Download Measles Epi

Epidemiology of Measles
Ahmed Mandil, MBChB, DrPH
Prof of Epidemiology
Dept of Family & Community Medicine
College of Medicine, King Saud University
Headlines
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Distribution
Clinical presentations / complications
Agent
Diagnosis
Reservoir
Portals of exit / entry
Modes of transmission
Susceptibility
Prevention and control
Distribution (I)
Measles is one of the typical viral diseases of
childhood. However, unlike other common viral
diseases i.e. Varicella-Zoster Virus, rubella, mumps,
and parvovirus infections, measles often leads to
severe complications that may be fatal. In the third
world, there may be up to 900,000 measles related
deaths per year. Therefore, there is a lot of pressure
on health in different countries in controlling the
disease through vaccination. Indeed, measles is
targeted by the WHO in its Expanded program of
immunization (EPI).
Distribution (II)
In the Northern Hemisphere, the incidence tends to
rise in the winter. In tropical regions epidemics are
less marked. In the pre-vaccination era, the
maximum incidence was seen in children aged 5 - 9
years. By the age of 20, approximately 99% of
subjects have been exposed to the virus. With the
introduction of vaccine, measles infection has shifted
to the teens in countries with an efficient
immunization programme.
Introduction (III)
In contrast, in third world countries, measles infection
has its greatest incidence in children under 2 years of
age. Here the disease is a serious problem with a
high mortality (10%) with malnutrition being an
important factor. In general, measles mortality is
highest in children < 2 years and in adults. Acute
encephalitis is a severe complication with a
frequency of around 1 in 1000-5000. The mortality
rate is around 15%, 20-40% are left with residual
neurological sequelae.
Clinical presentations (I)
After an incubation period of 10 - 11 days, the patient
enters the prodromal stage with fever, malaise,
sneezing, rhinitis, congestion, conjunctivitis and
cough. Koplik's spots, which are pathognomonic are
measles, appear on the buccal and lower labial
mucosa opposite the lower molars. The distinctive
maculo-papular rash appears about 4 days after
exposure and starts behind the ears and on the
forehead. From here the rash spreads to involve the
whole body.
Clinical Presentation (II)
Complications
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Secondary bacterial infection: e.g. otitis media, bronchitis and
pneumonia.
Measles Pneumonia: mainly in immuno-compromised patients
Subacute sclerosing panencephalitis (SSPE): in children and
young adults and occurs 6 - 8 years after the initial attack of
measles. The incidence is of the order of 1 in 100,000 cases of
acute measles. Half the SSPE patients have contracted
measles before the age of 2 years.
Acute measles encephalitis
Myocarditis
Thrombocytopenic purpura
Cycle of Communicable
Disease Transmission
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Agent
Reservoir
Portal of exit
Modes of transmission
Portal of entry
Susceptible host
Agent
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Member of the family of Paramyxoviruses ssRNA enveloped
virus, helical symmetry lacks virion neuraminidase and thus
grouped into a separate genus, the morbillivirus
Envelope consists of haemagglutinin protein and the fusion
protein embedded in the lipid bilayer M protein (membrane or
matrix protein) lies immediately below the membrane.
ssRNA is encased in a helix of N (nucleocapsid protein). The
ssRNA molecule is of negative sense. The HA protein acts as a
means of attachment to susceptible cells.
Measles is an antigenically stable virus. There is one serotype
only and there are very little differences between different
isolates.
Diagnosis
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History
Clinical examination
Laboratory investigations
– Microscopy
– Immuno-fluorescence: direct and indirect
immunofluorescence have been used extensively to
demonstrate MV antigens in cells from NPS specimens.
– Virus isolation: measles virus can be isolated form a
variety of sources, e.g. throat or conjunctival washings,
sputum, urinary sediment cells and lymphocytes.
– Serology: diagnosis of measles infection can be made if the
antibody titres rise by 4-fold between the acute and the
convalescent phase or if measles-specific IgM is found. The
methods that can be used include: Hemagglutination
lnhibition (HAI), Complement Fixation (CF), neutralization
and Enzyme-Labeled Immuno-Sorbent Assay (ELISA) tests.
Reservoir
Humans in the form of
 Carriers (sub-clinical, during the incubation period)
 Cases (through-out the course of the clinical
sydrome)
In contrast to the influenza virus, measles does not
have an animal reservoir, which makes it candidate
for “elimination” if we manage to successfully prevent
infection among human reservoirs.
