Infectious Diseases Affecting the Skin and Eyes Notes

The Skin and Its Defenses

Integument: Forms the boundary between the human body and the environment.
- Composed of skin, hair, nails, sweat glands, and oil glands.
Skin:
- Surface area of 1.5 to 2 square meters.
- Thickness varies: 1.5 mm at eyelids to 4 mm on the soles of the feet.

Layers of the Skin: Epidermis

Stratum corneum:
- 25 cells thick, composed of dead cells migrated from deeper layers.
- Millions of cells slough off daily, shedding microorganisms.
- Entire epidermis replaced every 25 to 45 days.
Keratin:
- Provides cells with the ability to withstand damage, abrasion, and water penetration.
Stratum basale:
- Several layers below the stratum corneum.
- Attached to the dermis, it's the source of cells that make up the epidermis.

Layers of the Skin: Dermis

Composed of connective tissue (not epithelium).
Rich matrix of fibroblast cells, collagen, macrophages, and mast cells.
Harbors a dense network of nerves, blood vessels, and lymphatic vessels.
Damage deep enough to penetrate the dermis results in broken blood vessels.
Blister Formation:
- The result of separation of the epidermis and dermis from friction, trauma, or burns.
Hair Follicles:
- "Roots" of hairs are housed in follicles.
Sebaceous (oil) Glands:
- Secrete sebum and are associated with the hair follicles.
Sweat Glands:
- Also found in the dermis.
All glands have openings on the surface of the skin that pass through the epidermis.

Defenses of the Skin

Antimicrobial peptides:
- Positively charged chemicals disrupt negatively charged membranes of bacteria.
- Many types exist.
- Chiefly responsible for keeping the microbial count on the skin relatively low.
Sebum:
- Low pH makes the skin inhospitable to microorganisms.
- Oily due to its high concentration of lipids.
- Lipids serve as nutrients for normal microbiota.
- Toxic by-products of fatty acid metabolism inhibit the growth of microorganisms not adapted to the skin environment.
Sweat:
- Inhibitory to microorganisms due to low pH and high salt concentration.
Lysozyme:
- Enzyme found in sweat, tears, and saliva.
- Specifically breaks down peptidoglycan found in the cell walls of bacteria.

Normal Biota of the Skin

Must be capable of living in dry and salty conditions.
Sparsely distributed over dry, flat areas of the body like the back.
Grow in dense populations in moist areas and skin folds such as the underarm and groin.
Also live in the protected environment of hair follicles and glandular ducts.
Recent microbiome studies have delivered:
- Hundreds of species of microbes, including some well-known pathogens, inhabit the epidermis, dermis, and subcutaneous layers.
- Different species favor different areas of the body.
- Different people have different species.
- An individual’s microbiota remains relatively constant over time.

Highlight Disease: MRSA Skin and Soft-Tissue Infection

Characteristics of MRSA:
- Common cause of skin lesions in non-hospitalized people.
- Strains are resistant to multiple antibiotics.
Staphylococcus aureus:
- Gram-positive coccus that grows in clusters; nonmotile.
- Can be highly virulent but is also considered "normal" biota of the skin in 1/3 of the population.
- Withstands 7.5 to 10% salt, extremes in pH, and high temperatures.
- Remains viable after air drying.
- Resists the effects of many disinfectants and antibiotics.
Signs and symptoms:
- Raised, red, tender, localized lesions.
- Features pus and feels hot to the touch.
- Occur easily in breaks in the skin and may localize around a hair follicle.
- Fever is common.
Transmission and epidemiology:
- Contaminant of all kinds of surfaces: gym equipment, airplane tray tables, electronic devices, razors, etc.
- Persons with active MRSA skin lesions should keep them covered to avoid transmission.
Pathogenesis and virulence factors:
- Coagulase is a diagnostic species characteristic.
- Also produce hyaluronidase, staphylokinase, DNase, and lipase.
Culture and/or diagnosis:
- PCR
- Isolation on blood agar or mannitol salt agar
- Catalase distinguishes S. aureus from other staphylococcal species.
- Coagulase test
Prevention and treatment:
- Prevention is only possible with good hygiene.
- Treatment through incision of lesion and pus drainage.
- Antimicrobial treatment should include more than one antibiotic (vancomycin is recommended in the U.S.).

