eMedicine Specialties > Physical Medicine and Rehabilitation > Medical Diseases

West Nile Virus

Jess D Salinas Jr, MD, Medical Director, Lake Mary Clinic, National Pain Institute, LLC; Associate Medical Director, Winter Park Clinic, National Pain Institute, LLC
Monica L Steiner, MD, Clinical Assistant Professor, Program Director, Department of Orthopedics and Rehabilitation, Loyola University Medical Center

Updated: Nov 6, 2009

Introduction

Background

The West Nile virus is one of the many members of the genus Flavivirus that are known to cause human disease. The life cycle of the West Nile virus involves the microbe's transmission from nonhuman animals to humans by way of Aedes, Culex, or Anopheles mosquitoes. The West Nile virus can infect horses, birds, dogs, and other mammals.^1,2 However, wild birds are apparently the optimal hosts for harboring and replicating the virus.

The West Nile virus has been reported in Africa, Asia, Europe, the Middle East, and North America. In 1999, the first cases of West Nile virus disease were reported in New York City, and the infection has been spreading throughout the North American continent ever since.³ In 2002, a total of 3,389 cases were reported in the United States.^4,5 Approximately 55% of these cases were from the Midwest (Illinois, Michigan, Ohio, Indiana).

The West Nile virus causes serious manifestations in approximately 1% of persons who are infected, with increased morbidity and mortality in individuals older than 50 years. In hospitalized patients in New York City, neurologic sequelae of the West Nile virus included severe muscle weakness, with approximately 10% of patients developing a complete flaccid paralysis.^6,7,8 One in 150 West Nile virus infections results in encephalitis or meningitis, and the mortality rate from severe illness is 3-15%. Individuals older than 75 years are at particular risk.¹

As the elderly population increases and the distribution of the West Nile virus spreads nationwide, a growing number of infected individuals may require comprehensive inpatient rehabilitation to overcome the virus's disabling effects.⁹

Pathophysiology

Examining the ways in which the West Nile virus may cross the blood-brain barrier to infect the nervous system, Verma et al infected primary human brain microvascular endothelial (HBMVE) cells with NY99, a neurovirulent strain of the virus.¹⁰ The authors noted that the virus did not have a cytopathic effect on the HBMVE cells. Increased mRNA (messenger ribonucleic acid) and protein expression of the tight-junction protein claudin-1 and of 2 cell adhesion molecules (vascular cell adhesion molecule and E-selectin) were seen 2-3 days after cellular infection, the same time at which West Nile virus replication had peaked.

The study provided evidence that infection of HBMVE cells by the West Nile virus enables the cell-free virus to enter the central nervous system without disturbing the barrier's integrity. In addition, the authors suggested that cell adhesion molecules may help to traffic West Nile virus – infected immune cells into the central nervous system.

Frequency

United States

In the Western hemisphere, West Nile virus infection originated in 1999, in New York City. Since then, the disease has occurred with greater frequency in the Southern, Midwestern, and Western states. Symptoms of the infection first appear in the population in early June, with the peak incidence occurring in late August and tapering through early November.

International

The West Nile virus is most commonly identified in Asia, Africa, and the Middle East and is endemic in those parts of the world. In the 1990s, outbreaks of West Nile virus encephalitis were reported in Algeria, the Czech Republic, France, Romania, Russia, and Israel.¹

Mortality/Morbidity

Reports indicate that less than 1% of persons who are infected with the West Nile virus develop severe illness; of individuals who have severe illness secondary to the infection, 3-15% die.
Severe disease particularly affects the elderly. Advanced age is by far the greatest risk factor for neurologic disease, long-term morbidity, and death, especially in persons older than 75 years.

Race

There is no known predisposition related to a particular ethnic group.

Sex

There is no known sex predilection. Men and women are affected equally.

Age

In the United States, the elderly are particularly disposed to illness from West Nile virus infection.

Clinical

History

Mosquito bites may or may not be present in an infected person. A history of travel to or from an area that is known to harbor the virus is common.

