Naegleria Fowleri Causes, Symptoms, Diagnosis

Naegleria Fowleri

Causes, Symptoms, Diagnosis and Treatment

Naegleria is a free-living, thermophilic amoeba that is commonly found in the environment in both water and soil. There is only one species of Naegleria that has been found to infect humans, Naegleria fowleri. Naegleria fowleri is found everywhere in the world -- specifically, the amoeba is found in warm bodies of freshwater -- like lakes and rivers, stagnant ponds, geothermal water such as hot springs (springs that are consistently below 80 degrees are considered safe, however, as the organism needs hotter water to thrive), warm water discharge from industrial plants, poorly maintained and minimally chlorinated swimming pools, and soil. These are free-living amoeba, which normally live as phagotrophs in aquatic environments where they feed on bacteria, but they are "opportunistic pathogens" (Pond 2005).

Factors that account for free-living amoeba expression of pathogenic characteristics are unknown (Khan 2008). It has not been determined whether amoebae that are found in water that is enriched in certain bacterial species, pollutants, or specific elements such as iron are more pathogenic (2008), but it is a possibility. Khan (2008) notes that compromised specific and nonspecific host resistance resulting from deficiencies in secretory IgA or complement components may account, at least in part, for disease manifestations. Temperature tolerance is not a factor in pathogenicity. Virulence of Naegleria fowleri has been shown to decline when the amoebae are passaged through experimental animals (2008). In vitro, Naegleria fowleri destroy nerve cells and other mammalian cells by piecemeal ingestion, a process called trogocytosis (2008). Naegleria fowleri also destroy mammalian cells by producing lytic substances upon contact with a target cell. Several researchers have posited that Naegleria fowleri releases cytolytic substances that account for invasiveness and tissue damage in vivo and cytopathogenicity in vitro. It has been suggested that the release of phospholipolytic enzymes by Naegleria fowleri results in the rapid destruction of brain tissue (Chang 1979; Khan 2008).

Although Naegleria fowleri is commonly found in the environment as aforementioned, infection occurs only very rarely. In fact, according to the CDC, there is an average of one to three infection in the U.S. each year (Levy et al. 1998). When infection does occur, it normally occurs during the dry, summer months, when the air temperature is hot, the water is warm, and the water levels are low. The number of infections often increased during the years when heat waves have occurred.

Naegleria infection happens when the amoeba enters the human body through the nose. It can occur when water is forcefully inhaled or splashed onto the olfactory epithelium (Pond 2005). Amoebae enter the nasal mucosa, cribriform plate and olfactory bulbs of the brain (2005). Infection most often occurs when people are playing water-related activities like swimming underwater, diving, or other activities in the water where water is prone to get up the nose. The amoeba then travels to the brain and the spinal cord where it destroys the brain tissue. Infection with Naegleria causes the disease known as primary amebic meningoencephalitis (PAM), which affects the central nervous system. Essentially PAM results in brain inflammation that leads to a destruction of brain tissue. It is highly uncommon but is almost always fatal (2005). It should be noted that though uncommon, the southern states of the U.S. have had the most deaths from PAM in recent years.

Most of the reports of PAM have been from developed rather than developing nations, however, this may be due more to the greater awareness of the infection rather than greater incidence (Pond 2005). The first case of PAM was recognized in Orange County, Florida in 1962 and since then there have been over 250 reported cases worldwide (Embrey et al. 2004). Over 46% of lakes surveyed in Florida have the pathogenic amoebae.

The onset of PAM is very abrupt, with rapidly progressing symptoms. An individual will see signs and symptoms of PAM and death will occur usually within 1 to 14 days after infection. The symptoms include headache, fever, nausea, vomiting, sore throat, decreased appetite, nosebleeds, rapid and shallow breathing, light sensitive eyes, muscle cramping, painful lymph nodes and a stiff neck. The patient normally falls into a coma and death occurs thereafter. Some other symptoms of Pam may include abnormalities of taste and smell; cerebellar ataxia; nuchal rigidity; photophobia; and increased intracranial pressure (Pond 2005). Healthy people may also experience subclinical infections where the protozoa colonize the nose and throat (2005). As the amoebae cause more extensive destruction of brain tissue, confusion will set in as will a lack of interest or attention to people and surrounding environment, loss of balance will also occur and seizures and hallucinations. After symptoms begin, the disease progresses very quickly and death most normally results within 3 to 7 days. Embrey et al. (2004) notes that PAM occurs in otherwise healthy, often active individuals, particularly adults and children. The symptoms are, for the most part, often indistinguishable from meningitis and death occurs typically within 10 days of symptoms onset, usually on day 5 or 6.

