A patient presents with a rash and fever after recent travel to Bolivia. She was there for one week and returned 3 days ago. She took her antimalarial medication, so could this be dengue fever?
Fever in the returning traveler relies on Bayesian thinking. We start with probabilities based on endemic diseases and then the history and physical examination raises or lowers the probability of each. In this case malaria was prevented with doxycycline, so dengue fever is more likely.
Dengue fever is caused by a virus spread by mosquitos bites. The virus is brought to regional lymph nodes and then spreads throughout the body. It presents commonly with typical viral symptoms of fever, headache, nausea and vomiting, myalgia, maculopapular rash, etc. These findings are not diagnostically specific. Although it is called “breakbone fever,” in fact the finding of myalgia has no value in differentiating dengue from other febrile illnesses.(Low PLoS Neglected Tropical Diseases 2011) In a cohort study, half of patients were misdiagnosed as having upper respiratory infections.(Sirivichayakul, PLoS Neglected Tropical Diseases, 2012) To decide what is specific, consider the pathology.
The pathology at the cellular level is microvascular permeability. The vessels are not necessarily damaged, just leaking.(Nelson Pathology of Emerging Infections, 1998) Plasma leakage presents as edema and petechiae. The more severe the disease (such as dengue hemorrhagic fever or dengue shock syndrome) the more likely these specific features will be prominent. One might see pleural effusions, ascites, and widespread edema. The tourniquet test is a way of identifying microvascular permeability. Inflate a blood pressure cuff to a level between diastolic and systolic blood pressure and leave it there for five minutes. Now the capillaries are stressed. A positive result is the emergence of 10 petechiae per square inch (2.5 cm) on the forearm.(Gregory PLoS Neglected Tropical Diseases 2011) It is around 50% sensitive, with wide variation depending on the study. Its specificity ranges from the high 80s to mid 90s.
Other more specific findings might include hepatomegaly. Hematologic findings such as leukopenia and thrombocytopenia are characteristic of dengue fever and do raise the probability of this disease.
Take home points:
-The differential diagnosis for fever in the returning traveler depends on endemic diseases
-The pathology of dengue fever is vascular permeability, so look for petechiae or edema
-The capillary fragility test is a way of eliciting this vascular permeability