Estimation of the thermal coefficient in the decline of a bacterial population under heat stress.

The "most probable number" (MPN) technique for estimating numbers of bacteria in suspensions is well known and has been used for decades by microbiologists, food researchers, and other laboratory scientists. A related procedure, involving an infinite number of serial dilutions at each of times ti, i = 0,1,...,n, is considered, and the joint probability law of the index numbers of the last tubes showing growth at these times is derived through use of probability-generating functions. Maximum likelihood estimates of the initial density lambda and the thermal death rate mu are computed. Another estimate, mu, of mu is given, using a simple weighting scheme. Finally, the thermal death time (TDT) is estimated by D = (1n 10)/mu.

Improved estimates of discordance ratios in twin studies.

In the study of monozygotic twins relative to disease and risk factors, particular interest focuses on the subset who are discordant for some suspected risk factor (for example, smoking), since such twins constitute a natural case-control pair. In such studies, questionnaires designed to identify the status of all twin pairs are sometimes error prone and can yield misleading estimates of the concordance-discordance ratios. Greater efforts to verify the characteristics of apparently discordant pairs than to verify those of apparently concordant pairs can result in the 'unequal ascertainment' fallacy. Using the results of a questionnaire with known error rates and the 'apparent' frequencies yielded, we present unbiased, maximum likelihood estimates of the 'true' proportions of concordant and discordant pairs. concordant and discordant pairs. We also present approximate covariances among these estimates.

Antibody response and resistance of turkeys to Newcastle disease vaccine strain LaSota.

Studies were undertaken to determine the influence of repeated revaccination on the immune response in immuno-competent turkeys as measured by humoral antibody and resistance to challenge. Protection was better in turkeys given the LaSota spray vaccine at 4 weeks and 30 days later than in turkeys given one vaccination by spray or intramuscular route or exposed 4 times at 10-day intervals by the aerosol route. The anamnestic response, as measured by the HI tests to revaccination with the same immunogen, was not evident by the 3rd day postrevaccination but was observed on the 7th day. The interval between primary and secondary vaccination was found to be important to a true and optimal anamnestic response. Response was greater, however, in vaccinated turkeys exposed to VVND, a more virulent virus antigenically different from the vaccine strain. Exposure to LaSota vaccine by the intramuscular route gave a poorer HI response than LaSota given by aerosol.