They are most active on warm days, when they seek cattle to lay their eggs. When the flies approach to lay eggs the cattle become nervous and attempt to escape the attack by running away, and will even go into water. Because the flies are persistent, the animals are constantly irritated and do not feed properly, which results in an apprecia ble loss of weight and decrease of milk yield. The animals may also hurt themselves severely, or at least become wounded and damage their skins. The larvae irritate the tissues around them, causing the flesh to become greenish-yellow and infiltrated, especially along the tracks where the larvae have wandered, and thus depreciated in value.
Great damage is caused to the hide by the perforation produced.
The annual loss due to this factor alone is very high in some countries. Except for poor growth in cases of severe infestation and decreased milk yield, the animals show no appreciable signs of harboring parasites until the larvae appear along the back. At this point the swellings can be felt and seen. The larva lies in a cyst, which also contains a yellow purulent fluid. Calves and young cattle are more frequently and more severely infected than older animals.