49.4 Vector-Borne, Tick-Borne & Rickettsial Illnesses

Lyme Disease (Borrelia burgdorferi): Staging & Management

Lyme disease, caused by the spirochete Borrelia burgdorferi (and less commonly Borrelia mayonii in the Upper Midwest), is the most frequently reported vector-borne infection in North America. It is transmitted through the bite of infected blacklegged deer ticks: Ixodes scapularis in the Northeastern, Mid-Atlantic, and North-Central United States, and Ixodes pacificus on the Pacific Coast.

Transmission Biology & Kinetics

Transmission of B. burgdorferi is not instantaneous upon tick attachment. In unengorged ticks, the spirochete resides dormant within the midgut. Upon ingestion of a blood meal, temperature and pH changes trigger spirochetal multiplication, surface protein remodeling (downregulation of OspA, upregulation of OspC), and migration across the hemolymph to the tick's salivary glands. This biological cascade requires a minimum of 36 to 48 hours of tick attachment and feeding. Ticks attached for <24 to 36 hours carry an exceedingly low risk of transmission.

Clinical Staging of Lyme Disease

Lyme disease progresses through three distinct clinical stages if untreated:

                     THE THREE CLINICAL STAGES OF LYME DISEASE

   Stage                 Timing Post-Bite     Cardinal Clinical Manifestations
   ═════════════════════════════════════════════════════════════════════════════════════════
   1. Early Localized    7 to 14 days         Erythema Migrans (EM) lesion at bite site (>=5 cm);
                         (Range: 3-30 days)   Mild flu-like illness (fever, fatigue, myalgias).

   2. Early Disseminated Weeks to months      Multiple secondary EM annular lesions;
                                              Neurologic: Cranial nerve VII palsy (Bell's palsy;
                                              unilateral or bilateral), lymphocytic meningitis;
                                              Cardiac: Lyme carditis (AV block: 1st, 2nd, 3rd deg).

   3. Late Disseminated  Months to years      Lyme Arthritis: Recurrent, asymmetric mono/oligo-
                                              arthritis of large joints (classically the knee);
                                              Late Neuro: Subacute encephalopathy, polyneuropathy.
   ═════════════════════════════════════════════════════════════════════════════════════════

Stage 1: Early Localized Disease & Erythema Migrans (EM)

• Morphology of Erythema Migrans:
  • Occurs in 70% to 80% of infected individuals, appearing at the site of the tick bite after an incubation period of 7 to 14 days (range: 3 to 30 days).
  • Begins as a red macule or papule that expands outward over days, reaching a diameter of >= 5 cm (often >10 to 15 cm).
  • The lesion is characteristically erythematous, flat or slightly indurated, warm to the touch, but typically non-pruritic and painless.
  • While the classic "bull's-eye" or targetoid appearance (central clearing with a red outer ring) is historically emphasized, uniform erythematous plaques without central clearing account for over 60% of EM lesions in North America.
  • Accompanied by systemic symptoms: low-grade fever, fatigue, malaise, arthralgias, myalgias, and headache.

[!CAUTION] THE CARDINAL DIAGNOSTIC PEARL: NEVER ORDER SEROLOGY FOR ERYTHEMA MIGRANS Erythema Migrans (>=5 cm) in an endemic area is DIAGNOSTIC OF LYME DISEASE.

Clinicians must NEVER order serologic testing in early localized disease. Specific IgM and IgG antibodies take 2 to 4 weeks to develop; testing during the acute EM phase has a false-negative rate >50%. A negative serology never excludes early Lyme disease. Empiric antimicrobial therapy must be initiated immediately based on clinical grounds alone.

• Antimicrobial Regimens for Early Localized Lyme:
  • First-Line: Oral Doxycycline 100 mg twice daily for 10 to 14 days (effective, well-tolerated, and concurrently covers co-transmitted Anaplasma phagocytophilum).
  • Alternatives in Pregnancy & Lactation:
    • Oral Amoxicillin 500 mg three times daily for 14 days;
    • Oral Cefuroxime axetil 500 mg twice daily for 14 days. (Note: Per AAP and CDC guidelines, short courses of doxycycline [<21 days] are now considered safe in young children <8 years; amoxicillin remains an acceptable alternative in young children).

