Taxonomy/Epidemiology

• Borrelia burgdorferi sensu lato complex – genetically diverse group of spirochaetes
• At least 18 different genospecies – not all are considered established human pathogens
• Clinically important species that cause Lyme disease:
  • B.garinii (neurotropic) – Europe, Asia
  • B.afzelii (dermatotropic) – Europe, Asia
  • B.burgdorferi sensu stricto (joints) – USA, Europe
• Less common genospecies causing disease:
  • B.bavariensis, B.spielmanii – Europe
  • B.maynoii – USA
• Dominance of one genospecies over another may vary in space and time across a landscape ( habitat, weather, climate, predator/prey interactions)
  • Host availability:
    • Reservoir hosts:
      • Mammal (B.afzelii, B.burgdoerferi s.s), notably rodents, and deer
      • Birds (B.garinii)
    • Non-reservoir host:
      • Deer sustain ticks via blood meals, but do not trasnmit Lyme to the tick (rodents do)

Transmission:

• Ticks by area:
  • Ixodes ricinus (Europe)
  • Ixodes scapularis, I pacificus (USA)
  • Ixodes persulcatus (Europe & Asia)
• Tick Life Cycle: Four stages: Egg – Larva – Nymph -Adult
• Humans infected by adult and nymphal ticks
• Ticks can be active all year round, mild winters may extend the season
• Cannot be transmitted via: breast feeding, sex, blood donation
• Pregnancy: No syndrome of ‘congenital Lyme disease’ or evidence of teratogenicity
  • Limited/mixed evidence of vertical transmission – mainly case reports some of questionable calibre. Consensus - untreated maternal infection potential risk to fetus so treat (usually amox)

Geographical distribution of ticks that transmit Lyme https://www.nature.com/articles/s41572-026-00691-0/figures/1

Clinical features

| Risk Factors | 50% of people do not recall a tick bite – not usually painful % of ticks infected UK ~4-6 %

Nymphal ticks mostly transmit (small, easy to miss), and need to be on for sevreal hours

CDC guidance states needs attachment >24h

Spring/Summer: Nymphs active Autumn: Adults active | | --- | --- | | Pathogenic mechanisms | Initial Spirochaete replication at site of tick bite following by local spread in skin and skin structures before dissemination | | Clinical syndromes | Early localised • Erythema migrans (EM)

Early disseminated • Borrelial lymphocytoma • Multiple erythema migrans • Early Lyme neuroborreliosis • Lyme carditis

Late disseminated • Lyme arthritis (LA) • Acrodermatitis chronica atrophicans (ACA) • Late Lyme neuroborreliosis

Can present with late manifestations without having noticed early stage symptoms |

Early localised - Erythema migrans

(Photo: NHS Lyme webpage)

• Incubation: 3-30d
  • US: shorter incubation, less central clearing of EM rash, more systemic Sx