It is said that anything injected into the blood of the fasting person breaks the fast. So when the saliva or poison from insect bites is injected into the body why isn't the fast broken?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Not everything that is injected in the blood disrupts the fast.  Only that which compensates the food or the drink invalidates fasting such as feeding injections like “Glucose”….

So, the insect bite does not affect the validity of fasting even if its saliva or venom spreads in the blood, for the reason mentioned above.  In addition, the insect bites the person with no prior notice.  So, the person’s will and choice are absent here.  And it is known that when the person is compelled to do something that normally breaks the fast, then his/her fast is valid.

The Prophet (Sallallahu Alaihi wa Sallam) said: “Allah has pardoned to my Ummah the mistake, the thing they forget and the thing they did under compulsion”. [Ibn Majah]

Allah knows best.