Stage 2: Early Disseminated Lyme Disease

Arises weeks to months after the initial bite as spirochetes disseminate hematogenously:

• Cutaneous: Multiple secondary annular erythematous lesions (smaller than the primary EM lesion, appearing across the trunk and extremities).
• Neurologic (Lyme Neuroborreliosis):
  • Cranial Neuropathy: Cranial nerve VII (facial) palsy is the most common neurologic manifestation. It may be unilateral or bilateral. In Lyme-endemic areas, bilateral facial palsy is virtually pathognomonic for Lyme disease.
  • Lymphocytic meningitis: Headache, photophobia, mild nuchal rigidity, and CSF lymphocytic pleocytosis.
  • Motor/sensory radiculoneuritis (Bannwarth syndrome): Agonizing radicular pain.
• Cardiac (Lyme Carditis):
  • Occurs in 1% to 4% of untreated patients. Spirochetal infiltration of the cardiac conduction system causes fluctuating atrioventricular (AV) nodal conduction blocks (first-, second-, or third-degree complete heart block).
• Management of Early Disseminated Disease:
  • Isolated Facial Nerve Palsy (normal CSF): Oral Doxycycline 100 mg twice daily for 14 to 21 days. (Corticosteroids provide no benefit and are not recommended for Lyme facial palsy).
  • Severe Lyme Carditis (PR interval >=300 ms, 2nd or 3rd degree AV block, or syncope): Immediate hospital admission, continuous telemetry monitoring, and intravenous Ceftriaxone 2 g IV daily until high-grade block resolves, followed by oral step-down to complete 21 to 28 days total.

Stage 3: Late Disseminated Lyme Disease

• Lyme Arthritis: Manifests months to years later in up to 60% of untreated patients. Characterized by recurrent, asymmetric, pauciarticular (mono- or oligo-) arthritis affecting large weight-bearing joints, overwhelmingly the knee (>90% of cases). The affected knee is markedly swollen with a massive, tense effusion, warm to touch, but characteristically only mildly to moderately painful (out of proportion to the dramatic swelling). Synovial fluid analysis reveals an inflammatory effusion (WBC 20,000 to 50,000/mcL with neutrophil predominance).
• Management: Oral Doxycycline 100 mg twice daily for 28 days (or amoxicillin 500 mg TID x 28 days). If arthritis persists or recurs after a full oral course, IV Ceftriaxone 2 g daily for 14 to 28 days is indicated.

CDC Two-Tiered Serologic Testing Algorithm

Serologic testing is indicated exclusively for patients with objective signs of disseminated or late Lyme disease (cranial neuropathy, carditis, meningitis, arthritis) or prolonged constitutional symptoms:

1. First-Tier Test: Highly sensitive Enzyme Immunoassay (EIA) or Immunofluorescence Assay (IFA).
   • If the first tier is negative, no further testing is indicated.
   • If the first tier is positive or equivocal, the laboratory automatically reflexes to the second tier.
2. Second-Tier Test: Western Blot (or second FDA-cleared EIA):
   • Illness Duration <= 30 Days: Both IgM and IgG Western blots are performed and interpreted. An IgM Western blot is positive if >= 2 of 3 specific bands are present (23, 39, 41 kDa). An IgG Western blot is positive if >= 5 of 10 specific bands are present (18, 23, 28, 30, 39, 41, 45, 58, 66, 93 kDa).
   • Illness Duration > 30 Days: ONLY THE IgG WESTERN BLOT IS INTERPRETED. An isolated positive IgM Western blot in a patient with symptoms lasting >30 days represents a false positive and must be disregarded!

                  CDC TWO-TIERED TESTING FOR LYME DISEASE

                      First-Tier: Sensitive EIA or IFA
                                     │
                       ┌─────────────┴─────────────┐
                       ▼                           ▼
                    Negative               Positive or Equivocal
                       │                           │
                  Stop Testing                     ▼
                  (No Lyme)               Second-Tier: Western Blot
                                                   │
                       ┌───────────────────────────┴───────────────────────────┐
                       ▼                                                       ▼
             Symptoms <= 30 Days                                      Symptoms > 30 Days
                       │                                                       │
         Perform IgM AND IgG Western Blot                           Interpret IgG Western Blot ONLY
         • IgM: >= 2 of 3 bands (23, 39, 41 kDa)                    • IgG: >= 5 of 10 bands
         • IgG: >= 5 of 10 bands                                    • DISREGARD IgM (High False-Positive!)

Post-Exposure Lyme Prophylaxis Criteria

Prophylactic antibiotics following a tick bite are NOT indicated for every tick encounter. Routine antimicrobial prophylaxis after tick bites is inappropriate.

