The tropics are a warm, wet and wonderful place for plant, animal and other life to flourish. In all of that diversity are some bizarre and fascinating parasites that make their living on—or at least find a temporary home in—us.
A session called "Adventures in Tropical Dermatology" last week at the American Society of Tropical Medicine and Hygiene's annual meeting (ASTMH) in Philadelphia featured a collection of slides depicting conditions that would be enough to anchor anyone to the temperate zone, come hell or high water. But the organisms that resulted in the frequently grotesque images were impressive examples of incredible adaptation or revealing of surprising gaps in human defenses.
The following are some of the highlights—sans photos, which, for the brave, are just a Google search away.
Four infective fungi
One of the more entrancing slides showed a man's shoulder covered in rounded, off-white concentric ovals, tracing a path that spread over the chest and back, almost like coral. The infection, known as tinea imbricata, is caused by the fungus Trichophyton concentricum, which lives primarily in Southeast Asia, Central and South America and the South Pacific.
A second fungus that has made its way up from the tropics as far north as North Carolina is Lacazia loboi, which causes the skin infection lobomycosis. The more northerly carriers of the fungal infections so far have been bottlenose dolphins; in humans, the infection is still primarily limited to Central and South America. The fungal spores can enter the body through a skin abrasion, such as a scratch or bug bite. The human immune system, however, does not seem to mount a defense against the invader, leaving it relatively free to spread in the body and create a fungal network of bumps under the skin.
Another tropical fungal escapee is chromoblastomycosis, which is often caused by one of four types of fungi that are commonly found breaking down plant matter in forests. This infection is often kicked off by a splinter and once inside the body, "we really elicit no defense or immunological response," Scott Norton, of Georgetown University Hospital, said at the ASTMH meeting. Although cases are mainly confined to warm climates, there have been some cases reported in northern Asia and Europe, including some from old Finnish saunas.
Splinters or other puncturing objects can also be the vector for eumycetoma, another fungal disease against which the human body does not mount much of an immune response, Norton noted. The implanted, unchecked fungus, which might be one of many species, "can grow so luxuriantly that it can become a whole colony," he said. Eventually, the infected area—often the foot or hand of agricultural workers—will swell and begin to extrude grains of different colors depending on the species of infecting fungus.
Nematodes are responsible for some of the classic tropical parasitic diseases. But a closer look reveals some fascinating adaptations to their hosts. Human hookworm might be the most well-known nematode, causing many cases of child illness and death in developing countries. But animal hookworms can also make their way into the skin. And even if they do not end up in the intestines as human hookworms do, they can get stuck between the upper and lower levels of the skin, causing an itchy infection that is marked by the wandering larvae's path.
Rather than entering the skin on their own, the Loa loa—or eye worm—relies on the deer fly or mango fly to bite into a suitable host and provide an entry for its larvae. Once mature, the worms can cause the infection known as loiasis, creating local skin swelling or, in an X-Files-esque flourish, crawl across the eyes.
One of eight types of nematode worm can infect the human lymphatic system and cause the disease filariasis, which can lead to severe swelling and thickening skin. Severe cases can end up in elephantiasis and painful physical disability, Noah Craft, of the University of California Los Angeles, explained.
Two burrowing bugs
On rare occasions, a lovely stroll down the beach in Central or South America can lead to tungiasis, an infection with a female sand flea Tunga penetrans. This burrower "enlarges with eggs and has lots of little babies—and unfortunately, this happens under the skin," Craft said. An itchy, painful lump develops, characterized by a single black dot. To add to what can be "a very horrifying experience," he said, "sometimes if you look closely, you can see the black dot moving around, and that's the female."
Smaller than the sand flea is the Scaroptes scabiei mite, responsible for the distressing condition called scabies. The affliction, which can be spread via close contact physical, causes red rashes. Females lay eggs into an upper layer of the skin, and hatched larvae and nymphs leave spots and trails of itchy redness in their wake. Although these mites are not confined to tropical climates, they can be prevalent where beds and bedding is shared among many people. A severe variation, known as crusted scabies, is more common among those with suppressed immune systems.
A host of other tropical (dermatological) diseases
Body lice—a skin biting bug, which, as opposed to head lice, live in clothing and can cause itchy red reactions
Chagas disease—transmitted by insects, this protozoan Trypanosoma cruzi can cause local swelling and, in some cases, cardiac disorders
Leprosy—the bacterial infection (from Mycobacterium leprae or M. lepromatosis) that can cause both skin, nervous system and respiratory damage
Onchocerciasis—also known as river blindness, is caused by a parasite Wolbachia pipientis transmitted by black flies
Paracoccidioidomycosis—infection by the Paracoccidioides brasiliensis fungus, which infects men 17 times more frequently than it does women
Pitted keratolysis—common on the soles of the feet, this bacterial infection is more frequent in warm wet climates, or in those with sweaty feet
Strongyloides stercoralis—caused by the parasitic threadworm that spends its larval period in the intestinal tract, and after defecation, matures on the surface of the skin, before returning back into the body through the skin
Tinea capitis—a fungal infection (usually from those in the Trichophyton or Microsporum genera) that appears on the scalp, most often in children