Physicians' fears that Nova Scotia's
troubling ongoing mumps outbreak would spread farther
afield have been realized. In mid-May a slew of new
cases cropped up in Ontario, Alberta and BC, on top
of a smattering already diagnosed in New Brunswick and
PEI. All the cases have been traced back to NS.
The Nova Scotia Department of Health
Promotion and Protection announced on May 11 that healthcare
workers will get a mumps vaccine booster, in response
to the outbreak. Forty thousand doses of MMR vaccine
will be made available, aimed at protecting workers
and preventing absenteeism. The virus, which can cause
parotitis, at its worst can bring on viral meningitis,
sterility and deafness; dozens of NS healthcare workers
have already been quarantined.
Mumps infections have been con—firm-ed
in roughly 250 people across Canada, the vast majority
of them in NS. Halifax's Dalhousie University has been
particularly hard hit.
This is NS's third outbreak in
two years, and now the province is exporting the virus.
What's going on?
CAMPUS
LIFE
"Several factors could have aided the spread of the
virus," says Dr Todd F Hatchette, director of virology
and immunology at Dalhousie. "Close contact between
students, in dorms and the bar scene would facilitate
its spread. And now, heightened awareness in NS could
mean better reporting."
Students returning home for visits
and for summer vacation are clearly contributing to
the spread outside the province. "[The mumps virus]
has an incubation period of roughly 14-25 days," according
to NS regional health officer Dr Gaynor Watson-Creed,
"and up to 40% of people infected with it won't show
symptoms."
THEORIES
ABOUND
No one's sure why the NS outbreak started, but there
are plenty of theories:
1. Vaccination coverage
Born between 1970 and the early 90s, the bulk of those
infected in NS constitute a "lost cohort," who are too
old to have received two mandated doses of MMR (the
measles-mumps-rubella combo), but too young to have
a natural immunity built with age. A single shot protects
roughly 80% of people from the virus. A booster brings
this to 90%. A huge 2004 outbreak in the UK, which infected
over 60,000 people, is thought to have been partly due
to poor vaccine coverage owing to suspicions of a MMR-autism
link.
2. Vaccine handling Another
theory involves the way the vaccine was handled and
administered. "You would assume that the single-dose
cohort, like those infected in NS, would have the same
susceptibility all around Canada. The students are from
all over the country," says Dr Hatchette. "This suggests
there could have been something wrong with their vaccinations."
Violations of cold chain protocols, necessary with this
fragile, live virus vaccine, are suspected. "This makes
a single dose more difficult to rely on," points out
Dr Ian Gemmill, an Ontario regional health officer.
"The vaccine must be maintained between 2-8íC,
but 20 years ago, not much attention was given to respecting
the cold chain. We know for a fact that insults to the
vaccine do occur."
3. Waning immunity Around
half of those infected in a recent Iowa outbreak had
received a double-dose of the vaccine and nine of 13
infected in one of the recent NS outbreaks had also
received boosters. Dr Hatchette thinks this could indicate
that the vaccine wears off in time. Dr Watson-Creed
adds that the worldwide trend towards mumps outbreaks
lends credence to this theory.
4. Antibody resistance An
even more alarming possibility is that this outbreak
strain may have developed a resistance to the antibodies
promoted by the Jeryl Lynn strain vaccine, which is
Canada's mumps vaccine. "Its been traditionally thought
that mumps was a monotypic virus, unlike, say, the flu
virus, and that a single vaccine could protect against
all strains," explains Dr Hatchette. "In the current
and last NS outbreaks, and the Iowa outbreak, it was
always the same strain, 'Genotype G.' The UK outbreak
virus clustered around the same genotype too." Dr Hatchette
notes that we'll better understand the resistance of
the outbreak strain once controlled neutralization studies
have been done comparing its invasiveness with that
of vaccine strains like Jeryl Lynn, using antisera from
vaccinated humans.
Both strains should be similarly
prevented from infecting tissue cultures, provided the
outbreak strains of the virus aren't acting contrary
to conventional immunological knowledge, becoming resistant
to the vaccine.
ON
THE LOOKOUT
For now, physicians will have to stay vigilant, keep
their patients' immunization schedules up to date and
watch out for symptoms. "Mumps is a disease which physicians,
by law, report to their local health authorities," notes
Dr Watson-Creed, adding that doctors are doing a good
job of this.
"MMR is a good vaccine, but field
studies have suggested that one dose doesn't provide
the same protection measured in clinical trials," says
Dr Gemmill. "National recommendations for measles coverage
are for two doses, which incidentally means that kids
now get two shots for mumps, too." A recommendation
specifically for two mumps shots would mean catch-up
vaccinations for the "lost cohort." Doctors in NS have
recommended university-aged young adults get a booster,
but nobody's calling for a nationwide campaign just
yet. "In Canada, we've had outbreaks mostly concentrated
in one area," says Dr Gemmill. "It might not be enough
of a critical situation to invoke a very widespread
and resource intensive catch-up program. But if it progresses
in numbers or geographical scope, there'll be a lot
more impetus to consider a catch-up program. If people
aren't now up to date with at least their one dose,
they should get that done."
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