Systematic Veterinary Virology

CULTIVATION

• DHV I grows well in ducklings, embryonated eggs and in cell cultures.
• Ducklings: Ducklings between 1 and 7 days of age are highly susceptible to DHV type I. The suspected materials are injected subcutaneously or intramuscularly. The characteristic clinical signs are followed by death of ducklings within 18–48 hours of inoculation, often in under 24 hours.
• Duck eggs (10–14 days) and chicken eggs (8–10 days) are used for culturing DHV I. The route of inoculation is allantoic sac route. Duck embryos die between 24 and 72 hours later, whereas chicken embryos are more variable and die between 5 and 8 days. Gross pathological changes in the embryos include stunting and subcutaneous haemorrhages over the whole body, with oedema particularly of the abdominal and hind limb regions. The embryo livers may be red and yellowish, swollen and may show some necrotic foci. In embryos that take longer to die, the greenish colour of the allantois is more pronounced, and both the liver lesions and stunting become more prominent.
• Primary cell cultures like duck embryo liver (DEL) cells are commonly used. DHV type I cause a cytopathic effect (CPE), which is characterised by cell rounding and necrosis. When overlaid with a maintenance medium containing 1% agarose (w/v), the CPE gives rise to plaques approximately 1 mm in diameter.
• DHV II (Astrovirus) are difficult to cultivate under laboratory conditions.
• DHV III (Pirconavirus) can be cultivated in the laboratory. The virus will not grow in embryonated chicken eggs.