Protozoan and Helminthic Infections of the Skin and Eyes

Many parasitic protozoans and helminths use the skin or eyes as a portal of entry. Some may physically burrow into the skin or the mucosa of the eye; others breach the skin barrier by means of an insect bite. Still others take advantage of a wound to bypass the skin barrier and enter the body, much like other opportunistic pathogens. Although many parasites enter the body through the skin, in this chapter we will limit our discussion to those for which the skin or eyes are the primary site of infection. Parasites that enter through the skin but travel to a different site of infection will be covered in other chapters. In addition, we will limit our discussion to microscopic parasitic infections of the skin and eyes. Macroscopic parasites such as lice, scabies, mites, and ticks are beyond the scope of this text.

Acanthamoeba Infections

Acanthamoeba is a genus of free-living protozoan amoebae that are common in soils and unchlorinated bodies of fresh water. (This is one reason why some swimming pools are treated with chlorine.) The genus contains a few parasitic species, some of which can cause infections of the eyes, skin, and nervous system. Such infections can sometimes travel and affect other body systems. Skin infections may manifest as abscesses, ulcers, and nodules. When acanthamoebae infect the eye, causing inflammation of the cornea, the condition is called Acanthamoeba keratitis. The figure below illustrates the Acanthamoeba life cycle and various modes of infection.

While Acanthamoeba keratitis is initially mild, it can lead to severe corneal damage, vision impairment, or even blindness if left untreated. Similar to eye infections involving P. aeruginosa, Acanthamoeba poses a much greater risk to wearers of contact lenses because the amoeba can thrive in the space between contact lenses and the cornea. Prevention through proper contact lens care is important. Lenses should always be properly disinfected prior to use, and should never be worn while swimming or using a hot tub.

Acanthamoeba can also enter the body through other pathways, including skin wounds and the respiratory tract. It usually does not cause disease except in immunocompromised individuals; however, in rare cases, the infection can spread to the nervous system, resulting in a usually fatal condition called granulomatous amoebic encephalitis (GAE) (see Fungal and Parasitic Diseases of the Nervous System). Disseminated infections, lesions, and Acanthamoeba keratitis can be diagnosed by observing symptoms and examining patient samples under the microscope to view the parasite. Skin biopsies may be used.

Acanthamoeba keratitis is difficult to treat, and prompt treatment is necessary to prevent the condition from progressing. The condition generally requires three to four weeks of intensive treatment to resolve. Common treatments include topical antiseptics (e.g., polyhexamethylene biguanide, chlorhexidine, or both), sometimes with painkillers or corticosteroids (although the latter are controversial because they suppress the immune system, which can worsen the infection). Azoles are sometimes prescribed as well. Advanced cases of keratitis may require a corneal transplant to prevent blindness.

Extended description

1. Cysts in water become trophozoites. 2. Trophozoites undergo mitosis to form more trophozoites. 3. Trophozoites can also become cysts. Cysts, trophozoites, and mitosis are all marked the infective stage. 4. Amoebae (cysts and trophozoites) can enter humans in various ways. 5. Amoebae entering through the eye can result in severe keratitis of the eye. 6. Amoebae entering through the nasal passages and infecting the lower respiratory tract can result in granulomatous amoebic encephalitis (GAE) and/or disseminated disease in individuals with compromised immune systems. 7. Amoebae entering through ulcerated or broken skin can cause granulomatous amoebic encephalitis, disseminated disease, or skin lesions in individuals with compromised immune systems. The resulting cysts and trophozoites in tissue (steps 6 and 7) are marked the diagnostic stage.

