Lessons from Nigeria: the role of roads in the geo-temporal progression of avian influenza (H5N1) virus.

The daily progression of the 2006 (January-June) Nigerian avian influenza (AI H5N1) epidemic was assessed in relation to both spatial variables and the generation interval of the invading virus. Proximity to the highway network appeared to promote epidemic dispersal: from the first AI generation interval onwards > 20% of all cases were located at < 5 km from the nearest major road. Fifty-seven per cent of all cases were located 31 km from three highway intersections. Findings suggest that the spatial features of emerging infections could be key in their control. When the spatial location of a transmission factor is well known, such as that of the highway network, and a substantial percentage of cases (e.g. > 20%) are near that factor, early interventions focusing on transmission factors, such as road blocks that prevent poultry trade, may be more efficacious than interventions applied only to the susceptible population.

Spatial distribution of acaricide profiles (Boophilus microplus strains susceptible or resistant to acaricides) in southeastern Mexico.

The ability of Boophilus microplus strains to be susceptible (-) or resistant (+) to amidines (Am), synthetic pyrethroids (SP), and/or organo-phosphates (OP) (or acaricide profiles) was investigated in 217 southeastern Mexican cattle ranches (located in the states of Yucatán, Quintana Roo, and Tabasco). Three questions were asked: (1) whether acaricide profiles varied at random and, if not, which one(s) explained more (or less) cases than expected, (2) whether the spatial distribution of acaricide profiles was randomly or non-randomly distributed, and (3) whether acaricide profiles were associated with farm-related covariates (frequency of annual treatments, herd size, and farm size). Three acaricide profiles explained 73.6% of the data, representing at least twice as many cases as expected (P<0.001): (1) Am-SP-, (2) Am+SP+, and (3) (among ranches that dispensed acaricides > or = 6 times/year) Am-OP+SP+. Because ticks collected in Yucatán ranches tended to be susceptible to Am, those of Quintana Roo ranches displayed, predominantly, resistance to OP/SP, and Tabasco ticks tended to be resistant to Am (all with P < or = 0.05), acaricide profiles appeared to be non-randomly disseminated over space. Across states, two farm-related covariates were associated with resistance (P < or = 0.02): (1) high annual frequency of acaricide treatments, and (2) large farm size. Findings supported the hypothesis that spatial acaricide profiles followed neither random nor homogeneous data distributions, being partially explained by agent- and/or farm-specific factors. Some profiles could not be explained by these factors. Further spatially explicit studies (addressing host-related factors) are recommended.

Human-mediated foot-and-mouth disease epidemic dispersal: disease and vector clusters.

Disease clusters were retrospectively explored at national level using a geo-referenced dataset from the 2001 Uruguayan Foot-and-Mouth Disease (FMD) epidemic. Disease location and time (first 11 epidemic weeks) were analysed across 250 counties (of which 160 were infected), without and with control for human mobility related factors (human population and road densities). The null hypothesis of random disease distribution over space and/or time was assessed with: (i) purely temporal; (ii) purely spatial; and (iii) space/time tests. At least within epidemic weeks 2 and 6, a principal disease cluster was observed in 33 contiguous counties (P < 0.01). Two secondary clusters, located at >100 km from each other, were also observed (P < 0.01). The purely spatial test that controlled for human population density identified two non-contiguous clusters (P < 0.01). Space and time analysis also revealed the same 33 counties as members of the principal cluster, of which 31 were also clustered when human population was controlled (P < 0.01). No clusters were reported by the spatial test when road density was assessed. The hypothesis that human mobility related factors autocorrelate with disease was empirically supported by two pieces of information: (i) removal of human population/road densities eliminated >93.9% of the counties included in the principal disease cluster; and (ii) statistically significant correlations (P < 0.05) were observed in the first three epidemic weeks between road density and the number of cases. Clusters where human population density was associated with 47% greater number of cases/sq. km than that of the principal cluster indicated possible roles as disease vectors (vector clusters). Selective control policy in vector clusters is recommended. Periodic (i.e. weekly) cluster and correlation analyses of both disease and other covariates may facilitate disease surveillance and help design space-specific control policy.

Early and cost-effective identification of high risk/priority control areas in foot-and-mouth disease epidemics.

Geo-referenced data from the 2001 Uruguayan foot-and-mouth disease (FMD) epidemic were explored to assess whether spatial analysis could lead to cost-benefit based policies. Four variables were analysed: (i) location and size of 4022 individual rural land parcels, of which 574 were infected over 60 days, (ii) animal density, (iii) percentage of dairy farms per county, and (iv) road density. Each variable was categorized into two to five classes (e.g. small/medium/large) and the proportion of cases per class reported at days 1-3 of the epidemic was compared with that reported at days 4-6. A higher proportion of cases was found at days 4-6 than at days 1-3 in areas with: small and medium size land parcels, high animal density, > 20% farms specialized in dairy production, and high road density (P < 0.03 for each). Each of these classes showed a greater proportion of cases at days 7-60 than the proportion of the total territory covered by each class's area (early case concentration ratios: 1.14-1.37). Land parcel clusters were indicated by Moran's I-test (P < 0.01). A new region was constructed by intersecting the four spatial classes associated with higher proportions of cases at days 4-6. At days 7-60, this region included 50.4% of all cases and represented 30.6% of the territory under study (final case concentration ratio: 1.65). The final area per case in this region was at least 33% lower and covered at least 45% less territory than any of the four single-variable approaches. Bio-statistical, multivariate spatial analysis of early cases may greatly increase the efficiency of epidemiologic policy.

