What Causes Ear Infections?

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An ear infection commonly refers to a bacterial or viral infection of the middle ear (acute otitis media). Many middle ear infections are caused by an upper respiratory infection that migrates to the ear or an allergy that causes the blockage of fluid in the middle ear.

Infections can affect the outer ear (otitis externa) and inner ear (otitis internal). They involve a virus, a bacterium, and sometimes a fungus (in outer ear infections), and the cause and route of the infections differ.

Ear infections are either acute (sudden, severe, and typically short-lasting) or chronic (persistent or recurrent).

This article describes the most common causes of ear infections, including why they happen.

middle ear infection causes

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Primary Causes of Middle Ear Infections

There are two main types of middle ear infection: acute otitis media, which involves an infectious agent, and otitis media with effusion, which does not.

Acute Otitis Media

Acute otitis media (AOM) is a middle ear infection caused either by a virus or bacterium. The infection can develop on its own, but most occur with upper respiratory tract infections (URTIs), including colds, flu, sinusitis, strep throat, laryngitis, and tonsillitis.

This is because the middle ear is connected to the nose and throat via a pair of canals called eustachian tubes. When you have a URTI, the inflammation in the nose and throat can extend to a eustachian tube, causing it to swell. This creates negative pressure that draws mucus out of the lining of the ear. The accumulated mucus, in turn, becomes a hotbed for infection.

AOM is characterized by the rapid onset of pain in the middle ear accompanied by other signs of infection, including redness, swelling, and a pus-like discharge. Muffled hearing is also common.

Common viral causes of acute otitis media are:

Common bacterial causes of acute otitis media are:

Many cases of uncomplicated AOM will clear on their own, so a healthcare provider may recommend treating the symptoms and waiting for up to 72 hours before prescribing oral antibiotics (typically amoxicillin).

Over-the-counter (OTC) painkillers like Tylenol (acetaminophen) or nonsteroidal anti-inflammatory drugs (NSAIDs) like Advil (ibuprofen) and Aleve (naproxen) to help ease swelling and pain.

Otitis Media With Effusion

Otitis media with effusion (OEM), also known as serous otitis media, is the presence of noninfectious fluid or mucus in the middle ear that can persist for weeks. It is common in younger children for many reasons but can also affect adults.

OEM can result from a URTI, severe allergies, or a rapid change in air pressure (barotrauma) following an air flight or scuba dive. In such cases, inflammation of the eustachian tube can also cause swelling, negative pressure, and the buildup of mucus or fluid—but without the presence of infection.

Less commonly, severe acid reflux can cause the inflammation of the soft pads at the back of the nose called the adenoids. Because they are situated near the opening of the eustachian tubes, adenoid swelling (called adenoiditis) can block the eustachian tubes.

OEM causes the buildup of fluid that places pressure on the eardrum (tympanic membrane). As a result, OEM causes a feeling of fullness in the ear rather than pain.

The treatment may involve antihistamines, decongestants, and steroid nasal sprays to reduce congestion and relieve the blockage. If this doesn't work, the fluid may need to be manually suctioned by an ear, nose, and throat (ENT) specialist. Recurrent OEM may require the temporary placement of a small pressure equalization (PE) tube inserted into the eardrum.

Secondary Risk Factors for Middle Ear Infections

Some people are affected by middle ear infections more than others. Chief among these are children under 2 years old. Environment, anatomy, and even socioeconomics can also play a role.

Risk Factors in Young Children

Acute otitis media is the most common infection in the world and one that affects 7 out of 10 children before the age of 2. Not only are children more likely to get middle ear infections than adults, but they are also more likely to have recurrent episodes.

There are several reasons for this, including:

  • Anatomy: Eustachian tubes are shorter, wider, and more horizontal in infants and children under 2. Because of this, fluids are more likely to accumulate and become infected, especially when the child is lying down on one side.
  • Immunity: Children under 2 don't have developed enough immune systems to fight common infections. In fact, most bacterial causes of otitis media are confined to young children and older people, while viruses like RSV are almost exclusively seen in kids.
  • Group care: Children in daycare generally are exposed to more viruses and germs than children who stay home, increasing the risk of both URTIs and middle ear infections.
  • Bottle-feeding: Bottle-feeding a baby in bed increases the risk of a middle ear infection since the child is in a horizontal position, where fluids can accumulate in the ear. Breastfeeding reduces the risk by keeping the baby's head upright.
  • Pacifiers: Sucking on a pacifier changes the air pressure in the eustachian tubes and can cause fluids to stay in the ear rather than being drained into the throat as they are meant to.

Other Risk Factors

Many of the risk factors for middle ear infections are non-modifiable, meaning you cannot change them. Others are modifiable, and changing them may help reduce your risk of ear infections.

