Monday, January 31, 2011

E. vermicularis: Nocturnal Nuisances

pinworm magnified

Intro- E. Vermicularis, also known as pinworm, is a thin, white nematode that lives in human intestines. Infection is seen worldwide in practically all economic groups, and is the most common worm infection in the US. The female emerges at night to lay eggs on the skin just outside the colon, which can cause itching. They can survive outside the body for 2-3 weeks, and can be transferred by touching a contaminated surface which can lead to transfer to the mouth and ingestion [1].

E. Vermicularis and Host Description- This parasite is a helminth (parasitic worm), of which humans are considered the only host. They predominantly affect children because thumb sucking is common among young children and eggs can easily be transferred to hands and then to the mouth. They are also common among institutionalized persons because they are easily spread when people are in close contact with each other [1]. E. vermicularis is a species of nematode, a group of helminthes that lacks a cardio-respiratory system, but has a digestive, nervous, excretory and reproductive system [3]. Males are 2-5 mm long and females are 8-13 mm long. This “pin” shaped tail is where the common name of pinworms comes from [4].

Image depicting clear picture of how the pinworms are found like cotton threads in the anus while the pin worms move to the perianal fold to deposit eggs.

Life Cycle- Humans can either ingest or inhale the parasite’s eggs because they are small and have a low enough density to become airborne [1]. Male and female E. vermicularis worms reproduce sexually, but their offspring only contain female larvae. The production of predominantly female larvae maximizes the reproductive fitness of the parasite because it maximizes the number of egg producing offspring. Male larvae are a product of parthenogenisis – the development of an unfertilized egg [2]. The larvae hatch in the small intestines and migrate to the large intestines where they mature and feed on blood and tissue of the host’s intestines. About a month after egg ingestion, females are sexually mature and can begin to produce eggs. Fertilization occurs in the large intestine and then at night the female migrates to the colon to deposit her eggs on the skin around the anus. After laying eggs, the female often dies, but occasionally returns to the large intestine. The male dies soon after fertilization of the female [4]. Once eggs are laid they become infective in six hours, but mortality is high because most larvae do not reach hosts. Eggs can survive outside the body for 2-3 weeks and can be spread by inhalation of airborne eggs, or hand-to-mouth ingestion after touching contaminated skin, clothing, or bedding [1]. The short, two month lifespan [4] and high fecundity both lead to high reproductive fitness of the parasite because these two life cycle characteristics both lend themselves to evolutionary efficiency and the ability to over-reproduce to ensure the continuation of the species.

Life cycle of Enterobius vermicularis

Ecology- An estimated 500 million cases of infection are reported globally, and it is the most common parasitic worm in the United States [5].The classical pathological effect of the parasite is perianal irritation and itching, but severe cases of infection can cause sleeplessness, weight loss, hyperactivity, abdominal pain, and vomiting. Peridiocally, after a female deposits her eggs near the anus, she migrates into a female host's vagina, instead of back into the anus. This produces irritative symptoms and can cause a vaginal discharge; infestation of the uterus can cause menstrual-like bleeding from the vagina [4]. Pyrantel pamoate and mebendazole are the primary drugs used to treat pinworms [5].

Example of parasite controlling time of redezvous- E. vermicularis nocturnally release eggs, which need 6 hours to become infective [1]. A person usually sleeps for more than 6 hours, giving pinworm eggs enough time to mature before host movement and potential spread of the parasite. If the eggs are ingested before they become infective they will not be able to grow into reproducing adults. So, it is evolutionarily advantageous for the parasite to lay eggs at night so that they have a sufficient amount of time to mature before being ingested.