Pertussis
 Current Case Definition
for Surveillance
The clinical case definition for endemic or sporadic
cases of pertussis is a cough illness lasting at least
two weeks with one of the following: paroxysms of coughing,
inspiratory "whoop," or post-tussive vomiting,
and without other apparent cause. In outbreak settings,
a case may be defined as a cough illness lasting at least
two weeks. Isolation of Bordetella pertussis from
a clinical specimen, or a positive Polymerase Chain Reaction
(PCR) is required for laboratory confirmation. Probable
cases must meet the clinical case definition and not be
laboratory confirmed nor epidemiologically linked to a
laboratory-confirmed case. Confirmed cases must be clinically
compatible and either laboratory confirmed or epidemiologically
linked to a laboratory-confirmed case. Both confirmed
and probable cases are reported.
Immunization
 Pertussis vaccines were used widely in the United States
in clinical practice by the mid-1940's and became standardized
in 1949. While the efficacy of three or more doses of
vaccine is estimated to be only 70% - 90%, breakthrough
disease is usually milder than disease that occurs in
unvaccinated individuals. Protection resulting from pertussis
vaccination is thought to decrease over time, but the
rate of decrease has not been well established.In Maryland,
pertussis immunization is required by law for entry into
pre-school programs, and kindergarten through grade two
for children under age 7 years. Based on the 1998/99 retrospective
kindergarten survey, an estimated 78% of children in Maryland
had received four doses of DTP by the age of 24 months
(compared to 60% in the 1988/89 survey)
Photo Coutesy of World
Health Organization
Historical Trends (see
graphs below)
In the early- to mid-1900's, pertussis was one of the
most common childhood diseases and a major cause of childhood
mortality in the United States. The average incidence
of reported pertussis in the United States has declined
from 150 per 100,000 population in 1922 to 1940, to 1.2
per 100,000 in 1980 to 1991. However, pertussis incidence
increased since the 1980’s for unknown reasons. In
1996, 7,796 cases were reported, the highest number of
cases since 1976. The disease remains serious, especially
among infants, with 41% of all reported cases in 1989-91
hospitalized, and 69% of all infants with pertussis requiring
hospitalization. In Maryland, there was substantial year-to-year
fluctuation in the number of reported pertussis cases
from the 1920's through the 1950's. There was also a steep
and uninterrupted decline in the five-year mean incidence
of pertussis in Maryland from the early 1940's through
the 1950's.
It appears likely that the dramatic peak in reported
pertussis cases in 1985 was more the result of a pertussis
study in Baltimore City and Baltimore County involving
active case detection, and thus greatly increased surveillance
program sensitivity, than the result of a real increase
in incidence. The study, which was conducted from July
1984 to July 1986, enrolled physicians and schools and
involved active case finding in the households of pertussis
cases. The increase in the numbers of reported pertussis
cases during the study period was mostly confined to the
Baltimore City/County study area, while there was a comparatively
much smaller increase in the numbers of cases reported
through the regular passive surveillance system in the
remaining 22 jurisdictions of Maryland.
Epidemiology, 1989 - 1999
(see graphs below)
Pertussis reached epidemic levels in
Maryland in 1993 and 1996. These increases in pertussis
incidence in Maryland coincide with national increases.
With regard to U.S. cases in 1993, the CDC reports that,
"of 1,347 cases of pertussis among children seven
months to four years of age with known vaccination status,
630 (46.8%) had received fewer than three doses of DTP
vaccine-the minimum number of doses necessary for clinical
protection." Reasons for the U.S. increase in 1996
are not clear, but may be a reflection of the 3-5 year
cyclicity observed with pertussis. In Maryland
outbreaks in Howard and Frederick counties and improved
surveillance and reporting accounted for some of the increase.
Incidence during the ten year period appears to have been
strongly associated with age, with the highest incidence
in children under seven months of age. Most of the cases
among infants less than three months of age, and among
adults received no pertussis vaccinations. The average
incidence during the first six months of life was approximately
one per thousand per year, a very substantial disease
burden, especially when the severity of pertussis in this
age group and the likely under-reporting of the disease
are taken into consideration. Among the pertussis cases
less than seven months of age with known admission status,
69% were reported to have been admitted to hospital. There
were 124 cases of pertussis reported in 1999, an 88% increase
from the 66 cases reported in 1998.
Figure
1. Reported Cases of Pertussis in Maryland, 1974-1999.
Figure
2. Pertussis Incidence, Maryland and United States. Reported
Cases, 1989-1998.
|
are PDF. Download