The spread of a brain-infecting parasitic worm in Florida (USA) in 2017 has raised public concern about this otherwise rare infection in humans. These worms are usually found in rats, snails or slugs. While humans are not a natural host, it can cause disease in humans by infecting the brain or sometimes the intestines. Most of the time these worms do not cause any serious disease but may lead to serious and even deadly complications.
What is Rat Lungworm?
Rat lungworm is a type of parasitic worm that usually infects rats. It is caused by Angiostrongylus worms, and specifically by Angiostrongylus cantonensis. The infection is therefore referred to as angiostrongyliasis. These parasites can also enter the human body where it may cause inflammation of the lining around the brain and spinal cord (meninges). This is known as eosinophilic meningitis.
Most of the time this is not a serious infection since the parasite cannot survive for long periods in the human body. However, in rare instances complications can arise in the humans leading to nervous system problems and even death. Mostly these infections were found in Southeast Asia and the Pacific Basin. However, these infections have also been reported in certain areas of Australia, the Carribean and the United States.
Why is it called rat lungworm?
The Angiostrongylus cantonensis parasitic worm is also known as rat lungworm because it resides in the pulmonary arteries. These are the arteries that carry low oxygen blood from the right ventricle of the heart to the lungs for oxygenation. However, these parasites are more likely to migrate to the brain in humans and rarely to the lungs where it dies. Although the term rat lungworm is also used to refer to Angiostrongylus costaricensis, this species of the worm do affect the intestines and not the lungs.
Causes of Rat Lungworm
Rat lungworm is caused by Angiostrongylus cantonensis, a type of roundworm (nematode). The infection is referred to as angiostrongyliasis. A similar species of worm known as Angiostrongylus costaricensis which also infects rats may be responsible for human angiostrongyliasis as well. Humans are not the natural hosts for these parasitic worms. It is mainly found in snails and rats where it completes its life cycle.
Read more on roundworms.
The larvae of the worms infects snails or slugs. It then matures to reach the infective stage (third stage larvae). The larvae may then be consumed by rats. The Angiostrongylus cantonensis worm migrates to the arteries of the lungs while the Angiostrongylus costaricensis worm resides in the arterioles of the ileocecal area (where the small intestine meets the large intestine).
Eggs hatch in the rat and then first stage larvae are passed out in the feces. Snails or slugs may then consume the rat droppings and become infected. Various organisms may transport the third-stage larvae between snails or slugs and rats. These hosts are called paratenic hosts or transport hosts. The larvae can complete its life cycle once the transport hosts are consumed by the definitive host.
How is rat lungworm spread?
Frogs or crustaceans (like shrimp) may become accidental hosts like humans. Eating raw or undercooked snails, slugs, frogs or crustaceans is how the larvae may then enter the human body. There is still a risk even if these organisms are not consumed. Sometimes eating raw or undercooked vegetables which may have snail or slug particles can allow the worm to enter the human body. These worms cannot spread from person to person.
Signs and Symptoms
The signs and symptoms arise after an incubation period varying from 1 to 3 weeks for A. cantonensis infection. The incubation period for A. costaricensis infection is not conclusively known but appears to be longer, ranging from several weeks to months. However, the incubation period can be as long as 6 weeks for A. cantonensis infection and up to 1 year for A. costaricensis infection.
A. cantonensis infection symptoms
The disease from A. cantonensis infection can last for anywhere between 2 to 8 weeks. The most common illness is eosinophilic meningitis which usually presents with the following signs and symptoms:
- Stiff neck
Read more on meningitis.
In severe cases where there is central nervous system (CNS) impairement, the following signs and symptoms may be seen:
- Muscle weakness
- Impaired reflexes
- Abnormal sensations
- Difficulty breathing
- Loss of consciousness
Damage to the central nervous system may be permanent. The infection can even lead to death but this is rare. Sometimes the worms may migrate to and invade the eye. It may cause the following signs and symptoms such as:
- Eye pain
- Impaired vision
- Corneal inflammation (keratitis)
- Retinal edema
A. costaricensis infection symptoms
A. costaricensis mainly causes an intestinal infection. Signs and symptoms include:
- Abdominal pain
Rare complications may include bleeding within the digestive tract, obstruction or perforation of the intestines.
Diagnosis of Rat Lungworm
There are no specific diagnostic investigations for Angiostrongylus cantonensis or Angiostrongylus costaricensis infections. Identifying the presence of these roundworms can be difficult. Diagnosis depends on a case history as well as blood or CSF (cerebrospinal fluid) tests revealing high levels of eosinophils, a type of immune cell. A CT or MRI scan may also be useful in some cases.
Treatment and Prevention
There is no specific treatment for Angiostrongylus cantonensis or Angiostrongylus costaricensis infections. Most of the time these infections will resolve spontaneously with no complications. Supportive measures may help to control and ease symptoms but cannot treat or eradicate the infection.
Despite the lack of specific treatment, it is important to consult with a medical professional. Surgery may necessary in some cases to the remove the worms like when there is invasion of the eyes, or when a portion of the intestinal tissue has to be removed.
Preventative measures that should be applied, especially in endemic areas, include:
- Practicing good hygiene by washing hands thoroughly, especially before meals.
- Clean fruits and vegetables thoroughly before consuming.
- Avoid handling snails or slugs, especially without protective handwear like gloves.
- Always thoroughly clean and fully cook snails, slugs, frogs and shrimp before eating.