Why is a Fever a Medical Emergency on JAK Inhibitors?

If your child with Aicardi-Goutières syndrome (AGS) is taking a JAK inhibitor, a fever of 100.4°F (38.0°C) or higher is considered a strict medical emergency. This is because the medication suppresses their immune system, which can hide or “mask” the typical warning signs of a severe illness. A fever could indicate either a rapidly progressing, life-threatening infection that your child’s body cannot fight off on its own, or a dangerous AGS inflammatory flare. Both scenarios require immediate medical evaluation, blood work, and often prompt treatment. While this is frightening, acting quickly and following a clear emergency plan ensures your child gets the targeted care they need for the best possible outcome.

How JAK Inhibitors Affect the Immune System

In AGS, the body produces too much type I interferon, causing chronic inflammation that damages the brain and other organs ^[1][2]. JAK inhibitors—such as baricitinib, ruxolitinib, or tofacitinib—work by targeting and calming down this overactive signaling pathway ^[3][4].

While these medications are highly effective at improving AGS symptoms like skin lesions and frequent fevers ^[5], they act as immunomodulators (drugs that alter how the immune system works) ^[6][7]. By turning down the immune system to stop it from attacking the body, JAK inhibitors also make it harder for the body to fight off everyday viruses, bacteria, and opportunistic infections ^[8][9].

The Danger of “Masked” Symptoms

Because JAK inhibitors alter immune function, your child might not look or act as sick as they actually are when an infection starts ^[10][11].

Furthermore, these medications can actively suppress the body’s warning signals. For example, some JAK inhibitors have been shown to potently lower the production of C-reactive protein (CRP) ^[12]. CRP is a key blood marker that emergency room doctors typically rely on to detect severe bacterial infections. If your child’s CRP response is blunted by their medication, an ER doctor unfamiliar with JAK inhibitors might falsely believe your child is not seriously ill, even while a dangerous infection is rapidly progressing.

Because fevers and CRP can both be masked, watch for other hidden signs of infection, such as:

• Unusual lethargy, extreme fatigue, or difficulty waking up.
• Changes in breathing (fast breathing, grunting, or working hard to breathe).
• Decreased urination or refusal to drink fluids.
• A sudden change in their neurological baseline (more seizures, increased irritability, or loss of recent skills).

Infection vs. AGS Flare

When a child with AGS develops a fever while on a JAK inhibitor, doctors face a critical diagnostic challenge. The fever could be caused by:

• A severe infection: Which requires immediate targeted treatments (like IV antibiotics) because the child’s immune system is suppressed ^[13].
• An AGS inflammatory flare: Which can also cause sudden fevers (febrile attacks) and widespread inflammation affecting multiple organs ^[14][15].

Because both situations can be life-threatening and require completely different treatments, doctors must perform a thorough clinical evaluation and blood work to rule out a hidden infection before assuming the fever is just a disease flare ^[16].

What to Do When Your Child Has a Fever (100.4°F / 38.0°C or higher)

If your child hits this temperature threshold, you must act quickly:

• Do not wait it out: Seek immediate emergency medical care. Do not wait until morning or until the fever gets higher.
• Do not give fever reducers: Unless explicitly instructed by your specialist, avoid giving acetaminophen or ibuprofen before heading to the ER. These medications can further mask the fever curve the doctors need to assess.
• Hand over an Emergency Letter: ER doctors are much more likely to adapt their standard protocols when presented with a signed letter. Ask your AGS specialist to provide a formal “Emergency Care Letter” that details the diagnosis, the specific JAK inhibitor, the immunosuppressive risks, and the fact that the drug blunts CRP levels.
• Bring the physical medication bottles: ER staff may be unfamiliar with specialty drugs like baricitinib or ruxolitinib. Having the physical bottles ensures exact dosing and name recognition.
• Contact your specialist: Call your child’s neurologist or immunologist on your way to the ER so they can coordinate care directly with the emergency room physicians.