Annual microfilarial counts have been determined for periods of 1, 2 and 3 years following the use of diethylcarbamazine (Hetrazan, Notezine) as a microfilaricide in a filariasis control program. Two dosage schedules have been employed: (1) Two mg. per kg. of body weight 3 times a day for 7 days, repeated at the end of the year on the individuals who remained positive, (2) 2 mg. per kg. of body weight 3 times a day, 1 day each month for a period ranging from 12 to 18 months. The results are recorded in terms of (a) percentage of the population positive for microfilariae, (b) average number of microfilariae per 20 cmm. of blood, and (c) the frequency distribution in the population of microfilarial counts per 20 cmm. of blood at six different levels. The results may be summarized as follows:
1.Of 145 positive individuals treated with schedule (1) the percentage of positives was reduced to 17.5 at the end of the second year; the average microfilarial count per 20 cmm. dropped from 79 to 0.8; and the frequency distribution results showed marked reductions at all levels, no positives occurring above the 11–30 group.
2.Field studies in areas where considerable migration of population occurs and where supervision of drug administration was not as rigid as in the above group, likewise exhibited marked reductions at the end of 2 years, but to a lesser extent.
3.The second dosage schedule of 2 mg. per kg. of body weight 3 times a day, 1 day a month, for 1 year gives promise of being a practical method of administrating diethylcarbamazine in a filariasis control program.
4.A possible “infection factor” based on the experimental results of Rosen in which mosquitoes were fed on man harboring different levels of microfilariae is suggested and a potential “infection rate” is determined from certain experiments in this program showing the frequency distribution at these same levels in man before and after the use of diethylcarbamazine.
5.All data observed following the administration of diethylcarbamazine as employed in this study show marked reduction in the microfilarial rates. Whether these reductions are sufficient to limit the transmission of W. bancrofti to a level where clinical cases will cease to occur can be determined only by observation over a period of years.