Acute otitis media (middle ear infection) is usually due to a combination of factors that increase susceptibility to bacterial and viral infections in the middle ear. The primary setting for middle ear infections is in a child's Eustachian tube, which runs from the middle ear to the nose and upper throat. The Eustachian tube is shorter and narrower in children than adults, and more vulnerable to blockage. It is also more horizontal in younger children and therefore does not drain as well
Bacteria. Many bacteria normally thrive in the passages of the nose and throat. Most are not harmful. However, certain types of bacteria are the primary causes of acute otitis media (AOM). They are detected in about 60% of cases. The bacteria that most commonly cause ear infections are:
- Streptococcus pneumoniae (also called S. pneumoniae or pneumococcus) is the most common bacterial cause of acute otitis media, causing about 40 - 80% of cases in the U.S.
- Haemophilus influenzae, the next most common culprit, is responsible for 20 - 30% of acute infections.
- Moraxellacatarrhalis is responsible for 10 - 20% of infections.
- Other bacteria include Streptococcus pyogenes and (rarely) Staphylococcus aureus.
Viruses. Viruses play an important role in many ear infections, and can set the scene for a bacterial infection. Rhinovirus is a common virus that causes a cold and plays a leading role in the development of ear infections. It is not the direct infecting organism, however. If a cold does occur, the virus can cause the membranes along the walls of the inner passages to swell and obstruct the airways. If this inflammation blocks the narrow Eustachian tube, the middle ear may not drain properly. Fluid builds up and becomes a breeding ground for bacteria and subsequent infection.
Review Date: 05/03/2011
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.