The incubation period for the West Nile virus is postulated to be approximately 5-15 days. Symptoms of mild infection may last 3-6 days and include fever in 20% of cases. Other symptoms include nausea, anorexia, malaise, myalgia, headache, backache, rash, eye pain, and vomiting.¹

Symptoms of more severe illness include severe muscle weakness, flaccid paralysis, photophobia, seizures, mental status changes, respiratory symptoms, and an erythematous, maculopapular, or morbilliform rash involving the neck, trunk, arms, or legs.^6,11 The severity of the illness is related to the degree of central nervous system invasion by the virus.

Physical

Signs of encephalitis and meningoencephalitis may be seen. These include mental status changes, such as confusion, stupor, or coma. Other findings include positive Brudzinski and Kernig signs, papilledema, cranial nerve involvement (eg, facial weakness, double vision, visual loss, decreased taste sensation), motor strength weakness, decreased sensation, hyperreflexia, and positive pathologic reflexes (eg, Babinski sign, Hoffman sign).

Causes

The West Nile virus is transmitted to humans by the bite of an infected mosquito. Typically, warm climates and the summer months provide an ideal environment for mosquitoes to breed.¹ Multiple mosquito bites and greater exposure to environments with a large mosquito population increase the risk of infection. However, some cases have been linked to organ transplantation, breastfeeding, and (possibly) blood transfusions.¹²

Differential Diagnoses

Acute Poliomyelitis
Guillain-Barre Syndrome
Multiple Sclerosis
Postpolio Syndrome
Vertebrobasilar Stroke

Workup

Laboratory Studies

The complete blood count (CBC) may show elevated or normal leukocytes values.
In cases of encephalitis, hyponatremia may be present. The complication of syndrome of inappropriate ADH (SIADH) secretion is a possibility.
Cerebrospinal fluid (CSF) analysis may reveal elevated protein and increased leukocyte levels, with predominant lymphocytes.¹³ Glucose levels are usually normal rather than decreased.
Serologic testing to detect immunoglobulin M (IgM) antibodies is currently the best means¹⁴ of diagnosing West Nile virus infection.
- IgM antibody-capture enzyme-linked immunosorbent assay (MAC-ELISA) has been used to detect IgM for the West Nile virus by using serum or CSF samples.¹
- False-positive results may occur because of the close relationship of the West Nile virus to other flaviviruses.
- In light of this limitation, the plaque reduction neutralization test (PRNT) may help to identify false-positive MAC-ELISA results caused by cross-reactions by other flaviviruses.

Imaging Studies

In acute disease, computed tomography (CT) scans do not show any evidence of abnormalities.
In an estimated one third of infected individuals, magnetic resonance imaging (MRI) scans show notable enhancement in the leptomeninges and periventricular areas.

Histologic Findings

Autopsy findings in some patients with West Nile virus infection reveal mononuclear inflammation that extensively involves the medulla, with some involvement of the cranial nerve roots.¹⁵ However, these findings are not diagnostic for the infection.

Treatment

Rehabilitation Program

Physical Therapy

Brain injury from West Nile virus encephalitis or meningitis can result in cognitive, gross motor, and fine motor delays. Because infected patients have varying degrees of functional deficits, treatment programs must be individualized. Comprehensive rehabilitation using a team consisting of a physiatrist, nurse, physical therapist, occupational therapist, speech therapist, social worker/case manager, and neuropsychologist achieves best outcomes.

The physical therapist can partially address the problems of increased muscle tone, weakness, decreased sensation, and poor endurance. Mobility training, transfer training, and gait training are usually implemented, with range of motion and proper positioning attended to as well. Physical therapists are also important in providing exercises for muscle reeducation and for the improvement of strength, endurance, coordination, and balance, with the goal of returning the patient to independent function.

Occupational Therapy

Occupational therapy focuses on the activities of daily living (ADLs), including bathing, dressing, feeding, and hygiene maintenance. Occupational therapists provide a program to maximize the use of the arms and hands with functional activities; they also address the cognitive issues that affect daily independent function. (See also Further Outpatient Care.)

Speech Therapy

Patients may develop dysarthria, dysphagia, or aphasia. A structured speech therapy program may improve their ability to swallow, help them recover speech and language function, and prevent complications, such as aspiration pneumonia. (See also Further Outpatient Care.)

