ABSTRACT
Since 2006 increasing numbers of laying hen flocks with decreased production have been reported in the Netherlands. At necropsy, birds from affected flocks showed multifocal areas of necrosis in the duodenum. Histologically the duodenum had moderate to marked villus atrophy and fusion with crypt hyperplasia and a mixed inflammatory infiltrate within the lamina propria underlying focal areas of degenerative epithelium. Multifocally, free within the intestinal lumen and associated with epithelial necrosis, were marked numbers of large rod-shaped bacteria. Anaerobic culturing and subsequent toxin typing revealed, in 19 out of 73 affected birds, the presence of Clostridium perfringens strains, either type A or type C harbouring the atypical allele of cpb2 and netB. Eighteen out of these 19 birds carried C. perfringens strains capable of producing beta2 toxin in vitro and all of these birds harboured C. perfringens strains capable of producing NetB toxin in vitro. In contrast, specific pathogen free (SPF) birds lacked gross or histological lesions in their duodenum, and C. perfringens type C was isolated from four out of 15 SPF birds tested. One of these isolates harboured the consensus three allele of cpb2 that produced beta2 toxin in vitro. None of the C. perfringens isolates originating from SPF birds harboured netB. These findings might indicate that the NetB toxin produced by C. perfringens is associated with subclinical necrotic enteritis in layers, whereas the involvement of beta2 toxin in subclinical necrotic enteritis, if any, might be variant dependent.
Subject(s)
Chickens/microbiology , Clostridium Infections/veterinary , Clostridium perfringens/isolation & purification , Enteritis/veterinary , Poultry Diseases/microbiology , Alleles , Animals , Bacterial Toxins/genetics , Bacterial Toxins/metabolism , Clostridium Infections/microbiology , Clostridium Infections/mortality , Clostridium Infections/pathology , Clostridium perfringens/genetics , Clostridium perfringens/metabolism , DNA, Bacterial/genetics , Duodenum/pathology , Enteritis/microbiology , Enteritis/mortality , Enteritis/pathology , Enterotoxins/genetics , Enterotoxins/metabolism , Female , Necrosis , Netherlands/epidemiology , Polymerase Chain Reaction/veterinary , Poultry Diseases/mortality , Poultry Diseases/pathology , Specific Pathogen-Free OrganismsABSTRACT
La enfermedad de Newcastle es una enfermedad de importancia económica de las aves de corral y la vacunación constituye una importante medida preventiva en muchos países. Sin embargo, se han notificado brotes en las poblaciones vacunadas. Esto sugiere que o bien el nivel de cobertura de vacunación es muy bajo o que la vacunación no proporciona inmunidad perfecta, permitiendo que el virus se propague en las poblaciones vacunadas parcialmente.En este artículo se estudian los requisitos de un programa de vacunación epidemiológicamente eficaz contra la enfermedad de Newcastle en las aves de corral, sobre la base de datos de estudios experimentales de transmisión. Los estudios de transmisión indican que las aves vacunadas con bajos o indetectables títulos de anticuerpos podrían no estar protegidas contra las enfermedades y la mortalidad, por lo tanto la infección y la transmisión puede ocurrir. De hecho, nuestros análisis cuantitativos muestran que el virus de la enfermedad de Newcastle es altamente transmisible de las aves de corral con bajo títulos de anticuerpos.Como consecuencia de ello, la inmunidad sólo puede lograrse si una proporción elevada de aves (mayor que 85 por ciento) tienen una alta valoración de anticuerpos (log2 título de inhibición de la hemaglutinación mayor o igual que 3) después de la vacunación. Se discuten las implicaciones para el control de la enfermedad de Newcastle en las aves de corral por la vacunación.
Subject(s)
Animals , Poultry , Newcastle Disease , Poultry Diseases , Immunity , Newcastle disease virusABSTRACT
Newcastle disease is an economically important disease of poultry for which vaccination is applied as a preventive measure in many countries. Nevertheless, outbreaks have been reported in vaccinated populations. This suggests that either the vaccination coverage level is too low or that vaccination does not provide perfect immunity, allowing the virus to spread in partially vaccinated populations. Here we study the requirements of an epidemiologically effective vaccination program against Newcastle disease in poultry, based on data from experimental transmission studies. The transmission studies indicate that vaccinated birds with low or undetectable antibody titres may be protected against disease and mortality but that infection and transmission may still occur. In fact, our quantitative analyses show that Newcastle disease virus is highly transmissible in poultry with low antibody titres. As a consequence, herd immunity can only be achieved if a high proportion of birds (>85%) have a high antibody titre (log(2) haemagglutination inhibition titre > or =3) after vaccination. We discuss the implications for the control of Newcastle disease in poultry by vaccination.
