A prospective clinical study of the mononucleosis syndrome in a developing country.

A prospective aetiologic analysis made of 70 children and adults with the clinical and haematologic features of infectious mononucleosis (IM) in Sao Paulo, Brazil, revealed 65.7% due to EBV (31 heterophile antibody positive (HA+), 15 HA negative (HA-) cases), 8.6% due to T. gondii and 4.3% due to CMV. One case was related to viral hepatitis and one to drug hypersensitivity. The other 18.6% were of unknown cause. In the 46 IM cases positive tests were found for EBV/IgM in 93.5%, for heterophile by the absorbed horse Rbc test in 64.2% and by sheep Rbc in 37%. The immune-adherence haemagglutination test was slightly more sensitive than the horse Rbc test in 39 IM sera tested. In 41 EBV/IgM positive sera, EBV/IgA was present in 17/25 containing heterophile antibody and in 2/16 lacking heterophile antibody; anti-EA was present in 85%. The average age of HA+ IM cases was 13.2 years and of HA- cases 4.7 years. Three HA+ and 5 HA- IM cases occurred in the 0-2 year old age group. Few clinical features differentiated between aetiological agents. Exudative tonsillitis was the most helpful one and occurred in 67.7% of HA+ IM cases, 26.7% of HA- cases, in 1 of 3 CMV cases, and not at all in 6 T. gondii infections.