Naegleria fowleri Carter, 1970
Naegleria fowleri is a free-living amoeba commonly found in warm freshwater environments such as hot springs, lakes, natural mineral waters, and resort spas frequented by tourists. These amoebae can tolerate temperatures of up to 45 C. Infection with this parasite can cause primary amoebic meningoencephalitis (PAM), an acute and generally fatal disease of the central nervous system that has a rapid onset and typically results in death within a week or less. Although PAM is generally viewed as a very rare condition, the number of reported PAM cases is increasing each year. PAM is difficult to diagnose because the clinical signs of the disease are similar to those of bacterial meningitis.
Because it typically presents in healthy individuals, N. fowleri is not categorized as an opportunistic amoeba, as are Acanthamoeba and Balamuthia, but rather as a pathogen. PAM often occurs in healthy immunologically intact children and young adults who were exposed during recreational activity in warm bodies of freshwater. Typically, PAM occurs in the hot summer months, when large numbers of people engage in recreational aquatic activities in freshwater bodies that may harbor these amoebae.
Naegleria fowleri amoebae enter the human host via the nasal route when they are splashed or inhaled into the nose. Forcing water into the nose by diving or jumping into water is common, but N. fowleri can become motile even if the victim is simply submerged under water. Upon entering the nasal cavity, N. fowleri infect the olfactory mucosa and ascend the olfactory nerve through the cribiform plate until they reach the olfactory bulbs of the central nervous system.
Naegleria fowleri occurs worldwide and has been isolated from soil and fresh water. More than 30 species of Naegleria have been described based on the sequence of the SS rRNA gene, but only N. fowleri is known to cause infection in humans. In nature, these amoebae feed upon bacteria and have been isolated from freshwater pools, puddles, lakes, rivers, swimming pools, hot springs, thermally polluted effluents of power plants, hydrotherapy pools, aquaria, sewage, irrigation canals, and even from the the nasal passages and throats of healthy individuals. However, they have not been recovered from seawater.
Naegleria fowleri has been found in aquatic environments frequented by tourists inThailand and Japan, in swimming areas along rivers in Italy, in swimming pools and streams in Belgium and the former Czechoslovakia, in swimming pools in England and New Zealand, in hot springs in California and New Zealand, and in warm freshwater lakes throughout the southern United States.
Visvesvara et al. (2007) reviewed the diagnosis and pathophysiology of N. fowler infection, as well as public health strategies for monitoring and control. Heggie (2010) reviewed the epidemiology, diagnosis, pathophysiology, and treatment of N. fowler infection.
(Visvesvara et al. 2007; Heggie 2010 and references therein; Centers for Disease Control Parasites and Health website)