease Detailed info on diagnosis

Canine Heartworm Disease: Prevention and Treatment

http://www.vetmed.wsu.edu/depts-VCPL/ Information on Ivermectin sensitivity and testing

Heartworm Treatment:
    Heartworm-positive dog requires tailored treatment
    Heartworm Treatment Aftecare