Highlight Disease: Maculopapular Rash Diseases

Maculopapular: Skin eruptions caused by a variety of microbes. Flat to slightly raised colored bumps
Measles
Rubella
Fifth Disease

Measles

Also known as rubeola.
About 367 children die of this disease per day even though an extremely effective vaccine has been available since 1963.
Before the vaccine was introduced, measles killed 6 million people every year.
Sharp increase in measles cases in the United States in 2019.
The year 2019 also saw a massive measles epidemic on the Pacific island of Samoa, population 200,000. In that year, there were more than 5,600 cases of measles, and 81 deaths
Signs and symptoms:
- Sore throat
- Dry cough
- Headache
- Conjunctivitis
- Lymphadenitis
- Fever
- Oral lesions called Koplik’s spots
- Red, maculopapular exanthem that erupts on the head and progresses to the trunk and extremities
Sequelae and complications:
- Laryngitis
- Bronchopneumonia
- Secondary bacterial infections such as ear and sinus infections
- Pneumonia
- Encephalitis resulting in CNS changes ranging from disorientation to coma
- Can result in permanent brain damage or epilepsy
Subacute sclerosing panencephalitis:
- Progressive neurological degeneration of the cerebral cortex, white matter, and brain stem
- Occurrence: 1 in less than 1,000 children who get measles before being vaccinated
- Involves a defective virus, unable to form a capsid and be released from an infected cell
- Spreads unchecked through the brain by cell fusion, destroying neurons and accessory cells, and breaking down myelin
- Causes profound intellectual and neurological impairment
- Eventually leads to coma and death
Pathogenesis and virulence factors:
- Virus implants on respiratory mucosa and infects the tracheal and bronchial cells
- It travels to the lymphatic system where it multiplies and enters the bloodstream (viremia), which carries the virus to the skin and various organs
- Cell membranes of adjacent host cells fuse into large syncytia (giant cells with many nuclei) and no longer perform their proper function
Transmission and epidemiology:
- One of the most contagious infectious diseases:
- Transmitted by respiratory droplets
- Humans are the only reservoir
- Person is infectious during the periods of incubation, prodrome phase, and the skin rash, but not during convalescence
Culture and diagnosis:
- Clinical presentation
- ELISA to test for IgM to measles antigen
Prevention:
- MMR vaccine contains live, attenuated measles virus that confers protection for up to 20 years
- Recommended for healthy children aged 12 to 15 months, with a booster before the child enters school
Treatment:
- Reducing fever
- Suppressing cough
- Replacing lost fluid
- Remedies to relieve neurological and respiratory symptoms and to sustain nutrient, electrolyte, and fluid levels
- Vitamin A supplements are recommended

