General Bacteriology

• Pathotypes of Newcastle Disease Virus (Avian Paramyxovirus-1):
• Classified based on pathogenicity indices (MDT, ICPI, IVPI):
• Velogenic Strains: Highly virulent, causing severe fatal Newcastle disease ($90-100\%$ mortality; viscerotropic or neurotropic).
• Mesogenic Strains: Moderately virulent, causing acute respiratory disease, neurological signs, and egg drop (used as secondary booster vaccines).
• Lentogenic Strains: Low-virulence, causing mild subclinical respiratory infection (used as primary live vaccines, e.g., LaSota, F-strain).
• Hence, virulent disease is caused by velogenic and mesogenic strains.
• nalysis:
• ((A) and (C) only): Correct.

Correct option: (3) (A) and (C) only.

• Lymphoid Leukosis in Domestic Poultry:
• Commonly referred to as "Big liver disease" due to massive diffuse nodular enlargement of the liver (hepatomegaly) and spleen. True.
• Caused by exogenous Avian Leukosis Viruses (ALV, family Retroviridae), transmitted both vertically through the egg albumen/yolk and horizontally. True.
• Neoplastic transformation specifically targets B lymphocytes originating in the bursa of Fabricius. (In contrast, Marek's disease transforms T lymphocytes!). Statement C is False.
• The disease has a long incubation period, manifesting clinically in sexually mature adult birds ($\gt 16-24\text{ weeks}$ of age). True.
• nalysis:
• Statements (A), (B), and (D) are correct. Hence, Option 2 is correct.

• Bacteriological Characteristics of Brucella Species:
• Catalase & Oxidase: Brucella organisms are Gram-negative, non-motile, non-spore-forming coccobacilli that are uniformly catalase positive and predominantly oxidase positive (except B. ovis and B. neotomae) [Statements A & B are TRUE].
• Urease Activity: They are characteristically urease positive, hydrolyzing urea rapidly (B. suis within minutes; B. abortus within hours), making 'urease negative' incorrect [Statement C is FALSE].
• Modified Ziehl-Neelsen (Stamp's MZN) Staining: Because their cell walls contain high mycolic-acid-like lipid complexes, Brucella are partially acid-fast; in Stamp's MZN staining, they resist decolorization with dilute ($0.5\%$) acetic acid and stain characteristically red (pink) against a blue/green background [Statement D is TRUE].

Correct option: (1) (A), (6) and (D) only.