[!IMPORTANT] THE FOUR MANDATORY CRITERIA FOR LYME POST-EXPOSURE PROPHYLAXIS A single dose of oral doxycycline is indicated ONLY when ALL FOUR of the following criteria are satisfied:

1. The attached tick can be reliably identified as an adult or nymphal blacklegged deer tick (Ixodes scapularis);
2. The tick is estimated to have been attached for >= 36 hours (based on engorgement index or known timing);
3. Prophylaxis can be initiated within 72 hours of tick removal;
4. The local infection rate of Ixodes ticks with B. burgdorferi is >= 20% (endemic areas: New England, Mid-Atlantic states, Minnesota, Wisconsin).

PROPHYLACTIC REGIMEN: A single oral dose of Doxycycline 200 mg (for children: 4.4 mg/kg up to 200 mg max). If any criterion is missing or if doxycycline is contraindicated, the recommended approach is watchful clinical observation for 30 days.

Rocky Mountain Spotted Fever (RMSF): The Life-Threatening Emergency

Rocky Mountain Spotted Fever (RMSF), caused by the obligate intracellular bacterium Rickettsia rickettsii, is the most lethal tick-borne infection in the United States. Despite its geographic name, RMSF is disproportionately concentrated in the Southeastern and South-Central states (North Carolina, Tennessee, Missouri, Arkansas, Oklahoma).

Vector & Transmission

Transmitted primarily by the American dog tick (Dermacentor variabilis) in the Eastern and Central US, the Rocky Mountain wood tick (Dermacentor andersoni) in the West, and the brown dog tick (Rhipicephalus sanguineus) in the Southwest and Mexican border region. Unlike Lyme disease, R. rickettsii can be transmitted within 6 to 10 hours of tick attachment.

Pathogenesis: Endothelial Tropism

R. rickettsii possesses an exclusive tropism for vascular endothelial cells lining small blood vessels, capillaries, and arterioles. Spirochetal invasion induces widespread endothelial cell necrosis, loss of vascular integrity, microvascular thrombosis, and diffuse microvascular leakage. This systemic vasculitis triggers non-cardiogenic pulmonary edema (ARDS), cerebral edema, acute kidney injury, cutaneous gangrene, and multiorgan failure.

Clinical Presentation & The Centripetal Rash

• Initial Symptoms: Following an incubation period of 3 to 12 days, patients develop abrupt, severe constitutional illness: high fevers (>38.9°C / 102°F), excruciating frontal headache, severe photophobia, prostrating myalgias, nausea, and vomiting.
• Evolution of the Classic Rash:
  • The rash is characteristically ABSENT on days 1 and 2 of illness (present in <15% of patients initially).
  • Develops on days 2 to 4 as small (1 to 5 mm), blanching, erythematous macules beginning symmetrically on the wrists, forearms, and ankles.
  • Over subsequent days, the rash spreads centripetally inward to the palms, soles, legs, arms, and trunk.
  • By days 5 to 6, the lesions evolve into non-blanching petechiae and purpura, coalescing into areas of skin necrosis and gangrene.
  • Up to 10% to 15% of patients never develop a rash ("spotless RMSF"), which carries the highest mortality rate due to delayed diagnosis.
• Laboratory Clues: Marked thrombocytopenia (platelets <100,000/mcL), hyponatremia (serum Na <135 mEq/L resulting from vascular leak and increased ADH release), elevated transaminases (AST/ALT), and leukocytosis with left shift.

[!CAUTION] CRITICAL CLINICAL MANDATE: DOXYCYCLINE FOR ALL AGES IN SUSPECTED RMSF Doxycycline is the FIRST-LINE DRUG OF CHOICE FOR PATIENTS OF ANY AGE, including children younger than 8 years and pregnant patients with severe illness.

TREATMENT MUST BE INITIATED IMMEDIATELY UPON CLINICAL SUSPICION without waiting for laboratory confirmation. Initial indirect immunofluorescence antibody (IFA) serologies are universally negative in the first 7 to 10 days of illness.

Delaying doxycycline administration beyond day 5 of illness causes mortality to surge from <2% to over 20% to 30%. The AAP and CDC confirm that short courses of doxycycline do not cause dental staining in young children.

• Dosing: Doxycycline 100 mg PO or IV twice daily (children: 2.2 mg/kg/dose twice daily up to 100 mg/dose) continued for at least 3 days after fever resolves and clinical improvement is evident (minimum course: 7 to 10 days).