Medical Issues/Complications

Pressure ulcers
- The development and progression of a pressure ulcer can deeply affect the type, length, and cost of a patient's rehabilitation.
- Pressure ulcers are caused by prolonged pressure, shear forces, friction, and maceration.
- Means of preventing this complication include close monitoring of potential ulcer sites, frequent repositioning to reduce pressure on vulnerable areas, ensuring that adequate nutrition is provided, and cleaning and drying sites of perspiration, urine, or feces.
- Once a pressure ulcer develops and progresses, more severe complications (eg, wound infection, bacteremia, osteomyelitis) may enter the clinical picture.
Deep venous thrombosis
- Elderly patients who are severely deconditioned because of West Nile virus encephalitis may be predisposed to deep venous thrombosis (DVT). The inherent risk of having DVT is the development of a pulmonary embolus that can cause death.
- Risk factors for DVT may include, among others, decreased mobilization, a history of smoking, and a history of premorbid medical conditions, such as coronary artery disease, diabetes mellitus, hypercoagulopathy, and peripheral vascular disease.
- Prevention strategies include the use of thigh-high compression stockings, pneumatic compression devices, and subcutaneous, unfractionated or low – molecular weight heparins. Early mobilization and ambulation also may decrease the risk of DVT.
- Doppler ultrasonography may be used to monitor for DVT, but its accuracy is limited, as has been shown in many studies.
Pulmonary complications
- Individuals with severe illness secondary to West Nile virus infection are at increased risk of pulmonary complications in the rehabilitation setting. Individuals with encephalitis may have a decreased level of consciousness, or they may suffer from dysphagia related to their neurologic injury, predisposing them to aspiration pneumonia.
- Swallow evaluation can be performed to identify the problem and to help in implementing the appropriate diet and feeding techniques to decrease the risk of aspiration.
- Phrenic nerve palsy has been described. This complication could lead to decreased expansion of the lungs, further increasing the risk of atelectasis and nosocomial pneumonia.
- Deep-breathing exercises, use of an inspiratory spirometer, and early mobilization and ambulation help to decrease the risk of these complications occurring.

Surgical Intervention

No surgical indications are reported at this time.

Consultations

Physiatrist
Neurologist
Infectious disease specialist
Psychologist or neuropsychologist

Medication

Ongoing research is being pursued into the direct treatment of West Nile virus meningoencephalitis with interferon alpha and intravenous immunoglobulin G (Omr-IgG-am).

Follow-up

Further Outpatient Care

Consultation with a psychologist or neuropsychologist may be helpful.¹⁶
The neuropsychologist's role includes developing behavioral treatment plans, assessing the patient's cognitive and emotional function, and providing individual, group, and family therapy.
The neuropsychologist also assesses the patient's attention, executive functions, personality, and memory, language, and visual-spatial abilities by using various specialized tests.
Although the literature concerning the neuropsychologic assessment and cognitive rehabilitation of patients with encephalitis is limited, neurocognitive testing may be useful in identifying deficits, and neurocognitive therapy to treat those deficits plays a significant part in the rehabilitation process.
The primary cognitive consequences of encephalitis involve attention, memory, information processing speed, and cognitive efficiency.
As a part of a rehabilitation team, the neuropsychologist also collaborates with the speech therapist and occupational therapist to maximize the interplay between cognitive rehabilitation and function.¹⁷

Deterrence

The Centers for Disease Control and Prevention (CDC) offer the following 3 major suggestions to help prevent West Nile virus infection¹⁸:
- Avoid mosquito bites.
- Mosquito-proof the home.
- Support community-based efforts in mosquito control and the prevention of West Nile virus infection.^19,20
Avoiding mosquito bites
- Individuals should apply insect repellent containing N,N -diethyl-meta-toluamide (DEET) to exposed skin whenever they go outdoors. In addition, using permethrin on clothing is very effective as an insecticide and as a repellent.
- When possible, people should wear long sleeves, long pants, and socks when outdoors.
- Mosquitoes can bite through thin clothing, so spraying clothes with repellent containing permethrin or DEET provides additional protection. However, repellents containing permethrin should not be applied directly to the skin, and repellent containing DEET should not be applied to skin under clothing.
- The peak hours for mosquito bites are from dusk to dawn. Individuals are advised to use repellent and protective clothing in the evening and early morning or to consider avoiding outdoor activities during these times.
Mosquito-proofing homes
- Because mosquitoes lay their eggs in standing water, containers or bodies of standing water should be drained if possible.
- People can help prevent mosquitoes from entering their homes by repairing or installing screens on their doors and windows.
Supporting community-based efforts
- Dead birds can indicate the presence of the West Nile virus in a community and should be reported to local health authorities.
- Some communities have initiated mosquito-control programs. Local governments should have additional information about these programs.
- Keeping the community clean by picking up garbage and draining standing water from vacant areas and parks can help to eliminate mosquito breeding grounds.

