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Pertussis
(whooping cough) is an acute respiratory disease caused by the bacterium Bordetella pertussis. It causes violent
coughing spells, which makes a person difficult to breathe. It becomes
paroxysmal between 1-2 weeks and often followed by a characteristic whooping
noise. B. pertussis is an aerobic,
gram-negative aerobic coccobacillus. It transmits from person to person via
airborne droplets containing the bacteria during sneezing or coughing.

B. pertussis produces a number of virulence factors that interact
with host cells and alter their functions. Filamentous hemagglutinin, pertactin
and agglutinogen serve as adhesion molecules help to anchor B. pertussis to the ciliated respiratory epithelial cells. While
tracheal cytotoxin releases nitric oxide which paralyze the cilia, stopping
them from beating. This hinders the ability to mechanically remove mucus and
debris. Mucus starts building up which triggers a violent cough reflex to clear
the airway. Besides, pertussis toxin (PT) is a protein exotoxin secreted
exclusively by B. pertussis. PT also
helps with the binding of host target cells. In addition, it induces
lymphocytosis and increases histamine sensitivity, which makes it easier for
fluid to seep out of the blood vessels and into the airways tissues. This makes
the airways swell up, making it harder to breathe, and causes the classic
“whopping” sound during a coughing fit. Next, adenylate cyclase toxin catalyzes
excessive CAMP production, compromising phagocytosis.

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The
incubation period of pertussis usually lasts about a week. Early symptoms at
catarrhal phase include sneezing, runny nose, low-grade fever and a mild,
occasional cough, similar to the common cold. Pertussis is highly contagious at
this phase and usually lasts about 2 weeks. The cough gradually becomes more
severe. The paroxysmal phase begins which lasts another 1-6 weeks or more. It
presents with uninterrupted fits of coughing followed by a whooping sound. This
violent coughing force can cause vomiting and exhaustion. While, young infants
often present with symptoms of apnea and cyanosis without cough. At
convalescent phase, which lasts between 2-3 weeks, the frequency and intensity
of cough decreases gradually.

The gold
standard to diagnose pertussis is bacterial culture due to its high
specificity. Nasopharyngeal (NP) swab specimens should be obtained during the first
2 weeks of illness when the viable bacteria are still present. Sensitivity reduces
and risk of false negative results increases at the later course of illness.
Bacteria DNA can also be detected by polymerase chain reaction (PCR). PCR has
fast turnaround time and high sensitivity. However, PCR tests vary in
specificity. Thus, culture confirmation of pertussis should be obtained for
diagnosis. PCR should be tested from NP specimens taken at 0-3 weeks following
cough onset, but it may give accurate results for up to 4 weeks. Serologic
testing is useful for diagnosis in the later stage of disease. It can be
performed from 2-12 weeks following cough onset. Antibody levels are at their
highest between 2-8 weeks following cough onset, which is the ideal time for
specimen collection.

 

There are 2
types of pertussis vaccines available: whole-cell (wP) vaccines based on dead B. pertussis bacteria and acellular (aP)
vaccines based on highly purified pertussis antigens. The wP vaccines were
initially introduced in the industry. Lately in the 80s, many high income
countries have replaced wP with aP vaccines as means of decreasing the
reactogenicity of the vaccine. According to World Health Organization (WHO),
the incidence of pertussis increasing despite childhood vaccination programmes.
Pertussis outbreaks involving adolescents and young adults have been reported.
This is likely being driven by the transition to aP vaccines. It was found that
aP vaccine works well but has a shorter duration of protection than the wP
vaccine. The aP vaccine immunity wanes rapidly over time. This leads to a
limited impact of aP vaccines on infection and transmission of pertussis to
others. Besides, the resurgence of pertussis may be due to increased
recognition and diagnosis of pertussis as well as reduced boosting immunity by
circulating B. pertussis.

Pertussis
continues to be an international public health concern as it has been a major
cause of childhood morbidity and mortality nowadays. Babies who get pertussis
are usually infected by their parents, siblings or caregivers who might not
even know they have the disease. Pertussis is highly contagious and dangerous
in infants especially those under 6 months of age. Babies infected with
pertussis will require treatment in the hospital. The most common and sometimes
deadly complications are pneumonia and apnea. It can also causes seizures and
encephalopathy due to decrease oxygen levels in the brain. Other complications
include weight loss, ear infection and dehydration. Adolescents and adults may also
develop less serious complications such as rib fracture difficulty sleeping, and
urinary incontinence.

Vaccination
plays an important role in preventing and reducing the risk of severe pertussis
in infants and young children. There are vaccines for babies, children, teens
and adults. DTaP is the childhood vaccine, which consists of 5 doses, typically
given at age of 2 months, 4 months, 6 months, 15-18 months and 4-6 years. Tdap
is the pertussis booster vaccine for teens and adults. Booster shot every 10
years is recommended to maintain the protection.

Some
strategies are implemented to increase immunity within the populations and
prevent early infant mortality. Pregnant women are advised to receive a dose of
Tdap in the 2nd or 3rd trimester and at least 15 days
before the end of pregnancy. This may help to develop antibodies and protect
the newborns until they can get their own DTaP vaccine at 2 months old. Vaccination
of healthcare personnel especially who have close contact with pregnant mothers
and infants should also be prioritized to prevent nosocomial transmission.

Last but
not least, the main treatment for pertussis is macrolide antibiotics, such as
erythromycin and azithromycin which can be used at the catarrhal phase of
disease.

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