Tick-Borne & Zoonotic Infections

Vectors and reservoirs

One tick, three diseases. The Ixodes tick transmits Lyme disease (Borrelia burgdorferi), babesiosis (Babesia microti) and anaplasmosis (Anaplasma phagocytophilum), so co-infection is common. A Lyme patient with unexplained hemolysis or cytopenias may have a second infection. Ixodes feeds on the white-footed mouse (the reservoir) and on deer, and it is concentrated in the Northeast and upper Midwest. Dermacentor transmits Rocky Mountain spotted fever (RMSF) and tularemia; Amblyomma (the Lone Star tick) transmits ehrlichiosis.

Doxycycline is the default. Doxycycline treats Lyme disease, RMSF, ehrlichiosis, anaplasmosis and Q fever. The main exceptions are babesiosis, a protozoan (atovaquone plus azithromycin), and tularemia and plague (streptomycin or gentamicin).

Lyme disease

Organism. Borrelia burgdorferi is a spirochete transmitted by Ixodes, and Lyme is the most common vector-borne disease in the US. The disease unfolds in three stages:

"FAKE a Lyme": Facial palsy, Arthritis, Karditis (AV block), Erythema migrans. Bilateral facial palsy after a bull's-eye rash is Lyme until proven otherwise.

Diagnosis and treatment. Erythema migrans is a clinical diagnosis; serology is more useful in the later stages. Doxycycline is first-line for early Lyme. Amoxicillin or cefuroxime is used in pregnancy and in children under 8. IV ceftriaxone is reserved for carditis with high-grade AV block or for CNS disease.

RMSF, ehrlichiosis, anaplasmosis and babesiosis

Rocky Mountain spotted fever. Rickettsia rickettsii, an obligate intracellular bacterium carried by Dermacentor, invades vascular endothelium and causes a widespread small-vessel vasculitis. Increased vascular permeability produces a petechial rash, edema and hypotension. Fever and headache come first, then a rash that starts on the wrists and ankles, involves the palms and soles, and spreads to the trunk. Despite the name, most cases occur in the southeastern US. Doxycycline is given at any age, including young children, because untreated RMSF carries a high mortality.

"Rickettsii on the wRists": the RMSF rash moves from the periphery inward, whereas the endemic typhus rash (Rickettsia typhi, carried by fleas) spreads from the trunk outward and spares the palms and soles.

Ehrlichiosis and anaplasmosis. Both organisms are obligate intracellular bacteria that form morulae, mulberry-like inclusions, inside white cells. Ehrlichia chaffeensis (Amblyomma) infects MONOCYTES, and Anaplasma phagocytophilum (Ixodes) infects GRANULOCYTES. Both cause fever, headache, leukopenia, thrombocytopenia and elevated LFTs, usually with no rash. Treat with doxycycline.

Babesiosis. Babesia microti is a protozoan carried by Ixodes that infects RBCs and causes fever and hemolytic anemia. The smear shows intraerythrocytic rings and tetrads arranged as a Maltese cross, without the hemozoin pigment of malaria, and the patient has no travel history. Disease is severe in asplenic patients. Treat with atovaquone plus azithromycin.

Zoonoses from animals and their products

Tularemia. Francisella tularensis is a gram-negative intracellular organism acquired by skinning or hunting rabbits or from a tick bite. It usually causes ulceroglandular disease: a skin ulcer with regional lymphadenopathy. Its very low infectious dose makes it a bioterror agent. Treat with streptomycin or gentamicin.

Brucellosis. Brucella, also gram-negative and intracellular, comes from unpasteurized dairy or livestock contact. It causes undulant (rising-and-falling) fever, night sweats, arthralgia and granulomas. Treat with doxycycline plus rifampin.

Q fever. Coxiella burnetii is the outlier of the rickettsial group: NO arthropod vector and NO rash. Hardy, spore-like particles from the birth products of cattle, sheep and goats are inhaled and cause high fever and atypical pneumonia, and the organism can cause culture-negative endocarditis. Treat with doxycycline.

Leptospirosis. Leptospira interrogans is a spirochete found in water contaminated with animal urine (surfers, the tropics, flooding). It causes fever, calf myalgias and conjunctival suffusion. Severe Weil disease adds jaundice, azotemia with AKI, and hemorrhage. Doxycycline or penicillin treats it.

Plague. Yersinia pestis is a gram-negative rod with bipolar ("safety pin") staining, carried by rodent fleas (Xenopsylla) from prairie dogs in the Southwest US. Bubonic plague produces fever with painful swollen lymph nodes (buboes) and can progress to pneumonic or septicemic disease. Treat with streptomycin or gentamicin.

Bites and scratches

Cat scratch versus bite. Bartonella henselae follows a cat SCRATCH and causes tender regional lymphadenopathy; treat with azithromycin. In AIDS the same organism causes bacillary angiomatosis, vascular skin lesions that mimic Kaposi sarcoma. Pasteurella multocida follows a cat or dog BITE and causes rapidly spreading, painful cellulitis within 24 hours; treat with amoxicillin-clavulanate. Capnocytophaga follows dog bites in asplenic patients and can cause overwhelming sepsis.