Zoonotic infections are a class of over 200 specific diseases and infections that are naturally transmitted between vertebrate animals and humans. Transmission may occur via direct contact with an infected animal or animal product, by ingestion of contaminated water or food products, by inhalation, or through arthropod vectors. Pets, farm animals, and common wildlife are the primary reservoirs. Arthropods, in particular ticks, are the primary vectors. These diseases may be caused by myriad organisms including bacteria (eg, Rickettsia, Borrelia), viruses, and parasites. The high morbidity and mortality rates often associated with these illnesses mandate their careful consideration in patients who present with fever, chills, myalgias, rash, and other nonspecific symptoms. In such cases, specific risk factors for zoonotic infection should be sought: exposure to animals; residence or recent travel in rural areas or underdeveloped countries; history of tick bites or exposure to tick habitat; dressing, skinning, or handling animal skins or raw flesh; animal bites or scratches; or ingestion of animal or dairy products. Worldwide, ticks are the most common vector of disease transmission. Unfortunately, many patients with tick-acquired infections do not recall a history of tick bite. Hence, clinical suspicion should remain high for patients in endemic areas. West Nile Virus infections are discussed in Chapter 91 “Disseminated Viral Infections.”
Rocky Mountain spotted fever (RMSF) is the most severe tick-borne disease in the United States. It is caused by Rickettsia rickettsii, a pleomorphic obligate intracellular coccobacillus. The primary vector for transmission is the Dermacentor tick; the usual animal hosts are deer, rodents, horses, cattle, cats, and dogs. Most cases occur between April and September. More than half of reported cases of RMSF occur in the south to mid-Atlantic states, but cases have been reported in the majority of the continental United States.
RMSF affects multiple organ systems, and its nonspecific initial presentation renders diagnosis difficult. The classic clinical triad of fever, rash, and history of tick bite is unreliable. Only 50% of infected patients can recall a tick bite and rash is absent in 20% of cases. Initial findings commonly include fever, headache, myalgias, and malaise. Additionally, patients may experience lymphadenopathy, petechiae/purpura, pulmonary infiltrates, jaundice, hepatosplenomegaly, abdominal pain, nausea, vomiting, diarrhea, meningitis/encephalitis, renal failure, and myocarditis.
Patients with RMSF often seek medical attention before onset of the rash, which is seen 2 to 4 days after initial fever. Initially, the rash is maculopapular. It typically begins on the hands, feet, wrists, and ankles (and may involve palms or soles). The rash spreads centripetally up the trunk, usually sparing the face. Infection can also result in a pulmonary capillary vasculitis and associated bronchiolitis; secondary bacterial pneumonia is common.
Diagnosis and Differential
The diagnosis is largely clinical, with confirmation coming after treatment is initiated. In the absence of an alternative explanation, a febrile patient with a headache who was exposed to known tick ...