Biphasic (Saddleback) Fever: Pattern, Causes & Clinical Significance

Biphasic fever (also saddleback fever) is a temperature pattern with two distinct peaks separated by a period of defervescence (return to normal or subnormal temperature).

Classic Pattern

1. Phase 1 – High fever: Rapid rise to 39–40.5°C (102–105°F); lasts 2–5 days.
2. Phase 2 – Defervescence: Temperature normalizes (or drops below baseline); lasts 1–3 days. Patient may feel better.
3. Phase 3 – Second fever spike: Temperature rises again, often to similar height; lasts 2–4 days before final resolution.

Pathophysiology

• Initial viremia/bacteremia → cytokine storm (TNF-α, IL-1, IL-6) → fever.
• Transient immune control → defervescence.
• Secondary viremia / immune complex dissemination / cytokine rebound → second peak.

Classic Etiologies

Disease	Pathogen	Notes
Dengue fever	DENV 1–4 (flavivirus)	Most classic; 2nd peak coincides with critical phase (plasma leakage).
Malaria	P. vivax, P. ovale (relapsing)	Periodic fevers; "saddleback" less regular than dengue.
Yellow fever	YFV (flavivirus)	Remission phase = "period of calm" before toxic phase.
Colorado tick fever	Coltivirus	Biphasic in ~50%.
Spirillum minus (Sodoku)	Rat-bite fever	Relapsing fever pattern.
Leptospirosis	Leptospira	Septicemic phase → immune phase (meningism).
Polio	Poliovirus	Minor illness → CNS invasion.

Other Causes

• Drug fever, lymphoma, Kawasaki disease, viral encephalitis, brucellosis.

Clinical Action

• Biphasic pattern + travel/exposure history → targeted testing (Dengue NS1/IgM, malaria smear/RDT, leptospira PCR, blood cultures).
• Monitor for warning signs during defervescence (dengue: abdominal pain, bleeding, effusions).
• Do not assume recovery during the afebrile interval.