Borrelia is usually transmitted via the tick bite or body louse to the human host. After entering the bloodstream, spirochetes replicate extracellularly and remain predominantly in the plasma space. Patients generally remain asymptomatic until high-level spirochetemia develops, at which time symptoms begin abruptly. Organisms are cleared predominantly by opsonizing antibodies with the resolution of symptoms ( afebrile period), followed several days or weeks later by the reemergence of a new antigenic strain, high-level spirochetemia, and recurrence of symptoms. There are multiple genes in the spirochete encoding variable membrane proteins( VMPs). These VMPs determine the antigenic serotype of the organism. At any given time, each spirochete has VMP genes that are expressed and others that are silent. An antigenic switch occurs when a given VMP gene transposes from silent to an expressed locus. This cyclical process of initially effective immune response followed by antigenic variation and immunologic escape is responsible for the relapsing nature of this illness.

• Borrelia is usually transmitted via the tick bite or body louse to the human host.
• Borrelia is able to induce cycles of disease by varying antigen expression and by displaying new outer-surface proteins(VMPs) during the disease course(antigenic variation). The antigenic variation contributes to the recurring nature of relapsing fever.There are multiple genes in the spirochete encoding variable membrane proteins( VMPs). These VMPs determine the antigenic serotype of the organism. At any given time, each spirochete has VMP genes that are expressed and others that are silent. An antigenic switch occurs when a given VMP gene transposes from silent to an expressed locus. This cyclical process of initially effective immune response followed by antigenic variation and immunologic escape is responsible for the relapsing nature of this illness.

Incubation period = time from tick bite to illness

• 7 days, range 2 to 18 days

Symptomatic period= Length of illness = time from symptom onset to resolution of symptoms

• 3 days, range 2 to 7 days
• During episodes of spirochetemia and replicate extracellularly and remain predominantly the plasma space. Borrelia invades the endothelium. After multiplying in the endothelial cells, the bacteria are released into the bloodstream and patients develop symptoms that include fever. However,the organisms may invade the brain, eye, inner ear, heart, liver. Splenic abscesses and infarcts, and hemorrhages within the central nervous system, a perivascular histiocytic interstitial myocarditis, conduction defects, arrhythmias, and myocardial failure, splenic rupture with massive hemorrhage, cerebral hemorrhage, and hepatic failure are seen in LBRF which can result in sudden death.Hepatitis with patchy hemorrhages and necrosis, meningitis, and perisplenitis, thrombi in small vessels also are found. Thrombi are occasionally found occluding small vessels.^[1]CNS involvement is more common in TBRF than LBRF, however eschars, ARDS, cranial nerve palsies, focal neurologic deficits, uveitis, iritis or iridocyclitis, splenic rupture and myocarditis may be seen in both TBRF and LBRF.^[2][3]Most information on the pathophysiologic aspects of fatal cases comes from autopsy data of louse-borne disease in humans or experimental animals.^[3]

Afebrile period= Length of time before reoccurrence = time from the resolution of symptoms to reoccurrence of symptoms

• 7 days, range 4 to 14 days
• Organisms are cleared predominantly by opsonizing antibodies with the resolution of symptoms ( afebrile period), followed several days or weeks later by the reemergence of a new antigenic strain, high-level spirochetemia, when high-level spirochetemia (104-108 organisms m!) develops, at which time symptoms begin abruptly.

• Most cases eventually resolve spontaneously. Occasionally, the crisis occurs after the resolution. During the crisis, patients may develop cerebral edema with seizures, cardiac failure, or death. This stage may result in death in up to 10% of patients.

• Specific serotypes can recur within an individual. Infection with a given strain of Borrelia may cause partial protection against subsequent infection by the same strain. In some highly endemic areas, relapsing fever is more severe in newcomers than natives.
  

The development of the cyclical process is the result of antigenic variation and subsequent immunologic escape:

• There are multiple genes in the spirochete encoding variable membrane proteins( VMPs) which are the outer membrane of spirochetes contains surface proteins. Attacks of relapsing fever end abruptly when specific bactericidal immunoglobulin M antibodies generated by the B1b cell subset lyse spirochaetes in the blood, independently of complement and T cells. These VMPs determine the antigenic serotype of the organism and at any given time, each spirochete has VMP genes that are expressed and others that are silent. An antigenic switch occurs when a given VMP gene transposes from silent to an expressed locus. This mechanism of antigenic variation presumably leads to immunologic escape and relapse. Because of the organism’s ability to undergo antigenic variation, several serotypes may be present in the bloodstream at one time.