Infection with Naegleria fowleri is extremely rare and early symptoms that are associated with Naegleria fowleri may actually be caused by other more common illnesses like meningitis. However, if one believes that they have been infected with Naegleria fowleri because of symptoms such as fever, headache, or stick neck -- the earlier symptoms -- he or she must seek medical treatment immediately.

Unfortunately, PAM is most usually diagnoses at autopsy, but in suspected cases diagnosis is made by examination of CSF which shows predominately polymorphonuclear leucocytosis, increased protein and decreased glucose concentrations, mimicking bacterial meningitis (Embrey 2004). Sometimes amoebae may be seen on Gram-stained smears. For diagnosis during life, there must be clinical suspicion based on exposure history (2004). If a previously health patient has swum in warm, fresh water within 7 days of symptoms onset and displays symptoms of bacterial meningitis with predominantly basilar distribution of exudates by head CT, Naegleria fowler infection should be suspected (2004). Examination of un-refrigerated CSF should be undertaken and care taken to examine further any atypical mononuclear cells, which may actually be amoebae (2004). Transformation from the amoeboid to the biflagellate form can be induced within 1 to 20 hours to aid identification and is undertaken by dilution of 1 drop CSF with 1 ml distilled water (2004).

As mentioned, the majority of cases of PAM are diagnosed post mortem and they are usually diagnosed by hematoxylin and eosin staining of brain tissue (Khan 2008). Since disease progression is so rapid with PAM, serological assays are not helpful, according to Khan (2008), as there is little possibility of measuring a rise in antibody titer. Also, computed tomography (CT) and magnetic resonance imaging (MRI) of the brain performed in response to symptoms of CNS infection and severe headache do not always indicate abnormalities. CT findings are usually nonspecific and may appear normal early in the infection process. However, CT may show evidence of brain edema and hydrocephalus (Kidney & Kim 1998; Shumaker 1995; Khan 2008) and of obliteration of cisterns with enhancing basilar exudates (Singh 2006; Khan 2008).

Khan (2008) notes that more recently, molecular techniques such as PCR and real-time PCR have been created that allow for a more rapid, sensitive, and specific laboratory diagnosis. PCR assays that have been developed for detection of Naegleria fowleri in environmental samples have been adapted for laboratory diagnosis of clinical samples. In that context, "amplification of repetitive DNA has been used for the identification of Naegleria fowleri from purified nucleic acid extracted from infected mouse brain tissue (McLaughlin 1991; Khan 2008).

Naegleria fowleri is cultured from environmental samples on non-nutrient agar plates seeded with Escherichia coli using prior concentration membrane filtration (Anon 1989; Embrey 2004). However, differentiation from other closely related but non-pathogenic organisms is difficult but vital for complete risk assessment (2004).

Phenotypic characterization, including morphology, thermo-tolerance, pathogenicity, detection of antigens by monoclonal antibodies and isoenzyme electrophoretic profiles, and genotypic characterization by PCR/PFLP have been sued to differentiate the pathogenic N. fowleri from non-pathogenic thermophilic species (Embrey 2004).

Marciano-Cabral and Cabral (2007) note that the pathogenesis of PAM and the role of host immunity to Naegleria fowleri are not well understood.

Strategies for combating infection are limited because disease progression is rapid and N. fowleri has developed strategies to evade the immune system. The medical significance of these free-living ameboflagellates should not be underestimated, not only because they are agents of human disease, but also because they can serve as reservoirs of pathogenic bacteria (Marciano-Cabral & Cabral 2007).

