Investigations
1st investigations to order
reverse transcription-polymerase chain reaction (RT-PCR)
Test
Specific diagnosis can be made in acute serum samples by detection of VEEV by RT-PCR.
Viraemia may be detected only up to three days after onset of illness; therefore, sensitivities may be low if done after three days have passed since disease onset (i.e., when viraemia is low).
May not be available in VEEV-endemic regions.
Result
positive for VEEV RNA
enzyme-linked immunosorbent assay (ELISA)
Test
ELISA (especially IgG ELISA) has low specificity for VEEV as it tends to cross-react with other alphaviruses (e.g., eastern equine encephalitis virus, Mayaro virus, chikungunya virus); therefore, further confirmation by plaque reduction neutralisation test is warranted if ELISA is used for serological diagnosis.
Serologies may be performed in paired acute and convalescent sera.
IgM and IgG become positive approximately after five days of onset of illness. IgM remains positive for at least one month, whereas IgG likely remains positive for decades.
May not be available in VEEV-endemic regions.
Result
positive for IgG or IgM for VEEV
Investigations to consider
plaque reduction neutralisation test (PRNT)
Test
Most specific serological test, allows differentiation between cross-reactive alphaviruses.
Does not differentiate between IgM and IgG.
Requires skill and is time consuming.
Only available at some reference laboratories.
Result
reduction of plaques (commonly used measure is 80% reduction)
viral isolation
Test
Specific diagnosis can be made using viral isolation in Vero cells.
Viraemia may be detected only up to three days after onset of illness; therefore, sensitivities may be low if done after three days have passed since disease onset (i.e., when viraemia is low).
Viral isolation requires a biosafety level 3 laboratory and expertise in virology. This is not routinely available in VEEV-endemic countries.
Result
positive for cytopathic effect
cerebrospinal fluid cell count, chemistries (glucose, protein), and viral isolation
Test
Lumbar puncture should be performed and cerebrospinal fluid (CSF) analysed for cell count and standard chemistries (e.g., glucose, protein) if central nervous system involvement is suspected.
If facilities and expertise for viral isolation are available, virus may be isolated from CSF during the early phase of infection. VEEV IgM and IgG may also be detected in CSF.
Result
pleocytosis with lymphocytic predominance; elevated protein; moderately depressed glucose; positive for VEEV or VEEV IgM or IgG in CSF
FBC (including haematocrit)
Test
Test can be ordered in resource-poor regions.
FBC may reveal leukopenia (lymphopenia in particular) or leukocytosis (less predominant than leukopenia), anaemia, and/or thrombocytopenia.
If there are any signs of haemorrhage, the haematocrit should be monitored frequently.
Result
leukopenia (i.e., <4 x 10^6 cells/mL) or leukocytosis (i.e., >11 x 10^6 cells/mL)
LFTs
Test
Test can be ordered in resource-poor regions.
Should be performed to evaluate for hepatitis if there is right upper quadrant tenderness, jaundice, ascites, or hepatomegaly.
Result
transaminitis
chest x-ray
Test
Not necessary for the diagnosis of VEEV infection; however, if neurological symptoms are present, a CXR may reveal infiltrates suggestive of interstitial pneumonia.[35]
May not be available in VEEV-endemic regions.
Result
interstitial infiltrates
head CT
Test
Central nervous system imaging is recommended if any neurological symptoms are present.
A head CT may show cerebritis, oedema, or haemorrhage.[35]
May not be available in VEEV-endemic regions.
Result
cerebritis, oedema, haemorrhage
brain MRI
Test
Central nervous system imaging is recommended if any neurological symptoms are present.
MRI of the brain may show cerebritis, oedema, or haemorrhage.[35]
MRI is a more sensitive diagnostic modality than CT for the detection of encephalitis due to its ability to discern early signs of oedema and infection.[36]
May not be available in VEEV-endemic regions.
Result
cerebritis, oedema, haemorrhage
rapid malaria antigen test or malaria smear
Test
If available, a rapid malaria antigen test or malaria smear could be carried out to rule-out malaria.
Result
negative for malaria/malaria antigen
electroencephalogram
Test
Electroencephalogram (EEG) is not useful for determining the aetiology of encephalitis in patients with VEEV infection, but can be done to identify non-convulsive seizure activity.
Result
shows non-convulsive seizure activity
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