What causes a Naegleria fowleri infection?
N. fowleri exposure occurs when people come into contact with warm freshwater through swimming, diving, water skiing, water toys, or other recreational activity. Public drinking water and well water also pose a risk. Although contact with infected water is common in the United States, symptomatic disease caused by N. fowleri is not often reported.
PAM occurs when N. fowleri is aspirated or forced high into the nasal cavity. The amoeba produces enzymes that digest mucus and protein, which it swallows up with its "feeding cups" or amoebastomes. N. fowleri is attracted to chemicals released by nerve cells. The olfactory nerves (nerves of smell) travel from the roof of the nasal cavity through perforations in the skull (cribriform plate) into the base of the brain. The amoeba consumes the nerve cells, migrating along these tracts until it reaches the brain. The brain is an especially rich food source, with high oxygen levels, glucose, and living cells. Damage to the brain is caused by severe inflammation, direct injury, and bleeding. Death is caused by the resulting severe brain swelling.
What are risk factors for Naegleria fowleri infection?
The source of nearly all of the Naegleria fowleri infections reported since 1962 in the U.S. has been exposure to untreated freshwater during the summer. Activities that pose a risk include submerging the head, jumping feet first, diving, wakeboarding, and water skiing. Water sources include freshwater lakes, rivers, drainage ditches, and ponds. Other freshwater sources have included hot springs, poorly chlorinated swimming pools, untreated well water, water heaters, neti pots, hose water, and warm water discharge from power plants.
Cases have historically occurred in the South, in Florida, and Texas. In recent years, cases have been reported as far north as Minnesota, from Lake Havasu City in Arizona, from Los Angeles in California, and other sites. Climate change is thought to be playing a role in its spread.
Of the 37 cases reported from 2005 to 2015, 33 people acquired PAM from recreational exposure to freshwater, three from using a neti pot with contaminated tap water, and one from playing on a hose-fed Slip 'N Slide toy. Most patients are young, healthy, and active, between 10 and 14 years of age. The full range of ages is 5-19.
Neti pots are sinus irrigation systems that are designed to flush water deep into the nasal cavity. They are often successfully used by allergy and sinus sufferers. Many people are not aware that public drinking water is not tested for amoeba and is only chlorinated enough to kill some diarrhea-causing bacteria; drinking water is not sterile and contains a living ecosystem of bacteria, fungi, and amoebae. These are usually harmless but occasionally are not. Higher levels of chlorine are needed to kill most infection-causing parasites like amoeba and other protozoa. In addition, private water cisterns and water storage tanks may pose a risk.
In parts of the world where chlorination is poor, nasal rinsing may be used to purify the body before prayer -- an act called ritual ablution. In Pakistan, recent years have seen reports from 10 to 15 cases per year of PAM, and chlorination of drinking water or pools has been lax.
It is not possible to give an accurate estimate of invasive Naegleria fowleri infection compared to the many more probable exposures. It is easy to consider, however, that cases are missed because health professionals lack awareness, there is misdiagnosis or trouble making a diagnosis before death, or there is a lack of diagnosis because autopsy is not routinely performed. Few medical situations require a legally mandatory autopsy by the medical examiner, and many facilities don't do them anymore. Autopsies are not covered by insurance and cost up to $5,000. Most death certificates are signed by a doctor based on a best guess. Underreporting may occur because there is no mandatory federal reporting, and states differ in the requirement to report diseases related to amoebae.
Other amoeba have been transmitted by transplanted tissue, but Naegleria fowleri has not caused disease. Between 1995 and 2012, 21 organs have been transplanted from individuals who died from PAM; however, no cases of transplant-related PAM have been recorded, which is reassuring. The risk is not zero, though, because Naegleria can spread in the blood to other organs, probably when the blood-brain barrier has been destroyed.
Swallowing the amoeba has not been associated with infection. Properly chlorinated and maintained recreational water systems or seawater have not been associated with infection.
Humidifiers or vaporizers do not pose a risk; droplets produced by these devices are very small and cannot carry an amoeba or cyst without the organism drying out.