While several drugs have been effective against Naegleria fowleri in the laboratory, some of these treatments have been used to treat infected individuals, but their effectiveness isn't very clear since most infections have still been fatal. In laboratory experiments, mice have been given the Cry1Ac protein, administered intransally, alone and with amoebic lysates, and it has been found that this increases protection against Naegleria fowleri meningoencephalitis, "apparently by eliciting IgA responses in the nasal mucosa" (Jarillo-Luna et al. 2008).

Naegleria fowleri is sensitive to amphotericin B (Fungizone) but only a handful of survivors have been documented (Embrey et al. 2004). In those cases, there was very early diagnosis and administration of intravenous and intrathecal or intraventricular amphotericin B. with intensive supportive care (2004). One survivor received miconazole intravenously and intrathecally and rifampicin orally (2004). Other treatment options include the drugs rifampicin and micoazole.

Khan (2008) notes that the mortality rate for PAM is 95%. Again, one of the major obstacles to effective treatment is the rapid progression of the disease. Another obstacle is the paucity of drugs that have the ability to cross the blood-brain barrier (Schuster & Visvesvara 2004; Khan 2008). Nevertheless, there have been documented recoveries from PAM (Seidel 1982; Wang 1993; Khan 2008). Early recognition and treatment of the disease appear to be the chief elements in successful outcomes (2008). At the time of Khan's (2008) writing, the drug of choice for treatment of human cases was amphotericin B. In conjunction with rifampin as well as other antifungal agents. The patients who have effectively recovered from PAM have been treated with amphotericin B. either alone or with the aforementioned combinations. Treatment with amphotericin B. And fluconazole intravenously followed by oral administration of rifampicin led to the successful treatment of a 10-year-old child who developed PAM (Vargas-Zepeda 2005; Khan 2008). Optimal therapy for PAM, however, has not yet been developed since not all patients treated with amphotericin B. survive. Poungvarin and Jariya (1991; Khan 2008) posited that a triple combination of low dose amphotericin B. administered intravenously for 14 days with oral rifampacin and oral ketoconazole for 1 month would result in a more favorable outcome than when a high dose of amphotericin B. was administered intrathecally.

Because of the limited amount of drugs available for treatment of human cases of PAM, a number of studies to assess the efficacy of therapeutic agents has been conducted in vitro and in vivo. For in vivo studies the mouse model of PAM has been used most extensively (Khan 2008). However, use of this animal model has translational limitations to the human (2008).

For example, due to a faster rate of metabolism in the mouse, one may not obtain a true indication of whether the drugs that are effective in the mouse will also be effective in the human. Thong et al. (1979( treated PAM in Balb/c mice with a combination of amphotericin B. And rifamycin. Rifamycin alone was found to be ineffective. However, a synergistic effect was observed when rifamycin was used in combination with amphotericin B. resulting in increased survival in mice (Khan 2008).

A number of animals have been used to study PAM including mice, guinea pigs, sheep, and rabbits. The mouse model has been used most extensively since it resembles the disease in humans and the immune system of the mouse is well characterized (Khan 2008). The nasal passages involving attachment to the olfactory epithelium can be used as the portal of entry by Naegleria fowleri to mimic natural exposure in humans (2008). Also, migration via the olfactory nerves across the cribriform plate to the brain is similar in mice and humans (Martinez 1973; Khan 2008). Lastly, mice infected intranasally develop a fatal disease resembling PAM in humans (2008). To produce PAM in the mouse, usually a trophozoite suspension (10:1 containing 104 to 105 amoebae) in water or medium, can be instilled into the nasal passages using Eppendorf pipette (2008). Mice display symptoms that include "ruffled fur, arched posture, loss of appetite, and a loss of equilibrium, within 4 to 5 days and up to 21 days post inoculation depending on the inoculum size and the virulence of the strain of amoebae" (2008). Naegleria fowleri strains that are low in virulence can cause "sub-acute or chronic encephalitis while highly virulent strains can cause death within 4 days post intranasal instillation" (Dempe et al. 1982; Whiteman & Marciano-Cabral 1989; Khan 2008).