Intradermal tests with antigen prepared from the dog heartworm, Dirofilaria immitis, gave positive reactions in all of 20 individuals infected with Onchocerca volvulus when used in a dilution of 1:2000 and positive reactions in 10 of 11 such patients when employed in a dilution of 1:4000. In addition to O. volvulus, all of the patients were infected with one or more other helminth parasites.
Similar tests were carried out on 20 control individuals, 18 of whom had never been exposed to onchocerciasis. Two of the control persons who had lived or worked intermittently in endemic zones gave a positive reaction. The other 18 individuals harbored intestinal parasites and 16 of them had intestinal helminths. The two with intestinal protozoa reacted negatively to the intradermal test with D. immitis antigen in both of the dilutions employed. With a 1:2000 dilution, 9 of the 16 individuals with intestinal helminths reacted positively, while 7 gave a similar reaction with the 1:4000 dilution of the antigen.
Control tests were carried out with canine serum antigen in the same dilutions as the D. immitis antigen. Two of the onchocerciasis patients reacted positively to the injection of the serum antigen, while 5 of the controls reacted similarly. However, the distribution and nature of these reactions indicated that the small amount of host protein which might be contained in the D. immitis antigen would not be likely to produce false positive reactions.
Results of the present study indicate that at least part of the positive reactions in the onchocerciasis patients were non-specific in character and due to a helminth group reacting factor in the antigen. Additional work may demonstrate the feasibility of using antigen dilutions capable of detecting cases of onchocerciasis without producing false positive reactions in the presence of other helminths.
Detailed from the United States Public Health Service.