West Nile Virus— Multiple Modes of Transmission
Abstracts & Commentary
Synopsis: Two means of transmission of West Nile virus are newly described—sharps injury and transplacental infection.
Sources: CDC. Laboratory-acquired West Nile virus infections—United States, 2002. MMWR Morb Mortal Wkly Rep. 2002;51:1133-1135; CDC. Intrauterine West Nile Infection—New York, 2002. MMWR Morb Mortal Wkly Rep. 2002;51:1135-1136.
Two microbiologists became infected with West Nile virus in the course of their work. One developed a febrile illness without clinically evident encephalitis after lacerating his thumb while removing the brain of an infected blue jay during a necropsy. Another punctured a finger with a needle contaminated in the course of working with West Nile virus-infected mouse brains. Although he had previously had antigenic exposure from at least 3 flaviviruses (he had dengue fever and had received yellow fever and Japanese encephalitis vaccines), he developed an influenza-like illness without clinically apparent encephalitis.
A woman in the 27th week of pregnancy had a febrile illness with severe headache, abdominal pain, and vomiting. She subsequently developed leg pain and weakness with hyporeflexia of all 4 limbs. Examination of cerebrospinal fluid (CSF) obtained approximately 6 weeks after the onset of illness found 11 WBC/mm3, mostly lymphocytes, protein of 63 mg/dL; IgM antibody to West Nile virus was present in CSF, as it was in serum, but a PCR for the virus was negative. Her infant, with an estimated gestational age of 38 weeks, was delivered approximately 5 weeks later, suffering from bilateral chorioretinitis, with a brain MRI demonstrating severe bilateral temporal and occipital white matter loss, as well as cystic change in 1 temporal lobe. Serological studies of cord blood, infant heel-stick blood, and CSF were consistent with acute West Nile virus infection.
Comment by Stan Deresinski, MD, FACP
A total of 3389 human cases of illness due to West Nile virus infection were reported from 619 counties in 37 states and the District of Columbia from January 1, 2002, through November 30, 2002.1 Approximately two-thirds were associated with meningoencephalitis, and 10% of these died. With rare exception, these infections were transmitted by the bite of infected mosquitoes.
These 2 cases of laboratory transmission demonstrate that percutaneous transmission does not require a mosquito bite if the sharp instrument causing injury has been contaminated by infected blood or tissues. It has also been demonstrated that West Nile virus is transmissible by blood transfusion and organ transplantation.2 During 2002, it was estimated that the maximum and mean risks of West Nile virus transmission from blood donors in Queens, NY, were estimated as 2.7 (95% CI, 0.9-5.6) and 1.8 (95% CI, 1.4-2.2) per 10,000 donors, respectively, with the peak risk in late August and very low risk before August and after September.3 It is likely that infection could also be transmitted by sharing of needles used for illicit injection of drugs.
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Intrauterine transmission of other flaviviruses has been described: Intrauterine infection with Japanese encephalitis virus has been associated with spontaneous abortion, while intrauterine dengue virus infection has caused severe infantile infection. The presence of congenital abnormalities in this single case of intrauterine infection does not, of course, prove causality. In addition to transplacental infection, a single case is suggestive of the possibility that transmission may also occur via infected breast milk.4
The primary means of prevention of infection with West Nile virus remains the avoidance of mosquito bites. Avoidance may be especially important for pregnant women for whom the CDC recommends that preventive measures include protective clothing and the use of DEET. Because of the risk of transmission by transfusion of blood products and transplanted organs, the US FDA has issued a guidance for deferral of donors with suspect or diagnosed West Nile Virus infection.5 Screening of donors may be contemplated in the future; a workshop on the development of donor screening assays for West Nile virus was held on November 5, 2002.6
Dr. Deresinski is Editor of Infectious Disease Alert, Clinical Professor of Medicine at Stanford, and Associate Chief of Infectious Diseases at Santa Clara Valley Medical Center.
References
1. CDC. Provisional surveillance summary of the West Nile virus epidemic—United States, January-November 2002. MMWR Morb Mortal Wkly Rep. 2002; 51:1129-1133.
2. CDC. Update: Investigations of West Nile virus infections in recipients of organ transplantation and blood transfusion. MMWR Morb Mortal Wkly Rep. 2002; 51:833-836.
3. Biggerstaff BJ, Petersen LR. Estimated risk of West Nile virus transmission through blood transfusion during an epidemic in Queens, New York City. Transfusion. 2002;42:1019-1026.
4. CDC. Possible West Nile virus transmission to an infant through breast-feeding—Michigan, 2002. MMWR Morb Mortal Wkly Rep. 2002;51:877-878.
5. Guidance for industry. http://www.fda.gov/cber/gdlns/wnvguid.htm.
6. Workshop on development of donor screening assays for West Nile virus. http://www.fda.gov/cber/minutes/wnv110502.pdf.
Two means of transmission of West Nile virus are newly describedsharps injury and transplacental infection.
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