Seventy-eight patients under the surveillance of joint IndiaW.H.O. Smallpox Eradication activities in Bihar State, India, were visited and virological specimens were collected by the authors in February and March, 1975. Rapid diagnosis by direct immunofluorescence (IF) staining was attempted against smallpox (SPX), varicella-zoster (VZ) and herpes simplex (HS), type 1, viruses in a field laboratory composed of facilities brought from authors' home laboratories. A procedure applying IF staining to scabs was developed by releasing and smearing the antigen-containing epithelial cells and leukocytes. IF results in the field were compared with virus isolation in the home laboratory with specimens sent back through W.H.O. under rigid freezing in liquid nitrogen:
1) All 50 cases clinically diagnosed of smallpox ranging from macular (2 days after rush onset; vesicular smear) to late scabbing (30 days after rush onset; scab) stages, gave positive SPX.IF staining.
2) Three cases out of four of clinical varicella and three out of nine suspected varicella cases were demonstrated to be VZ.IF positive.
3) Eleven cases were demonstrated to be SPX.IF positive from cases of suspected smallpox (2/3), varicella (2/9), undecided suspects of smallpox or varicella (4/5), combined smallpox and varicella (1/1), suspected measles (1/1) and uncharacteristic eruptive cases (1/5) .
4) Variola virus was isolated from 55 out of 61 SPX.IF-positive cases. No variola virus was isolated from specimens of SPX.IF-negative cases.
From above observations, it was concluded that IF staining should be the first choice of smallpox diagnostic procedure which has both versatility and rapidity in the field with high sensitivity to have no possibility of false negative results.
