Lingering Cough

Dr. Greene's Answer

Picture a child who for 3 or 4 days feels ill with a fever and perhaps a headache. She doesn’t have much of a runny nose, but a cough has developed. The cough has become increasingly prominent even though the fever and other symptoms have gone away. Initially, the cough was dry and nonproductive but begins to produce sputum as the weeks go by. And the weeks do go by–the cough lingers 1, 2, 3, 4 weeks, with no sign of improvement. This little girl has pneumonia caused by Mycoplasma sp. (although because she is acting fairly well, no one has made this diagnosis). Mycoplasmas are the leading cause of pneumonia in school-aged children and adolescents.

Mycoplasmas are the smallest free-living creatures ever discovered. Dwarfed by amebas, they are smaller even than bacteria (or perhaps than “other bacteria”–sometimes they are classified as the smallest bacteria). They are the original “morphers”–changing shape from long, thin filaments to little spheres to intricate branching patterns to bulbs to an endless variety of asymmetric shapes, like the latest science-fiction special effects. They grow and reproduce very slowly for such tiny organisms.

Almost 100 species of mycoplasmas have been identified and are plentiful everywhere in nature. Fifteen species are known to live in humans, most as normal and helpful intimate neighbors. The species you asked about, Mycoplasma pneumoniae, is a very common cause of human disease.

Many pediatricians still consider mycoplasmas an uncommon cause of disease. Infectious diseases are often identified by “culturing” them–taking a small sample of tissue or fluid and keeping it in an incubator for a while to see what grows. Mycoplasmas are very tiny, shape-shifting, slow-growing critters that don’t often show up on cultures.

But newer, more sensitive antibody tests prove mycoplasmas to be a common cause of infection. Over 40% of children have had a mycoplasma infection by their first birthday, more than 65% by age 5 years, and 97% by adulthood. Repeat infections, even severe repeat infections, are common (Textbook of Pediatric Infectious Diseases, WB Saunders, 1998, pp 2259–2286).

In addition to pneumonia, mycoplasmas can cause a variety of other respiratory illnesses, including ear infections (about 12% of all ear infections), sinus infections, bronchitis, croup, bronchiolitis, severe sore throats, infectious asthma, and even a version of the common cold. Most mycoplasma infections are not very severe but last for quite a long time.

About 25% of kids with mycoplasma infections have nausea, vomiting, or diarrhea. Uncommonly, mycoplasmas can cause hepatitis, pancreatitis, arthritis, heart disease, anemia, or neurologic disease.

Most of the commonly prescribed antibiotics are completely ineffective against mycoplasma infections. Suspect a mycoplasma infection when a cough, sore throat, ear infection, or wheezing doesn’t clear up as expected from treatment. Erythromycin, azithromycin (Zithromax), and clarithromycin (Biaxin) work very well to treat patients with diseases caused by mycoplasmas.

The most widely available laboratory test for mycoplasma infection is to test specific antibody levels in the blood. IgG, IgA, and IgM antibodies can be found. Because IgM and IgA antibodies disappear after months to years, their presence indicates that an infection was present in the not-too-distant past. IgG antibodies begin forming at the beginning of the infection, peak at about 1 month, and then decline slowly but persist throughout life. The presence of IgG on a test only tells that a person was infected at some time.

Two blood tests, taken 5 days or so apart, will show a significant increase in antibody levels if infection is present. This is a solid way to confirm the diagnosis.

Scientists are hard at work to find even better tests for these infections including looking for protein markers (RNA) of these organisms. (Journal of Clinical Microbiology. 46(1):185-91, 2008 Jan.)

Mycoplasma infections are often overlooked in children. This is a situation in which a short course of the properly selected antibiotic can save weeks or months of illness and perhaps even severe disease.