Chemotherapy is indicated in asymptomatic as well as symptomatic cases. Treatment against macrofilariae (adult filaria) and microfilariae has to be differentiated. In symptomatic cases supportive treatment is additionally necessary.

Adulticidal therapy

The trivalent arsenical melarsamine dihydrochloride is the drug of choice in the therapy against macrofilariae. It is potential against adult and fifth stages of D. immitis, but does not possess any activity against microfilariae. It is better tolerated than formerly used drugs, but still has a low drug security index. After adulticidal therapy thromboembolic complications regularly occur, especially in high worm burdens.

Microfilaricidal therapy

After the elimination of adult female worms microfilariae may persist for a long time. Therefore after macrofilaricidal therapy, 5 to 6 weeks later a therapy against microfilariae should be added. The most commonly used heartworm chemotherapeutics are macrocyclic lactones. These drugs have exceptionally high therapeutic/toxic ratios, and possess anthelmintic activity against microfilariae, 3rd and 4th stage larvae, and at least young adult heartworms.

An additional complication after microfilarial therapy is an anaphylactic reaction against liberated microfilarial antigens. The risk is especially high in small dog breeds with high microfilarial density (> 10,000/ml). Measurement of microfilarial density and a fractionated treatment over several days may help to prevent those anaphylactic reactions.

The filaricidal effect of oral and topical formulations on precardiac larvae is achieved by brief pulsing at very low doses, which makes these safe drugs virtually 100% effective at the prescribed doses and intervals of administration. The single dose retroactive efficacy of all these macrocyclic lactones is assured for one month, and remains high for at least an additional month. However, efficacy against older larvae declines and requires progressively longer-term administration as the worms age to achieve a high level of protection.

Caval syndrome (Vena cava syndrome)

Immediate surgical extraction of the worms occluding the vena cava and/or the right atrium is the treatment in the ‘vena cava syndrome’. Worms are extracted by way of right jugular venotomy using flexible alligator forceps. After a recovery period of two weeks and thromboembolic prophylaxis, regular treatment with adulticidal drugs may be initiated.

Prevention

Canine heartworm infection is preventable, despite the inherent susceptibility of dogs. Since most dogs living in heartworm endemic areas are at risk, chemoprophylaxis is a priority. Continuous, year-round, chemoprophylaxis may not be necessary throughout areas, in which the prospects for transmission are limited to the months of May through October, but it is important to note that successful seasonal prophylaxis depends on proper timing of heartworm preventative administration. In regions where heartworm transmission occurs more than half the year, seasonal chemoprophylaxis may not be the most effective method and year around treatment may be considered to enhance compliance. In advance of a prophylactic therapy a current Dirofilaria immitis infection should be excluded. The main chemoprophylactics belong into the group of macrocyclic lactones (like moxidectin in Advocate^® or others).

The risk of infection can be also minimized by applying topical drugs which repel and kill the transmitting mosquitoes like advantix^®. Especially for regions, which harbor not only mosquitoes but also sand flies and ticks, it may be more suitable to use a broad-spectrum ectoparasiticide instead of an endoparasiticide. Additionally travel of dogs from non-endemic areas into D. immitis endemic regions should be carefully planned or even prevented. Furthermore dogs in endemic areas should be kept inside during the main diurnal vector activity.