To detect and diagnose a Bordetella pertussis infection
When you have persistent, sharp spasms or fits of coughing (paroxysms) that the doctor suspects is due to pertussis (whooping cough); when you have symptoms of a cold and have been exposed to someone with pertussis
A nasopharyngeal (NP) swab or a nasal aspirate; occasionally, a blood sample taken from a vein in your arm
No test preparation is needed.
This is a group of tests that are performed to detect and diagnose a Bordetella pertussis infection. B. pertussis is a bacterium that targets the lungs, typically causing a three-stage respiratory infection that is known as pertussis or whooping cough. It is highly contagious and causes a prolonged infection that is passed from person to person through respiratory droplets and close contact.
The incubation period for pertussis varies from a few days to up to three weeks. The first stage of the disease, called the catarrhal stage, usually lasts about two weeks and symptoms may resemble a mild cold. It is followed by the paroxysmal stage, which may last for one or two weeks or persist for a couple of months and is characterized by severe bouts of coughing. Eventually, the frequency of the coughing starts to decrease and the infected person enters the convalescent stage, with coughing decreasing over the next several weeks. Pertussis infection, however, can sometimes lead to complications such as pneumonia, encephalitis, and seizures, and it can be deadly. Infants tend to be the most severely affected and may require hospitalization.
Pertussis infections used to be very common in the United Kingdom, averaging over one hundred thousand cases per year, with major epidemics occuring every few years. Since the introduction of a pertussis vaccine and widespread vaccination of infants, this number drastically decreased to less than a thousand cases per year in the early 2000s. However, since neither the vaccine - nor the pertussis infection - confers lifetime immunity, health professionals are still seeing periodic outbreaks of pertussis in young unvaccinated infants, in adolescents, and in adults, most recently in 2012 when there were over 6000 recorded cases in England and Wales.
Pertussis testing is used to diagnose these infections and to help minimize their spread to others. Several different types of tests are available to detect pertussis infection. Some of these include:
- Detection of pertussis genetic material (Polymerase Chain Reaction, PCR)
- Direct fluorescent antibody (DFA)
- Blood tests for pertussis antibodies, IgA, IgG, IgM
- Detection of anti-pertussis toxin IgG antibodies in oral fluid
Pertussis can be challenging to diagnose at times because the symptoms that present during the catarrhal stage are frequently indistinguishable from those of a common cold or of another respiratory illness such as bronchitis, influenza, and, in children, Respiratory Syncytial Virus (RSV). In the paroxysmal stage, many adults and vaccinated patients who have pertussis will present with only persistent coughing. Suspicion of pertussis infection is increased in patients who have the classic “whoop,” in people who have cold symptoms and have been in close contact with someone who has been diagnosed with pertussis, and when there is a known pertussis outbreak in the community. A swab for pertussis culture and/or PCR test will usually be sent from these patients but should not be performed on close contacts that do not have symptoms.
How is the sample collected for testing?
Sample collection technique is critical in pertussis testing. For a culture and/or a test for genetic material, a nasopharyngeal (pernasal) swab or aspirate is used. The nasopharyngeal swab is collected by having you tip your head back and then a Dacron swab (like a long cotton bud with a small head) is gently inserted into one of your nostrils until resistance is met. It is left in place for several seconds, then rotated several times to collect cells, and withdrawn. A nasopharyngeal aspirate is taken by using a syringe or other suction device to insert and immediately withdrawing a small amount of saline into your nose while your head is tipped back. Neither procedure should be painful, but it may tickle a bit, cause your eyes to water, and provoke a coughing reaction. Whenever possible, samples should be collected before you are given any antibiotics.
For antibody testing, a blood sample is obtained by inserting a needle into a vein in the arm.
Oral fluid is collected by brushing a swab along the gums. This can be done by a health professional, by the patient themselves or the patient’s parent or guardian.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
Pertussis tests are used to detect and diagnose a Bordetella pertussis infection. Early diagnosis and treatment may lessen the severity of symptoms and help limit spread of the disease.
There are several tests that may be used when a pertussis infection is suspected:
- Culture – this test has been the “gold standard” for identifying pertussis and is used to diagnose a pertussis infection. The sample is put into nutrient media and the bacteria are grown and identified. Results are reported in one to two weeks.
- Polymerase Chain Reaction (PCR) – this test amplifies the genetic material of the bacteria in a sample and is available within a couple of days.
- Direct Fluorescent Antibody (DFA) – this test is not as widely used as it once was. It is less specific and sensitive than the pertussis culture and PCR.
- Antibodies, IgA, IgG, IgM – these blood tests measure the body’s immune response to a pertussis infection.
- In January 2014, Public Health England introduced a test to detect anti-pertussis toxin IgG in oral fluid.
Other tests that may occasionally be used include:
- Toxin antibodies, IgA, IgG – these blood tests measure the body’s immune response to toxins released by B. pertussis.
- B. pertussis molecular sub-typing – this test may be requested not to benefit an individual patient, but so that health professionals can better understand the strain and severity of the B. pertussis present in a community during an outbreak.
Typically a pertussis culture will be requested as early in the illness as possible. Cultures are less likely to grow the organism 2-3 weeks into the illness, and cultures will be affected by some antimicrobial agents if the patient has been treated.
Since the introduction of PCR testing, the use of direct fluorescent antibody testing has significantly decreased. When used, it should be requested along with a pertussis culture to recover the organism in order to investigate potential outbreaks and perform antimicrobial susceptibility testing.