Critical response time (time available to implement effective measures for epidemic control): model building and evaluation.

The time available to implement successful control measures against epidemics was estimated. Critical response time (CRT), defined as the time interval within which the number of epidemic cases remains stationary (so that interventions implemented within CRT may be the most effective or least costly), was assessed during the early epidemic phase, when the number of cases grows linearly over time. The CRT was calculated from data of the 2001 foot-and-mouth disease (FMD) epidemic that occurred in Uruguay. Significant regional CRT differences (ranging from 1.4 to 2.7 days) were observed. The CRT may facilitate selection of control measures. For instance, a CRT equal to 3 days would support the selection of measures, such as stamping-out, implementable within 3 days, but rule out measures, such as post-outbreak vaccination, because intervention and immunity building require more than 3 days. Its use in rapidly disseminating diseases, such as FMD, may result in regionalized decision-making.

Breastfeeding behavior and infant survival with emphasis on reverse causation bias: some evidence from Nigeria.

The possibility of selection bias in the estimation of the effects of breastfeeding on subsequent survival is implied by the clinical evidence that children who are healthier at birth are more likely to be breastfed than their less healthy counterparts who may be prone to difficulties in sustaining breastfeeding. This paper addresses an important problem in understanding the association of breastfeeding and child survival with regard to reverse causation. It utilizes data on the reported reason for weaning to assess the degree to which reverse causality may be responsible for observed associations. The analysis indicates that children who are weaned in the neonatal period because of illness or weakness to suckle, experience a much higher risk of dying than others. This is not mainly because of the cessation of breastfeeding, but because of the original factor, being their illness. Any biases imparted by an initial selection mechanism appear, therefore, to have influence on the effectiveness of breastfeeding behavior.

Interaction between milk yield of Holstein cows in Mexico and the United States.

Genotype by environment interaction for milk yield was investigated by analyzing 55,162 mature equivalent, first lactation records of daughters from 1339 Holstein sires in Mexico and 499,401 daughters from 663 Holstein sires in the northeastern US. There were 474 US sires in common. Herd-year standard deviation was used to define non-overlapping high (> or = 1600 kg) and low (< or = 1300 kg) Mexican environments and a low (< or = 1025 kg) US environment. Variance components across Mexican environments were about 40% less than those of the US environment. Genetic correlation coefficients between milk yield in various Mexican environments and all US environments ranged from 0.60 to 0.71 and were different from unity (P < 0.001). Genetic correlation coefficients with low environment in the US ranged between 0.69 and 0.93; the largest correlation was between the low US and high Mexico environments. Both reductions in the size of genetic variance in Mexican environments relative to the US and genetic correlation coefficients less than unity were indicative of genotype by environment interaction. A significant rank change in estimated breeding values (EBV) of sires in Mexican environments relative to the US was another indicator of genotype of EBV of a sire estimated from daughters performances in low and high environments in Mexico were 0.46 and 0.62 against EBV of sires estimated from all data in the US. Against EBV estimated from the low environment in the US they were 0.57 and 0.83. The US low environment was a better predictor of performance in Mexican environments.

Effects of maternal nutritional status and maternal energy supplementation on length of postpartum amenorrhea among Guatemalan women.

To investigate the extent to which better maternal nutrition leads to reduction in length of postpartum amenorrhea, multivariate-logistic and linear-regression analyses were applied to data on 339 mother-infant pairs from the longitudinal Guatemalan Four Village Study, 1969-1977. Maternal triceps skinfold thickness was negatively associated with length of amenorrhea when infant supplementation (a proxy for reduced suckling) was accounted for. However, its effect was small: amenorrhea was only 0.5 mo shorter among women at the 75th percentile than among those at the 25th, equivalent to less than even one additional child during the women's reproductive years. Maternal supplementation was not associated with length of amenorrhea when infant supplementation was controlled. This is in contrast to previous studies in which breast-feeding or infant supplementation was not controlled. These results suggest that infant, not maternal, supplementation influences length of postpartum amenorrhea, and that maternal nutritional status has minimal influence.

Determining the minimum sample size required to obtain sufficient progeny with a desired genotype at two quantitative trait loci.

