The in vitro uptake of gamma-emitting radionuclides by microfilariae of Dirofilaria immitis was investigated. Radionuclides tested were 133Ba, 207Bi, 82Br, 109Cd, 51Cr, 60Co, 59Fe, 203Hg, 125I, 54Mn, 32P, 125Sb, 46Sc, 75Se and 65Zn. Only 207Bi, 59Fe, 203Hg, 54Mn and 46Sc showed more than 2% of the available radioactivity to bind to the microfilariae. When tested for retention in vitro only 203Hg showed retention levels approaching 90%. Moreover, when dimethyl-sulphoxide was incorporated into the medium at levels of 1% (v/v) the uptake of 203Hg could be increased by 3-5 times; no other radio-isotope tested responded in this manner. The uptake of 203Hg was directly related to temperature and time of incubation. Mercury, as mercuric chloride, was toxic to the microfilariae and represents an impediment to the incorporation of high levels of 203Hg in microfilariae.

Microfilariae of Dirofilaria immitis were labelled with 203Hg2+ in vitro and injected into irradiated mice and Beagle dogs. With irradiated mice it was possible to demonstrate microfilariae present in the blood and to detect 203Hg by external counting as long as 28 days after dosing. The 203Hg2+ label had a half-time of 4-5 days; the amount of stable mercury in the labelling medium strongly influenced the survival of microfilariae in vivo. In dogs, external counting showed the lungs to be a major location of the microfilariae soon after reinjection into the host. Evidence was obtained that labelled microfilariae can circulate; however, the detection of dispersed microfilariae is difficult because of the relative insensitivity of the detecting system. For radiomercury the accumulation of the inorganic form in the liver and kidneys limits the long-term usefulness of 203Hg2+ as a label if the organism being studied also accumulates in these organs.