Larval Development of Dirofilaria immitis (Leidy) in Fleas

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Abstract
446 of the 1203 Ctenocephalides felis collected from 71 dogs were infected with D. immitis larvae in advanced stages of development. Development of D. immitis microfilariae to the infective stage was traced in both males and females of C. felis, C. canis, Xeno-psylla cheopis, Pulex irritans, Echidnophaga gallinacea, and Orchopeas wickhami. Prelarval forms of D. immitis entered the abdominal haemocoele of the flea as early as 1 hr. after having been ingested with a blood meal. The 1st and 2d stages of development of IX. immitis took place in the hemocoele of the abdomen; 3rd stage larvae were found in the hemocoele of the abdomen and thorax. Dissections and serial sections of fleas showed that no major tissue damage or apparent injury of any organs could be attributed to developing larvae. Fertility of the male fleas and egg production of the females were not affected by the larvae infecting them. Infective stage larvae were shown to be thermotropic. Infective stage larvae proved incapable of penetrating intact skin. Because of the high percentage of infected fleas found under natural conditions and because of the close association between dogs and fleas, these insects are most likely the natural vectors of canine filariasis. It is very apparent that filarial transmission is far less efficient than the transmission of malaria where every bite results in a positive inoculation. Under these conditions the rate of infection of the intermediate host can be high and the rate of the definitive host low.

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