January 2005
Feature Story
Nancy Cornish, MD
CAP TODAY published an article in November 2000 about Nancy Cornish,
MD, director of microbiology, Methodist Hospital and Children’s Hospital,
Omaha, who is working to improve physician test-ordering. She teaches
Methodist’s physicians about lab tests through periodic clinical briefs,
which she writes and distributes. The response of CAP TODAY readers
to Dr. Cornish’s work was so enthusiastic and the requests for copies
of her briefs so numerous that we asked her to share the clinical briefs
she writes as they become available. Here, this month, is her word on
Bordetella pertussis.
Whooping cough is caused by the gram negative coccobacilli Bordetella
pertussis. The organism enters the body through the mouth and nose
and preferentially infects the ciliated respiratory epithelium in the
nasopharynx and bronchial tree. It also has been identified in blood and,
therefore, can cause invasive infection as well.
The incubation period for B. pertussis is four to 21 days after
exposure to the organism. The classic symptoms are as follows: catarrhal
stage—runny nose, sneezing, mild cough, and low-grade fever (lasts
for one to two weeks), and paroxysmal stage—staccato cough, whoop,
and post-tussive vomiting (lasts for one to 10 weeks). The convalescent
stage lasts two to four weeks or more.
Patients also may be asymptomatic or have a prolonged nondescript cough,
and infants may present with choking and apnea. The differential diagnosis
of atypical symptoms includes adenovirus, parainfluenza and respiratory
syncytial virus, Chlamydia pneumoniae, and Mycoplasma pneumoniae.
Identifying people infected with B. pertussis allows public
health departments to identify and prophylax those individuals who have
contact with the infected person, some of whom may be in high-risk groups.
The populations most at risk for serious infections are infants and older
adults, whose complications may include seizures, pneumonia, encephalopathy,
and death.
B. pertussis is a human pathogen with a high attack rate, infecting
80 to 90 percent of susceptible contacts. It is spread by respiratory
droplets; close contact (less than three feet) or direct contact, such
as touching, is needed for disease transmission. Having caregivers and
patients wear masks and wash their hands frequently can help prevent transmission.
Pertussis can be contracted at any age, regardless of immunization status.
Immunization protects young children from the serious side effects of
the disease but does not prevent infection. Immunity to pertussis wanes
after five to 12 years.
The Centers for Disease Control and Prevention and Nebraska public health
officials recommend culture and polymerase chain reaction testing in patients
who have symptoms consistent with Bordetella infection. Our laboratory
will replace direct fluorescent antibody testing with PCR testing. The
combination of PCR and culture will give maximum sensitivity, specificity,
and speed of diagnosis. Asymptomatic patients should not be tested.
The sensitivity and specificity of all laboratory testing for B.
pertussis depends on a variety of factors, including prior antibiotic
therapy, duration of symptoms, age of patient, vaccination status, specimen
transport conditions, growth media type/quality, and specimen collection
technique.
For best results, collect the specimen properly and early in the disease,
before antibiotics are administered, and transport the specimen promptly.
The best specimen is a posterior nasopharyngeal specimen obtained using
a Dacron minitip swab slowly inserted into a nostril and pushed back until
the posterior nasopharynx is reached. Leave the swab in place for 15 to
30 seconds and rotate it in place to collect the cells. Remove the swab
and inoculate the growth media at the patient’s side. Repeat the procedure
with the other nostril. Another approach is to obtain an aspirate. These
procedures can be done at the hospital by respiratory therapy or at Methodist’s
laboratory. The on-site pertussis kit with instructions can be obtained
from the laboratory.
If distance from the hospital is an issue, posterior nasopharyngeal swabs
can be collected bilaterally, placed in specialized transport media obtained
from the laboratory, and transported to the laboratory immediately at
4°C. The off-site pertussis kit with instructions can be obtained
from the laboratory.
Culture and PCR testing can be done from the same specimen.
Serology is not recommended for diagnosis. The tests are not standardized
between laboratories and are not approved for diagnostic use. The tests
are difficult to interpret because antibody response differs based on
age and previous exposure to the organism or its antigens. The tests require
that acute and convalescent specimens be compared for maximum specificity.
Asymptomatic people can have rises in titer after exposure.
Antibiotic therapy is administered to prevent spread of the organism
or as a prophylaxis to prevent disease. It may or may not modify symptoms
in patients with the disease. Patients are considered noninfectious after
taking a full five days of appropriate antibiotics. The antimicrobial
agents and dosages used for chemoprophylaxis of contacts are the same
as those recommended for treating clinical cases.
Initiating laboratory testing, treatment, or prophylaxis three weeks
after onset of cough is of limited value in cases or their contacts and
is not routinely recommended. The exceptions to this are for pregnant
women, who should receive treatment up to six weeks after cough onset,
and infants, who should receive prophylaxis up to six weeks after exposure.
In conclusion, both PCR and culture should be ordered for B. pertussis.
The turnaround time after the specimen is received is 24 hours for PCR
and seven to 12 days for culture. Culture and PCR tests may yield useful
information up to five days after antibiotic treatment has been started.
References
- Centers for Disease Control and Prevention; www.cdc.gov.
- Douglas County (Neb.) Health Department; www.co.douglas.ne.us/dept/health/.
- American Academy of Pediatrics’ 2003 Red Book.
To view Dr. Cornish’s other clinical briefs, go to www.thepathologycenter.org. |
