Arthropod-borne Bacterial Diseases

 

• Background

  Bacterial infections spread by arthropods, usually ectoparasites – such as ticks, lice, mites, and fleas – which feed on mammalian blood and transmit the bacteria through their saliva.

• Lyme disease

  Pathophysiology and epidemiology

  • Infection with various Borrelia species (e.g. B. burgdorferi), a spirochete transmitted by the Ixodes tick which lives on small mammals and deer.
  • Tick must usually be attached for >24 hrs for infection to occur.
  • Found throughout northern Europe and much of the US (esp. northeast). In UK, common sites are the Highlands, Lake District, and New Forest.
  • Commonest in summer.

  Presentation

  Common features:

  • Erythema migrans rash – well-demarcated, expanding, usually round, ± clear inner ring ('bull's-eye') – appears within 1-2 weeks (range 3 to 30 days) at the site of the bite. May initially just look like cellulitis.
  • Constitutional symptoms: fever, fatigue, myalgia, arthralgia.

  Rarer presentations:

  • Early disseminated Lyme: neuroborreliosis (meningitis, neuropathies inc. CN7), carditis (heart block, very rare).
  • Late Lyme: arthritis (esp. knee), acrodermatitis chronic atrophicans (fibrosing skin disease of extremities), late neuroborreliosis (progressive encephalomyelitis, very rare).

  Investigations

  • Clinical diagnosis for most: erythema migrans plus endemic area.
  • Serology – ELISA (IgM and IgG) then confirmatory Western blot – only needed if diagnosis unclear or there is disseminated disease.

  Management

  • Doxycycline PO 10-21 days. Can also be given as single dose prophylaxis within 72h of bite if tick attached >36 hours in endemic area.
  • Ceftriaxone IV for meningoencephalitis or severe carditis (2nd-3rd degree AV block).

• Relapsing fever

  Pathophysiology and epidemiology

  • Infection with various Borrelia species which cause recurrent febrile episodes. Most (e.g. B. hermsii) are tick-borne (TBRF), while B. recurrentis is louse-borne (LBRF).
  • TBRF is found in parts of every continent except Antarctica and Australia. LBRF is endemic in Ethiopia and Sudan.

  Presentation

  • ≥2 episodes of febrile illness (usually >39°C).
  • First episode 1-2 weeks post-bite, lasting 3-5 days, followed by first relapse around 1 week later.
  • Other symptoms include myalgia, arthralgia, headache, and nausea. May have splenomegaly on examination.
  • First episode is usually the worst, and may end with crisis phase of rigors and fluctuating BP and HR.
  • Complications include myocarditis and meningoencephalitis.

  Investigations

  • Diagnosis: thick and thin blood films to directly visualise the spirochetes.
  • Other bloods: ↓Hb, ↓PLT, ↑LFTs.

  Management

  • Doxycycline or tetracycline.
  • Treatment may trigger Jarisch-Herxheimer reaction (esp. in LBRF): rigors, fever, and hypotension in response to release of bacterial endotoxins.

• Rickettsial infection

  Pathophysiology and epidemiology

  Rickettsia is a genus of Gram -ve pleomorphic bacteria (cocci or bacilli).

  Common species, their vectors, and distribution:

  • R. africae (African tick bite fever): cattle ticks in sub-Saharan Africa.
  • R. conorii (Mediterranean spotted or tick bite fever): dog ticks in the Mediterranean, India, and Africa.
  • R. rickettsii (Rocky Mountain spotted fever): Dermacentor tick on various large mammals in North America.
  • R. typhi (murine typhus): from rat fleas in tropical areas.
  • Oriental tsutsugamushi (scrub typhus): mites in south and southeast Asia.

  Presentation

  • Fever, headaches, and myalgia.
  • Signs: inoculation eschar (i.e. necrotic bite site), rash, lymphadenitis.
  • Complications include CNS infection, pneumonia, and fatality rates up to 30% (R. conorii).

  Investigations

  Paired acute and convalescent serology (3-6 weeks).

  Management

  Doxycycline