Tetrad is understood as the set of four interlinked elements. In the pathology are considered symptoms or injuries linked to each other and although it is sufficient to obtain a triad in the case of avian infectious toxic hepatic enteritis (A.I.T.H.E.) is easily possible to obtain a consistent tetrad in injuries of yolk, blind, liver and gizzard of newborn birds and even in embryos of 18 days.

The yolk appear superior size to 8% of live weight and greenish color, the blind filled with a green liquid presents the gas hydrogen bubbles SH2 , the enlarged and jaundiced liver and gizzard stratum corneum with green injuries bile regurgitation and growth of producing bacteria SH2.