Sudden chicken death???

[HowellChicken123]

Hello, I went out about 30 minutes ago to let my chickens out to free range and do my daily behavior checks. Everyone seemed completely normal. Shortly after it started raining so I ran out to make sure everyone was getting in okay and I found one of our 5 month old silkies layed out flat on the ground panting (looked almost like when they hide from a hawk). After observing her for a few minutes I decided to bring her inside where she very quickly deteriorated. Began flailing around and lost complete control of all of her limbs. Was still breathing so I made the decision to put her down. Does anyone have any idea what may have caused this? Can Mareks be so fast acting? Really don’t want to lose more of my flock.

 

[sourland]

What is the temperature where you are?

 

[HowellChicken123]

What is the temperature where you are?

In mid Michigan, so it has been low 80’s the past week

 

[HowellChicken123]

Sorry not trying to spam, just really hoping someone may have some insight. Coop is fully shaded and we do measures to try to prevent heat stroke