Testing for anti-pertussis toxin IgG in oral fluid is used to detect cases of pertussis where the person may have had the infection for more than two weeks, when culture and PCR are less likely to be positive. Oral fluid is used because it is easier to collect than a blood sample.
Pertussis antibody testing on blood samples is not used frequently. Acute and convalescent samples, collected several weeks apart, are sometimes requested on a person who has not sought treatment until late in their illness or on an adult who has had a cough for an extended period of time. They are requested to help determine if the person has had a recent pertussis infection. Pertussis IgG antibodies will be present in anyone who has been vaccinated. Pertussis IgM and IgA antibodies will usually only be present a short time after vaccination or infection. These tests may sometimes be requested to help evaluate and study the spread of pertussis in the community. Rarely, an antibody test may be performed to evaluate the adequacy of a person’s immune response to a pertussis vaccine.
When is it requested?
Pertussis tests are requested when your doctor suspects that you have a Bordetella pertussis infection. A pertussis culture and PCR are typically performed when you have symptoms suggestive of pertussis, and as early in the illness as possible. Oral fluid testing is used only when a B. pertussis infection is suspected in people between 5 and 16 years old who have had a cough for more than two weeks and not received a pertussis vaccine in the past year.
PCR testing should not be used to diagnose outbreaks of the disease. False positive results may occur when PCR is used to screen people who may have been exposed but have no symptoms of disease.
Symptoms during the first stage of the infection, called the catarrhal stage, may include typical cold symptoms such as a runny nose, sneezing, mild cough and/or a low-grade fever. After about two weeks, the paroxysmal stage begins and may include symptoms such as:
- Frequent severe bouts of coughing sometimes followed by vomiting
- Several rapid coughs followed by a whooping sound as the person inhales; affected adults may cough but not whoop, and infants may have trouble breathing and may choke more than whoop
These symptoms may last for one or two weeks or persist for a couple of months. During the convalescent stage, the severity of symptoms lessens, with the frequency of coughing gradually decreasing over the next several weeks.
What does the test result mean?
A positive culture is diagnostic for a B. pertussis infection, but a negative culture does not rule it out. Culture results are dependent on proper specimen collection and transport, duration of symptoms when the sample is collected, and prior antimicrobial therapy administered before the culture is taken.
A positive PCR test means that it is likely that the patient has pertussis. However, the PCR test may also be positive with other members of the Bordetella genus. A negative PCR test means that it is less likely that the person has pertussis but does not rule it out. If there are an insufficient number of organisms in the sample, then they may not be detected. Both culture and PCR tests are less likely to be positive as the illness progresses.
The direct fluorescent antibody test is not as sensitive or specific as other methods. If it is positive, then the person may have pertussis, but this should be confirmed with a culture. A negative direct fluorescent antibody test does not rule out pertussis.
The presence of IgG B. pertussis antibodies in blood or oral fluid may be seen with a recent infection but also after vaccination. A rise in the quantity of IgG B. pertussis antibodies between the acute and convalescent samples and the presence of IgM and IgA antibodies are evidence of a recent pertussis infection.
People who are diagnosed with a B. pertussis infection should receive antibiotics. Children and infants should be excluded from school or nurseries until they have completed the course of antibiotics.
Is there anything else I should know?
In the UK, a pertussis-containing vaccine is routinely offered to babies at two, three and four months old and a fourth dose is included with the pre-school booster when they are three-and-a-half years old. Those children who have not completed the series of pertussis vaccinations are at a higher risk of becoming infected. Even some people who have been vaccinated may be infected by B. pertussis, but they will tend to have a less severe illness.
People who have been in contact with someone who has a confirmed B. pertussis infection may also be given prophylactic antibiotics to prevent them from developing the infection.
International travellers should be aware that many less developed countries do not have widespread vaccination for pertussis. Infants who have not completed their series of vaccinations and people who have not had a booster vaccination in many years may be at an increased risk of contracting pertussis.
Can a throat culture be used instead of a nasopharyngeal sample from my nose?
A throat culture is not generally acceptable. During a pertussis infection, the organism is found in the tissues in the back of the nose, not in the throat or the front portion of the nose.
Can pertussis testing be done in my doctor's surgery?
No. There is no simple, rapid diagnostic test for pertussis. It requires specialised equipment and is typically performed in laboratories. Not every laboratory performs this testing – samples may need to be sent to a specialist laboratory.
Why did my doctor report my child's pertussis infection?
Doctors are required to report pertussis to their local Department of Public Health. Outbreaks are tracked and interventions, such as vaccination and appropriate antimicrobial therapy, can be used to stop the outbreak.
Why do I hear so little about pertussis?
The number of people affected has dropped since widespread vaccination was instituted in the United Kingdom. Infants are routinely vaccinated, reducing the population who are susceptible. Pertussis outbreaks are sporadic instead of seasonal like influenza and RSV and may be underreported and under-diagnosed, especially in adults who may not seek treatment when they have cold symptoms or a persistent cough.
My doctor said I have Bordetella parapertussis. Is this the same as whooping cough?
B. parapertussis is a bacterium that can infect humans in the same manner as B. pertussis, but the infection usually causes a milder respiratory illness. Culture methods and PCR tests can detect and distinguish B. parapertussis from B. pertussis, and both agents are commonly tested for since the clinical presentation may be similar in patients with either infection. There is no vaccine to prevent B. parapertussis infections.