Western Equine Encephalitis Virus – Serology and PCR

Testing Indications

Testing for WEEV may be indicated if a patient has compatible clinical information, symptoms and exposure history (e.g. residence in or travel to an area with active local WEEV transmission or a risk of WEEV transmission in animals or humans).² Testing should not be performed on asymptomatic individuals. Given the overlap in the distribution of disease vectors, testing for other potential co-pathogens may be considered, where applicable. Serology is the preferred test method for WEEV.

Acceptance/Rejection Criteria

Specimens received without the appropriate forms (Refer to Submission and Collection Notes) will not be tested.

Timing of Specimen Collection

Serology:
Acute and convalescent clotted blood or serum specimens for serology should be collected 2-3 weeks apart from patients with clinical illness.

Molecular (PCR):
Specimens for WEEV PCR testing should be collected ASAP after the onset of symptoms. This should be discussed on a case-by-case basis with a PHO Microbiologist. CSF should only be collected in cases of suspected neuroinvasive disease.¹

Limitations

Haemolysed, icteric, lipemic or microbial contaminated sera or plasma are not recommended for testing.

Storage and Transport

Centrifuge if using SST. Place specimen in biohazard bag and seal. Specimens should be stored at 2-8°C following collection and shipped to the PHO’s laboratory on ice packs.

Specimens for molecular testing should be frozen and shipped on dry ice.

Test Frequency and Turnaround Time (TAT)

Serology is performed once a week at PHO. Turnaround time is up to 8 days from receipt at PHO.

Molecular testing by RT-PCR is performed at NML after approval by a PHO Microbiologist. As this is not a routine test, NML will determine TAT after approval of testing.

Serology:
Specimens submitted for WEEV serology are tested using a laboratory-developed Hemagglutination Inhibition (HI) Assay.

Molecular (PCR):
Although WEEV PCR testing is available, it is not considered a first line test. Under special circumstances, the National Microbiology Laboratory (NML) can perform WEEV PCR testing on patients with suspected neuroinvasive disease or immunocompromised individuals.

Algorithm

Submitted serum specimens will first be analyzed by HI Assay to determine if the patient has developed antibodies.

PCR testing is not performed routinely and will be considered on a case-by-case basis.

Interpretation

All results should be interpreted in the context of the specific clinical scenario.

Serologic testing for WEEV:
The HI test does not distinguish the presence of WEEV IgM or IgG antibodies. Titres of ≥1:10 may indicate a recent or prior WEEV infection. Cross reactions (a reactive HI result) may occur with other related arboviruses such as Chikungunya virus or Eastern Equine Encephalitis virus.

Molecular (PCR) testing for WEEV:
A positive PCR result indicates that WEEV nucleic acids were detected in the specimen and an acute/recent infection.

A negative PCR result indicates that WEEV virus nucleic acids were not detected in the specimen. This does not exclude WEEV virus infection.

Reporting

Results are reported to the ordering physician, authorized health care provider (General O. Reg 45/22, s.18) or submitter as indicated on the requisition.

Positive results from patients with encephalitis are also reported to the Medical Officer of Health as per the Ontario Health Protection and Promotion Act.³

References

1. Western equine encephalitis - Argentina – World Health Organization
2. Data and Maps for Western Equine Encephalitis – Centers for Disease Control and Prevention
3. Reportable Diseases of Public Health Significance – Toronto Public Health