Pertussis

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 a special type of 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.
• 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.

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.

• 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 presence of IgG B. pertussis antibodies in blood or oral fluid at least 14 days after the illness started can be used as a sign of recent infection. However, this type of antibody is also produced in response to pertussis part of the DPT (diphtheria, pertussis, tetanus) vaccine. 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.

In the UK, a vaccine against pertussis and two other bacteria which produce toxins is routinely offered. This is the diphtheria, pertussis, tetanus (‘DPT’) vaccine. It is given 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.

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.