Modifiable and non-modifiable risks factor for otitis media include:

  • Family history of otitis media: Having a first-degree family member (parent or sibling) with chronic otitis media increases your risk of recurrent middle ear infections. Genetic and environmental factors within the home are thought to contribute to the condition.
  • Atopy: Atopy describes allergy-related conditions like eczema, food allergies, allergic rhinitis, and asthma that commonly co-occur. People with atopic disorders are at greater risk of recurrent and severe otitis media.
  • Tobacco smoke: The effects of tobacco smoke can increase the risk of middle ear infections not only in younger kids but also adolescents exposed to secondhand smoke. Smog and poor air quality appear to have the same effect on adults and children.
  • Cleft palate: Contractions of muscles in the roof of the mouth (palate) allow air into the middle ear and the drainage of fluids from the middle ear. A cleft palate, a condition in which the palate doesn't close during fetal development, disrupts this function and increases the risk of otitis media.
  • Seasonal allergies: Perennial allergic rhinitis caused by allergens like tree pollen and grass tends to be chronic. The persistent nature of these allergies increases the risk of otitis media, particularly if hay fever symptoms are not controlled.
  • Socioeconomic factors: Poverty can influence the frequency of otitis media in certain populations. For instance, the lack of access to running water among rural Alaska Natives is independently linked to a two-fold increased risk of otitis media and childhood hearing loss.

What Causes Outer or Inner Ear Infections?

Outer ear infections (otitis externa) and inner ear infections (otitis interna) are usually caused by infections. The causes and symptoms of these infections vary significantly.

Otitis Externa

Otitis externa is when the outside ear canal, otherwise known as the external auditory canal, becomes infected with bacteria and occasionally fungi. It is often referred to as swimmer's ear because repeated exposure to water can inflame the tissues of the ear canal, increasing their vulnerability to infection.

Otitis externa is less common than otitis media, affecting around one in 10 people over a lifetime. Children, adolescents, and adults are equally at risk.

Outer ear infections are often caused by contaminated water that gets into the ears while swimming. They can also occur when foreign objects, such as cotton swabs, earplugs, hearing aids, or fingernails introduce bacteria into the canal.

The infection is usually caused by one of two bacteria: Pseudomonas aeruginosa, which lives in soil and water, or Staphylococcus aureus which commonly resides on the skin.

Most cases of otitis externa are treated with eardrops that either contain an antibiotic to fight bacteria, an antifungal medications to fight fungus, a corticosteroid (steroid) to reduce inflammation, an acidic solution to restore the ear's normal pH, or a combination of these.

If the ear canal is blocked due to a severe infection, a small absorbent wick called an otowick may be inserted into the canal to help deliver the ear drops. As the drops are applied, the wick will expand and help open the canal (generally over a period of one to three days).

Otitis Interna

Otitis interna is the inflammation of the inner ear, the part of the ear made up of a maze of fluid-filled channels (called the labyrinth) and a nerve that transmits sensory information from the labyrinth to the brain (called the vestibulocochlear nerve).

The two types of otitis interna characterized by the part of the inner ear they affect, are:

  • Labyrinthitis: This involves the labyrinth and both branches of the vestibulocochlear nerve (called the vestibular nerve, which helps maintain balance, and the cochlear or auditory nerve, which enables hearing). Common symptoms include nausea, vomiting, and often debilitating vertigo (spinning sensations). Temporary hearing loss and tinnitus (ringing in the ears) can also occur.
  • Vestibular neuritis: This involves the vestibular nerve only. Because the cochlear nerve is not involved, a person can experience vertigo, nausea, and vomiting, but without the hearing problems associated with labyrinthitis.

Otitis interna most often develops in the aftermath of a viral URTI. In such cases, the inflammation caused by the URTI indirectly affects the organs of the inner ear (rather than there being an infection of the inner ear). Actual infections of the inner ear are rare.

Less commonly, a bacterial infection can cause otitis interna. In some cases, the bacteria will release toxins that cause inner ear inflammation. In others, an infection in the bones surrounding the inner ear can also release toxins that cause inflammation (referred to as labyrinthitis ossificans).

Otitis interna is far less common than otitis media, affecting less than 8% of people over a lifetime. It is most common in adults 30 to 60, although certain childhood infections can cause inner ear inflammation that leads to hearing loss in kids.

Viral causes of otitis interna include:

Bacterial causes of otitis interna include:

Treatment can involve antiviral drugs to treat viral infections or antibiotics to treat bacterial infections. Oral steroids may be prescribed to reduce inflammation, while antinausea medications like Compazine (prochlorperazine) can also help ease symptoms.


An ear infection commonly refers to a middle ear infection (otitis media). Most are caused by a virus or bacteria. Allergies and other noninfectious conditions can also cause the buildup of fluid in the middle ear. Children are affected more than adults.

There are also outer ear infections (otitis externa) commonly seen in swimmers that develop as a result of bacteria or fungi that enter the ear canal. Both children and adults are affected.

Inner ear infections (otitis interna) are mostly caused by agents that indirectly trigger inflammation of the central chamber (labyrinth) and/or nerves of the inner ear. Adults have more of these infections than children.

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By James Myhre & Dennis Sifris, MD
Dennis Sifris, MD, is an HIV specialist and Medical Director of LifeSense Disease Management. James Myhre is an American journalist and HIV educator.