Rubella

Also known as German measles, its name is derived from Latin for “little red.”
Relatively minor rash disease with few complications.
Serious damage can occur to the fetus in utero.
Women of childbearing years must be sure to have been vaccinated before they plan to conceive.
Postnatal rubella:
- Rash of pink macules and papules
- Appears first on the face and progresses down the trunk and toward the extremities
- Advances and resolves in about 3 days
- Milder than measles rash
- Adult rubella characterized by joint inflammation and pain rather than rash
Congenital rubella:
- Teratogenic: causes harm to the fetus
- Virus is transmitted from the mother to fetus in utero, even if the mother is asymptomatic
- Infection in the first trimester can result in miscarriage or multiple permanent defects.
- Most common defect is deafness, but cardiac abnormalities, ocular lesions, deafness, and mental and physical retardation can occur in varying combinations
- Other sequelae include anemia, hepatitis, pneumonia, carditis, and bone infection
Causative agent:
- Rubivirus from the family Togaviridae
- Virus has the ability to stop mitosis, an important process for a developing embryo and fetus
- Induces apoptosis of normal tissue cells, causing irreversible harm to organs
- Damages vascular endothelium, leading to poor development of many organs
Transmission and epidemiology:
- Worldwide distribution
- Initiated through contact with respiratory secretions and occasionally urine
- Virus is shed through prodromal phase and up to a week after the rash appears
- Congenitally infected infants are contagious for a much longer time
- Virus is only moderately communicable; close living conditions are required for its spread
- Declared eliminated from the U.S. in 2004 and from the Americas in 2015
Culture and diagnosis:
- Because it mimics other diseases, rubella should not be diagnosed on clinical grounds alone
- IgM antibody detected through ELISA or latex agglutination
Prevention and treatment:
- Attenuated rubella vaccine is given in MMR at 12 to 15 months and a booster at 4 to 6 years
- Postnatal rubella is usually benign and requires only symptomatic treatment
- No specific treatment is available for congenital manifestations

Fifth Disease

Also known as erythema infectiosum:
- Fifth of the diseases recognized by doctors to cause rashes in children: scarlet fever, measles, rubella, and another rash thought to be distinct
Signs and symptoms:
- Mild disease that results in a “slapped cheek” appearance that begins on the face
- Within 2 days, rash spreads on the body but is most prominent on arms, legs, and trunk
- Rash may reoccur for several weeks and may be brought on by any activity that increases body heat
Causative agent: Parvovirus B19
Diagnosis:
- Clinical presentation
- Rule out rubella by testing for IgM antibodies
Transmission and epidemiology: Very contagious
Prevention and treatment:
- No vaccine and no treatment
- Usually mild disease

Roseola

Common disease in young children and babies; sometimes known as “sixth disease.”
Signs and symptoms:
- Can result in a maculopapular rash, but up to 70% of cases proceed without the rash stage
- Usually accompanied by a high fever ( $41°C$ or $105°F$ ) that comes on quickly and lasts up to 3 days:
  - Seizures may occur during this period.
- On the fourth day, the fever disappears and the rash appears, first on the chest and trunk, then less prominently on the face and limbs
Causative agent:
- Human herpesvirus 6 (HHV-6)
- Can remain latent in the host indefinitely after the disease has cleared
- Occasionally reactivates in childhood or adulthood, causing mononucleosis-like or hepatitis-like symptoms
Transmission and epidemiology:
- It is thought that 100% of the U.S. population is infected with the virus by adulthood
Prevention and treatment:
- No vaccine and no treatment exist for roseola

Impetigo

Characteristics of impetigo:
- Superficial bacterial infection that causes the skin to flake or peel
- Can be caused by either Staphylococcus aureus or Streptococcus pyogenes, or may be caused by a mixture of the two:
  - S. pyogenes begins most cases of the disease, but later S. aureus takes over and produces a bacteriocin (toxin) that destroys S. pyogenes
Signs and symptoms:
- Lesion: peeling skin, crusty and flaky scabs, or honey-colored crusts
- Lesions found most around the mouth, face, and extremities
Impetigo caused by Staphylococcus aureus:
- Exfoliative toxins A and B cause characteristic blistering and facilitate the spread of the bacterium
Impetigo caused by Streptococcus pyogenes:
- Gram-positive coccus, beta-hemolytic on blood agar
- Causes streptococcus pharyngitis, scarlet fever, pneumonia, puerperal fever, necrotizing fasciitis, serious bloodstream infections, and poststreptococcal conditions
Pathogenesis and virulence factors:
- Possesses a huge arsenal of enzymes and toxins
- Rarely, impetigo caused by S. pyogenes can be followed by acute poststreptococcal glomerulonephritis
- Strains that cause impetigo never cause rheumatic fever.
- S. pyogenes is more often the cause of impetigo in newborns; S. aureus is more often the cause of impetigo in older children; both can cause infection in either age group

