Likelihood ratio estimation without gold standard: a case study evaluating brucellosis c-ELISA in cattle and water buffalo in Trinidad

The likelihood ratio (LR) is a measure of association that quantifies how many more times likely a particular test result is from an infected animal compared to one that is uninfected. They are ratios of conditional probabilities and cannot be interpreted at the individual animal level without information concerning pretest probabilities. Their usefulness is that they can be used to update the prior belief that the individual has the outcome of interest through a modification of Bayes' theorem. Bayesian analytic techniques can be used for the evaluation of diagnostic tests and estimation of LRs when information concerning a gold standard is not available. As an example, these techniques were applied to the estimation of LRs for a competitive ELISA (c-ELISA) for diagnosis of Brucella abortus infection in cattle and water buffalo in Trinidad.Sera from four herds of cattle (n = 391) and four herds of water buffalo (n = 381) in Trinidad were evaluated for Brucella-specific antibodies using a c-ELISA. On the basis of previous serologic (agglutination) test results in the same animals, iterative simulation modeling was used to classify animals as positive or negative for Brucella infection. LRs were calculated for six categories of the c-ELISA proportion inhibition (PI) results pooled for cattle and water buffalo and yielded the following estimations (95% probability intervals): <0.10 PI, 0.05 (0ùC0.13); 0.10ùC0.249 PI, 0.11 (0.04ùC0.20); 0.25ùC0.349 PI, 0.77 (0.23ùC1.63); 0.35-0.499 PI, 3.22 (1.39ùC6.84); 0.50ùC0.749 PI, 17.9 (6.39ùC77.4); ­í0.75 PI, 423 (129ùC­è). LRs are important for calculation of post-test probabilities and maintaining the quantitative nature of diagnostic test results.

Ear-tag retention and identification methods for extensively managed water buffalo (Bubalus bubalis) in Trinidad

Thirty-two young domestic water buffalo were studied to evaluate ear-tag retention during an epidemiologic field trial. Plastic ear-tags were placed in both ears before the start of the trial, which was implemented in an extensively managed domestic water buffalo herd of approximately 1000 animals in Trinidad from 1999–2001. The presence or absence of ear-tags was recorded at the times of animal handling. The rate of ear-tag loss was modeled using a parametric survival analysis assuming an exponential rate of loss. A gamma distribution was used to estimate the amount of time that each animal would be positively identified based only on the presence or absence of one or more ear-tags. The overall median ear-tag retention was 272 days. The estimated rate of ear-tag loss was 0.0024 ear-tags lost per day. The use of ear-tags alone might not be sufficient for long-term identification of extensively managed animal populations.

Seroprevalences of brucellosis, Q-fever and toxoplasmosis in slaughter livestock in Trinidad

Serum samples obtained from livestock (cattle, chicken, pigs, sheep, goats and water buffaloes) slaughtered at various slaughter houses in Trinidad

Veterinary public health: the importance of animal disease to public health in the Caribbean, with special reference to Jamaica

A survey of the Caribbean area by questionnaire, reference to official reports, personal contacts and correspondence, was made in order to find out the incidence of brucellosis. It appears that while the incidence is low in animals and man in most territories, it can assume dangerous proportions in animals if given suitable conditions and can become a formidable public health problem. It is evident that brucellosis in the West Indies is a disease introduced from Europe and the U.S.A. by importation of animals to improve local breeds. It is emphasised that strict precautions must be taken in future to ensure that such animals are brucellosis free (AU)