Rickettsia/Bartonella

Question 1

Major biological characteristics of Rickettsiae and related bacteria

Answer 1

• Rickettsiae
  
  • Obligate intracellular bacteria
  • Gram-negative coccobacillus (but stains poorly)
• Usually have animal reservoirs and are arthropod-borne (louse/tick)
• Humans generally incidental-only hosts
  
  • Except louse-borne typhus
• Very small genome; uses host metabolites

Question 2

Intracellular growth cycles of Rickettsiae and related bacteria

Answer 2

• Replicate in cytoplasm rather slowly
  
  • 9-12 hours
• Can’t be cultured on artificial media
• Lose infectivity outside cells

Question 3

Pathogenesis of Rickettsiae

Answer 3

• Most species are pathogenic
• More or less all of them do the same thing: invade endothelial cells and spread through vascular system
• Cause increased vascular permeability –> edema, hypovolemia, ischemia
• Presents with: high fever, headache, petechial rash
• Can have severe consequences if untreated

Question 4

Rickettsiae: 3 main groups

Answer 4

• Spotted fever group
  
  • Transmitted through wood ticks = Rocky Mountain spotted fever
  • Presents with rash on extremities spreading to trunk (centripetal spread)
• Typhus group
  
  • Transmitted mainly through lice
  • Epidemic typhus, endemic typhus, Brill-Zinsser
  • Presents with rash on trunk spreading to extremities (centrifugal spread)
• Scrub typhus group
  
  • Transmitted through chiggers
  • Doesn’t present with rash

Question 5

Rocky Mountain spotted fever

Answer 5

• Caused by Rickettsia rickettsia
• Most common rickettsial infection in US
• Presents with petechial rash that starts on extremities and moves to trunk
• Spread by tick bites
• Reservoir: small rodents
• Incidence not actually the Rockies but is instead largely Arkansas and Missouri (like tularemia)
• Usually hits children (60% of patients under 15)

Question 6

Epidemic typhus

Answer 6

• Caused by Rickettsia prowazekii
• Reservoirs: humans, flying squirrels
• Transmission through human body louse
  
  • Doesn’t bite, but itching smashes louse into skin –> inoculation
• Nits (lice eggs at hair follicles) are a sign of louse infection
• Tends to pop up during wars and other natural disasters
• Presents as petechial rash on trunk (can become necrotic or hemorrhagic), high fever and prostration, renal failure, stupor
• 20-70% mortality rate if untreated (pneumonia and circulatory collapse)
• Infection + survival = lifelong immunity
• Effective live attenuated vaccine (usually for military only)

Question 7

Brill-Zinsser disease

Answer 7

• Caused by Rickettsia prowazekii
• Similar symptoms as endemic typhus, but milder
• Probably caused by reactivation of endemic typhus

Question 8

Endemic typhus

Answer 8

• Caused by Rickettsia typhi (from fleas that have bitten rats) or *Rickettsia felis *(from fleas that have bitten cats)
• Milder than epidemic typhus
  
  • Low-grade fever, mild headache, joint pain

Question 9

Scrub typhus

Answer 9

• Caused by Orientia tsusgamushi
• Transmitted by chiggers who’ve bitten rats
• Presents without rash, but eschar develops at site of bite like tularemia or plague
• Immunity is short-lived

Question 10

Q fever

Answer 10

• Caused by Coxiella burnetti
  
  • Obligate intracellular bacteria that multiply in phagolysosomes (fusogenic)
  • Extremely resistant to dessication (dries up and then just drifts around waiting to get inhaled)
• Reservoir: ruminants (particularly fetal membranes of cows/sheep)
• Transmitted by aerosolized membranes/dust
• Super virulent: just 1 organism can be infectious
• Causes self-limited influenza-like illness
  
  • Doesn’t present like other rickettsial infections and isn’t caused by Rickettsia species

Question 11

Characteristics of Erlichia spp. & their major infections

Answer 11

• Ehrlichia:
  
  • Obligate intracellular bacteria
  • Infect phagocytic cells & multiply inside vacuoles
• Can see on blood smear - filled vacuoles inside leukocytes (morulas)
• Ehrlichia more or less = Anaplasma
• Clinical manifestations:
  
  • Similar to rickettsial infections - high fever, prostration, aches and pains, but no rash
• Cause leukopenia & thrombocytopenia, high ALT/AST
• Transmitted by lone-star tick
• # of cases increasing in last 10 years - may just be due to better diagnostics

Question 12

Characteristics of Bartonella spp. & major infections

Answer 12

• Bartonella
  
  • Facultative intracellular bacteria
  • 3 pathogenic species - most common in US is Bartonella henselae (cat-scratch disease)
• Manifestations:
  
  • Oroya fever (B. bacilliformis)
  • Trench fever (B. quintana)
  • Cat-scratch disease (B. henselae)
• In immunosuppressed patients, B. henselae or B. quintana can cause bacillary angiomatosis
  
  • ​Normally self-limited trench fever or cat-scratch disease runs amok and causes painless, angioproliferative skin lesions (look like Kaposi’s sarcoma) and hepatic blood cysts (abdominal pain)

Question 13

Oroya fever

Answer 13

• Caused by B. bacilliformis
• Can pick up through sandfly in South America
• Invades RBCs, causes anemia & skin lesions

Question 14

Trench fever

Answer 14

• Caused by B. quintana
• Transmitted by louse bite
• Presents similarly to rickettsia but with relapsing fever every 5 days

Question 15

Cat-scratch disease

Answer 15

• Caused by B. henselae
• Transmitted through cat scratches, bites, or licks
• Cat is asymptomatic
• Infects capillary endothelial cells
• Causes fever, painful lymphadenopathy
• Usually self-limited