In this paper we determine the minimum progeny sample size n needed to obtain, with probability α, at least m individuals of a desired two-locus genotype [Symbol: see text] affecting quantitative traits. The two quantitative trait loci (QTLs) of interest may be linked or independent, with or without epistatic interaction between them. Parental genotypes may be known or unknown, and gene action at either locus may range from additive to overdominance. To reduce the required sample size, mating patterns that will produce a high proportion of desired progeny are suggested for different progeny genotypes and dominance levels. Based on the assumption of normally distributed quantitative trait expression, individuals can be classified into a genotype or genotypic group according to their phenotypic expressions. This technique is used to select both parents and progeny with unknown genotypes. Choice of parental classification criteria for a given quantitative trait affects classification accuracy, and hence the probability of obtaining progeny of the desired genotype. The complexity of this probability depends on the dominance level at each locus, the recombination fraction, and the awareness of parental genotypes. The procedure can be expanded to deal with more than two loci.

The effect of linkage on sample size determination for multiple trait selection.

Sufficient sample sizes are needed in breeding programs to be confident, with a specified probability α, of obtaining a specified number of plants of a desired genotype in segregating populations. We develop a method of determining the minimum sample size needed to produce, with specified probability α, at least m individuals of a desired genotype. This method takes into consideration factors affecting differential selection of gametes, segregation at a single locus, and linkage among the loci of interest. We first consider the effects in the gametophyte (haploid level) of fitness and linkage on the frequencies of alleles at two linked loci, then at three or more linked loci. The probability of obtaining at least m successes, or occurrences of the desired allele, among n gametes is given by a formula based on the binomial distribution. This probability is affected by fitness and linkage through their impact on the probability that a single randomly chosen gamete is of the desired type. Using an extension of this approach, we examine the effects of the altered allelic frequencies on the likelihood of obtaining the desired genotype from a randomly chosen pair of gametes in the sporophyte (diploid level). A table and a figure show the sample size required to produce, with probability 0.95, m individuals of the desired g enotype or phenotype, as a function of m and the probability that a randomly selected individual is of the desired type.

Mapping of ripening-related or -specific cDNA clones of tomato (Lycopersicon esculentum).

Nineteen ripening-related or -specific clones from Lycopersicon esculentum were mapped via RFLP analysis using an F2 population from the cross L. esculentum x L. pennellii and cDNA or genomic clones of known map location. The map produced using cDNA and genomic clones of known map location corresponded well with previously published maps of tomato. The number of loci detected for each ripening-related or-specific clone varied from one to seven. These loci were located on all 12 chromosomes of the tomato genome. There was no significant clustering of ripening-related or-specific genes. Regions of very low recombination were observed. The clone for polygalacturonase (TOM6) mapped to a single region on chromosome 10, the same chromosome as the nor and alc ripening mutants. To fine map this chromosome, two backcross populations were produced from the cross of L. esculentum x L. pimpenillifolium, in which the esculentum parents used were homozygous for either the alc or the nor. The coding region for polygalacturonase is functionally unlinked to either of these two ripening mutants.

Prevalence of patent Baylisascaris procyonis infection in raccoons (Procyon lotor) in Ithaca, New York.

The prevalence of patent Baylisascaris procyonis infection in raccoons was determined by examining fecal samples collected between July 1986 and May 1987 in Ithaca, New York. September, October, and November had the highest prevalence of infection (35-48%). Significant differences (P less than 0.001) were found when months were grouped by season to test the hypothesis that a fecal sample's probability of being positive does not vary from month to month. Fall was the season contributing most to the overall chi-square statistic. Host sex/age class and prevalence of patent infection were investigated. The raccoons were aged as either juveniles or adults. A significantly higher prevalence of patent infection (P less than 0.001) was found in juveniles when compared to adults. No statistically significant difference was found in other comparisons of host sex and age. Contingency analysis tested the independence of sex/age class/season and presence of eggs. The results of the test were significant (P less than 0.001).

Single-Locus Inheritance in the Tetraploid Treefrog Hyla versicolor with an Analysis of Expected Progeny Ratios in Tetraploid Organisms.

The recently evolved autotetraploid frog, Hyla versicolor , was examined electrophoretically for evidence of genomic restructuring leading to diploidization. Loci were tested against the progeny ratios expected if inheritance was disomic vs. tetrasomic. Two loci (Mpi and Sod-2) appeared to be inherited tetrasomically, one (Mdh-2) appeared to be inherited disomically, and one (Tpi) appeared to be inherited disomically in one family and tetrasomically in another family, when tested conventionally against 1:2:1 and 1:4:1 segregation ratios. The minimum number of progeny required for this type of analysis for codominant alleles is shown to be 92. Progeny resulting from double reduction were observed, and the occurrence of a null allele class at Mpi was noted. A reexamination of expected progeny ratios in tetraploid organisms reveals that tetrasomic inheritance patterns cannot be predicted without adequate knowledge of the amount of crossing-over, the proportion of tetravalents vs. random bivalents that are formed, and the ratio and types of centromere segregation (alternate and adjacent) that occur from tetravalents in the species being studied. However, disomic inheritance can be unambiguously confirmed only by the production of all heteroallelic gametes from homobivalent, symmetrically heterozygous individuals. In addition, a method is described for estimating genecentromere distances using the ratio of progeny genotypes in certain crosses in tetraploid species.