Studies on the Dog Heartworm, Dirofilaria Immitis, with Special Reference to Filarial Periodicity

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  • Department of Tropical Medicine, Louisiana State University Medical Center, New Orleans, La.


  1. 1.A report is presented of data on Dirofilaria immitis infestation obtained by blood examination and subsequent autopsy of 93 dogs.
  2. 2.It has been found that there is no correlation between the numbers of microfilariae in the peripheral blood stream and the numbers of adult female filarial worms present. A single female may account for several million embryos or may be responsible for only a few thousand at 5 p.m.
  3. 3.The writer has been unable to demonstrate the presence of a substance in the salivary glands of four species of mosquitoes which possesses a chemotactic effect upon the embryos of Dirofilaria immitis.
  4. 4.A dog under observation, in screened animal quarters, continued to exhibit a constantly high number of microfilariae over a period of three years.
  5. 5.Aedes aegypti will develop the larvae of Dirofilaria immitis to an infective stage but in very carefully controlled experiments this species of mosquito was unable to transmit the infection to three dogs.
  6. 6.The young larvae, after inoculation into a dog by mosquitoes, must undergo development in some location other than the heart since this organ contains only adult worms or nearly mature ones.
  7. 7.It is believed that in the case of the dog heartworm cyclical parturition of the adult female filarial worms is not a major factor in the production of periodicity. This is substantiated by the long life which microfilariae may show following transfusion into an uninfected host and also by the enormous numbers of embryos per female worm which may appear in the dog's blood.
  8. 8.Probably normal physiological processes in the dog account for the phenomenon of periodicity of microfilariae.