Which lab value would you expect to be abnormal?
A fully vaccinated 7-year-old male presents to the emergency department with three days of fever and malaise after camping with his family in the woods of North Carolina.
Yesterday, his mother noticed an erythematous, blanching, macular rash on the bilateral hands and feet including palms and soles. Today it seems to be spreading up the arms and legs. On examination, he has mild conjunctival injection and cervical lymphadenopathy. Which of the following lab abnormalities is most likely present?
A. Anemia
B. Eosinophilia
C. Hypoglycemia
D. Hyponatremia
E. Thrombocytosis
Correct answer: D. Hyponatremia
The patient’s clinical presentation is consistent with Rocky Mountain spotted fever (RMSF), a tick-borne illness caused by Rickettsia rickettsii. It is a pleomorphic, obligate intracellular organism transmitted by Dermacentor and Rhipicephalus genus ticks.
Most cases in the US are transmitted in North Carolina, Tennessee, Oklahoma, Missouri and Arkansas (ironically not in the Rockies), but they may also occur in the Southwest. The disease has higher incidence in communities with free-roaming dogs. Deer, rodents, horses, cattle, dogs and cats are all zoonotic hosts.
Early signs and symptoms are nonspecific and include fever, malaise, arthralgias, headache, lymphadenopathy, abdominal pain, vomiting and diarrhea. A characteristic rash develops in 80% of patients, usually two to four days after fever onset. It is a blanching, erythematous maculopapular rash that starts on the distal extremities and spreads centripetally. It later becomes petechial.
The rash is more common and appears earlier in children then in adults, and may be missed in patients with darker skin. Other features may include nonexudative conjunctivitis and periorbital edema, which can cause the diagnosis to be confused with Kawasaki disease arteritis or toxic shock syndrome. Typical laboratory findings include normal RBC and WBC counts but thrombocytopenia. Mildly elevated LFTs and hyponatremia also occur.
Treatment is with doxycycline in all ages, which is a change from prior practice.
Incorrect answer choices:
Anemia (Choice A) may be seen in infections with malaria, parvovirus and many other viral and bacterial illnesses that directly impact the RBCs.
Eosinophilia (Choice B) is seen in various parasite and helminth infections as well as hematologic malignancies and allergic and atopic conditions.
Hypoglycemia (Choice D) is not a typical finding of RMSF. It may be seen in overwhelming sepsis due to any pathogen as well as with any infection that causes fulminant hepatic failure, but is not secondary to a specific pathogen.
Thrombocytopenia, rather than thrombocytosis (Choice E) is a characteristic of RMSF. Thrombocytosis may be seen in Kawaski’s disease, which may have multiple other symptoms in common with RMSF such as the bilateral nonexudative conjunctivitis, rash, fever and lymphadenopathy. The camping history in the vignette makes RMSF more likely.
References:
Smith, Lane M and Mahler, Simon A. Chapter 160: Food and Waterborne Illnesses. In Tintinalli JE, Ma, O Stapczynski J, Yealy DM, Meckler GD, Cline DM., Thomas, Stephen H, eds. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 9e New York, NY: McGraw-Hill; 2020