Cellulitis

Caused by a fast-spreading infection in the dermis and in the subcutaneous tissues
Signs and symptoms:
- Pain, tenderness, swelling, and warmth
- Fever and swelling of the lymph nodes in the area may also occur
- Lymphangitis: red lines leading away from the area are visible, the result of microbes and inflammatory products being carried by the lymphatic system
- Bacteremia could develop, but uncomplicated cellulitis has a good prognosis
Causative agents:
- Healthy individuals: Streptococcus pyogenes and occasionally Staphylococcus aureus
- Immunocompromised individuals: almost any bacterium and some fungi
- Infants: group B streptococci
Transmission and epidemiology:
- Generally follows introduction of bacteria or fungi into the dermis:
  - Trauma
  - Subtle means; no obvious break in the skin
Culture and diagnosis:
- Diagnosis through clinical signs and symptoms
Prevention and treatment:
- Mild cellulitis responds well to oral antibiotics effective against S. aureus and S. pyogenes
- More involved infections and infections in immunocompromised patients require intravenous antibiotics
- Surgical debridement is required with extensive tissue damage

Staphylococcal Scalded Skin Syndrome (SSSS)

Dermolytic condition caused by S. aureus:
- Affects mostly newborns and babies
- Can infect children and adults
Signs and symptoms:
- Bullous lesions appear first around the umbilical area in neonates or in the diaper or axilla area
- Split occurs in the epidermal tissue layers just above the stratum basale
- Widespread desquamation of the skin follows
Transmission and epidemiology:
- Transmission may occur when caregivers carry the bacterium from one baby to another
- Adults in the nursery can directly transfer S. aureus
- 30% of adults are asymptomatic carriers, harboring the organism in the nasopharynx, axilla, perineum, and vagina
Culture and diagnosis:
- Diagnosis through clinical signs and symptoms
Prevention and treatment:
- Eliminate carriers in contact with neonates
- Immediate treatment with systemic antibiotics

Vesicular or Pustular Rash Diseases

Chickenpox: very common and mostly benign
Smallpox: even a single case constitutes a public health emergency
Both are viral diseases that present as generalized rashes
Pox: individual lesions that contain fluid

Chickenpox

Signs and Symptoms
- Fever and abundant rash appear after an incubation period of 10 to 20 days:
  - Rash begins on the scalp, face, and trunk and radiates in sparse crops to the extremities
- Skin lesions progress quickly from macules and papules to itchy vesicles filled with a clear fluid:
  - Encrust and drop off after several days; heal completely or may leave a small scar
  - Lesions can number from a few to hundreds; more abundant when they are in adolescents and adults
- Lesion distribution is centripetal, more lesions in the center of the body than in the extremities
- Usually lasts 4 to 7 days:
  - New lesions stop appearing after 5 days
  - Patients considered contagious until all of the lesions have crusted over
- Approximately 0.1% of cases are followed by encephalopathy, which can be fatal, but in most cases, recovery is complete