Check Your Understanding

Loiasis

The helminth Loa loa, also known as the African eye worm, is a nematode that can cause loiasis, a disease endemic to West and Central Africa, as shown in the figure below. The disease does not occur outside that region except when carried by travelers. There is evidence that individual genetic differences affect susceptibility to developing loiasis after infection by the Loa loa worm. Even in areas in which Loa loa worms are common, the disease is generally found in less than 30% of the population (Garcia, A., et al., “Genetic Epidemiology of Host Predisposition Microfilaraemia in Human Loiasis,” Tropical Medicine and International Health 4 [1999] 8:565–74). It has been suggested that travelers who spend time in the region may be somewhat more susceptible to developing symptoms than the native population, and the presentation of infection may differ (Spinello, A., et al., “Imported Loa loa Filariasis: Three Cases and a Review of Cases Reported in Non-Endemic Countries in the Past 25 Years,” International Journal of Infectious Disease 16 [2012] 9: e649–e662, DOI: 10.1016/j.ijid.2012.05.1023).

The parasite is spread by deerflies (genus Chrysops), which can ingest the larvae from an infected human via a blood meal, as shown in the figure below. When the deerfly bites other humans, it deposits the larvae into their bloodstreams. After about five months in the human body, some larvae develop into adult worms, which can grow to several centimeters in length and live for years in the subcutaneous tissue of the host.

The name “eye worm” alludes to the visible migration of worms across the conjunctiva of the eye. Adult worms live in the subcutaneous tissues and can travel at about 1 cm per hour. They can often be observed when migrating through the eye, and sometimes under the skin; in fact, this is generally how the disease is diagnosed. It is also possible to test for antibodies, but the presence of antibodies does not necessarily indicate a current infection; it only means that the individual was exposed at some time. Some patients are asymptomatic, but in others the migrating worms can cause fever and areas of allergic inflammation known as Calabar swellings. Worms migrating through the conjunctiva can cause temporary eye pain and itching, but generally there is no lasting damage to the eye. Some patients experience a range of other symptoms, such as widespread itching, hives, and joint and muscle pain.

Worms can be surgically removed from the eye or the skin, but this treatment only relieves discomfort; it does not cure the infection, which involves many worms. The preferred treatment is diethylcarbamazine, but this medication produces severe side effects in some individuals, such as brain inflammation and possible death in patients with heavy infections. Albendazole is also sometimes used if diethylcarbamazine is not appropriate or not successful. If left untreated for many years, loiasis can damage the kidneys, heart, and lungs, though these symptoms are rare.

Extended description

Fly stages (left) and human stages (right) of the Loa loa life cycle. 1. A fly of the genus Chrysops takes a blood meal from a person, and infective (L3) larvae enter the bite wound. 2. The larvae develop into adult worms living in the subcutaneous tissue. 3. The adult worms produce sheathed microfilariae found in spinal fluid, urine, sputum, peripheral blood, and the lungs; this stage is marked diagnostic. 4. A fly takes a blood meal from an infected person and ingests the microfilariae. 5. The microfilariae shed their sheaths, penetrate the fly’s midgut, and migrate to its thoracic muscles. 6. The microfilariae develop into L1 larvae. 7. The L1 larvae develop into infective L3 larvae. 8. The L3 larvae migrate to the fly’s head and proboscis, ready to infect another person on the fly’s next blood meal.

Check Your Understanding

Summary

• The protozoan Acanthamoeba and the helminth Loa loa are two parasites that can breach the skin barrier, causing infections of the skin and eyes.
• Acanthamoeba keratitis is a parasitic infection of the eye that often results from improper disinfection of contact lenses or swimming while wearing contact lenses.
• Loiasis, or eye worm, is a disease endemic to Africa that is caused by parasitic worms that infect the subcutaneous tissue of the skin and eyes. It is transmitted by deerfly vectors.

Key terms

• Acanthamoeba keratitis — a condition characterized by damage to the cornea and possible blindness caused by parasitic infection of the protozoan Acanthamoeba.
• loiasis — a disease caused by the parasitic Loa loa worm, which is transmitted by deerflies; adult worms live in the subcutaneous tissue and cause inflammation, swelling, and eye pain as they migrate through the skin and the conjunctiva of the eye.

