Insighst on a "new"
respiratory virus
The season for metapneumovirus is approaching, and even though
this "emerging" respiratory virus was probably with
us long before being isolated in 2001, recent surveys indicate
that it accounts for a substantial proportion of culture-negative
respiratory illness. Anyone who treats young children will probably
see it this winter or spring.
Human metapneumovirus, or hMPV, is a close cousin of respiratory
syncytial virus (RSV). How hMPV spreads is unclear, and there
is no clinically available test, vaccine or specific treatment.
"Not a lot is known about the presentation, since the virus
is hard to find without special technology," says Kenneth
McIntosh, MD, director of Clinical Research in Children's
Hospital Boston's Division
of Infectious Diseases. "We can say that it's a wintertime
virus, like RSV; it causes primary bronchiolitis, like RSV; and
it tends to infect small babies, though not quite as much as RSV
does."
Much of what we know about hMPV's epidemiology comes from two
retrospective reviews: a Vanderbilt University study of otherwise
healthy children at a primary care clinic, and a study of relatively
sick children at Children's.
The Children's study, led by Alexander
McAdam, MD, PhD, of the Department of Laboratory
Medicine, tested respiratory specimens from more than 800 patients,
age 18 and under, who were seen over a two-year period. Two-thirds
had underlying illness, and most were hospitalized. As shown in
Table 1, hMPV was the second most common
virus detected: it was found in 6 percent of patients, well behind
RSV but ahead of influenza (Journal of Infectious Diseases,
July 2004). The Vanderbilt study, which focused on lower-respiratory-tract
illness, found a similar pattern, and hMPV accounted for 20 percent
of all previously culture-negative cases.
Like RSV and influenza, hMPV surfaces mainly in winter, peaking
from January to March and trailing off in April and May. But Children's
also saw cases in September and October, and the Vanderbilt researchers
found cases in August. hMPV-related illness usually lasts five
to 10 days. In both care settings, bronchiolitis was the most
common diagnosis, recorded in 39 percent of the Children's patients
and 59 percent of clinic patients.
hMPV also causes upper respiratory infection, respiratory distress
and pneumonia. "There's a great spectrum of severity, but
this disease usually isn't that severe on the whole," Dr.
McIntosh says. "If kids are otherwise healthy, they probably
won't get that sick. But if there's an underlying abnormality-congenital
heart disease, bad lungs, prematurity-metapneumovirus can be very
bad, much like RSV."
A Children's pathology team was the first to describe the changes
hMPV causes in the cells and tissues of the respiratory tract
(Journal of Pediatric and Developmental Pathology, Sept./Oct.
2004). They located pathology specimens for six of the hMPV-positive
patients in McAdam's study, and identified features that will
help pathologists diagnose hMPV. "We've corroborated the
clinical impression that the airways, rather than the lungs, are
the main target of infection," says Sara
Vargas, MD, the study's lead author.
Unlike RSV, there are currently no FDA-approved virologic tests
for hMPV, but they should be available for routine diagnostic
use within a year or two, according to Dr. McAdam, whose research
lab has had to turn down outside requests for testing. Usually,
however, the virologic diagnosis isn't critical.
"Until we have a test that's widely available, it's not
worth pursuing testing in most cases," Dr. McIntosh says.
"The children for whom this information might be useful are
those at the severe end of the spectrum; kids with underlying
disease."
Nor is there an approved treatment for hMPV. Ribavirin is licensed
for treatment of RSV, but is rarely used at Children's. "It's
difficult to give, has some toxicity and evidence for its efficacy
is pretty marginal," says Dr. McIntosh. "So for all
but a very few children, treatment is supportive." This means
simple, time-tested measures like suctioning mucus from the nose,
small frequent feedings, and propping the baby up at night so
he can breathe easier are the best ways to combat the virus. Hospitalized
children will need more intense supportive therapy, including
oxygen, IV fluids, and possibly intubation and ventilation.
