Transmission of infectious bronchitis virus within vaccinated and unvaccinated groups of chickens.
The aim of this study was to determine whether vaccination against infectious bronchitis virus (IBV) reduces virus transmission, i.e. to test whether IBV transmission among vaccinated chickens is significantly reduced compared to that among unvaccinated chickens. In two vaccinated and two unvaccinated groups of SPF chickens, a standard measure for virus transmission, the reproduction ratio (R) was determined. R is defined as the average number of new infections caused by one typical infectious individual during its entire infectious period. A single vaccination by eye-drop with IBV H120 reduced the transmission of the IBV challenge virus among the vaccinated chickens (estimated R = 0.69, s.e. = 0.33) significantly (P < 0.05) compared to the transmission among the unvaccinated chickens (estimated R = 19.95, s.e. = 12.41). The possible implications for further study, including selection or development of vaccines are discussed.
Transmissibility of Infectious Bronchitis Virus H120 Vaccine Strain Among Broilers Under Experimental Conditions
Abstract The aim of this study was to quantify transmission of infectious bronchitis virus (IBV) H120 vaccine strain among broilers, and to assess whether birds that have been exposed to vaccine strain-shedding birds were protected against clinical signs after infection with a virulent strain of the same serotype. A transmission experiment and a replicate were carried out, each with six groups of commercial broilers. At day of hatch (n  =  30) or at 15 days of age (n  =  20), half of each group was inoculated with either IBV H120 vaccine (H120 group), virulent IBV M41 (M41 group), or were mock-infected, thereby contact-exposing the other half of each group. Nasal discharge was recorded, and antibody response and virus shedding were measured. To measure clinical protection, four weeks after inoculation all birds, in all groups, were challenged with IBV M41. The reproduction ratio (R; the average number of contact infections caused by one infectious bird) was determined to quantify virus transmission. All contact-exposed birds, except for one in an H120 group, became infected with either IBV H120 or IBV M41. Almost all birds contact-infected with IBV H120 or IBV M41 were subsequently protected against clinical signs after challenge with IBV M41. The lower limits of the 95% confidence interval (CI) of the R of IBV H120 vaccine, and of IBV M41, were significantly <1. For both IBV H120 and IBV M41, the 95% CI was [2.1–∞] following inoculation at day of hatch and [1.8–∞] after inoculation at 15 days of age. This finding demonstrates that IBV H120 vaccine is able to spread extensively among broilers. This implies that this vaccine strain might be able to become endemically present in the poultry population. It also implies that, even if not all birds received vaccine during spray application, due to the ability of the vaccine to spread in the flock, they will most likely be protected against clinical signs after a subsequent field virus infection. Transmisibilidad de la cepa vacunal H120 de bronquitis infecciosa entre pollos de engorde bajo condiciones experimentales. El objetivo del presente trabajo fue cuantificar la transmisión de la cepa vacunal H120 de bronquitis infecciosa entre pollos de engorde y evaluar si las aves que han sido expuestas a aves diseminando la cepa vacunal del virus, estaban protegidas contra los signos clínicos luego de un desafío con una cepa virulenta perteneciente al mismo serotipo. Se realizó un experimento de transmisión viral y una réplica, cada uno con seis grupos de pollos de engorde comerciales. Al día de edad (n  =  30) o a los 15 días de edad (n  =  20) la mitad de cada grupo se inoculó con la vacuna H120 (grupo H120), con virus virulento de bronquitis infecciosa cepa M41 (grupo M41) o no fueron infectadas, en consecuencia exponiendo por contacto a la otra mitad de cada grupo. Se llevó registro de las descargas nasales y se midió la respuesta de anticuerpos y la diseminación del virus. Cuatro semanas después de la inoculación, todas las aves se desafiaron con la cepa virulenta de bronquitis infecciosa M41 para evaluar la protección clínica. Cuantificando la transmisión del virus, se determinó la tasa de reproducción (promedio de infecciones por contacto causadas por un ave infecciosa). Todas las aves expuestas por contacto, excepto una en el grupo H120, se infectaron con el virus H120 o M41. Casi todas las aves expuestas por contacto con el virus H120 o con M41mostraron protección contra signos clínicos luego de un desafío con la cepa M41 de bronquitis infecciosa. Los limites inferiores de los intervalos de confianza 95% de la tasa de reproducción de la vacuna H120 y del virus M41 fueron significantes <1. Para ambos virus el intervalo de confianza 95% fue [2.1–∞] luego de la inoculación al día de edad y [1.8–∞] después de la inoculación a los 15 días de edad. Este hallazgo demuestra que la vacuna de bronquitis infecciosa H120 es capaz de diseminarse ampliamente entre pollos de engorde. Esto implica que la vacuna puede tornarse endémica en la población avícola. Esto a su vez implica que si no todas las aves recibieron vacuna durante la aplicación por aspersión, debido a la capacidad de la vacuna de diseminarse en el parvada, estas aves probablemente esten protegidas contra signos clinicos luego de una infeccion viral en el campo. Abbreviations: CI = confidence interval; HI = hemagglutination inhibiting; IB = infectious bronchitis; IBV = infectious bronchitis virus; MLE = maximum likelihood estimate; R = reproduction ratio
Estimating Crop Growth Rate with Land Treatment
A new and more comprehensive procedure for estimating crop growth rate is illustrated with data from the literature for coastal bermudagrass. Yield data are described by the Gaussian probability function. The effect of irrigation and harvest frequency is shown. A plot of relative yield for each combination of irrigation and harvest frequency on probability paper shows that all the data follows a single line. By assuming and average composition, it is possible to estimate weekly nutrient uptake.