Egg candling is defined as a technique that is used to determine the condition of the air cell, yolk, and white component of an embryonated egg prior to its usage for viral culture. Candling detects bloody whites, blood spots, or meat spots, and enables observation of germ development. And it is mainly conducted in a dark area or room – so that the interior components of the egg to be candled can be properly illuminated and seen. The embryonated egg sample(s) to be candled is usually held before a light preferably one coming from a torch light in a vertical or horizontal direction as determined by the researcher (Figure 1). The light penetrates the embryonated egg and makes it possible to observe the inside of the egg. Preferably, the torch light or illuminating source is placed on the embryonated egg is held in a slanting position with the large end of the egg placed against the hole in the candler. The candler is a hollow-cylindrical tube or box that covers the lighted candle used for candling technique.
And the candler should be set on a box or table at a convenient height (about 38 to 44 inches from the floor), so that the illuminating light will not shine directly into the eyes of the researcher – thereby affecting his or her vision. The egg is grasped by the small end and, while held between the thumb and tips of the first two fingers, is turned quickly to the right or left. This moves the contents of the egg and throws the yolk nearer the shell. Because of the color of their shells, brown eggs are more difficult to candle than white eggs. Candling is generally used to dedifferentiate fresh embryonated egg samples from old egg samples; and it helps to detect such abnormalities as bloody whites, blood spots, meat spots, and cracked shells as aforementioned prior to viral cultivation using embryonated sample(s). Embryonated egg samples with bloody whites, blood or meat spots, and cracked shells should not be used for viral cultivation. Only those with plain air space and whose “white” moves freely (inclusive of the other aforementioned factors that candling helps to decipher) should be used for viral cultivation.
Figure 1: Candling of embryonated chicken eggs before inoculation with virus suspensions. CDC
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