A 12-year-old boy developed fever to 38.9ºC and felt ill. Over the next 2 days, he developed a red spots on his hands and arms, which became petechial (Figure 188-1) and spread to involve his entire upper extremities and trunk. He also developed abdominal pain and a headache. History was significant for a recent camping trip with his family to the Southeastern coast of the US. The parents report that he did sustain several tick bites while camping. He was treated presumptively for Rocky Mountain Spotted Fever (RMSF) with doxycycline, and his symptoms resolved over several days.
Zoonoses are infectious agents spread to humans by animals. Infection may occur through direct contact with the animal or via vectors such as ticks (Table 188-1). Examples of zoonotic diseases include RMSF, Ehrlichiosis, Tularemia, Cat-scratch disease, and Rat-bite fever.
| Disease | Infectious Agent | Seasonality | Reservoir | Vector/Exposure | Risk Area |
| RMSF | Rickettsia rickettsia | Late spring through early fall | Wild mammals, such as squirrels, opossums, rabbits, dogs, and mice | Ticks: Dermacentor andersoni, in western US; Dermacentor variabilis in the eastern US; Rhipicephalus sanguineus in Arizona and Mexico | Southeastern US |
| Ehrlichiosis/Anaplasmosis | E. chaffeensis; E. ewingii; Anaplasma phagocytophila | Late spring through early fall | Deer | Ticks: Ixodes scapularis and Amblyomma americanum | Eastern seaboard, South Central, Midwest and Northern California |
| Tularemia | Francisella tularensis | Late spring through early fall | Rabbits, hares and small rodents | Direct exposure or Ticks | Central US |
| Rat-bite Fever | Streptobacillus moniliformis in the US Spirillum minus | None | Rats, mice, squirrels | Direct exposure/bite | |
| Cat-scratch disease | Bartonella henselae | Cats | Direct exposure |
RMSF
Incidence is highest (19 to 77 cases/1 million people in 2008) in Southeastern (Virginia, Carolinas) and Central (Tennessee, Missouri, Arkansas and Oklahoma) US (Figure 188-2).1
Approximately 2,000 cases are reported annually in the US.
Ehrlichiosis
Incidence is highest (14 to 33 cases/1 million people in 2008) in Central States (Missouri, Arkansas and Oklahoma) with moderate incidence (1.7 to 14 cases/1 million people in 2008) in Upper Midwest and Southeastern states (Figure 188-3).1
Approximately 2500 cases were reported in 2010.
Number of reported cases as well as size of endemic regions appears to be growing with spread of tick vectors.
Tularemia
Rare; only 124 cases were reported in the US in 2010.1
Cat-scratch
Uncommon, although not routinely reported through National Notifiable Disease Surveillance System (NNDSS) at the Centers of Disease Control and Prevention (CDC).
Rat-bite fever.
Rare, but not routinely reported to NNDSS.
FIGURE 188-3
A. Ehrlichiosis. Number of reported cases, by county, US, 2010. Anaplasma phagocytophilia and B. Ehrlichia chaggeensis (Used with permission from the National Notifiable Disease Surveillance System, Centers for Disease Control and Prevention. MMWR Morb Mortal Wkly Rep. 2012;59(53):1-111.)
RMSF.
Etiologic agent is Rickettsia rickettsii, which produces endothelial cell infection resulting in a systemic small-vessel vasculitis.
Ticks are the natural hosts, reservoirs and vectors. Rocky Mountain wood ticks (Dermacentor andersoni) are the most common vector for RMSF in Western North America (Figure 188-4). There are 3 other ticks that have been identified as vectors of Rickettsia rickettsii in North America.
Ehrlichiosis.
Etiologic agents are obligate intracellular gram-negative bacteria Ehrlichia chaffeensis, E. ewingii, and Anaplasma phagocytophilum, which have tropisms for different white blood cells.
Tularemia.
Francisella tularensis is the etiologic agent.
Cat scratch disease.
Bartonella henselae is the etiologic agent.
Rat-bite fever.
In the US, Streptobacillus moniliformis is the etiologic agent, whereas Spirillum minus is found in Asia.
