Encephalitozoonosis is a parasitic disease caused by the microsporidian organism Encephalitozoon cuniculi, which has a worldwide distribution. This disease is particularly significant in domestic rabbits, where it can cause serious neurological and renal illness, though many animals remain carriers without showing symptoms. While the infection is widespread in animal populations, it is considered a zoonotic disease, meaning it can be transmitted to humans. Clinical disease in people is rare and almost exclusively occurs in individuals with compromised immune systems.
The Parasite and Its Primary Hosts
The causative agent, Encephalitozoon cuniculi, is a highly specialized, single-celled organism belonging to the phylum Microsporidia, which is now classified as a group closely related to fungi. It is an obligate intracellular parasite, meaning it must invade a host cell to survive and replicate. The infectious stage is a tiny, environmentally resistant spore.
Rabbits are considered the primary reservoir host for the most common strain, Genotype I, and a high percentage of pet rabbit populations globally have been exposed to the parasite. Once the spore is ingested, it extrudes a specialized polar tube to inject its sporoplasm into the host cell. The parasite then multiplies extensively within the host’s cells, showing a strong predilection for the tissues of the central nervous system, the kidneys, and the eyes.
This parasite has a broad host range beyond rabbits, infecting various other mammals, including rodents, dogs, primates, and occasionally birds. While other animals can be affected, the clinical disease is most frequently observed in rabbits. The presence of the parasite in these target organs often results in granulomatous inflammation.
How the Infection Spreads
The most frequent method of transmission, known as horizontal transmission, occurs when an animal ingests the resilient spores shed by an infected host. Spores are typically excreted in the urine of infected rabbits, contaminating food, water, and bedding. These spores possess a thick protective wall that allows them to survive in the environment for weeks or even months, making disinfection challenging.
Another important route, particularly within rabbit colonies, is vertical transmission, where the infection passes directly from the mother to her offspring while still in the uterus. This transplacental route is responsible for the early infection of the lens in young rabbits, often leading to ocular disease. Less commonly, infection can occur through the inhalation of aerosolized spores, though ingestion remains the primary cause of environmental spread.
After the initial ingestion, the parasite is taken up by cells in the intestine, which then carry the infection through the bloodstream and lymphatic system to the target organs. Spore shedding in the urine typically begins about 35 days after the initial infection and can continue for several months, perpetuating the cycle of contamination. The parasite’s ability to hide within host cells contributes to the chronic nature of the infection.
Recognizable Clinical Signs
A significant majority of infected rabbits are asymptomatic carriers, meaning they harbor the parasite without ever developing any noticeable illness. Clinical signs usually emerge when the host’s immune system is weakened by stress, age, or concurrent disease, allowing the parasite to replicate rapidly. The resulting disease is often characterized by a triad of manifestations affecting the nervous system, kidneys, and eyes.
Neurological signs are the most commonly recognized form of active Encephalitozoonosis, resulting from inflammation and damage to the brain and spinal cord. These manifestations often present as vestibular disease, including head tilt and a loss of balance leading to circling or rolling. Other central nervous system issues include nystagmus, hindlimb weakness, and in severe cases, partial paralysis or seizures.
Renal involvement is a common consequence of chronic infection, where the parasite causes inflammation. Early signs of kidney dysfunction may be subtle but can progress to polyuria and polydipsia. Over time, severe kidney damage can lead to weight loss and general lethargy.
The third major presentation involves the eyes, which occurs most frequently in young rabbits that were infected before birth. The parasite invades the developing lens, leading to the formation of a cataract or a white spot within the eye. This can cause phacoclastic uveitis, a severe inflammatory reaction that occurs when the lens capsule ruptures, often leading to blindness in the affected eye.
In humans, clinical Encephalitozoonosis is generally restricted to individuals who are immunocompromised. The infection in these patients is typically disseminated, often affecting the gastrointestinal tract, brain, or respiratory system. Symptoms can include diarrhea, encephalitis, or sinusitis, with the potential for severe, life-threatening multi-organ failure if left untreated.
Management and Prevention Strategies
Treatment for active Encephalitozoonosis in rabbits focuses on reducing the parasite load and managing the resulting inflammatory damage. The anti-parasitic medication Fenbendazole is commonly used. While this drug works to suppress the replication of the parasite, it cannot always eliminate the spores already present in the host’s tissues, and it may not reverse existing neurological damage.
Supportive care is an important component of management, especially for rabbits with severe neurological signs. This may involve the use of anti-inflammatory medications to address the inflammation in the central nervous system, and in some cases, anti-sickness drugs to help animals struggling with severe head tilt. For human patients, a different benzimidazole compound, Albendazole, is the medication typically used to treat disseminated microsporidial infections.
Prevention centers on minimizing exposure to the resilient spores, particularly in environments shared by rabbits and immunocompromised humans. Strict hygiene protocols are recommended, which include daily cleaning and disinfection of rabbit enclosures and frequent hand washing after handling the animals or their bedding. Since the spores are shed in urine, reducing stress factors in rabbits may also help limit the recurrence of active disease and subsequent spore shedding.
Immunocompromised individuals should take extra precautions. Prophylactic administration of Fenbendazole to rabbits has been shown to be effective in preventing the establishment of the infection and in limiting spore shedding.