Shingles: Herpes Zoster

Signs and symptoms:
- Characteristic asymmetrical distribution on the skin of the trunk or head
- Virus enters the sensory endings of cutaneous spinal nerve branches after recovery from chickenpox:
  - Becomes latent in the ganglia and may reemerge as shingles abruptly after reactivation by psychological stress, immunosuppressive and other drug therapy, surgery, or developing malignancy
- Virus migrates down the nerve ganglion to the skin
- Multiplication of virus resumes
- Produces crops of tender, persistent vesicles
- Postherpetic neuralgia: inflammation of the ganglia and nerve pathways that cause pain and tenderness and can last for several months
- Involvement of cranial nerves can lead to eye inflammation and ocular and facial paralysis
Causative agent: Human herpesvirus 3 (HHV-3) AKA varicella, an enveloped DNA virus
Pathogenesis and virulence factors:
- HHV-3 enters the respiratory tract, attaches to respiratory mucosa, and invades and enters the bloodstream
- Viremia disseminates the virus to the skin, where it causes adjacent cells to fuse and lyse, causing characteristic lesions
- The virus then enters sensory nerves and dorsal root ganglia
- Ability to remain latent in nerve ganglia is an important virulence factor
Transmission and epidemiology:
- Humans are the only natural host for HHV-3
- Harbored in the respiratory tract:
  - Communicable from both respiratory droplets and fluid from active lesions
  - People can acquire chickenpox from exposure to fluid of shingles lesions
- Infected persons are infectious a day or two prior to the development of the rash
- Chickenpox is so contagious that if you are exposed to it, you will almost certainly get it
Prevention:
- Live attenuated vaccine licensed in 1995
- Zostavax approved by the FDA in 2006 to prevent shingles
Treatment:
- Uncomplicated varicella is self-limiting and requires no therapy aside from alleviation of discomfort
- Secondary bacterial infections are treated with topical or systemic antibiotics
- Oral acyclovir should be administered to those considered at risk for complications
- Do not administer aspirin as this may lead to Reye’s syndrome

Smallpox

After a comprehensive global effort by the World Health Organization, smallpox is largely a disease of the past:
- After the anthrax bioterrorism in 2001, the U.S. began taking the threat of smallpox bioterrorism seriously
- Vaccination had been discontinued when the disease was eradicated but is now offered to certain U.S. populations
Signs and symptoms:
- Fever, malaise, and a rash that begins in the pharynx, spreads to the face, and progresses to the extremities
- Rash:
  - Macular
  - Papular
  - Vesicular
  - Pustular
  - Crusts over, leaving nonpigmented sites pitted with scar tissue
- Variola major: highly virulent form causing toxemia, shock, and intravascular coagulation
- Important to recognize signs and symptoms of smallpox:
  - Prodrome period of high fever and malaise
  - Rash emerges, first in the mouth
  - Severe abdominal and back pain
  - Rash spreads throughout the body within 24 hours
  - Diagnosis of a single case must be treated as a health and law enforcement emergency
- Variola major (cont’d):
  - After the 3rd or 4th day of the rash, bumps become larger and are filled with a thick, opaque fluid
  - Pustules are indented and begin to scab over within a few days
  - After 2 weeks, most lesions have crusted over, but the patient remains contagious until the last scabs fall off
  - During the rash, the patient is very ill
  - Lesions heal on the dermal level, leaving scars
- Variola minor:
  - Minor, less dense rash
  - Patient is less ill than with variola major
Causative agent:
- Variola virus:
  - Orthopoxvirus
  - Enveloped DNA virus
  - This group also includes monkeypox and vaccinia
  - Hardy virus, surviving outside the host longer than most viruses
Transmission and epidemiology:
- Spread primarily through droplets
- Fomites such as contaminated bedding and clothing can also spread the disease
- Endemic in 31 countries in the 1970s:
  - At that time, 10 to 15 million contracted the disease every year and 2 million people died from it
  - A massive vaccination effort lasting 11 years eradicated the disease
  - Last case was in Somalia in 1977
Prevention:
- Vaccination available for postexposure prophylaxis
Treatment:
- Two drugs have been approved by the Food and Drug Administration for smallpox
  - Tecovirimat and cidofovir

Large Pustular Skin Lesions: Leishmaniasis

Signs and symptoms:
- Cutaneous leishmaniasis is a localized infection of the capillaries of the skin
- Mucocutaneous leishmaniasis:
  - Endemic to Central and South America
  - Affects both the skin and mucous membranes
  - Also causes systemic leishmaniasis
Causative agents:
- Cutaneous leishmaniasis: L. tropica
- Mucocutaneous leishmaniasis: L. brasiliensis
Transmission and epidemiology:
- Zoonosis transmitted among mammalian hosts by female sand flies
- Endemic to equatorial regions that provide favorable conditions for the sand fly
- Travelers and immigrants are at particular risk
Culture and diagnosis:
- Microscopic visualization
Prevention and treatment:
- No vaccine
- Avoiding the sand fly is the only prevention

