The world around us is populated with often unseen organisms that depend on us for survival. Scanning electron photography gives us a glimpse into some of the organisms that live in and around the "micro-universe of you".

A symbiotic relationship between a parasite and a host can have beneficial effects. Unfortunately, as in the examples below, many of these parasites have no regard for their human hosts, consuming our food and nutrients, destroying tissue and producing toxic waste products that can make us seriously ill.

Head Louse

Humans host three different kinds of lice: head lice, body lice (which live mainly in clothing), and pubic lice. Head lice are wingless insects spending their entire life on human scalp and feeding exclusively on human blood. A female louse can lay anywhere from 6 to 100 eggs per night!

Image: Pediculus humanus capitis. Scanning electron micrograph of a head louse clinging to a hair. The head louse attaches to hair using well-developed claws. The mouthparts are adapted for sucking blood. Credit: Garry Hunter, Wellcome Images

Image: Pediculus humanus capitis. Wider view of a head louse clinging to a hair. Credit: Mantis

Infection is a common problem among school children.The primary symptom is an itchy scalp. You may also notice nits stuck to the hairs and pillows being dirtier than usual due to louse droppings.

Image: Pediculus humanus capitis. SEM of human head louse x55 (Colour Enhanced). Credit: Tim Flach 

Pubic Louse

Pubic lice are tiny parasitic insects that live in pubic hair, underarm hair, hair on the body and, occasionally, in eyebrows and eyelashes. They are yellowy-grey and about 2mm long. They have a crab-like appearance, so are often known as 'crabs'.

Pubic lice or crabs are unpleasant but, once diagnosed, easily treated.

Image: Phthirus pubis. SEM, Scanning electron micrograph of ventral view of pubic louse (computer-coloured green with red claws). Credit: David Gregory&Debbie Marshall, Wellcome Images

The primary signs of pubic lice are; itching in the affected areas, black powdery droppings from the lice in underwear, brown eggs on pubic or other body hair. There may also be irritation and inflammation in the affected area, sometimes caused by scratching.

Image: Scanning electron micrograph of close-up of head of pubic louse, showing mouthparts, computer-coloured green. Credit: David Gregory&Debbie Marshall, Wellcome Images

Body Louse

Body lice are found only in homeless, transient populations who don't have access to changes of clothes or bathing facilities. Infestation is unlikely to persist on anyone who bathes regularly and who regularly has access to freshly laundered clothing and bedding. In rare cases, body lice may carry uncommon diseases such as relapsing fever or trench fever.

Image: Pediculus humanus var. corporis. This 2006 scanning electron micrograph (SEM), magnified 152x, revealed the distal tip of the abdominal region of a female body louse. Credit: CDC Janice Carr

Trichomonas vaginalis

This parasite causes the sexually transmitted disease called trichomoniasis, an under-diagnosed global public health problem. The parasite has some foul habits latching onto vaginal tissue and forming tendril-like projections into it. The pathogen also secretes a series of proteins which destroy the vaginal epithelial cells, the cells that make up the vaginal tissue surface.

Image: An electron micrograph depicts the Trichomonas vaginalis parasite adhering to vaginal epithelial cells collected from vaginal swabs. A non-adhered parasite (right) is pear-shaped, whereas the attached parasite is flat and amoeboid. Credit: Antonio Pereira-Neves and Marlene Benchimol, Santa Ursula University, Rio de Janeiro, Brazil

Tapeworms

Although relatively uncommon, humans and especially infants and young children can contract tapeworms.

Tapeworms can survive and thrive indefinitely inside the human body, ranging in length from 1/250 of an inch to an incredible 30 feet!

In the early 1900's the Tapeworm diet was popular. The practice involved swallowing beef tapeworm eggs and then taking a medicine to kill the tapeworms after reaching your target weight.

Image: Canine Tapeworm Taenia pisiformis. The scolex of the Dog Tapeworm (Taenia), showing the hooks that allow this endoparasite to attach to the digestive tract of its host. SEM X75 Credit: Boston Museum of Science.

Image: Canine Tapeworm Taenia pisiformis. While this particular parasite doesn't typically infect humans the image was too spectacular to leave out.  Image of the head of a dog tapeworm (Taenia pisiformis), a parasite that lives in the intestine. It shows the hooks used to attach to the intestine. The image was taken using Rheinberg illumination.  Credit: M I Walker, Wellcome Images

While the symptoms can be unpleasant and painful, they are not usually life-threatening. They tend to revolve around stomach complaints such as; diarrhoea, cramping, abdominal pain, nausea and vomiting, loss of appetite and sometimes, rectal or anal itching.

Image: Rhinebothriidea Rhinebothrium sp. This is a scanning electron micrograph of the scolex (i.e. anterior attachment organ) of Rhinebothrium sp., a tapeworm in the new order Rhinebothriidea. Again don't panic - this particular parasite doesn't infect humans. Credit: Claire J. Healy

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