The way it is explained in WT 2006-1-15 WT
When Pharaoh was told to let God’s people go, he sneered: “Who is Jehovah, so that I should obey his voice to send Israel away? I do not know Jehovah at all and, what is more, I am not going to send Israel away.” (Exodus 5:2) As a result, Egypt experienced these blows: (1) water changed to blood, (2) frogs, (3) gnats, (4) gadflies, (5) pestilence upon livestock, (6) boils upon man and beast, (7) hail, (8) locusts, (9) darkness, and (10) the death of Egypt’s firstborn, including Pharaoh’s son. Finally, Pharaoh let the Hebrews go. He even urged them to leave!—Exodus 12:31, 32 The diff is a Pestilence in Plague 5, and Death in Plague 10. They failed in elaborating the animals being affected in the 10th plague in the article.