Diagnosing Measles in the Post-Elimination Era
For most of human history, the diagnosis of measles was made on clinical grounds: a doctor or parent could recognize the disease, also known as rubeola, simply by observing a miserable-appearing child who, after a few days of high fever, runny nose, cough, and conjunctivitis, developed a blanching, morbilliform rash on the face that subsequently spread over the rest of the body (Figure 1). No laboratory tests were available to confirm the diagnosis, but neither was there any great need for such confirmation: with no specific treatment and, in the pre-intensive care and pre-antibiotic era, few useful supportive care modalities, children were cared for at home regardless of the identity of the virus causing their symptoms. As the English pediatrician Thomas Phaer wrote of measles in his 1545 The Boke of Chyldren, “The best and moste sure helpe in this case, is not to meddle with anye kynde of medicines, but to let nature worke her operacion.” In a time when contraction of measles was a universal rite of childhood, moreover, there was no role for public health or infection control measures that rely on accurate diagnosis.
Scientists were, however, interested in finding a way to identify and propagate measles in the laboratory, in part because this capacity would be essential in the development of a vaccine. In the 1930s and 1940s, investigators working to isolate the measles virus succeeded in cultivating it in chick embryos, and in 1954 John Enders and Thomas Peebles isolated the virus in tissue culture and were able to observe its cytopathic effects (Figure 2). These developments also facilitated the introduction of serological tests for measles infection. In 1959, a method was described for rapid cytological identification of measles-induced epithelial giant cells in nasopharyngeal samples from children with suspected measles prior to the development of the rash – that is, in the catarrhal phase of measles during which symptoms of cough, runny nose, and conjunctivitis predominate. Nevertheless, measles remained by and large a clinical diagnosis as long as it continued to be a universal and routine infection.
The Introduction of the Measles Vaccine
In the pre-vaccine era, approximately 4 million people are estimated to have been infected with measles annually in the U.S.; about 1 in 1000 of them died. The epidemiology of measles changed dramatically with the introduction of the measles vaccine in the United States in 1963. Measles was declared eliminated in the U.S. in 2000, meaning that there had been no continuous disease transmission for over 12 months. However, it remains a highly prevalent disease worldwide, with an annual estimated 7 million annual cases and more than 100,000 deaths – and recent years have seen an increase in cases. As long as vaccination rates in the U.S. remained high, sporadic cases imported by travelers from endemic countries did not generally lead to outbreaks. However, vaccination rates in some regions of the U.S. have been declining recently due to vaccine refusal, resulting in a dramatic increase in cases: as of May 24, 940 measles cases have been reported in 2019 in the U.S.—already more than twice as many as in all of 2018.
The classic presentation of measles is distinctive, but as with all infections, its manifestations don’t always conform precisely to the expected course. Measles can be difficult to distinguish clinically from other ubiquitous childhood viral exanthems including parvovirus B19 and roseola, which are now vastly more common than measles in the US. The accurate identification of measles cases is critical for outbreak control, however. Measles is one of the most contagious infectious diseases in existence, so the prompt institution of appropriate infection control and post-exposure prophylaxis measures are critical in preventing its spread to vulnerable populations, including infants too young to have been vaccinated and immunocompromised individuals. A doctor who encounters a patient with suspected measles should promptly contact the local health department to discuss procedures for submitting samples for testing at the regional laboratory. If an Infection Control program is in place at the hospital or medical center where the patient is evaluated, an Infection Preventionist from this team will typically assist the clinician and coordinate communication with the local and state public health department, laboratory, and epidemiologist.
Diagnosis of Measles in the Post-Elimination Era
The 3 main components of the laboratory diagnosis of measles are:
- Serum IgM testing.
- Viral culture.
- Reverse-transcription PCR (RT-PCR).
Viral culture is less sensitive than RT-PCR and is not offered by all public health laboratories. RT-PCR and culture are usually performed on a throat or nasopharyngeal swab specimen. (Paired acute and convalescent IgG titers collected 10-21 days apart can also be used in cases of diagnostic uncertainty, with a rise in titer indicating infection, but the delayed nature of this result is not as helpful for immediate infection control measures.)
The most common method for measles IgM testing is a capture enzyme immunoassay (EIA), although indirect EIAs may also be used. IgM testing should be sent as soon as the patient presents with symptoms, but levels may not be detectable until 3 days after the rash appears, so a negative IgM result obtained earlier than this should be repeated. By contrast, direct identification of measles by RT-PCR or culture is most likely to be successful during the first three days of the rash. Although nasopharyngeal or throat swabs are usually the recommended specimen type for these tests, measles virus can also be identified and recovered in urine samples.
The combination of IgM and RT-PCR is usually sufficient to establish or rule out a diagnosis of measles. However, uncertainty about the diagnosis may occur when PCR and culture are negative but were obtained too late in the disease course to be considered definitive and IgM results are suspected of being either falsely negative (e.g., in a patient with a clinical and exposure history that are highly suggestive of measles) or falsely positive (e.g., in a patient at low risk whose presentation has subsequently evolved to be more consistent with an alternative diagnosis). The diagnosis of measles is particularly challenging in previously infected or vaccinated individuals. Although the measles vaccine is highly effective, infection in previously vaccinated or naturally infected individuals does occur and can account for approximately 10% of cases in highly vaccinated populations, simply because unvaccinated individuals are so much fewer in number.
Measles in previously immunized or infected hosts can represent 2 scenarios. The first is primary vaccine failure (PVF), in which a protective antibody response to vaccination or infection was never mounted. (A second dose of MMR vaccine was added to the immunization schedule in the U.S. in 1989 to reduce cases of measles due to PVF.) The other possible scenario is secondary vaccine failure (SVF), in which an appropriate antibody response was initially mounted, but this response has waned over time and the host has been reinfected.
An IgG avidity assay can be useful in distinguishing between these two groups: IgG avidity will be low in a person with PVF recently infected with measles, whereas it will be high in a person with SVF. However, while a high IgG avidity result demonstrates that a person does not have PVF, it does clarify whether current symptoms are the result of measles reinfection or of some unrelated infection. Unfortunately, the usual tests for diagnosis of acute measles have limitations in reinfection cases: viral shedding may be reduced, decreasing the likelihood of successfully detecting virus by PCR or culture, IgM results may be falsely negative, and the clinical manifestations often diverge from the classic measles presentation. In the case of a patient with suspected measles reinfection (due to suggestive symptoms and an appropriate exposure) who has a high measles IgG avidity but negative IgM, PCR, and culture, a plaque reduction neutralization (PRN) titer can distinguish those with measles reinfection, who will have a high PRN titer, from those with lower titers whose symptoms are due to a different cause.
Once a disease of primarily historical interest in the United States, measles now represents a growing re-emerging threat both in the U.S. and throughout the world. The clinical microbiology laboratory has a critical role in assisting clinicians, infection preventionists, and epidemiologists in controlling the spread of this highly contagious and potentially lethal infection.
The above represents the views of the author and does not necessarily reflect the opinion of the American Society for Microbiology.