The Use of Doxycycline in Micro- filaremic Dirofilaria immitis (Leidy, 1856) Naturally Infected Dogs

Heartworm is a mosquito-borne disease that affects different mammalian species with worldwide distribution. The agent, Dirofilaria immitis (Leidy, 1856), infects dogs from all continents and, in the Americas, the parasite has been detected in 5 of the 13 countries. The species D immitis harbors an endosymbiont intracellular bacteria of the genus Wolbachia, known to be vital for the worms. These bacteria are sensitive to tetracycline-like drugs, therefore the use of antibiotics can affect the worms. The effect of the use of doxycycline on microfilaremia of naturally infected microfilaremic dogs was studied by treating dogs with 3 cycles of doxycycline (10 mg/kg SID for 21 days) at 6-month intervals. Blood samples were collected on Days 0, 7, 21, and 111 of each cycle and microfilariae detected and counted. Thirteen dogs completed 1 cycle, 10 dogs completed 2 cycles, and 6 dogs completed all 3 cycles. Five dogs could be sampled at 7 months after the end of the third cycle. All 13 dogs were still microfilaremic while they were given doxycycline treatment. On Day 111 of the first cycle, 40% became amicrofilaremic and from then on, amicrofilaremic dogs were always detected. Overall, treatment lowered the microfilariae mean counts, which can be observed when the 3 cycles are compared. The microfilariae mean counts tended to be lower at Day 111 of each cycle, suggesting that the effect of the antibiotic on Wolbachia organisms and the consequent impact on the fitness of the worms is a slow process. The lowered microfilariae stock provided by doxycycline-treated dogs (amicrofilaremic dogs and lowered microfilariae concentrations) may have impaired heartworm transmission once the chances for the vectors to become infected were reduced.