Embryonated egg inoculation technique is one of the common techniques employed for virus isolation and identification. Embryonated chicken eggs provide an ideal environment for the cultivation of pathogenic viruses and other forms of virus for various pharmaceutical, medical and research purposes. Since its development and use in the early 1930’s, embryonated egg inoculation technique for viral cultivation has played tremendous role in the study of virology especially in their application for the production of vaccines (Figure 1).
Figure 1: Scientists working on vaccine production for the dreaded H1N1 influenza virus strain at a pharmaceutical company in Hangzhou in East China’s Zhejiang province.
Developing chick embryo eggs that are about 10-14 days old are often the most preferred for embryonated egg inoculation technique. This is because such category of embryonated eggs provides a variety of differentiated tissues or cells such as the chorioallantoic membrane, amniotic fluids, allantoic cavity and the yolk sac region – which alls serve as suitable substrates for the growth of a wide array of viruses including those that cause infections in humans. Inoculation of embryonated chicken eggs is still the most convenient method for cultivating high concentrations of viruses and, thus, this technique of viral cultivation (i.e. the embryonated chicken egg technique) is still being used today for the production of vaccines and even for other research purposes even though that the cell culture technique of viral cultivation as aforementioned is replacing this method of viral cultivation in some quarters.
Embryonated chicken eggs are prepared prior to their usage for viral cultivation. Egg candling as shall be seen later is an important process of this preparation; and the major regions for the inoculation of the embryonated eggs with the viral particle or samples suspected of containing pathogenic virus(s) includes the chorioallantoic membrane, yolk sac, allantoic cavity and amniotic cavity (Figure 2). One of the major disadvantages associated with the use of embryonated eggs for viral cultivation is that the site of inoculation varies for different types of viruses. For example, herpes simplex virus is inoculated in the yolk sac while the allantoic cavity is for the inoculation of influenza virus, avian adenovirus, mumps virus and Newcastle disease virus. Poxvirus is inoculated in the chorioallantoic cavity. The advantages of using embryonated chicken eggs for viral inoculation are enormous.
- Embryonated chicken egg technique is cost effective and requires less labour to undertake.
- The embryonated eggs provide the ideal environment for the growth and replication of viruses – which only replicate in living cells.
- This technique is free from contamination especially from bacteria and other viral agents.
- It is widely used to cultivate viruses meant for the production of vaccines.
- Embryonated chicken egg technique is ideal for the isolation and cultivation of avian viruses.
- It provides a wide range of tissues and fluids foe the cultivation of viruses.
- The growth environment provided by this technique is sterile, and it does not produce immunity against the cultivated viral agent as is applicable in the use of animal models.
Figure 2: Routes of inoculation of embryonated egg with virus.
Embryonated chicken eggs are usually used for the isolation of some avian viruses. However, other viruses can also be cultured using this medium. Eggs provide a suitable means for the primary isolation and identification of viruses; for the maintenance of stock cultures; and for the production of vaccines. Depending on the virus of interest to be isolated or cultured, the specimen suspected to contain a virus is inoculated in the amniotic cavity, the allantoic cavity, and the yolk sac or on the chorioallantoic membrane. Viral growth is usually determined by haemagglutination (HA) technique or by immunoflourescence. The air sac is used for respiration and for pressure adjustments. The egg shell and shell membrane function both as a barrier and as an exchange system for gases and liquid molecules. The chorioallantoic sac and the allantoic fluid remove waste products produced by the developing embryo. The yolk sac is the source of nourishment for the developing embryo. The amnion is a thin membrane that encloses the embryo and protects it from physical damage.
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