During the summer months of 2007, seven fatal infections caused by Naegleria fowleri occurred in Florida, Texas, and Arizona. All cases were in boys and young men who participated in recreational water sports at freshwater lakes during the summer. At the time, Florida was suffering from drought conditions through much of the state, leading to lowered water levels and increased temperature of the water. Arizona was in moderate to severe drought conditions during most of 2007, while Texas experienced typical high temperatures and had drought conditions present in 2005, 2006, and 2007. What connection, if any, exists between drought and these tragic fatalities?
Naegleria fowleri and primary amebic meningoencephalitis
Naegleria fowleri is a small ameba with an unusual appearance under the microscope (Fig. 1). This organism is a protozoan. Protozoans are grouped in the Kingdom Protista and consist of single-celled eukaryotic cells with typical organelles such as nuclei, mitochondria, and so on. Naegleria fowleri is motile through the use of pseudopodia (“false feet”); it extends its membrane outward and its cytoplasm flows to follow, allowing it to ooze along a surface or in water. The structure and microscopic appearance of a typical ameba is shown in Fig. 2.
Naegleria fowleri has two structural forms in its life cycle: a trophozoite form, which is the amebic cell that infects humans; and a rounded, thick-walled cyst, which is resistant to temperature extremes and mild chlorination. Most cases of Naegleria infection that have been reported occurred in people who have recently been swimming in warm, natural bodies of freshwater. Naegleria fowleri is a free-living ameba that is an accidental invader of humans, causing these infections only when victims inadvertently become exposed to the ameba. This ameba feeds on bacteria and algae that live in the sediment of freshwater lakes and ponds. However, when individuals swimming in these waters stir up the sediments and allow water to enter their nose, the ameba can enter and attach to the nasal epithelial cells. At this point, the ameba burrows into the olfactory mucosa, starts to multiply, and eventually travels into the brain.
Once in the brain, multiplication of the ameba occurs in the meninges (the membranes surrounding the brain) and in the neural tissue. Eventually, primary amebic meningoencephalitis (PAM) develops. Hemorrhage and edema occur; biopsies of brain tissue exhibit trophozoite forms of the ameba when examined by microscopy. Tissue necrosis is found in response to the infections. However, little can be done clinically to distinguish PAM from acute bacterial meningitis, which is why many clinicians do not consider PAM when diagnosing patients with severe meningitis. Another factor leading to the fatalities associated with this pathogen is that treatments for PAM and for infectious bacterial meningitis are extremely different, and what works for one infection will do nothing for the other.
The onset of symptoms may occur any time from 2 to 14 days after exposure to N. fowleri. Symptoms may include changes in the patient's sense of taste or smell, followed by fever, headache, anorexia (loss of appetite), nausea, and vomiting.
Most patients with PAM exhibit confusion, progressing to coma, and the infection is rapidly fatal within a week. PAM should be considered whenever acute meningitis occurs in a patient with a history of swimming in freshwater that is potentially contaminated with ameba. Cerebrospinal fluid (CSF), when taken from these patients, frequently has an increased number of erythrocytes (red blood cells), giving it a bloody appearance. The most important diagnostic tool involving CSF is examination of the fluid using a wet-mount slide preparation. This allows the detection of motile trophozoites, which are usually not recognized by Gram stain due to disruption of the amebas when the slide is fixed. Treatment is usually with amphotericin B, an antifungal agent that is given both intravenously and directly into the spinal canal (intrathecally). This drug has been used in all cases where patients survived the infection. However, its use is not always successful, even when administered within 24 h of patient admission and diagnosis.
Reasons for infections
Naegleria fowleri is a thermophilic organism, meaning it prefers warm temperatures. It can tolerate water as warm as 45°C (113°F). It is found worldwide in naturally warm waters, as well as in waters polluted with heat from energy production or other sources. The concentration of the ameba in warm water can be greater than one cell for every 25 mL of water. Consequently, it is obvious that millions of people have been exposed to this pathogen. Despite this apparent high level of exposure, only 33 cases have been reported in the United States during 1996–2007.
Does immunity to this ameba exist? Antibodies specific for N. fowleri have been demonstrated in human populations, but the role they play in protection is uncertain. Children often can carry the ameba as normal biota, or microbes, present in the body, especially during the summer months. Whether this leads to specific antibody production is unknown.
In 2007, seven boys and young men died from infection with N. fowleri. Why only male victims? And why are most cases found in boys? The tendency of boys to undertake more boisterous activities in water may be a possibility. People become infected when they wade through shallow water, stirring up the bottom. If water is allowed to enter and shoot up the nose, perhaps by someone doing a somersault in shallow water, the ameba can enter the nasal passages and latch onto the host cells.
How is infection with this ameba connected to drought conditions? As drought continues in a geographic area, standing water levels in bodies of freshwater drop, thereby concentrating the number of ameba found in the water. Therefore, where once the ameba was found in low concentrations during a period of adequate rainfall, it is now found in much higher levels—and exposure rate can increase significantly. High temperatures in the summer only exacerbate the situation because bacteria and algae are multiplying exponentially. Thus, conditions are conducive for amebic disease: food, warmth, and swimmers.
In order to avoid N. fowleri exposure, the Centers for Disease Control and Prevention (CDC) in the United States recommends staying out of bodies of warm freshwater, hot springs, and thermally polluted water (such as found around power plants). The CDC also advises avoiding water-related activities in warm freshwater during periods of high water temperature and low water levels, such as those seen during drought conditions. If individuals are taking part in water-related activities in these kinds of freshwater bodies, they are advised to hold their nose shut or use nose clips to close the nose and prevent exposure of the nasal passages. Swimming pools should be properly maintained and appropriately treated to prevent multiplication of the ameba. Finally, the CDC recommends that recreational swimmers avoid digging in or stirring up the sediment of these lakes or ponds, so as to avoid increasing the concentration of ameba in the water.
In the future, more cases of infection with N. fowleri are likely to be seen. El Niño conditions, changes in weather patterns, increases in human populations, and the effects of global warming may all play a role in whether this disease will become a more common occurrence in the United States and worldwide. Infections due to this ameba have occurred in 15 of the southern tier states, including the ones mentioned in this article: Texas, Florida, and Arizona. This disease is of concern to health authorities and to the general population. The ameba prefers warmer temperatures. As worldwide temperatures increase, this ameba is bound to thrive—and cause more cases of infection in recreation seekers.