Dear Princess Celestia

Celestia was resting in the peaceful embrace of the blanket formerly resting on her bed, her head resting on her pillows until a magical burst formed before her which left a scroll plopping to the ground with the familiar scent of Twilight's magic emanating in the air, "Twilight? Another scroll already? Hm, she's never been this quick to send scroll before" Celestia mumbled softly as her horn inflamed with her dazzling yellow aura, she raised the scroll and unrolled it so she may gazed upon the contents until her eyes widened at the photos displaying the confusing yet horrific imagery proving Twilight's claims to be true. "Spike...What has happened to you? I have never seen something this...Horrible, in my life..." Celestia spoke softly but the words felt like acid burning through her tongue as each word emerged from her throat, her eyes burned with tears as she stared into Spike's lifeless yet still lively eyes indicating some level of sapient or at least functional anatomy. Celestia narrowed her eyes as she turned to look away from the horrible imagery, she then levitates a quill and untouched scroll before her and began to write her response without a moment of hesitation to stop her.

Dear Princess Twilight Sparkle

I have received your letter and seen the evidence you have provided, I must confess I apologize for my lack of belief in you, but I must also confess that I have truly never seen this before, nor have I ever heard of anything like this happening anywhere in the entirety of Equestrian borders.
This unnatural affliction has not yet breached the borders of Canterlot to my knowledge, I will be informing my sister of this as soon as possible, so we can both quarantine Canterlot to avoid the potential affliction spreading here.
I will also inform the other kingdoms including the other pony cities to enforce similar quarantine guidelines to avoid potential spread, this situation is more dire than I suspected

This affliction does however seen to share one key similarity, aggression. You mentioned that the ponies who were originally afflicted began displaying increased levels of agitation and aggression, I would suggest gathering any uninfected ponies or creatures and taking them to your castle so they can be kept protected.
This means you will be unable to go and find Big Macintosh, you cannot risk anypony becoming afflicted by this ailment when we are just starting to plan for it.

Take care of yourself my faithful friend, and may the universe be in your favour until our next communication

-Your friend, Princess Celestia

"This grows more dangerous, aggressive ponies with an infectious affliction? Could critters become afflicted. Can Alicorns? What would happen if we are afflicted?...Oh, take care of yourself Twilight..." Celestia thought to herself with her eyes widening then she glanced down towards the note that was hovering in her magic, Celestia cleared her throat and rolls the scroll up which she then clipped into place with her royal seal so she could safely send the scroll away. Celestia shot the scroll with her spell making the scroll disappear into the wisp yet again which swiftly escaped from the room as she returned to her four hooves and unfurled her wings to stretch them out for the inevitable flight to find her own sister, "Hopefully, we can help you in Ponyville after we've handled Canterlot, then we can see if this affliction can be undone." Celestia thought to herself yet again, she then galloped over to the doorway leading out to her balcony so she could jump over the railing, Celestia began to descend until she was pulled up into the air by her wings flapping in the air then she used to head towards Luna's royal spire with Luna stepping back into her spire in order to begin her royal duties within the ream of dreams.