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Introduction of Human Metapneumovirus (hMPV) Testing at JHH
In the 2007-2008 respiratory virus season, the Clinical Virology Laboratory in the Divison of Medical Microbiology at JHH will perform testing for hMPV. Qualitative detection and identification will be performed on nasopharyngeal specimens using hMPV-specific antibodies in a microscopy-based direct fluorescent antibody stain test (DFA).
hMPV Clinical Significance
hMPV is a relatively recently described respiratory virus. It is related to RSV virologically and clinically. Behind RSV, it appears to be the second most common viral cause of lower respiratory infection in children. Similar to RSV, it is increasingly recognized as a cause of respiratory illness in adults, particularly transplant patients and the elderly.
The spectrum of hMPV disease is similar to RSV, including bronchiolitis and other respiratory presentations ranging from mild upper airway disease to severe pneumonia and respiratory failure. Like RSV, it is believed to be a trigger for asthma. Studies have shown that most children have been infected by age 5 and about 98% of them will have a detectable hMPV antibody level by age 10.
Epidemiology and Transmission
In temperate climates, the incidence of hMPV infection overlaps with that of RSV, occurring mostly during winter and early spring (December to April). However, sporadic infections can arise at any time. hMPV and RSV are transmitted by large particle aerosols requiring close physical contact. The incubation period is about 3-5 days however the duration of viral shedding has not yet been defined. Humans are the exclusive reservoir of infection.
hMPV Laboratory Tests
hMPV can be identified by DFA, culture and molecular amplification methods. The advantages of DFA include ability to perform random access testing and rapid turnaround time. Molecular amplification technology is likely the most sensitive assay format however no FDA approved tests or random access formats are available. Molecular testing for hMPV is currently not available at JHH.
hMPV can be challenging to grow in conventional tube cultures. Efficiency of recovery can be low, growth is slow, and the virus fails to produce distinctive cytopathic effect. Serologic testing for acute hMPV infections may not be helpful since diagnosis necessitates a comparison of acute- and convalescent-phase titers. Direct detection by electron microscopy is possible but lacks sufficient sensitivity or reproducibility.
Nasopharyngeal aspirates and washes provide the best specimens for direct testing since they contain large numbers of epithelial cells. Endotracheal aspirates and BALs are also acceptable. Although the performance of a new generation swab that has been reported to improve detection of respiratory viruses is being studied by the JHH Clinical Virology in collaboration with the Pediatric ED, swabs from nasal, throat, and nasopharyngeal areas are currently not accepted because they do not contain sufficient numbers of columnar epithelial cells. Nasopharyngeal aspirates should be transported directly to the Microbiology Laboratory rapidly after collection. hMPV and RSV are labile viruses that are unstable in respiratory specimens. Prolonged transport times can negatively impact on detection rates. Aspirates and traps SHOULD NOT be sent to the laboratory via the hospital tube system since tube system carriers do not hold nasopharyngeal aspirate traps properly. Inappropriate transport of nasopharyngeal aspirate traps in the tube system can lead to breakage of specimen collection devices with resultant contamination of the specimen and tube carrier system.
Testing will be performed as part of the Respiratory Virus Antigen/Culture panel (test code 7960). For sites still using paper requisitions, test code 7960 should be marked on the Pathology 6 order sheet. The turnaround time for the DFA test is 4 hours for specimens submitted from 7 A.M to 9 P.M., Monday-Friday, and from 8 A.M. to 2 P.M. on Saturdays and Sundays. DFA results will be available the following morning for specimens submitted Mon-Fri after 9 PM and on weekends after 2 PM.
Currently, there are no licensed treatments for hMPV infections. Several in vitro studies show inhibition of hMPV activity with ribavirin, IVIg, heparin, sulfated sialyl lipid (NMSO3), and low-molecular-weight benzimidazole but none of these has been tested clinically.
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José Ordás, José Antonio Boga, Marta Alvarez-Argüelles, Laura Villa, Cristina Rodríguez-Dehli, María de Oña, Julián Rodríguez, and Santiago Melón. Role of Metapneumovirus in Viral Respiratory Infections in Young Children J Clin Microbiol. 2006 Aug;44(8):2739-42
Jeffrey S. Kahn. Epidemiology of Human Metapneumovirus Clin Microbiol Rev. 2006 July; 19(3): 546–557.
Murray P R, Baron E J, Jorgensen J H, Landry M L, Pfaller M A, editors. Manual of clinical microbiology. 9th ed. Washington, D.C.: American Society for Microbiology; 2007, 1370-1373.
Diagnostic Hybrids D3Ultra DFA Product Insert, 2006.