College Health May Be Full of Surprises: International Travelers and Tropical Diseases

DIAGNOSIS
Malaria
The clinical presentation of malaria is nonspecific, so it is important to identify patients with a travel history and perform testing when this diagnosis is suspected.^7,8 The gold standard for diagnosing malaria remains microscopy of thick and thin films of the patient’s peripheral blood.⁷ In patients with blood-stage malaria, a blood slide will show multiple infected RBCs and the appearance of the classic “headphone” form within the cells (see Figure 3). This test allows for efficient detection of malaria parasites, determination of parasite species, and calculation of percent parasitemia. Early differentiation between falciparum and nonfalciparum malaria is required, since P falciparum is the most life-threatening form of malaria.⁷ All of these factors are key to determining the best treatment plan for each patient.⁸

Rapid diagnostic tests (RDTs) and polymerase chain reaction (PCR) for malaria are increasingly available for use in US laboratories. In comparison to microscopy, these newer diagnostic tools are slower, more costly, and less readily available. For this reason, microscopy remains the most common means of diagnosis.⁷ PCR is helpful for species confirmation of malaria parasites and can be used to confirm a positive result on microscopy.⁸

It is possible for the first malaria test to be negative, and performing a repeat test the following day in a stable patient is recommended.⁷ However, more than three tests are not needed as long as the patient’s symptoms are not changing.

Dengue fever
Laboratory diagnosis of dengue can be confirmed with detection of DENV genomic sequences through PCR or nonstructural protein 1 antigen by immunoassay.¹⁴ Virus isolation in cell culture, detection of viral RNA by nucleic acid amplification tests, and detection of viral antigens by rapid tests or enzyme-linked immunosorbent assay (ELISA) are most useful if a patient presents within five days of fever onset.¹³

After five days of febrile illness, dengue viruses and antigens disappear from the blood as the specific antibody levels rise.¹³ Therefore, ELISA testing for immunoglobulin (Ig) M anti-DENV is a more effective lab study for dengue in patients presenting after one week. Testing for IgG anti-DENV is not recommended for making a diagnosis, however, because this antibody remains elevated for life after any DENV infection, leading to many false-positive test results. ¹⁴

A useful diagnostic aid for detecting severe dengue is the tourniquet test, which assesses for microvascular fragility. To perform the test, inflate a blood pressure cuff on the arm to midway between systolic and diastolic blood pressures, and maintain pressure for five minutes. After releasing the pressure, count the number of petechea in one square inch of skin; if 20 or more are found, the test is positive.¹²

Chikungunya
Chikungunya laboratory testing is limited in the US due to lack of availability. Testing is available only at the CDC, one commercial laboratory, and a few state health departments.¹⁷

The only reliable method for diagnosing CHIKV infection is through testing of blood samples. Chikungunya should not be diagnosed clinically because of the difficulty in differentiating it from dengue fever and other viral illnesses. The laboratory diagnosis of CHIKV infection can be obtained through detection of the virus, viral RNA, or specific antibodies related to chikungunya.²⁰ Serologic detection of IgM or IgG antibodies is the most common method of diagnosis and is recommended by the CDC.²⁰ If initial IgM and IgG testing is negative but clinical suspicion remains high, repeat testing should be done during the convalescent phase of the illness (≥ 7 days after symptom onset).²¹ Reverse transcriptase PCR is an effective diagnostic laboratory method for chikungunya and can be used in the first seven days of illness.²⁰ ELISA and the hemagglutination inhibition assay can also provide diagnostic information.²⁰

Lab findings associated with malaria, dengue fever, and chikungunya are summarized in the Table.

Continue for differentiating between dengue, malaria, and chikungunya early in presentation >>