• Clinical science

Collection of zoonotic diseases


Zoonotic diseases are infections that are transmitted from animals to humans. While animals may transmit infection directly, they usually serve as hosts for a pathogen that is then transmitted to humans by a vector (e.g., ticks, fleas). Zoonoses are usually endemic to certain geographical regions, and peaks in incidence often correlate with the life cycle of the transmitting vector. Common diseases include Q fever, Rocky mountain spotted fever, epidemic typhus, ehrlichiosis, and babesiosis. Although these conditions differ in their exact presentation, symptomatic cases typically present with fever, flulike symptoms, and possibly skin rashes. In some cases of fulminant disease, there may be complications such as disseminated intravascular coagulation, shock, and organ failure. Most zoonoses are treated with antibiotics and respond well to treatment.

Q fever

  • Definition: notifiable; zoonotic disease with cattle, sheep, and goats as the primary reservoir
  • Pathogen: Coxiella burnetii (gram-negative, intracellular)
  • Epidemiology
    • Worldwide occurrence
    • 28–54 cases per year in the US
    • 70% of cases occur in men
    • Peak incidence from April to June
  • Route of transmission
    • Inhalation of aerosols from the secretions of infected livestock or animals about to give birth
    • Ingestion of raw milk produced by infected animals
  • Risk groups: slaughterhouse workers, farmers, shepherds, veterinarians
  • Pathophysiology: development of antigens
    • Phase I antigens; : seen when C. burnetii is highly infectious
    • Phase II antigens: seen when C. burnetii is less infectious
      • Antigenic shift essential to differentiating acute Q fever from the chronic variant (see below)
    • Virulence factors of C. burnetii
      • C. burnetii escapes macrophage phagocytosis by:
        • Producing superoxide dismutase to inactive phagolysosomal enzymes
        • Inhibiting cathepsin fusion
    • C. burnetii infection induces a range of immune responses, ranging from autoantibody production to immunosuppression
    • One of two syndromes develop depending on the host's immune response:
      • Acute Q fever: few C. burnetii organisms and stronger cell response to the pathogen
      • Chronic Q fever: multiple C. burnetii organisms and weaker cell response; C. burnetii survives in monocytes and macrophages
Type of Q fever Acute Q fever Chronic Q fever
Incubation period
  • 2–6 weeks
  • Months to years
Clinical features
  • Only develops in 1–5% of individuals
  • Low-grade fever
  • Culture-negative endocarditis
  • Culture-negative osteomyelitis
Diagnostics Serology via IFA (best initial test)
  • Anti-phase II antibody IgG titers ≥ 200 and IgM titers ≥ 50
  • In cases of negative IFA but high clinical suspicion, perform PCR on serum or tissue samples before administering antibiotics.
  • IgM antibody titers to phase II antigens much higher than titers to phase I antigens
  • Anti-phase I antibody IgG titers > 800 or persistently high levels of anti-phase I antibody 6 months after completing therapy
  • Antibodies with titers to phase I antigens are much higher than the titers to phase II antigens.
Additional findings
  • Avoid consuming unpasteurized milk products


Rocky Mountain spotted fever (RMSF)

  • Pathogen: Rickettsia rickettsii; (aerobic, gram-negative, obligate intracellular bacteria )
  • Epidemiology: Rocky mountains, southeastern, and south central US
  • Reservoir: dogs, rodents, ticks
  • Route of transmission: bite of Dermacentor variabilis (dog tick)
  • Pathophysiology
    • Infection of endothelial cells is a classic feature of all rickettsial diseases → vasculitis and subsequent endothelial dysfunction → SIRS → shock, peripheral and/or pulmonary edema, DIC, renal failure
  • Clinical features (incubation period ∼ 7 days, or 2–12 days)
  • Diagnosis: : empiric diagnosis based on clinical and epidemiological features
  • Treatment: doxycycline



Epidemic typhus Endemic typhus Scrub typhus
  • Rickettsia typhi
  • Orientia tsutsugamushi
  • Humans, flying squirrels
  • Rats, mice, cats
  • Rodents
  • Human body lice (feces)
  • Rat and cat fleas , flea feces
  • Trombicula mites larvae (chiggers)
Geographical distribution
  • Worldwide (esp. the Andes, Africa, Southeast Asia)
  • Worldwide, warm coastal regions, southern US
  • Central, eastern, and southeast Asia; northern Australia, South Pacific
Incubation period
  • 7–14 days
  • 8–16 days
  • 7–10 days
Clinical findings
  • Fever, severe headache, malaise
  • Maculopapular or petechial rash erupts on the trunk → spreads to extremities
  • No eschar (scab at site of flea bite)
  • Relative bradycardia despite fever
  • Brill-Zinser disease: exacerbated reccurence many years after the primary episode
  • Serology (four-fold rise in antibodies)
  • Positive Weil-Felix reaction



  • Pathogen: Ehrlichia chaffeensis, Ehrlichia ewingii (intracellular, gram-negative bacteria)
  • Epidemiology: southeastern and south central US, mid-Atlantic States
  • Reservoir: : white tail deer
  • Route of transmission: bite of the lone star tick (Amblyomma americanum) → infection of monocytes and macrophages
  • Clinical infection (incubation period of 1–2 weeks)
    • Fever, headaches, malaise, myalgias
    • Similar to RMSF, but usually without a rash (“spotless” RMSF)
    • Possibly neurologic symptoms (altered mental status, stiff neck, clonus)
    • May cause renal failure; and GI bleeding
  • Diagnosis
    • Leukopenia, thrombocytopenia
    • Wright-Giemsa stain; of blood smear: detection of morulae inside the infected monocytes
    • Serology via IFA: IgG Ehrlichia titer
  • Treatment: PO doxycycline or tetracycline






Previously important diseases

  • Smallpox is a highly contagious disease caused by the virus Variola major or Variola minor. The last natural case occurred in the seventies. Since that time, the disease has officially been eradicated by smallpox vaccine programs.