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Anaplasmosis, Ehrlichiosis & Babesiosis

                  COMPARATIVE SUMMARY OF TICK-BORNE INFECTIONS

   Illness / Organism   Primary Vector & Region   Target Cell / Smear Clue   Key Laboratory Clues    First-Line Drug
   ═════════════════════════════════════════════════════════════════════════════════════════════════════════════════
   Lyme Disease         Ixodes scapularis         Extracellular spirochete   High false (-) serology Doxycycline
   (Borrelia burg)      Northeast / Midwest       (No smear visualization)   in early disease        100 mg PO BID

   RMSF                 Dermacentor variabilis    Vascular Endothelial       Thrombocytopenia,       Doxycycline
   (Rickettsia rick)    Southeast / Central US    Cells (Vasculitis)         Hyponatremia, high AST  100 mg PO/IV BID

   Anaplasmosis (HGA)   Ixodes scapularis         Neutrophils / Granulocytes Leukopenia, severe      Doxycycline
   (Anaplasma phag)     Northeast / Midwest       Intracytoplasmic morulae   thrombocytopenia        100 mg PO BID

   Ehrlichiosis (HME)   Amblyomma americanum      Monocytes / Macrophages    Leukopenia,             Doxycycline
   (Ehrlichia chaff)    (Lone Star) Southeast     Intracytoplasmic morulae   thrombocytopenia        100 mg PO BID

   Babesiosis           Ixodes scapularis         Erythrocytes (Intra-RBC)   Hemolytic anemia,       Atovaquone +
   (Babesia microti)    Northeast (Islands)       "Maltese cross" tetrads    high LDH, low haptogl   Azithromycin
   ═════════════════════════════════════════════════════════════════════════════════════════════════════════════════

Human Granulocytic Anaplasmosis (HGA) vs. Human Monocytic Ehrlichiosis (HME)

• Anaplasmosis (Anaplasma phagocytophilum):
  • Vector: Ixodes scapularis (blacklegged tick); co-infection with Borrelia burgdorferi and Babesia microti occurs in up to 10% to 20% of patients.
  • Target Cell: Invades and replicates within polymorphonuclear neutrophils.
  • Diagnosis: Peripheral blood smear with Wright-Giemsa stain reveals intracytoplasmic morulae (clusters of microcolonies) within neutrophils in 20% to 70% of acute cases. Confirmed via PCR.
• Ehrlichiosis (Ehrlichia chaffeensis):
  • Vector: Amblyomma americanum (lone star tick); concentrated in Southeastern and South-Central states.
  • Target Cell: Invades and replicates within monocytes and macrophages.
  • Diagnosis: Peripheral blood smear demonstrates morulae in monocytes (rare, visible in <10%). Confirmed via PCR.
• Clinical Presentation: Both present with acute high fevers, shaking chills, severe headache, and myalgias. Rashes are rare in anaplasmosis (<10%) and occur in ~30% of adults with ehrlichiosis. Laboratory hallmarks: profound leukopenia (WBC <3,000/mcL), thrombocytopenia (platelets <100,000/mcL), and elevated liver transaminases (AST/ALT).
• Treatment: Oral Doxycycline 100 mg twice daily for 10 to 14 days is first-line for both.

Babesiosis (Babesia microti)

• Microbiology: An intraerythrocytic protozoan parasite transmitted by Ixodes scapularis in coastal New England (Nantucket, Martha's Vineyard, Cape Cod, Rhode Island, Connecticut), New York (Long Island), and Minnesota/Wisconsin.
• Pathogenesis: Sporozoites invade host erythrocytes, multiplying and lysing red blood cells. Severe, life-threatening babesiosis occurs predominantly in asplenic individuals, immunocompromised hosts, and the elderly.
• Clinical Presentation: Malaise, fatigue, high fevers, drenching sweats, followed by overt hemolytic anemia (jaundice, scleral icterus, dark hemoglobinuria, elevated indirect bilirubin, high lactate dehydrogenase [LDH], undetectable haptoglobin, and reticulocytosis) with thrombocytopenia.
• Diagnostic Hallmark:
  • Thin and thick Giemsa-stained peripheral blood smears reveal intraerythrocytic parasites: pleomorphic ring forms (resembling Plasmodium falciparum) and the pathognomonic "Maltese cross" (tetrad of four merozoites budding together).
• Antimicrobial Therapy:
  • Mild-to-Moderate Disease: Oral Atovaquone 750 mg twice daily PLUS oral Azithromycin 500 mg on day 1 then 250 mg daily for 7 to 10 days.
  • Severe / Hospitalized Disease (parasitemia >=10%, severe hemolysis, shock): Intravenous Clindamycin 600 mg IV every 6 hours PLUS oral Quinine 650 mg every 8 hours, accompanied by urgent red blood cell exchange transfusion.