The pathogens of the horse plague were transmitted through insects such as mosquitoes and flies, and the immune system of the host directly affected the length of the incubation period.
The northern kingdom held the custom of sky burial in high esteem, and they let the corpses of the Ferghana horses rot on the ground, becoming the food of insects, birds, and beasts.
Ranches were not spotlessly clean, so the slaves naturally wouldn't take any precautions against the small mosquitoes and insects.
One could imagine how terrifying it would be when the horse plague spread everywhere.
The horse dealers of Shalan conned the royal court of the northern kingdom of the huge sum of 10 million strings of coins. Fortunately, all the mares were done breeding, and thousands of mares were gradually found to be pregnant, so they managed to recover some losses. Otherwise, the royal court of the northern kingdom would have lost their shirt.