Cutaneous Anthrax

Signs and symptoms:
- Papule that becomes necrotic and later ruptures to form a painless, black eschar
Causative agent: Bacillus anthracis
Transmission and epidemiology:
- Endospores enter skin through cuts or abrasions:
  - Mail workers and others contracted the infection when endospores were sent through the mail
  - Can be naturally transmitted by contact with hides of infected animals
Culture and diagnosis:
- Culture on blood agar
- Serology
- PCR performed by CDC
Prevention and treatment:
- Left untreated, the cutaneous form is fatal in 20% of cases
- Vaccine exists but is recommended only for high-risk persons and the military

Cutaneous and Superficial Mycoses: Ringworm

Dermatophytes:
- A group of fungi that cause a variety of body surface conditions
- Confined to the nonliving epidermal tissues (stratum corneum) and their derivatives (hair and nails)
- All conditions have names beginning with tinea, derived from the erroneous belief that they were caused by worms
Ringworm of the Scalp (Tinea Capitis):
- This mycosis results from the fungal invasion of the scalp and the hair of the head, eyebrows, and eyelashes.
Ringworm of the Beard (Tinea Barbae):
- This tinea, also called barber’s itch, affects the chin and beard of adult males. Although once a common after effect of unhygienic barbering, it is now contracted mainly from animals.
Ringworm of the Body (Tinea Corporis):
- This extremely prevalent infection of humans can appear nearly anywhere on the body’s glabrous (smooth and bare) skin.
Ringworm of the Groin (Tinea Cruris):
- Sometimes known as jock itch, crural ringworm occurs mainly in males on the groin, perianal skin, scrotum, and occasionally the penis. The fungus thrives under conditions of moisture and humidity created by sweating.
Ringworm of the Foot (Tinea Pedis):
- Tinea pedis has more colorful names as well, including athlete’s foot and jungle rot. Infections often begin with blisters between the toes that burst, crust over, and can spread to the rest of the foot and nails.
Ringworm of the Nail (Tinea Unguium):
- Fingernails and toenails, being masses of keratin, are often sites for persistent fungus colonization. The first symptoms are usually superficial white patches in the nail bed. A more invasive form causes thickening, distortion, and darkening of the nail.
Causative agents:
- Trichophyton, Microsporum, and Epidermophyton
  - Causative agent varies by geographic location and is not restricted to one genus or species
Pathogenesis and virulence factors:
- Dermatophytes have the ability to digest keratin:
  - Fungi do not invade deeper epidermal layers
Transmission and epidemiology:
- Direct contact and indirect contact with other humans or infected animals
- Therapy is a topical, antifungal ointment applied for several weeks:
  - Several drugs work by speeding up loss of the outer skin layer

Superficial Mycoses

Signs and symptoms:
- Involve the outer epidermal surface:
  - Innocuous infections with cosmetic rather than disease-causing effects
Causative agent:
- Tinea versicolor is caused by the yeast Malassezia, a genus that has at least 10 species living on the skin:
  - Feeds on the high oil content of the skin glands
  - Also implicated in folliculitis, psoriasis, and seborrheic dermatitis (dandruff)
Transmission and epidemiology:
- Carried by nearly 100% of humans tested