FIGURE 188-4
Rocky Mountain wood ticks (Dermacentor andersoni). A North American vector of Rickettsia rickettsii, the etiologic agent of Rocky Mountain spotted fever. Note the smaller size of the female’s scutum (shield) compared to the male’s larger scutum. The dorsal shield covers only a small part of the female’s dorsal surface enabling her abdomen to expand and becoming engorged during feeding. (Used with permission from CDC/Dr. Christopher Paddock.)
RMSF/Ehrlichiosis.
Tick exposure in endemic areas and younger age are risk factors for infection.
Tularemia.
Exposure to infected rabbits, including skinning, is a risk factor for infection.
Tick bite in endemic area is common mode of spread.
Cat-scratch disease.
Exposure to cats, particularly kittens, is the main risk factor.
Can occur following a recent scratch or bite.
About 20 to 30 percent of patients who have cat scratch disease have no cat or kitten exposure.
Rat-bite fever
Infected rodents serve as vectors.
May also be acquired from ingestion of unpasteurized milk, water or other food contaminated with S. moniliformis (Haverhill fever).
RMSF.
Definitive diagnosis is difficult, as there are no widely available sensitive laboratory assays to confirm the diagnosis.
Treatment should be presumptive, pending testing results, as a delay in therapy is associated with a poor outcome.4
A confirmed case, as defined by the CDC,4 includes the appropriate clinical symptoms and:
Serological evidence of a fourfold change in immunoglobulin G (IgG)-specific antibody titer reactive with R. rickettsii or other spotted fever group antigen by indirect immunofluorescence assay (IFA) between paired serum specimens (one taken in the first week of illness and a second 2 to 4 weeks later)., OR
Detection of R. rickettsii or other spotted fever group DNA in a clinical specimen (skin biopsy) via amplification of a specific target by polymerase chain reaction (PCR) assay, or demonstration of spotted fever group antigen in a biopsy or autopsy specimen by Immunohistochemical staining, OR
Isolation of R. rickettsii or other spotted fever group rickettsia from a clinical specimen in cell culture.
Ehrlichiosis.
A confirmed case, as defined by the CDC,4 includes the appropriate clinical symptoms and:
Serological diagnosis with a fourfold change in specific antibody titers between acute (at illness onset) and convalescent sera (4 weeks after illness), OR
Detection of Ehrlichia or Anaplasma DNA in a clinical specimen via PCR assay, OR
Demonstration of Ehrlichia antigen in a biopsy or autopsy sample by immunohistochemical methods, OR
Isolation of E. chaffeensis from a clinical specimen in cell culture.
A probable case, as defined by the CDC,4 includes:
Presence of morulae (clusters of phagocytized ehrlichial organisms in vacuoles) in cytoplasm of peripheral blood granulocytes, monocytes, or macrophages (Figure 188-5), OR
Serologic evidence of IgG or IgM reactive to Ehrlichia antigen by IFA, enzyme-linked immunosorbent assay (ELISA), dot-ELISA, or assays in other formats.
Tularemia.
Growth of the organism in culture is definitive.
Appropriate specimens include swabs or scrapping of skin lesions, lymph node aspirates or biopsies, pharyngeal washings, sputum specimens, or gastric aspirates, depending on the form of illness.
Paradoxically, blood cultures are often negative.
Serological diagnosis with a fourfold change in specific antibody titers between acute (at illness onset) and convalescent sera (4 weeks after illness) can confirm the diagnosis. However, this is not helpful in acute clinical management because of the delay in diagnosis.
A presumptive diagnosis of tularemia can be made when there is:
Evidence of an elevated serum antibody titer to F. tularensis antigen without evidence of a four-fold increase,5 OR
A positive result using direct fluorescent antibody, immunohistochemical staining, or PCR.
Cat-scratch disease.
The diagnosis is usually confirmed by demonstrating serological evidence of antibodies to Bartonella antigens using an indirect immunofluorescent antibody (IFA) assay.6
PCR of body fluids is available at CDC or reference laboratories.
Histopathologic evidence of organisms on Warthin-Starry silver staining is not specific for B. henselae infection.
Rat-bite fever.
The diagnosis of S. moniliformis infection is made via culture of blood, synovial fluid, or other body fluids using specific media; cultures should be held for 3 weeks because of the slow growth of this agent.
The diagnosis of S. minus infection is made with darkfield microscopy of blood or infected body fluids.
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