Heartworm Prevention
Heartworm Treatment
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.
How long and how much doxycycline to give 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 study done on wolbachia in the lab 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. Therefore, 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:
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."
Interpet
CanadaVet
(ships from Australia)
Pets
Megastore
Vet-Pet-Supplies-Online
MavLab
(manufacturer)
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.
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
This page last updated 7/30/2008