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BETA v.3.0

2025 ICD-10-CM code A83.9

Mosquito-borne viral encephalitis, unspecified.

Use additional code to identify resistance to antimicrobial drugs (Z16.-). Do not use this code if the specific type of mosquito-borne viral encephalitis is known.

Medical necessity is established by the signs, symptoms, and clinical findings consistent with mosquito-borne viral encephalitis.

Providers diagnose the condition based on the patient’s history, physical examination, and neurological examination. Imaging techniques such as CT, EEG, and MRI of the brain, and a spinal tap may be used. Treatment may include corticosteroids, rest, fluids, pain relievers, and sedatives.

In simple words: This code indicates encephalitis (brain inflammation) caused by a virus transmitted through a mosquito bite. The exact virus isn't specified in the medical record.

Mosquito-borne viral encephalitis refers to inflammation and/or swelling of the brain due to a mosquito bite. The mosquito acquires the viral infection when it bites infected birds or animals. The most common mosquito-borne viruses are West Nile virus (WNV), St. Louis encephalitis (SLE), and western equine encephalomyelitis (WEE). This code is used when the specific type of mosquito-borne viral encephalitis is not documented.

Example 1: A patient presents with fever, headache, stiff neck, and lethargy after being bitten by mosquitoes. The provider suspects mosquito-borne viral encephalitis but does not specify the type of virus., A patient develops seizures and altered consciousness after a mosquito bite. Imaging studies show brain inflammation consistent with encephalitis. The specific viral cause is not identified., A patient experiences nausea, vomiting, and confusion following a trip to an area with known mosquito-borne illnesses. Clinical findings suggest encephalitis, but the virus is not documented.

Documentation should include details of the patient's symptoms, history of mosquito bites, physical and neurological examination findings, and results of imaging studies and laboratory tests, if performed.

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