Portals of exit / entry
Respiratory system
 Exit (from reservoir): exhalation
 Entry (into susceptible host): inhalation
Modes of Transmission
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Direct – Droplet
Indirect – Airborne
The virus spreads by the respiratory route via aerosol
droplets and respiratory secretions which can remain
infectious for several hours. The infection is acquired
through the upper respiratory tract or conjunctiva.
Susceptibility
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Natural immunity to measles is known to last at least
65 years (i.e. for those who get the infection itself)
In nations with efficient immunization programmes,
measles infection has shifted to the teens in countries
with an efficient immunization programme.
In others, measles infection has its greatest incidence
in children under 2 years of age.
Cases of measles have been seen in partially
immunized children, in babies with residual
antibodies, and in people who have been given
serum immune globulin for protection.
Non-immunized and immune-compromised are most
susceptible
Prevention (I)
The 2 vaccines currently available
 Inactivated Vaccine - this vaccine was intended for use in young
children less than 1 year of age who are most prone to severe
complications, to avoid the use of a live vaccine, but it was found that:
– At least 3 doses were needed to elicit a protective antibody
response but the antibody levels soon waned. This leave the
vaccinees open to attack by the natural virus.
– In some cases, the nature of the partial immunity led to serious
hypersensitivity reactions to infection (Atypical measles). The exact
mechanism is still uncertain but it was thought that the vaccine
lacked an important Ag of the virus, thus immunity was incomplete.
– antibody levels decline rapidly after administration
 In view of the above and the fact that, live vaccination is now generally
recommended and individuals previously immunized with the killed
vaccine should be re-immunized with the live vaccine. The killed
vaccine has now been withdrawn.
Prevention (II)
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Live vaccine - live vaccines are now usually used. The seroconversion rate is 95% and the immunity lasts for at least 10 years or
more, possibly lifelong. The virulence of the attenuated strain now in
use is so low that encephalitis has only been noted in 1 in 1 million
recipients. SSPE has been reported in children given the live vaccine.
However, the rate is lower than that following natural infection.
Therefore the vaccine is safe for use in very young children. The live
vaccine is now incorporated as part as the MMR vaccine. As vaccineinduced measles antibody develops more rapidly than following natural
infection, MMR vaccine can be used to protect susceptible contacts
during a measles outbreak. To be effective, the vaccine must be
administered within three 3 days of exposure. If there is doubt about a
child’s immunity, vaccine should be given since there are no ill effects
from immunizing individuals who are already immune. Immunoglobulin
should be given to those for whom the vaccine is contraindicated.
Control: Management of Cases
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In the majority of patients, measles is an acute self-limiting
disease that will run its course without the need for specific
treatment. However, it is far more serious in the immunocompromised, the undernourished, and children with chronic
debilitating diseases. Such patients can be protected by the
administration of human anti-measles gamma-globulin if given
within the first 3 days after exposure. Alternatively, the exposed
individual can simply be vaccinated within 72 hours of exposure.
Pneumonia - antibiotics may be indicated in cases of secondary
bacterial pneumonia or otitis media.
Encephalitis - treatment of acute measles encephalitis is only
symptomatic and supportive. A wide variety of treatment has
been tried for SSPE but no convincing effects have been
demonstrated.
Control of outbreaks
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Measles outbreaks are most deleterious in wards with immunocompromised children or adults e.g. children with leukaemia and bone
marrow transplant recipients. Measles is definitely as dangerous as
VZV in that setting. HNIG should be given to all severely immunocompromised children irrespective of their immunization status since it
has been reported that severe measles infection can occur in those
who had been immunized and had a documented low-level antibody
response. Therefore, the routine screening of children for measles
antibody before admission is probably unjustified since there would be
no difference in the management. The same argument applies to the
screening of patients for immunity before the administration of HNIG.
The use of live-attenuated vaccine for post-exposure prophylaxis is
contraindicated. The same protocol applies to immuno-compromised
adults who come into contact with measles. Immuno-competent
children under 12 months in whom there is a particular reason to avoid
measles, such as a recent severe illness, can also be given
immunoglobulin. MMR vaccine should then be given after an interval of
at least 3 months, at around the usual age.
Headlines
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Distribution
Clinical presentations / complications
Agent
Diagnosis
Reservoir
Portals of exit / entry
Modes of transmission
Susceptibility
Prevention and control
References
1.
2.
3.
Porta M. A dictionary of epidemiology. 5th edition.
Oxford, New York: Oxford University Press, 2008.
Heymann DL. Control of communicable diseases
manual. 18th edition. Washington DC: APHA, 2005
Wong L. Wong’s virology. http://virologyonline.com/index.html
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