Ear Infections - Causes

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Causes

Acute otitis media (middle ear infection) is usually due toa combination of factors that increase susceptibility to infections by specific organisms in the middle ear. The infection typically evolves as follows:

The primary setting for 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 smaller in children than adults, and therefore more vulnerable to blockage.
Changes in middle ear pressure occur in about two-thirds of children with colds. Colds are caused by viruses, such as the rhinovirus. Viruses are considered important in many ear infections, but usually only set the scene for a bacterial infection.
However, many bacteria normally thrive in the passages of the nose and throat. Most arenot harmful. In fact,some can even block harmful bacteria from getting out of control. An additionaldefense system in the airways, such asmucus,prevents the harmful bacteria from spreading and infecting deeper passages, such as those in the ear.
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.The defense systems described above become inefficient, and the fluid becomes a breeding ground for bacteria and subsequent infection.

Respiratory viruses may also contribute directly to the infection. Allergens can also produce inflammation and blockage in the Eustachian tube, which creates an environment favorable to bacteria.

Infections and Triggers

Bacteria. Certain bacteria are the primary causes of acute otitis media (AOM). They are detected in about 60% of cases. The bacteria most commonly causing 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 isresponsible for 10 - 20% of infections.
Other bacteria include Streptococcus pyogenes and Staphylococcus aureus.

About 15% ofAOM-causing bacteriaare now believed to be resistant to the first-choice antibiotics.

Viruses. Rhinovirus is a common virus which causes a cold, and plays a leading role in the development of ear infections. It is not the direct infecting organism, however. But other viruses, such as respiratory syncytial virus (RSV, avirus responsible for childhoodrespiratory infections) and influenza (flu), may be the actual causes of some ear infections. Increasing evidence suggests that such viruses play a greater role in ear infectins than previously believed. Thetheory rests on the significantly lower rates of ear infections in children who have received flu vaccines.