Sensitivity, specificity and predictive probability values of serum agglutination test titres for the diagnosis ofSalmonellaDublin culture-positive bovine abortion and stillbirth

Abstract

The objective of this study was to determine the diagnostic value of maternal serology for the diagnosis of Salmonella Dublin bovine abortion and stillbirth. A retrospective, unmatched, case-control study was carried out using twenty year's data (1989-2009) from bovine foetal submissions to an Irish government veterinary laboratory. Cases (n = 214) were defined as submissions with a S. Dublin culture-positive foetus from a S. Dublin unvaccinated dam where results of maternal S. Dublin serology were available. Controls (n = 415) were defined as submissions where an alternative diagnosis other than S. Dublin was made in a foetus from an S. Dublin unvaccinated dam where the results of maternal S. Dublin serology were available. A logistic regression model was fitted to the data: the dichotomous dependent variable was the S. Dublin foetal culture result, and the independent variables were the maternal serum agglutination test (SAT) titre results. Salmonella serology correctly classified 87% of S. Dublin culture-positive foetuses at a predicted probability threshold of 0.44 (cut-off at which sensitivity and specificity are at a maximum, J = 0.67). The sensitivity of the SAT at the same threshold was 73.8% (95% CI: 67.4%-79.5%), and the specificity was 93.2% (95% CI: 90.3%-95.4%). The positive and negative predictive values were 84.9% (95% CI: 79.3%-88.6%) and 87.3% (95% CI: 83.5%-91.3%), respectively. This study illustrates that the use of predicted probability values, rather than the traditional arbitrary breakpoints of negative, inconclusive and positive, increases the diagnostic value of the maternal SAT. Veterinary laboratory diagnosticians and veterinary practitioners can recover from the test results, information previously categorized, particularly from those results declared to be inconclusive.

Review of Diagnostic Procedures and Approaches to Infectious Causes of Reproductive Failures of Cattle in Australia and New Zealand

“…Salmonella infects and proliferates in the placenta and, subsequently, focal necrosis is often observed in the fetal villi after abortion ( 132 ). Diagnosis can be achieved by culture of fetal stomach contents or organs, as well as serology ( 133 ).…”

Role of Infection and Immunity in Bovine Perinatal Mortality: Part 1. Causes and Current Diagnostic Approaches

“…Salmonella species are not commonly detected in PM studies being more commonly detected in aborted foetuses [78] and associated with neonatal calf mortality [79]. The most commonly detected Salmonella species is S. Dublin [78]. Infected foetuses are often autolysed (Figure 3).…”

“…A single blood sample from the affected dam can have moderate utility as a proxy sample for foetal material. For example, a single blood sample from the non-vaccinated dam of a stillborn or aborted foetus can be up to 85% accurate in predicting a foetal culturepositive result for Salmonella Dublin [78]. However, the primary value of maternal serum is as an exclusionary test for maternal antibody.…”

“…However, repeat sampling may be necessary to rule out those cases whose antibody status fluctuates between seronegative and seropositive (false negatives): 'serologically elusive animals' [93]. Paired maternal sera (more than two weeks apart) may detect increased antibody values (e.g., two to four-fold titre rise, + to ++++, or increased absorbance) for some foetopathogens (e.g., S. Dublin SAT) but not for others (e.g., Leptospira spp., Neospora) due to the lag phase between infection and foetal mortality [78,91]. Higher positive rates in maternal serology than in foetal antigen detection is not uncommon given sero-endemicity with common pathogens, e.g., Vidal et al [25].…”

Investigation of bovine abortion and stillbirth/perinatal mortality - similar diagnostic challenges, different approaches

“…These can have moderate utility as proxy samples for foetal material. For example, a single blood sample from the non-vaccinated dam of an aborted or stillborn foetus can be up to 85% accurate in predicting a foetal culture-positive result for Salmonella Dublin, [25]. However, the primary value of maternal serum is as an exclusionary test for maternal antibody, i.e.…”