Introduction

Parasitic meningitis, while rare, presents a significant health threat with potentially severe consequences. This condition, characterised by inflammation of the protective membranes surrounding the brain and spinal cord, is caused by parasitic infections. Despite its rarity, parasitic meningitis highlights attention due to its life-threatening nature and the challenges it poses for diagnosis and treatment. Understanding the sources of contamination, recognising the symptoms, and implementing effective prevention strategies are essential for mitigating the risk of parasitic meningitis.

According to the United Nations’ report on levels and trends in child mortality (2017), meningitis stands as the second most significant infectious cause of mortality among infants and children under the age of five globally. Moreover, a UK-based meningitis charity has reported approximately 8,000 cases of meningitis annually in the UK. It's notable that parasitic meningitis is a rare ailment, primarily affecting animals rather than humans.¹

What is parasitic meningitis?

Parasitic meningitis, also referred to as eosinophilic meningitis or eosinophilic meningoencephalitis, represents a rare variant of non-contagious meningitis resulting from parasites infecting the meninges, the protective membranes enveloping the brain and spinal cord. This condition is characterised by inflammation and an elevated presence of eosinophils, a type of white blood cell, in the cerebrospinal fluid.² Three main parasites are responsible for Parasitic meningitis/ PM or eosinophilic meningitis (EM). These include: Angiostrongylus cantonensis, Baylisascaris procyonis, Gnathostoma spinigerum. Among these, A. cantonensis, the rat lung worm, is the principal etiologic agent of human eosinophilic meningitis.³

Amebic meningitis, an exceptionally uncommon form of parasitic meningitis, poses a grave risk to health. Among the various species associated with this condition, Naegleria fowleri is the most frequently identified.⁴

Sources of contamination

Parasitic meningitis does not spread directly from person to person. Instead, these parasites typically infect animals or contaminate food, which humans may subsequently ingest. If the parasite or its eggs are infectious at the time of ingestion, an infection can ensue. People get infected by ingesting something that has the infectious form or stage of the parasite. These parasites are mostly found in faeces, dirt, and on some animals and food like raw fish, snails, poultry, or produce.⁴ 

Scientists have found that people get infected with Baylisascaris procyonis by accidentally ingesting infectious parasite eggs in raccoon faeces or in something (such as dirt) contaminated with raccoon faeces. On the other hand, people get infected with Angiostrongylus cantonensis and Gnathostoma spinigerum by eating raw or undercooked freshwater fish or eels, frogs, poultry, or snakes, as well as by ingesting raw or undercooked snails or slugs or by eating contaminated produce.

It is also seen that people get infected with amebic meningitis by swimming in contaminated water bodies, where the parasite enters the body through the nose. The parasite can destroy brain tissue and may eventually lead to seizures, hallucinations, and other serious symptoms.

Individuals in various regions worldwide have contracted A. cantonensis, primarily observed in parts of Asia and the Pacific Islands, including Hawaii.

B. procyonis is prevalent among raccoons in select areas of the United States, notably in the mid-Atlantic, northeastern, and midwestern regions, as well as certain parts of California. Individuals residing in these regions, especially young children, who engage in activities such as ingesting dirt or animal waste or encountering raccoons, face an elevated risk of Baylisascaris infection.

The neurologic manifestation of G. spinigerum infection is most frequently reported in Southeast Asia, particularly in Thailand.⁵ 

Diagnosis

When meningitis is suspected, blood and cerebrospinal fluid (CSF) samples are typically obtained and sent for laboratory analysis to detect evidence of infection with these parasites and to exclude other potential causes. Identifying these parasites in the CSF or other bodily tissues can be challenging. However, valuable insights can be gleaned from the individual's travel history and exposure, alongside clinical assessments, laboratory examinations, and imaging studies such as brain scans. Other tests may also be performed to diagnose meningitis. Common tests include blood cultures to identify bacteria in the blood.

