Nematodes are a type of roundworm, and while many species are harmless, some are parasitic and can cause health problems in humans. The larval, or developing, stage of these worms is a primary source of infection. This article explores how parasitic nematode larvae infect humans and the health issues that result.
The Nematode Life Cycle and Infection
The life cycle of a parasitic nematode involves three main stages: egg, larva, and adult. Infection begins when humans encounter the infective stage, which can be either eggs or larvae depending on the nematode. The journey into the human body follows one of two pathways: ingestion or direct skin penetration.
Ingestion is a common route for species like the Ascaris roundworm, occurring when a person consumes food, water, or soil contaminated with nematode eggs. Once ingested, these eggs travel to the small intestine and hatch. The larvae then penetrate the intestinal wall to enter the bloodstream or lymphatic system.
The other route of infection is direct skin penetration by larvae, a method used by hookworms. Eggs passed in the feces of an infected person hatch in warm, moist soil and develop into infective larvae. When a person walks barefoot on this soil, the larvae can burrow through the skin and begin their migration through the body.
Diseases Caused by Larval Migration
The movement of nematode larvae through the body causes conditions known as larva migrans. Because humans are often not the natural host, the larvae cannot mature into adults and instead wander through tissues, causing inflammation and damage.
When larvae migrate within the skin, it results in cutaneous larva migrans, or “creeping eruption.” The movement of the larvae creates visible, intensely itchy tracks that advance by one to two centimeters daily. This is commonly caused by the larvae of dog and cat hookworms, such as Ancylostoma braziliense.
Visceral larva migrans is a more serious condition where larvae invade internal organs like the liver, lungs, and heart. Caused frequently by the dog roundworm, Toxocara canis, it can lead to fever, organ enlargement, and coughing.
Ocular larva migrans is a specific form of the visceral type where larvae invade the eye, potentially causing inflammation and vision loss. Toxocara larvae are the primary cause of this condition, which affects school-aged children more often than the visceral form.
Diagnosis and Treatment
Diagnosing a larval nematode infection combines clinical evaluation with lab tests. Blood tests detect antibodies produced in response to the parasites, while imaging scans can identify organ damage in visceral larva migrans. The characteristic skin tracks are often sufficient for diagnosing the cutaneous form, and stool samples may be examined for eggs from adult worms.
Treatment involves anti-parasitic medications like albendazole and mebendazole, which are effective against both larval and adult worms. In cases with significant inflammation, such as ocular larva migrans, corticosteroids may also be prescribed to manage symptoms.
Prevention and Control Measures
Preventing larval nematode infections centers on interrupting their life cycle and reducing exposure to contaminated environments. Key practices include:
- Thoroughly washing hands with soap and water, especially after contact with soil or pets and before handling food.
- Ensuring the sanitary disposal of human and pet feces to prevent eggs from contaminating soil and water.
- Washing all fruits and vegetables to remove any soil that may contain nematode eggs.
- Cooking meats to temperatures that kill any potential larvae.
- Regularly deworming pets, particularly dogs and cats, to control parasites like Toxocara.
- Wearing shoes when walking on soil or sand to prevent larvae from penetrating the skin.