Aproximación epidemiológica para medir y entender el aborto bovino / Epidemiologic approaches for measuring and understanding bovine abortion

El objetivo del presente artículo es ilustrar algunos métodos para medir la tasa de aborto en bovinos. Así mismo, se describirán algunas de las asociaciones que se han encontrado entre aspectos demográficos, características reproductivas y la subsecuente presentación de aborto. El método de las tablas de vida permite calcular el riesgo de pérdida fetal (aborto) en unos intervalos predefinidos (ej. días o meses) durante la gestación, y la proporción de vacas que abortan. Por otro lado, el método de la densidad de aborto permite calcular el riesgo de aborto en un período calendario definido (ej. por mes) como una medida del número de abortos durante los días en que la vaca estuvo a riesgo. Ambos métodos estandarizan la información y toman en cuenta el cambio en el número de vacas a riesgo de abortar ya que excluye aquellas que ya no lo están porque fueron vendidas, murieron, abortaron o parieron; por ello proveen un resultado más alto, pero también más real de lo que normalmente se calcula. Estas medidas y modelos estadísticos han sido utilizados para calcular la magnitud esperada de aborto en fincas lecheras (cuyo rango varía entre 8-19%) y también para identificar asociaciones entre factores demográficos o características reproductivas de la vaca y el subsecuente riesgo de aborto. Algunos de esos factores serán discutidos incluyendo el efecto de abortos anteriores, número de partos y edad, días abiertos al momento de la concepción y el efecto de la estación. Otros métodos analíticos de utilidad también serán presentados para el diagnóstico de hato de aborto relacionado con la exposición a agentes infecciosos o a otros factores. Estos métodos permiten estimar el riesgo o la proporción de abortos atribuibles a la exposición a un agente infeccioso o a cualquier otro agente que pueda producir aborto.

The purpose of the paper is to offer methods for use in measuring abortion rates and in undertaking abortion diagnostic investigations. In addition, some of the associations found between demographic and reproductive features of the dam and subsequent abortion will be described. The cohort life table method calculates the risk of fetal loss (abortion), for pre-defined time intervals (eg. days or weeks) during the gestation period, and the overall proportion of cows that abort. In contrast, the abortion density method calculates abortion risk for a defined calendar time (eg. per month) as the number of abortions per cow-days-at risk. Both methods are standardized to account for the changing number of cows at risk of abortion, as a result of culling, death, abortion, and calving, thus tend to produce higher, but more realistic, estimates for abortion than rates typically calculated. These measures and statistical modeling have been used to estimate the expected magnitude of abortion for dairies (ranging from 8% to 19%) and to identify associations between demographic or reproductive features of the dam and subsequent risk of abortion. Some of the factors to be discussed include effects of a previous abortion, dam gravidity and age, days open at the time of conception, and season. Analytic methods also will be presented for herd-based diagnosis of abortion related to infectious agents or other exposures. These methods permit estimates of the risk or proportion of abortions attributable to exposure to an infectious agent or to other putative abortifacient exposures.

Temporal and spatial distributions of foot-and-mouth disease under three different strategies of control and eradication in Colombia (1982-2003).

Outbreaks of foot-and-mouth disease (FMD) from January 1982 through December 2003 were used to examine variations in serotype- and species-specific risk for three control programmes in Colombia: (1982-1983) vaccination, using an aluminium hydroxide, saponin adjuvant, required but not enforced; (1984-1996) vaccination, using an oil double-emulsion adjuvant, required but not enforced; and (1997-2003) enforced vaccination, using an oil double-emulsion adjuvant, restricted animal movement enforced, and slaughter of infected animals. Hypotheses were tested for trend, cyclicity and seasonality in FMD occurrence, and for species- and serotype-specific differences in morbidity and case-fatality. The spatial density of outbreaks was estimated by kernel smoothing. The frequency of outbreaks decreased most between 1984 and 1996 (p < 0.01) for serotype A and between 1997 and 2003 (p < 0.01) for serotype O. Outbreaks occurred in cycles of 3-4 years for both serotypes (p < 0.05). Morbidity was not significantly different in pigs from that in cattle for serotype A-associated outbreaks (p = 0.314), but was higher in pigs than in cattle (p = 0.019) for serotype O-associated outbreaks. For both serotypes, case-fatality was higher for pigs than for cattle (p < 0.009). Temporal variation in FMD incidence provided insight into the expected evolution of FMD control for countries with similar conditions and where FMD is endemic.