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ENHANCED SURVEILLANCE ALERT
Maricopa County Department of Public Health (MCDPH)
Public Health Arboviral Surveillance Alert for Healthcare Providers: May 2026
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Arboviral Notification for Maricopa County – West Nile Virus, St. Louis Encephalitis, and Dengue
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Maricopa County Department of Public Health (MCDPH) has confirmed an adult case of West Nile virus (WNV) for the 2026 arboviral season. Routine mosquito surveillance has yielded increased mosquito activity and WNV and St. Louis encephalitis (SLE) positive mosquitoes in Maricopa County.
MCDPH retrospectively identified one locally acquired case of dengue virus that occurred in November 2025. Mosquito surveillance performed by Maricopa County Environmental Services Department did not detect dengue virus in mosquitoes. However, Aedes mosquitoes capable of spreading the virus are present throughout the county.
Healthcare providers should be aware that WNV and SLE are circulating in Maricopa County and there is potential for dengue to be introduced through travel-associated cases. Providers should consider all three in their differential diagnoses, as appropriate.
It is recommended that Maricopa County providers:
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Consider WNV and SLE in the differential of individuals who present with either viral encephalitis, acute flaccid paralysis or Guillain-Barré syndrome of unknown etiology, aseptic meningitis, or febrile illness of unknown etiology.
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Order WNV and SLE IgM antibody testing of serum or CSF.
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Consider dengue in the differential of individuals who present with fever and who live in or recently traveled to areas with risk of dengue, or who endorse recent mosquito bites without travel within two weeks of symptom onset.
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Order a nucleic acid amplification test (NAAT) and an IgM antibody test or an NS1 antigen test and an IgM antibody test for acute infections.
Providers Should Report Suspect Cases of Locally Acquired Dengue to MCDPH by calling (602) 506-3747
Other arboviral suspect cases may be reported through normal reporting processes.
WNV and SLE Laboratory Testing:
Both serum and cerebrospinal fluid (CSF) can be collected for diagnostic testing commercially.
Healthcare providers should strongly consider WNV and SLE for patients fitting the following profile:
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All cases of viral encephalitis.
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All cases of acute flaccid paralysis or Guillain-Barré syndrome of unknown etiology, with or without presence of viral meningitis or viral encephalitis.
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Cases of aseptic meningitis, especially those with at least one of the following:
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Cases of febrile illness of unknown etiology, with or without presence of other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash.
Laboratory diagnosis is recommended and is accomplished by testing of serum or CSF to detect WNV-specific and SLE-specific IgM antibodies.
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WNV-specific & SLE-specific IgM antibodies are usually detectable three to eight days after onset of illness.
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If serum is collected within eight days of illness onset, the absence of detectable virus-specific IgM does not rule out the diagnosis of WNV or SLE infection, and the test may need to be repeated on a later sample.
Dengue Laboratory Testing:
Healthcare providers should strongly consider dengue for patients fitting the following profile:
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Cases of febrile illness of unknown etiology and who live in or recently traveled to areas with risk of dengue, or who endorse recent mosquito bites without travel within two weeks of symptom onset.
Dengue testing is widely available through commercial laboratories. Laboratory diagnosis should be made using the following guidance.
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Acute phase: 0-7 days after symptom onset
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During this period, laboratory diagnosis should be made on serum samples using either one of these test combinations:
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A nucleic acid amplification test (NAAT) (e.g., RT-PCR) and an IgM antibody test OR
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An NS1 antigen test and an IgM antibody test
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Convalescent phase: >7 days post symptom onset
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IgM antibody testing is recommended as the primary test after day 7 of symptom onset.
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Some patients may be positive on NAAT or NS1 antigen tests after day 7 of illness.
Thank you for your continued collaboration and partnership.
Maricopa County Department of Public Health
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