Snakes, spiders, and sharks may give you nightmares, but many of the world's scariest animals are a little smaller scale.

We're talking parasites – worms that cause blindness, amebas that make your brain swell, and ticks that can trigger facial paralysis. Here are five of the world's nastiest, most frightening human parasites. 

Warning: There are some graphic images ahead. 

Filarial worms

Filarial worms are a particularly nasty group of roundworms that can cause lymphatic filariasis, a tropical disease better known as elephantiasis. The most common, Wuchereria bancrofti, is responsible for roughly 90 percent of all cases.

The parasites are transmitted by mosquito, but once inside a new human host, filarial worms wedge themselves into the lymphatic vessels, causing havoc to the lymphatic system (I.e. the part of the body responsible for disposing of toxins, waste, and other unpleasant substances). They can live in the human body for up to eight years, all the while spewing millions of microscopic larvae into the bloodstream.

In most cases, the disease is asymptomatic. This means most patients show no visible signs of infection, though their lymphatic system, immune system, and kidneys may still be compromised. Those that do tend to show signs do so years after infection. Physical symptoms can include lymphoedema (chronic swelling of the body’s tissues, usually in the legs), elephantiasis (a hardening and thickening of the skin), and, in men only, scrotal swelling (fairly self-explanatory). Not only are these patients physically disabled, but they are often heavily stigmatized and have to cope with the financial hardships and social costs that come with the disease. In summary, it’s not a lot of fun.

Australian paralysis tick

Australia is not known for its cute, fluffy animals – kangaroos and koalas being the obvious exceptions. Instead, it seems that everything in the land down under is out to kill you. One of the smallest potentially life-threatening bugs is the Australian paralysis tick.

The tick, which is found in long grasses and bushland close to Australia’s eastern coast, attaches itself to its new host with its teeth, sticking its large, sharp mouthparts into the skin before injecting anticoagulant saliva to prevent the blood from clotting. When feeding, the parasite swells, transforming into a 1-centimeter-long, grey-blue blob.

In the majority of cases, a tick bite results in nothing more than localized swelling and some itching. In rare incidences, however, it can trigger tick paralysis and severe allergic reactions. There is also a new and bizarre syndrome where patients suddenly develop anaphylactic reactions to meat and animal by-products (a condition called “tick-induced mammalian meat allergy”). The earliest symptoms of tick paralysis can include rashes, headaches, fever, sensitivity to bright light, flu-style symptoms, unsteady gait, weakening of the limbs, and partial facial paralysis. If the tick is not removed, the patient can die of respiratory failure.

Screwworm fly

Myiasis is a maggot-based infection, which sounds generally unpleasant whichever species of fly might be involved, but the New World screwworm (Cochliomyia hominovorax), the primary type of screwworm found in Mexico, South America, and occasionally the US, might just be the worst. After all, “hominovorax” is literally Latin for “man-eater”.

Unlike other maggots, screwworms love to gorge on living tissues, so female blowflies lay eggs – sometimes as many as 400 of them – in the open wounds of living animals. Here, they hatch and fester, living off the flesh, before digging themselves further into the host using their powerful jaws and screw-shaped bodies, which also just so happen to be covered in little spines to help them cling onto the body’s tissue. Not only can this cause the body damage, but it can also trigger secondary bacterial infections. After a while, the maggots will exit the host and fall to the ground, where they transform into flies.  

Fortunately, cases of screwworm myiasis in humans are extremely rare, but when they do happen things can get very gory very quickly. Take the example of one woman who developed an infection while holidaying in the Dominican Republic. She woke up with irritation in her ear, which turned into pain and a bloody discharge. Doctors found screwworms nesting in her external ear canal, which had to be removed manually before her ear canal could be reconstructed.

Onchocerca volvulus

Onchocerca volvulus is a parasitic worm responsible for onchocerciasis (aka river blindness), a disease that can cause extreme disfigurement of the skin or blindness. It’s mainly found in the tropics, where the World Health Organization (WHO) estimate more than 25 million people are infected and 123 million people are at risk.

The parasite is transmitted as microfilariae through the bite of infected blackflies. These take roughly a year to grow into adult worms, which can live for a further 10 to 15 years inside the human body. During this time, a female worm can produce millions of offspring, which journey through the host's body to the skin, eyes, and organs via the subcutaneous tissue and cause severe inflammatory reactions when they die.

The microfilariae can cause skin rashes, swelling, inflammation, lesions, and nodules under the skin, which if not properly treated can result in severe dermatitis. This can cause the skin to lose its elasticity (giving it the appearance of “lizard skin”), and, in some cases, its pigment (“tiger skin”). It is also the fourth leading cause of preventable blindness. Roughly 300,000 people have been blinded by the parasite and a further 800,000 have been visually impaired. 

Naegleria fowleri

Naegleria fowleri is a parasite you really, really do not want to get. Infection rates are low – up until 2012, there had been only 310 known cases worldwide – but it is the most deadly creature on the list with a fatality rate of over 97 percent.

Also known as “brain-eating ameba”, the freshwater-dwelling parasite usually dines on bacteria in lakes, rivers, and soil. On rare occasions, it comes across a human host and enters its unassuming victim by swimming up the nose. From here, it wiggles its way up into the brain where it destroys the organ’s tissue, resulting in brain swelling and, ultimately, death.

Early symptoms are close to those caused by bacterial meningitis. Think: headache, fever, nausea, and/or vomiting. As the disease advances into its later stages, patients will develop a stiff neck, a loss of balance, seizures, feelings of confusion and anxiety, and hallucinations. Death, caused by primary amebic meningoencephalitis (PAM), usually occurs within about five days of infection.