Allan, 1974) and even in infections of specific pathogen
free (SPF) birds, considerable overlapping may be seen. In addition, exacerbation of the clinical signs induced by
the milder strains may occur when infections by other organisms are superimposed or when adverse
environmental conditions are present. As signs of clinical disease in chickens vary widely and diagnosis may be
complicated further by the different responses to infection by different hosts, clinical signs alone do not present a
reliable basis for diagnosis of ND. However, the characteristic signs and lesions associated with the virulent
pathotypes will give rise to strong suspicion of the disease.