West Nile Virus

West Nile virus (WNV) is a single-stranded RNA virus of the family Flaviviridae; it belongs to the Japanese encephalitis virus serocomplex and is endemic to Europe, Australia, Asia, and Africa. The virus is transmitted from birds (the natural host) to humans through the bite of an infected mosquito. The peak onset of the disease occurs in late summer, but it can occur anytime between July and December. The incubation period ranges from 3 to 14 days. Eighty percent of WNV infections are subclinical. Twenty percent of infections present as a febrile illness, often accompanied by myalgia, arthralgia, headache, conjunctivitis, lymphadenopathy, and a maculopapular or roseolar rash. Severe neurologic disease (meningitis or encephalitis) may occur, frequently found in association with diabetes mellitus and advanced age.

Presenting ocular symptoms include pain, photophobia, conjunctival hyperemia, and blurred vision. A characteristic multifocal chorioretinitis is present in most patients, together with nongranulomatous anterior uveitis and vitreous inflammatory cells. Chorioretinal lesions vary in size (200–1000 μm) and number and may affect the midzone and/or posterior pole, often in linear arrays following the course of retinal nerve fibers. Active lesions appear whitish to yellow, are flat and deep, and evolve with varying degrees of pigmentation and atrophy.

Fluorescein angiography reveals central hypofluorescence with late staining of active lesions, and early hyperfluorescence with late staining of inactive lesions. Inactive or partly active lesions may show a target-like appearance with central hypofluorescence caused by blockage from pigment and peripheral hyperfluorescence due to atrophy (Fig 11-15). Indocyanine green angiography reveals hypofluorescent spots, more numerous than those apparent on fluorescein angiography or funduscopy.

Other findings may include intraretinal hemorrhages, disc edema, optic atrophy, and, less commonly, focal retinal vascular sheathing and occlusion, cranial nerve VI palsy, and nystagmus. Congenital WNV infection has been reported in an infant presenting without intraocular inflammation but with chorioretinal scarring.

Figure 11-15 West Nile virus chorioretinitis. A, Fundus photograph showing multiple active (arrow) and partially active (arrowhead) discrete cream-colored chorioretinal spots (100–300 μm) in a linear pattern. B, The corresponding fluorescein angiogram shows early hypofluorescence (arrow) on the active chorioretinal spots (acute stage) and focal hypofluorescence with a surrounding hyperfluorescent ring (arrowhead) on the partially active lesions (subacute stage). The late-phase angiogram (not shown) shows subsequent staining.

(Reprinted with permission from Chan CK, Limstrom SA, Tarasewicz DG, Ling SG. Ocular features of West Nile virus infection in North America: a study of 14 eyes. Ophthalmology. 2006;113[9]:1539–1546.)

In most patients, intraocular inflammation associated with WNV infection has a self-limiting course, with a return of visual acuity to baseline after several months. Loss of vision may occur because of CNV, foveal scar, ischemic maculopathy, vitreous hemorrhage, tractional retinal detachment, optic nerve pathology, and retrogeniculate damage. Diabetes mellitus has been implicated as a risk factor for WNV-related death and may increase the risk of WNV-associated ocular involvement.

Ocular findings in patients with systemic symptoms suggestive of WNV infection or with meningoencephalitis may suggest the diagnosis and may prompt serologic testing. The differential diagnosis includes syphilis, MCP, histoplasmosis, sarcoidosis, and tuberculosis.

Currently there is no vaccine or specific antiviral treatment for WNV infection. Patients receive supportive therapy. Anterior uveitis may be treated with topical steroids. The efficacy of systemic and periocular corticosteroids for chorioretinal manifestations is unknown. Public health strategies directed at prevention are the mainstays of WNV infection control.

• Chan CK, Limstrom SA, Tarasewicz DG, Lin SG. Ocular features of West Nile virus infection in North America: a study of 14 eyes. Ophthalmology. 2006;113(9): 1539–1546.

• Garg S, Jampol LM. Systemic and intraocular manifestations of West Nile virus infection. Surv Ophthalmol. 2005;50(1):3–13.

• Khairallah M, Ben Yahia S, Attia S, Zaouali S, Ladjimi A, Messaoud R. Linear pattern of West Nile virus–associated chorioretinitis is related to retinal nerve fibres organization. Eye (Lond). 2007;21(7):952–955.

• Khairallah M, Ben Yahia S, Letaief M, et al. A prospective evaluation of factors associated with chorioretinitis in patients with West Nile virus infection. Ocul Immunol Inflamm. 2007;15(6):435–439.

Figure 11-16 Rift Valley fever. Fundus photograph of a 44-year-old male farmer from Saudi Arabia who presented with decreased vision and macular retinitis sparing the fovea; he had a history of fever and contact with animal abortus.

(Courtesy of Albert T. Vitale, MD.)

Excerpted from BCSC 2020-2021 series: Section 9 - Uveitis and Ocular Inflammation. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.