Naegleria fowleri

Naegleria fowleri infection facts

• Naegleria fowleri is an amoeba that lives predominately in warm, fresh water.
• Naegleria fowleri is acquired by people when infected water is forcibly aspirated into the nose. This can occur through recreational swimming, diving, or during sports like water skiing.
• Once acquired, the amoeba travels into the brain, causing primary amoebic meningoencephalitis (PAM).
• PAM is very rare, and there are only a few cases reported each year in the United States.
• People with PAM initially experience changes in smell or taste. The disease advances rapidly, causing fever, stiff neck, and coma.
• Infection is diagnosed by examining spinal fluid under the microscope to identify the amoeba. Naegleria fowleri may also be grown in the laboratory, although this takes several days. Newer tests based on PCR technology are being developed.
• The treatment of choice is an intravenous drug called amphotericin B. Amphotericin B may also be instilled directly into the brain.
• More than 95% of cases of PAM are fatal despite treatment.

What is Naegleria fowleri?

Naegleria fowleri is an amoeba that lives in fresh water and soil. The organism goes through three stages in its life cycle: cysts, flagellates, and trophozoites. It is the trophozoite form that causes human disease. Naegleria are "thermophilic," meaning that they prefer warmer water. However, the cysts are able to survive for months in very cold water. Thus, Naegleria infection is found both in tropical and temperate climates.
Although there are many species of Naegleria, including Naegleria gruberi, only Naegleria fowleri causes human infection. There are other free-living amoebas that cause human disease, including Acanthamoeba.

Picture of Naegleria fowleri surrounded by white cells in spinal fluid; Source: CDC

What causes a Naegleria fowleri infection?

N. fowleri is a water-borne disease. Exposure occurs when people come into contact with warm, fresh water usually through swimming, diving, water skiing, or other recreational activity. Although contact with infected water is common, symptomatic disease caused by N. fowleri is rare. It is common for people in the southern U.S. to have antibodies showing evidence of past exposure even when they have no history of symptoms or disease.
The danger of serious infection comes when water containing Naegleria fowleri is forced in into the nose and nasal mucosa. The parasite then migrates through the olfactory nerves and enters the brain. The initial step of infection can occur when diving or inadvertently aspirating water during swimming. Rarely, under-chlorinated swimming pools have been implicated in transmission. Because Naegleria fowleri can be present in untreated well water, there is a small but real chance of transmission to young children during bathing. Naegleria fowleri has also caused disease in adults who inject water into the nose as part of ritual ablutions related to religious practices.

What are risk factors for Naegleria fowleri infection?

The major risk factor for infection is recreational exposure to warm fresh water, especially if there is a history of aspiration of water into the nose. A review of cases in the U.S. by Yoder, et al. showed that there were 111 reported cases of primary amoebic encephalitis between 1962 and 2008. Living in the southern States is a risk factor for infection, because the water is warmer and more conducive to growth of the amoeba. A frequently asked question is when infections most commonly occur. The answer is that the organism is most active in summer months, even in the southern states. Most cases occur in previously healthy young males (median age of 12 years).

What are symptoms and signs of a Naegleria fowleri infection?

After entering the nose, the amoeba travels into the brain along the olfactory nerve and through membranes to enter the brain. Once there, it causes primary amoebic meningoencephalitis (meaning inflammation of the brain and the lining around the brain). In the popular press, Naegleria fowleri is sometimes called the brain-eating amoeba, and meningoencephalitis is sometimes referred to as Naegleriasis.
Symptoms usually appear within five days after exposure but can be delayed up to two weeks. Patients may initially notice changes in smell or taste. Fever, headache, loss of appetite, and nausea follow quickly. The patient becomes confused or semiconscious and finally comatose. Physical examination shows fever and a stiff neck (meningismus).

How is a Naegleria fowleri infection diagnosed?

A spinal tap will be done to examine the spinal fluid. Infection is diagnosed by seeing the amoeba under the microscope. Traditional Gram staining is not used to detect Naegleria because the heat used in the fixation process destroys the organism. Although not specific to Naegleria, the spinal fluid often has a mild elevation in the levels of proteins and a mild decrease in glucose, along with a high white cell count, and is often bloody in the later stages of disease.
Because rapid diagnosis is critical, examination of the spinal fluid is imperative. The organism can also be cultured in the laboratory using on a plate that is coated with bacteria for the amoeba to eat. The culture takes a few days. Newer tests are becoming available that use polymerase chain reaction (PCR) technology to detect amoebic DNA in spinal fluid.

What is the treatment for a Naegleria fowleri infection?

Because Naegleria meningoencephalitis is rare, there are no studies comparing one treatment regimen to another.
The treatment of choice is amphotericin B, which is an intravenous drug usually used for fungal infections. In addition to intravenous treatment, amphotericin B can be instilled directly into the brain (intrathecally).
Other drugs such as rifampin (Rifadin), voriconazole (Vfend), or miltefosine (Miltex) have activity against Naegleria fowleri and may sometimes be used in combination with amphotericin B. However, there is no scientific data available to determine the clinical efficacy of these medications and no official recommendation for their use.

Can Naegleria fowleri infections be prevented?

Infection with Naegleria fowleri can be prevented by avoiding aspiration of fresh water into the nose. Although there have been investigations of how to reduce the risk of disease, it is not possible to eliminate the amoeba from all freshwater sources. Standard chlorination of swimming pools is sufficient to eliminate the organism. Untreated well water should not be forced into the nose or used to irrigate the nose.

What is the prognosis of a Naegleria fowleri infection?

The prognosis for infected patients is very poor. More than 95% of infections are fatal despite treatment. Survivors may have residual neurological problems, such as seizure disorders.

Where can people find additional information about Naegleria fowleri infections?

The CDC is a good source of information on Naegleria infections: http://www.cdc.gov/parasites/Naegleria/faqs.html.

REFERENCE:

Yoder, J.S., B.A. Eddy, G.S. Visvesvara, L. Capewell, and M.J. Beach. "The Epidemiology of Primary Amoebic Meningoencephalitis in the USA, 1962-2008." Epidemiol Infect. 138.7 July 2010: 968-975.

Medical Author:

Mary D. Nettleman, MD, MS, MACP

Medical Editor:

Charles Patrick Davis, MD, PhD

Source:Medicinenet.com.