Practice

Identify two parasites that commonly cause infections of the skin and eyes

Identify the major characteristics of specific parasitic diseases affecting the skin and eyes

This section is adapted from Microbiology, Section 21.5: Protozoan and Helminthic Infections of the Skin and Eyes by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: all three of the module’s ordinary figures are re-encoded as WebP and rendered as mediafigures after image and PDF inspection; kind="diagram" is set on the Acanthamoeba life-cycle figure and the Loa loa life-cycle figure, overriding the media manifest’s JPEG-based photo guess, because both are drawn flowcharts with numbered steps rather than photographs; kind="photo" is kept on the three-panel Acanthamoeba cyst/trophozoite/eye figure, all three of whose panels are genuine micrographs or a clinical photo; the Acanthamoeba life-cycle figure carries eager="true" as the page’s first figure; a longdesc walk-through is added to both life-cycle figures because their numbered, multi-step content is not fully carried by the caption alone; the Disease Profile summary table is transcribed from the source’s table image (OSC_Microbio_21_05_parasiticInf), checked against the PDF page, and the image itself is not vendored into the page, per this book’s Disease Profile rule; the module’s two footnotes are rendered as inline parenthetical citations after the sentences they support, with the Garcia et al. citation’s bare PubMed access URL dropped and the Spinello et al. citation’s DOI kept; the Loa loa life-cycle figure’s source alt and caption describe a photograph of an eye with a visible worm and a close-up photo of the worm in addition to the life-cycle chart, but the vendored source file and the published PDF page contain only the life-cycle chart, so this page’s alt and longdesc are authored fresh from the delivered image rather than the source alt’s claims — a source defect; the Disease Profile box’s own intro sentence is corrected from “fungal infections of the skin” to “parasitic infections of the skin and eyes,” with a visible source note, since the sentence otherwise contradicts the table it introduces (a copy-paste holdover from the neighboring Mycoses of the Skin Disease Profile box); the module’s four Multiple Choice and two Fill in the Blank items are rendered exactly as keyed, in source order and source option order; of the module’s two body Check Your Understanding bullets, “Describe the most common way to diagnose loiasis” is graded as a multiplechoice because a single module sentence (“They can often be observed when migrating through the eye, and sometimes under the skin; in fact, this is generally how the disease is diagnosed”) fixes the whole answer, with distractors built from the module’s own antibody-testing and Acanthamoeba-diagnosis sentences, and “How are Acanthamoeba infections acquired?” stays a body self-check, since the honest answer assembles two separate paragraphs (the eye/contact-lens route and the skin-wound/respiratory-tract route) rather than one sentence; the module’s two unkeyed Critical Thinking questions are rendered as selfchecks in Practice, per the run’s decision — the traveler-risk question’s model answer restates the module’s own hedged “it has been suggested” sentence rather than asserting a mechanism the module does not give, and the loiasis-prevention question’s model answer is restricted to what the module actually says about deerfly transmission, since the module gives no prevention or repellent advice for loiasis; one textin term-recall filler ("Acanthamoeba is a genus of…") is added under the first, thinner objective, built strictly from the section’s own opening body sentence, since the module’s source exercises otherwise cluster under the second objective; no source exercise, table, or Check Your Understanding bullet is omitted; key terms are compiled from the module’s 2 <term> elements and the book’s Glossary appendix, giving 2 distinct bullets, both taken verbatim from the Glossary appendix — “loiasis” and “Acanthamoeba keratitis” (the latter’s appendix headword is nested inside markup, but resolves to a matching entry once the markup is read); the source’s cross-reference to Fungal and Parasitic Diseases of the Nervous System is left as plain italic text, since that module is not authored in this run. The Acanthamoeba life-cycle artwork spells “amoebic” at step 6 and “amebic” at step 7; the page’s alt and longdesc use “amebic” throughout, as the source alt does.