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West Nile Virus
The premiere episode features Dr. Margaret Hessen covering West Nile virus, including the following:
- Clinical presentation of patients
- Diagnosing West Nile virus
- Virus complications and effective treatment
- Infection prevention options
Dr. Margaret Hessen
Editor-in-chief of Elsevier’s Point of Care content
Dr. Margaret Hessen is board certified in internal medicine and infectious disease, and was in clinical practice for 17 years. She has also worked with a number of professional organizations on a variety of public health initiatives relating to infectious diseases and disaster preparedness.
Download Podcast Transcript (PDF)
1. This year has seen the largest number of reported cases of West Nile Virus infection since its arrival in the United States. What are the reasons for this dramatic increase?
This year’s weather has presented the perfect conditions for an early and robust crop of the mosquito vectors that transmit the disease. Many areas in the country experienced a mild winter followed by an early spring of unusual warmth, allowing mosquitos an early and long breeding season. There has also been speculation that the economic recession has had an impact, as unattended standing water in the yards and swimming pools of foreclosed and abandoned properties adds uncontrolled breeding territory for mosquitoes.
2. What is the clinical presentation of patients with infection due to West Nile Virus?
The vast majority of patients (about 80%) with West Nile infection are actually asymptomatic. Among those who develop symptoms, most experience the West Nile fever syndrome, a mild flu-like illness, characterized by fever, malaise, anorexia, mild headache and arthralgias. Some patients complain of eye pain. Generalized lymphadenopathy and a mild maculopapular rash may be seen on exam. The illness generally subsides after several days to a week.
A small minority of infected patients (about 1%) have neuroinvasive disease, a much more serious form of infection that presents as meningo-encephalitis, with fever, altered mental status, headache, photophobia and stiff neck. Neurologic abnormalities often include tremors and gait disturbance; parkinsonian features such as bradykinesia may be seen. Myoclonus is common. Weakness is characteristic, and may be generalized or focal (including cranial nerve dysfunction in about 20%). Flaccid paralysis has been reported. Neuroinvasive disease is more common in persons over the age of 60 and immunocompromised persons, including diabetics and patients with chronic renal disease. As might be expected, neuroinvasive disease is associated with significant sequelae in many cases. Mortality exceeds 10%.
3. How does one make the diagnosis?
The work-up depends upon the clinical presentation. The diagnosis requires a high degree of suspicion in patients who present with West Nile fever. The presence of generalized lymphadenopathy and rash may trigger suspicion, especially under appropriate epidemiologic circumstances such as season, history of outdoor activity or documented mosquito bites. Rare cases are also associated with transfusion (although donated blood is tested for West Nile infection) or organ transplantation. The disease should be considered in all patients who present with a picture of meningoencephalitis, particularly now, when the disease is known to be epidemic.
Routine labwork is nondiagnostic, but may demonstrate a mild leukocytosis. The diagnostic test most commonly used to confirm West Nile fever is a serum ELISA for IgM antibodies to West Nile Virus. All positives in the US are reported to public health for confirmation by state or CDC laboratories by PCR or plaque reduction assay.
Patients with meningoencephalitis should undergo neuroimaging studies. Half or more show abnormalities on brain MRI, characteristically in the thalamus, basal ganglia and brainstem. In patients with flaccid paralysis, spinal MRI may reveal anterior horn changes.
Cerebrospinal fluid is characteristic of viral infection, including a pleocytosis of about 100 to 400 white cells, usually but not invariably with a lymphocytic predominance. CSF protein is usually elevated and glucose is normal. CSF tested by ELISA for IgM antibody to West Nile Virus is usually positive. PCR is less sensitive than the ELISA, but may be helpful very early in disease before significant antibody activity has developed, or in immunocompromised patients.
Further studies may be indicated in some patients. For example, EMG testing may be helpful in patients with flaccid paralysis to differentiate from Guillain-Barre and other similar syndromes.
4. Has any specific treatment been shown effective?
Unfortunately not yet. Various agents have been tested, including interferon alfa and IVIG with a high titer of West Nile Virus antibodies, but early results were not definitive and further trials are ongoing. A monoclonal antibody to the virus has also been developed and is being tested in clinical trials. Treatment thus is primarily supportive, including fluids and antipyretics. Severely affected patients, particularly those with flaccid paralysis, may require ventilator support.
5. You mentioned previously that some patients experience incomplete recovery or complications, particularly with neuroinvasive disease. Can you elaborate on that?
Neuroinvasive disease follows a longer time course than West Nile Fever, which usually resolves within a few days to a week, although it might take longer for patients to resume fully normal activity. Of patients who survive neuroinvasive disease, however, about half have prolonged cognitive dysfunction lasting several months or longer. A significant number also have residual motor deficits, tremors, parkinsonism and other symptoms that persist for a year or more. Worse, as already discussed, about 10-12% die during the acute phase of the illness.
6. What can be done to prevent infection?
A vaccine is being developed, but is not yet available. Therefore, prevention revolves around efforts to control mosquito populations and to avoid being bitten. The first is largely a public health endeavor. Mosquito populations are monitored and tested for prevalence of infection. In some areas, public health officials also monitor for die-off of birds, who serve as an amplifying host. Spraying of pesticides may be undertaken by public health authorities to reduce adult mosquito populations. Elimination or treatment of standing water is an important and effective means of removing breeding sites.. Standing water that can’t be removed or drained can be treated, with larvicidal agents. These include two Bacillus species (abbreviated and sold as Bti and Bsp) that are pathogenic for mosquitos but not for humans, animals, or aquatic plants. Rain barrels, wading pools, old tires and so forth should be drained. Swimming pools and hot tubs should be chlorinated and maintained. Other household efforts should include use of window screens, which should be kept in good repair. Personal protection includes reduction of exposure to mosquitos, such as avoiding outdoor activity at dawn and dusk, when mosquitos are most active. Mosquito repellants can be applied to skin or clothing. The CDC advises use of repellants containing DEET or picardin. The percentage correlates with length of protection, up to about 50% of the active ingredient, which confers about 8h or protection. Other acceptable and effective agents include oil of lemon eucalyptus (or PMD, the synthetic version) or IR3535. Permethrin-containing repellants may be applied to clothing and camping gear. These products impregnate fabric and remain effective through several washings or rain exposures. Small children may be more susceptible to side effects from repellants, and parents should check the product label for instructions pertaining to children. Repellants should not be used on infants under the age of 6 months. Mosquito nets may be fitted around baby carriages. Lastly, although a minority of cases are transmitted by transfusion or organ transplantation, testing of blood and tissue donors for evidence of West Nile Virus infection is now undertaken routinely.