Estimation of numbers of organisms is often made using dilution series, for example when examining water samples for coliform organisms. In this paper the most probable numbers (MPNs) are calculated for a 15-tube series consisting of five replicates at three consecutive tenfold dilutions. Exact conditional probabilities are computed to replace previous approximations.

When growth is observed in several of the tubes it is not realistic to select a single MPN. Instead a most probable range (MPR) should be reported. But using an MPR creates problems when comparison has to be made with a legislated, single-valued Standard. It is suggested that the wording of the Standards should be expressed differently when the multiple tube method is used.

Although an effective whooping cough vaccine has been available in the UK since the 1950s, its current association with neurotoxicity has resulted in poor uptake: as a consequence major epidemics (with significant morbidity and mortality) arc still being experienced.

Component (sub-unit) vaccines, which incorporate those antigens thought to be concerned with generating a protective effect, have been developed and are now available for field testing. This paper addresses how such a vaccine might be evaluated, the organization of a trial and the difficulties to be expected.

The efficacy of measles and pertussis vaccines was investigated using several different procedures based upon data routinely available at Local Health Authority level in England and Wales. It is demonstrated that such estimates can be derived by methods which can be carried out simply and routinely by local health department staff. Several theoretical and practical difficulties in the procedures are discussed. It is recommended that Health Authorities consider monitoring by a routine procedure based on crude case-control analysis of recorded vaccination status of notified cases compared with that of the population in the Child Health computer file, or of matched controls drawn from Child Health Registers. A simple protocol for such studies is provided.

Previous studies of boys at Christ's Hospital school have indicated that annual immunization with influenza virus vaccines did not significantly reduce the total incidence of influenza infection compared to unimmunized subjects. In view of the implications of this result, a similar study was conducted in ferrets to clarify these findings. Groups of ferrets were immunized or infected with a series of influenza A (H3N2) viruses over an 18-month period, and the immunity to subsequent live virus challenge was measured after each virus or vaccine exposure. The results indicated that live virus infection gave a more solid immunity than immunization with inactivated vaccine; and the serum haemagglutination-inhibiting antibody response was greater following immunization than following infection. In addition, differences in immunity could not be explained by measurements of cross-reacting and specific antibody, since the incidence of these antibodies was similar in both infected and immunized animals. The results do not suggest an explanation for the different levels of immunity induced following infection or immunization or the results obtained from the Christ's Hospital study. However, the relative contribution of various immune responses to virus or virus antigen is discussed, and it is suggested that the difference in immunity may lie in the ability of live virus infection to stimulate local antibody.

Rift Valley fever (RVF) causes abortion in sheep and cattle. However, the teratogenic and abortogenic potential of RVF in humans is not known. Sera from a total of 1163 pregnant women in Mozambique were tested for RVF virus antibodies by ELISA and 28 (2%) were found to be positive. Mothers experiencing fetal death or miscarriage (155) had the same RVF virus antibody prevalence as those with normal deliveries. Analysis of maternity histories showed some indication of increased fetal wastage among women positive for RVF virus antibody.

The ELISA used in this study was compared with a plaque reduction neutralization test and found to be equally sensitive and specific for the detection of RVF virus IgG antibodies.

During the 7-year period from September 1978 to August 1985, smear specimens of nasopharyngeal secretions from 3132 patients mainly hospitalized children, taken in different regions in Norway, were examined for respiratory viruses by the rapid immunofluorescence (IF) technique. A positive diagnosis for respiratory syncytial virus (RSV), parainfluenza virus type 1, 2 and 3 or influenza A and B virus was made for 896 patients (29%). The greatest prevalence for all these viruses was observed during the colder months with only sporadic cases during the summer months. A relative increase in parainfluenza virus activity, involving several parainfluenza virus types, was observed in every second autumn and during these periods only sporadic cases of RSV infection were diagnosed. Also both RSV and parainfluenza viruses were less frequently found during influenza virus epidemics and regional differences in RSV activity were observed. During the four autumn periods 1982–85 the monthly number of positive virus identifications by IF followed an epidemic curve, while the corresponding number of negative samples was relatively constant. The results of this study suggest interference between RSV, parainfluenza viruses and influenza virus in reaching their epidemiological peaks. It is suggested that interferon might be a mediator of this effect.

Using two sentinel calf herds, the seasonal incidence of bluetongue in central Sudan was shown to be a predictable event related to the rainy season and to vector population. Seroconversions to bluetongue virus (BTV) were detected only between July and December. Neutralizing antibodies to 16 BTV serotypes (1–10, 12, 14, 17, 20–22) were present in animals from both sentinel herds. The use of a blocking enzyme-linked immunosorbent assay made possible the distinction between infections due to BTV and those due to infections with other orbiviruses. Sera from 13 calves at Shambat and from 17 at Um Benein gave agar-gel tests indicative of BTV infections not attributable to any of the 22 serotypes, suggesting the presence of new serotypes of BTV. Sera from 30 calves gave results interpreted as evidence of infection with BTV-related viruses, rather than BTV serotypes. Comparing these results with those reported from Kenya, we suggest that the pattern of the disease and the types involved are similar in both countries and that the movements of the Intertropical Convergence Zone (I.T.C.Z.) may carry infected midges northwards into the Sudan at the start of the rainy season and southward at the end of this period.

Six hundred and fifty-seven children aged between 0 and 12 months were randomly chosen and studied for measles antibody titres by the haemagglutination inhibition (HAI) test. The results showed significant variations in HAI antibody titres. Children between 0–3 months showed high measles HAI antibody titres which declined to a trough between 4 and 0 months. A rise in HAI antibody titre was observed from 10 to 12 months age. Multiparity and higher age in the mothers were noted as probable factors influencing the titres of measles HAI antibody in children. The results indicated that with increased maternal parity, measles HAI antibody titres in the children declined.

The antigenic relationship of ten South African West Nile isolates, the South African prototype virus H442, the Egyptian strain EG101 and the Indian strain G2266 were compared using titre ratios. The titre ratios or ‘R’ values were calculated from heterologous and homologous neutralization titres and expressed as a percentage.

Substantial antigenic differences were demonstrated between the South African prototype strain and the majority of the recently obtained South African isolates, seven of which were fairly closely related and possibly form a distinct antigenic sub-set. The recent isolates also differed from the Egyptian and Indian West Nile isolates. The heterologous results between the South African West Nile strains and the Indian strain G2266 suggest that prior infection with an Indian West Nile virus would give poor protection against the South African viruses, whereas the reverse would not be so.

The incidence of nephropathia epidemica (NE) was compared with the NE antibody prevalence in normal population in a highly endemic area of Sweden (Västerbotten county). The antibody prevalence rate in the oldest age groups were found to be 14 and 20 times higher than the accumulated life-risk of being hospitalized with NE for men and women respectively. Whether the infection in persons not admitted to hospital is subclinical, atypical or clinically fulminant, remains to be determined.

Restriction endonuclease analysis of molluscum contagiosum virus DNA revealed two subtypes. In a study of 46 isolates from 41 patients, some with no other disorder and some with atopic dermatitis, the ratio of MCV I isolates to MCV II was 34:12. Multiple clustered lesions removed at the same time from an individual patient yielded only one type of MCV. Lesions induced by MCV I or MCV II were indistinguishable on the basis of size and form. Neither subtype was associated exclusively with lesions at certain sites or with other clinical features. Heterogeneity of DNA restriction endonuclease cleavage patterns amongst isolates of the same subtype was observed, this being greatest for MCV II.