A complete blood count with differential diagnosis for white blood cells, whose count is typically high in meningitis, serves as an important indicator. Moreover, chest X-rays have the capability to identify pneumonia, tuberculosis, or fungal infections, conditions that may precede the onset of meningitis. Additionally, a head CT scan can detect issues such as brain abscesses or increased intracranial pressure, which may be associated with meningitis.⁶ 

Prevention strategies

Individuals with weakened immune systems, a compromised spleen, those residing in group settings, individuals with travel histories to regions where meningitis is prevalent, individuals with chronic conditions, and those with alcohol use disorder are more susceptible to meningitis. While practising safe food handling and hand washing are essential for reducing the risk of infectious diseases, it is crucial to understand and implement effective measures for preventing each of these parasites.¹ 

For instance, education is key in preventing parasites like Angiostrongylus cantonensis and G. spinigerum. This involves raising awareness among individuals residing in or travelling to areas where these parasites are endemic. Additionally, precautions such as avoiding consumption of raw or undercooked snails, slugs, freshwater shrimp, land crabs, frogs, and lizards, as well as potentially contaminated vegetables or vegetable juice, are recommended. Removing snails, slugs, and rats from households and gardens can also mitigate the risk. Thoroughly washing hands and utensils after handling raw snails or slugs is advised, and vegetables should be washed thoroughly if consumed raw.⁷ 

Similarly, Baylisascaris infection can be prevented by avoiding contact with raccoons and their faeces. Practising good hand hygiene after outdoor activities helps prevent various diseases. It is advised not to keep, feed, or adopt wild animals, including raccoons, as pets. Raccoons may not exhibit symptoms of infection, making it difficult to discern whether they are carriers. To deter raccoons from inhabiting residential areas or parks, measures such as preventing access to food, securely sealing trash containers, and eliminating water sources are recommended.

Individuals should avoid areas and materials that may be contaminated by raccoon faeces, as raccoons typically defecate in specific locations such as the base or forks of trees, raised horizontal surfaces like fallen logs or rocks, and areas like woodpiles, decks, rooftops, and attics. Additionally, it is advised to refrain from consuming undercooked or raw freshwater fish, eels, frogs, birds, and reptiles in regions where parasites are prevalent. Marinating freshwater fish in lime juice, as done in ceviche, does not eliminate the parasite. Other preventive measures include avoiding contaminated freshwater, practising proper hand hygiene before and after food preparation, and wearing gloves when handling raw animal tissue that may be infected. While these precautions are recommended, their effectiveness in reducing the risk of infection has not been conclusively proven.⁸ 

Precaution measures

Frequent hand-washing with soap and water is essential. Thorough hand-washing is particularly crucial after using the bathroom, before and after meal preparation or consumption, following contact with faeces (whether animal or human), and after engaging in gardening or handling sand or dirt.

Covering your mouth and nose when coughing or sneezing is advised, along with disinfecting frequently touched surfaces. It is advisable to avoid close contact with others when either party is ill with a contagious disease. If close contact cannot be avoided, wearing a mask may help prevent the spread of illness, and personal items such as drinking glasses should not be shared.

Avoid swimming in or consuming potentially contaminated water. Use distilled or treated water for nasal irrigation. Practising safe food preparation is crucial, including freezing and cooking foods to safe temperatures, peeling or thoroughly washing all fruits and vegetables, and cleaning food preparation surfaces and utensils with soap and water after use. The FDA provides guidelines for fish preparation or storage to kill parasites, including adequate cooking temperatures and specific freezing methods. [Cooking: Cook fish adequately (to an internal temperature of at least 145° F [~63° C]).Freezing (Fish): At -4°F (-20°C) or below for 7 days (total time), or at -31°F (-35°C) or below until solid, and storing at -31°F (-35°C) or below for 15 hours, or at -31°F (-35°C) or below until solid and storing at -4°F (-20°C) or below for 24 hours.)].¹⁰

Undercooked or raw meat and seafood should be avoided. Reducing the risk of insect bites by using approved bug repellents for ticks and mosquitoes and covering exposed skin when outdoors is recommended. Tick checks should be conducted after being in wooded areas or places with tall grass.

Pregnant individuals should follow their healthcare provider's recommendations regarding foods to avoid and inquire about testing for group B Streptococcus, as well as methods to prevent passing infections to their baby during childbirth.

When travelling, it is essential to learn how to minimise the risk of infectious diseases prevalent in the destination.⁹ 

Summary

Preventing parasitic meningitis requires a multifaceted approach encompassing awareness, education, and proactive measures. With meningitis standing as a significant global health concern, understanding the sources of contamination and implementing effective prevention strategies is paramount. Whether it's through education about endemic parasites, promoting proper hand hygiene, or adopting safe food handling practices, individuals can mitigate their risk of infection. Additionally, precautionary measures such as avoiding contact with raccoons and their faeces, practising safe swimming habits, and following healthcare provider recommendations during pregnancy contribute to overall prevention efforts. By embracing these strategies, individuals can take proactive steps to safeguard their health and well-being against parasitic meningitis and related infections.