Surface of the Eye and Its Defenses

Conjunctiva:
- Thin, membranelike tissue that covers the eye (except for the cornea) and lines the eyelids
- Secretes an oil- and mucus-containing fluid that lubricates and protects the eye surface
Cornea:
- Dome-shaped central portion of the eye lying over the iris
- Has 5 to 6 layers of epithelial cells that regenerate quickly if they are damaged
- Called "the windshield of the eye"
Tears:
- Consist of an aqueous fluid, oil, and mucus
- Formed in the lacrimal gland at the outer and upper corner of each eye
- Drain into the lacrimal duct at the inner corner
- Aqueous portion consists of lysozyme and lactoferrin
- Mucus layer contains proteins and sugars
- Flow of tear film prevents attachment of microorganisms to the eye surface
Eye’s Primary Function Is Vision
- Anything that hinders vision would be counterproductive
- Inflammation does not occur in the eye as readily as it does elsewhere in the body
- Flooding the eye with light-diffracting objects such as lymphocytes and phagocytes would blur vision
- Immune privilege: evolution of the vertebrate eye has, of necessity, favored reduced innate immunity

Normal Biota of the Eye

Previously thought to be only sparsely populated by microbiota
16s rRNA analysis revealed a robust population of diverse bacterial species:
- In many cases, Corynebacterium is the dominant genus
- To a large extent, the eye microbiome resembles that of the skin

Highlight Disease: Conjunctivitis

Infection of the conjunctiva is relatively common and can be caused by:
- Specific microorganisms that have a predilection for eye tissues
- Contaminants that proliferate due to the presence of a contact lens or an eye injury
- Accidental inoculation of the eye by a traumatic event
Signs and symptoms:
- Most bacterial infections produce a milky discharge
- Viral infections produce a clear, watery exudate
- Patients often wake up in the morning with the eye “glued” shut by secretions that have accumulated and solidified overnight
- Some conjunctivitis cases are caused by an allergic response and a clear, watery fluid is formed
- Common name is pinkeye
Causative agents and their transmission:
- Neonatal eye infections caused by:
  - Neisseria gonorrhoeae or Chlamydia trachomatis
  - Transmitted vertically from a genital tract infection in the mother
  - Can lead to serious eye damage if not treated promptly
  - Herpes simplex can also cause neonatal conjunctivitis but is accompanied by a generalized herpes infection
- Bacterial conjunctivitis in other age groups is caused by:
  - Staphylococcus epidermidis, Streptococcus pyogenes, Streptococcus pneumoniae, Haemophilus influenzae, or Moraxella
  - N. gonorrhoeae and C. trachomatis conjunctivitis result from autoinoculation from a genital infection or sexual activity
  - A wide variety of bacteria, fungi, and protozoa can contaminate contact lenses and lens cases and be transferred to the eye
- Viral conjunctivitis is commonly caused by adenoviruses, although other viruses may be responsible
- Both bacterial and viral conjunctivitis are transmissible by direct contact and are usually highly contagious
Prevention and treatment:
- Treatment of non-neonatal cases of conjunctivitis:
  - Ciprofloxacin is a common choice because it covers all possible bacterial infections
  - Physicians may prescribe prophylactic antibiotic eye drops even if a viral case is suspected
  - If symptoms do not improve in 48 hours, more extensive diagnosis may be performed

Keratitis

Signs and symptoms: Invasion of deeper eye tissues can lead to complete corneal destruction
Causative agents:
- Miscellaneous bacteria
- Herpes simplex virus
Transmission and epidemiology:
- “Misdirected” reactivation of (oral) herpes simplex virus type 1 (HSV-1): virus travels to the ophthalmic rather than the mandibular branch of the trigeminal nerve
- Blindness due to herpes is the leading infectious cause of blindness in the United States
- Infections with HSV-2 can result from a sexual encounter or transfer of the virus from the genital to the eye area
Culture and diagnosis:
- Clinical diagnosis
- Viral culture or PCR, if needed
Treatment:
- Trifluridine or acyclovir, or both
Acanthamoeba keratitis:
- Amoeba causes keratitis in people who wear contact lenses:
  - Free-living amoeba live in tap water, freshwater lakes, etc.
  - Associated with less-than-rigorous contact lens hygiene or previous trauma to the eye