Prognosis

The prognosis is generally good.
However, the elderly and persons with multiple medical complications have a poorer prognosis.

Patient Education

See the above section Deterrence.
For excellent patient education resources, visit eMedicine's Bacterial and Viral Infections Center, Bites and Stings Center, and Brain and Nervous System Center. Also, see eMedicine's patient education articles West Nile Virus, Insect Bites, and Encephalitis.

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Keywords

West Nile virus, West Nile symptoms, West Nile virus symptoms, WNV, , West Nile fever, West Nile encephalitis, West Nile virus mosquitoes, West Nile fever, West Nile infection, mosquito bites, mosquitoes, mosquitoes, mosquitoes

Contributor Information and Disclosures

Author

Jess D Salinas Jr, MD, Medical Director, Lake Mary Clinic, National Pain Institute, LLC; Associate Medical Director, Winter Park Clinic, National Pain Institute, LLC
Jess D Salinas Jr, MD is a member of the following medical societies: American Academy of Pain Management, American Academy of Pain Medicine, American Academy of Physical Medicine and Rehabilitation, American Medical Association, American Society of Interventional Pain Physicians, and Physiatric Association of Spine, Sports and Occupational Rehabilitation
Disclosure: Emedicine Honoraria Other

Coauthor(s)

Monica L Steiner, MD, Clinical Assistant Professor, Program Director, Department of Orthopedics and Rehabilitation, Loyola University Medical Center
Monica L Steiner, MD is a member of the following medical societies: American Academy of Physical Medicine and Rehabilitation
Disclosure: Nothing to disclose.

Medical Editor

Elizabeth A Moberg-Wolff, MD, Associate Professor and Pediatric PM&R Fellowship Director, Department of Physical Medicine and Rehabilitation, Medical College of Wisconsin; Program Director, Tone Management and Mobility, Department of Physical Medicine and Rehabilitation, Children's Hospital of Wisconsin
Elizabeth A Moberg-Wolff, MD is a member of the following medical societies: American Academy for Cerebral Palsy and Developmental Medicine and American Academy of Physical Medicine and Rehabilitation
Disclosure: Medtronic Neurological Grant/research funds Speaking and teaching

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: eMedicine Salary Employment

Managing Editor

Kat Kolaski, MD, Assistant Professor, Departments of Orthopedic Surgery and Pediatrics, Wake Forest University School of Medicine
Kat Kolaski, MD is a member of the following medical societies: American Academy for Cerebral Palsy and Developmental Medicine and American Academy of Physical Medicine and Rehabilitation
Disclosure: Nothing to disclose.

CME Editor

Kelly L Allen, MD, Regional Medical Director, IMX-Medical Management Services
Disclosure: Nothing to disclose.

Chief Editor

Consuelo T Lorenzo, MD, Consulting Staff, Department of Physical Medicine and Rehabilitation, Alegent Health Care, Immanuel Rehabilitation Center
Consuelo T Lorenzo, MD is a member of the following medical societies: American Academy of Physical Medicine and Rehabilitation
Disclosure: Nothing to disclose.

Further Reading

Clinical guidelines:
Interim guidelines for the evaluation of infants born to mothers infected with West Nile virus during pregnancy. Centers for Disease Control and Prevention - Federal Government Agency [U.S.]. 2004 Feb 27. 4 pages. NGC:003471

The management of encephalitis: clinical practice guidelines by the Infectious Diseases Society of America. Infectious Diseases Society of America - Medical Specialty Society. 2008 Aug 1. 25 pages. NGC:007083

Clinical trials:
Treatment of West Nile Virus With MGAWN1 (PARADIGM)

VRC 300: Screening of Healthy Volunteers for Clinical Trials of Investigational Vaccines to Prevent Infectious Diseases