Gnathostoma spinigerum is a parasitic roundworm, a type of nematode, that causes gnathostomiasis in humans. This zoonotic disease is transmitted from animals to humans.
Understanding How Infection Occurs
The Gnathostoma spinigerum parasite has a complex life cycle involving multiple hosts. Adult worms reside in the stomach walls of definitive hosts like cats, dogs, and other wild carnivores, where they lay eggs. These eggs pass in feces and embryonate in freshwater, releasing first-stage larvae.
Small crustaceans, such as copepods, ingest these larvae, becoming the first intermediate host. When an infected copepod is consumed by a second intermediate host, like a freshwater fish, frog, or snake, the larvae develop into advanced third-stage larvae within their muscle tissue.
Humans become infected by consuming these undercooked second intermediate or paratenic hosts, ingesting the infective third-stage larvae. Once ingested, the larvae penetrate the gastric or intestinal wall and migrate through the human body. Humans are accidental hosts, meaning the larvae do not mature into adult worms. This infection is common in Southeast Asia, particularly Thailand, Japan, and Vietnam, and also in parts of Central and South America, such as Mexico and Ecuador.
How the Parasite Affects the Body
The symptoms of gnathostomiasis in humans are primarily due to the migration of the larval worms through various tissues and organs. Within 24 to 48 hours of ingesting the larvae, individuals may experience initial non-specific symptoms as the parasite moves through the stomach or intestinal wall and the liver. These early signs can include fever, general malaise, skin rashes, loss of appetite, nausea, vomiting, diarrhea, and pain in the upper abdomen, which may persist for two to three weeks.
The most common manifestation is cutaneous gnathostomiasis, characterized by intermittent, migratory swellings under the skin, often on the trunk or limbs. These swellings, also known as “wandering swelling” or “Yangtze edema,” can be painful, itchy, or red, and may recur over months or even years if the infection is not treated. Sometimes, the larva may migrate close enough to the skin’s surface to be visible as a creeping eruption or spontaneously emerge.
Beyond the skin, the larvae can migrate deeper into the body, leading to visceral gnathostomiasis. This form can affect various internal organs, and symptoms depend on the migration site. For example, pulmonary involvement can cause coughing, chest pain, coughing up blood, or even lung collapse.
Neurological gnathostomiasis occurs when the larvae invade the central nervous system (CNS), including the brain and spinal cord. This can lead to severe headaches, neck stiffness, radicular pain in the limbs, and conditions like eosinophilic meningitis or encephalitis. Neurological involvement can result in varying degrees of paralysis, sensory loss, cranial nerve palsies, and subarachnoid hemorrhage, which carries a risk of permanent neurological deficit or death. Ocular gnathostomiasis involves the migration of the parasite into the eye, causing pain, redness, blurred vision, or even vision loss.
Identifying and Treating the Infection
Diagnosing gnathostomiasis involves a combination of clinical evaluation, laboratory tests, and imaging. A healthcare provider will consider a patient’s history, especially travel to endemic areas and consumption of raw or undercooked foods. Laboratory tests often include blood work to look for eosinophilia, an increase in a type of white blood cell, which is commonly seen during the parasite’s initial migration.
Serological tests, such as Enzyme-Linked Immunosorbent Assay (ELISA) or Western blot, detect antibodies against Gnathostoma antigens in blood or cerebrospinal fluid, providing evidence of exposure. Imaging techniques like MRI or CT scans can help visualize the migrating larva or the damage it causes. A definitive diagnosis can also be made by surgically removing the worm from a superficial lesion or through a biopsy of affected tissue for microscopic identification.
Treatment primarily involves antiparasitic medications like Albendazole, often administered for 21 days, and Ivermectin, an alternative often given as a single dose. While both are effective, albendazole may sometimes cause larvae to migrate outwards, and ivermectin can temporarily worsen cutaneous symptoms. Surgical removal may also be performed to extract the parasite, especially for localized ocular or cutaneous forms. For neurological gnathostomiasis, antiparasitic drug use is more complex due to concerns about potential worsening of neurological symptoms as the parasite dies and releases antigens.
Preventing Gnathostomiasis
Preventing gnathostomiasis largely centers on modifying dietary habits and ensuring food safety. The most effective method to avoid infection is to thoroughly cook all freshwater fish, poultry, frogs, and snakes. This ensures that any larval stages of the Gnathostoma parasite present in the meat are killed.
Consuming raw or undercooked dishes made from these animals does not eliminate the parasite and should be avoided. Cooking or freezing fish to recommended temperatures kills parasites. Adopting general food safety and hygiene practices can also help reduce the risk of parasitic infections.