Western Equine Encephalitis Virus – Serology and PCR

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Background
This page provides information on the molecular and serology testing available for Western Equine Encephalitis Virus (WEEV) at Public Health Ontario.

Western Equine Encephalitis is a rare mosquito-borne disease caused by the WEEV which is a member of the Togaviridae family of viruses. The main mechanism of transmission is through the bite of an infected mosquito. In humans, most WEEV infections are asymptomatic. In severe cases, particularly those among the extremes of age, individuals may develop aseptic meningitis or encephalitis.¹

Testing for WEEV infection can be performed by serology or PCR depending on the specific clinical scenario.

* Copie d'impression non contrôlée. Valide uniquement le jour de l'impression : 21 nov. 2024.

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.

Specimen Collection and Handling

Specimen Requirements

Test Requested	Required Requisition(s)	Specimen Type	Minimum Volume	Collection Kit
Western Equine Encephalitis (WEE) serology	General Test Requisition Arbovirus (Non-Zika) Testing Intake Form	Whole blood or serum	5.0 ml blood or 1.0 ml serum	Clotted blood – Serum Separation Tubes (SST)
Western Equine Encephalitis (WEE) PCR	General Test Requisition Arbovirus (Non-Zika) Testing Intake Form	Plasma	1.0 ml	Plasma with non-heparin anticoagulant (or serum)
Western Equine Encephalitis (WEE) PCR	General Test Requisition Arbovirus (Non-Zika) Testing Intake Form	CSF	400 µl	Sterile container

Submission and Collection Notes

Effective October 3, 2022, the Arbovirus (Non-Zika) Testing Intake Form is a mandatory requirement for Western Equine Encephalitis virus testing. PHO uses the information on the requisition and the mandatory intake form to assess testing criteria, assign appropriate tests, and provide mandatory information required by the National Microbiology Laboratory for relevant testing performed there.

Clinical information, including any risk factors such as exposure to mosquitoes, and any recent travel, must be provided.

Testing for WEEV PCR is not performed routinely and must be pre-approved by a PHO laboratory Microbiologist. Contact PHO laboratory Customer Service at 416-235-6556 or 1-877-604-4567 to request approval.

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.

Requisitions and Kit Ordering

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.

Test Methods

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