Embryonated chicken eggs have been used for decades as a medium for virus propagation, especially for viruses that cause human and animal infections. This method is particularly famous for the production of influenza vaccines. The propagation of viruses within the embryonated chicken eggs happens in the following manner:
- Selection of Eggs: Fresh, fertilized chicken eggs are incubated for a specific period, usually 9-12 days, allowing the chicken embryo to develop sufficiently.
- Inoculation: A small hole is made in the eggshell, usually over the allantoic cavity or the chorioallantoic membrane, depending on the virus. The virus suspension is then inoculated into the egg. For example:
- Influenza viruses are typically inoculated into the allantoic cavity because they replicate well in avian cells present there.
- Mumps and several other viruses might be inoculated into the amniotic cavity because they replicate better in the embryonic tissues.
- Incubation: After sealing the hole (usually with paraffin wax), the inoculated eggs are incubated for a specific period, allowing the virus to infect the embryo and replicate.
- Harvesting: After incubation, the eggs are chilled, and the allantoic fluid or other parts of the egg, where the virus has been replicating, are harvested. The fluid is then tested for the presence and amount of virus.
- Purification: If being used for vaccine production, the harvested viral suspension is then subjected to several purification steps to extract the viral particles and remove egg proteins and other impurities.
- Testing: The viral suspension is then tested to determine the virus titer and to ensure the absence of contaminants.
It’s important to note that while embryonated chicken eggs offer a good medium for many viruses, not all viruses can be cultured this way. The utility of this method depends on the virus’s tropism or its preference for specific cells or tissues.