Systematic Veterinary Virology

SYMPTOMS

• Signs are predominantly seen in nervous, respiratory or reproductive systems. Morbidity is usually high and mortality varies from 0–100%. Higher mortality is seen in velogenic disease in unvaccinated stock.
• The clinical signs are highly variable and depend on the nature of the infecting virus (pathotype), the infective dose and the degree of immunity birds possess from previous infection or vaccination.
• The first clinical sign in laying chickens is usually a marked drop in egg production followed within 24 to 43 hours by high death losses. At the onset, 10-15 percent of a flock may be lost in 24 hours. After 7 to 10 days, deaths usually subside, and birds surviving 12 to 14 days generally do not die but may display permanent paralysis and other neurologic signs.
• The reproductive system may be permanently impaired, and thus egg production does not return to previous levels. In vaccinated chickens, or chicks protected by parental antibodies, the clinical signs are less severe and are proportional to the level of protective antibodies.
• With viscerotropic strains (VVND), edema of the head, especially around the eyes may become apparent after birds have been sick for 2 or 3 days. This edema usually does not involve the comb and wattle as in case of highly pathogenic avian influenza (HPAI).
• A dark ring sometimes forms around the eye, probably due to cyanosis and poor blood circulation in the edematous tissue. This "black eye" appearance is especially visible in white chickens. Bile-stained, greenish-dark diarrhea may be noted 2 to 3 days after onset of illness. Some birds in an affected flock usually have diarrhea throughout the course of the disease.
• The most noteworthy clinical sign in unvaccinated flocks is sudden death without prior indications of illness. Neurotropic strains cause respiratory signs followed by neurologic signs, including muscular tremors, paralysis of legs or wings, torticollis, and opisthotonos.
• The peracute onset often causes the owner to suspect poisoning.  Fall in egg production leading to complete cessation of egg laying and deaths.