The immune phase is associated with the appearance of circulating antibody, the disappearance of organisms from the blood and CSF, and the appearance of additional signs and symptoms. Despite the presence of circulating antibody, leptospires may persist in the kidney, urine, and aqueous humor. The immune or leptospiruric phase may last for several weeks. Most cases of human leptospirosis are subclinical, with inapparent infection particularly common in high-risk occupational groups such as farmers and their families. The onset of the initial or septicemic phase is abrupt, with fever, shaking chills, severe headache, malaise, nausea, vomiting, and severe, often debilitating muscular pain.
Circulatory collapse is uncommon, but some patients have bradycardia and hypotension.
Typically, the child is lethargic, with mild to moderate dehydration. Additional physical findings include extreme muscle tenderness, which is most prominent in the lower extremities, the lumbosacral spine, and the abdomen. Less common manifestations include pharyngitis, pneumonitis, arthritis, carditis, cholecystitis, and orchitis. The second or immune phase may follow a brief asymptomatic interlude and is characterized by recurrence of fever. Aseptic meningitis is the hallmark of this phase.