What is being tested?
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 more commonly whooping cough. It is highly contagious and causes a prolonged infection that is passed from person to person through respiratory droplets such as during sneezing or coughing and close contact. Most cases of pertussis in Australia occur between August and February each year, infecting mostly children under 2 years of age.
The incubation period for pertussis varies from a few days to up to a couple of 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 convulsive or paroxysmal stage, which may last for one or two weeks or persist for a couple of months and is characterised by severe bouts of coughing with possible vomiting. Night attacks also can occur. 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 can be deadly. Infants tend to be the most severely affected and may require hospitalisation.
Pertussis infections used to be very common in Australia affecting a peak of 49,500 people in 1930 (800 cases per 100,000 population). Improved living conditions and the introduction of mass vaccination with the pertussis vaccine in 1953, had decreased this number to less than 5,000 in 2007 (3 cases per 100,000). However, neither the vaccine nor previous whooping cough infection confers lifetime immunity and there are still periodic outbreaks of pertussis in young unvaccinated infants, in adolescents and in adults. Adults and adolescents are now seen as reservoirs of transmission to young children. Improved diagnostic tests for pertussis have also contributed to the greater number of cases reported. In 2014, 11,866 cases of pertussis were notified nationally to public health units (70 cases per 100,000).
Pertussis testing is used to diagnose these infections and to help minimise their spread to others. Several different types of tests are available to detect pertussis infection. Some of these include:
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 pertussis PCR test will usually be ordered on these patients but should not be performed on close contacts that do not have any symptoms.
How is it used?
Pertussis tests are used to detect and diagnose a Bordetella pertussis infection. Early diagnosis and treatment is critical to help limit spread of this highly contagious disease.
There are several tests that may be used when a pertussis infection is suspected:
Typically a pertussis culture and PCR test will both be ordered, 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.
Serology is commonly used by medical practitioners in Australia but may be tricky to interpret. PCR and/or culture is preferred in the early stages of pertussis infection; serology on the other hand is best used in situations where a person has not sought treatment until late in their illness, or in an adult who has had a cough for an extended period of time. At present there is still significant variation in the types of serology tests used for the diagnosis of pertussis, therefore for an accurate interpretation of the serology results, your doctor may need to find out from the laboratory the type of antigen used in the serology test and liaise with a microbiology specialist.
Serology tests for diagnosis of pertussis should not be performed within 1 year of vaccination. Serology tests cannot differentiate between vaccine and infection induced immune responses. Other antibody therapies can also impact results.
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 ordered 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 ordered when your doctor suspects that you have a Bordetella pertussis infection. A pertussis PCR and/or culture are typically performed when you have symptoms suggestive of pertussis, and as early in the illness as possible.
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:
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 result mean?
A positive culture is diagnostic for a B. pertussis infection, but a negative culture does not rule out infection. 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 family. A negative PCR test means that it is less likely that the person has pertussis but is not definitive. 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 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.
Is there anything else I should know?
The pertussis vaccination is given to infants as a series of injections. 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 Bordetella pertussis, but they will tend to have a less severe illness. Currently in some states of Australia a free pertussis vaccine is offered to parents and carers of young children.
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.
If you are suspicious of whooping cough to sure to cover your nose and mouth when coughing and sneezing. If a household member is diagnosed it might be advised if in the late stages of pregnancy or an infant is present, that preventative antibiotics be taken. Diagnosed individuals should abstain from work or school on a doctor's recommendation to prevent spreading the bacteria.
Common questions
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.
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 public health laboratory.
Laboratories and doctors are required to report pertussis to state health departments. Outbreaks are tracked and interventions, such as vaccination and appropriate antimicrobial therapy, are used to stop an outbreak.
The number of people affected has dropped since widespread vaccination was instituted in Australia. Infants are routinely vaccinated, reducing the population who are susceptible. Pertussis outbreaks are sporadic instead of seasonal like influenza and RSV and may be under-reported and under-diagnosed, especially in adults who may not seek treatment when they have cold symptoms or a persistent 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.
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