Periodicity of Dirofilaria immitis in Longterm Infections

The concentration of Dirofilaria immitis microfilariae in peripheral blood samples was measured in four experimentally infected dogs. Samples were collected at hourly intervals from 6.30h to 17.30h from all dogs at 11, 22, and 27 months post-infection, and at 39 months post-infection for two dogs only. Microfilarial periodicity follows the form of a simple harmonic wave over a 24h period, and concentration data was fit to sine wave for each sample date to characterize changes in periodicity over time. We found the periodicity index (i.e. wave amplitude) to decrease with time (p = 0.016, R-2 = 0.97) dropping from 74.57 (95% CI, 63.79 to 85.34) at 11 months post-infection to 5.55 (95 % CI, 0 to 14.82) at 39 months post-infection. The time of peak microfilaremia was calculated to be 17.36 h (95 % CI, 17.01h to 18.08 h) at 11 months post-infection and did not change significantly with time (p = 0.17, R-2 = 0.70). No significant trend was observed in total microfilarial count for individual dogs (p > 0.10). The data presented here indicate a gradual but significant loss of periodicity over the two-year study period despite maintenance of overall microfilarial levels.