Eye infections Flashcards
What is the principle cause of eye infections and why?
S. aureus, due to high carrier rate in humans
What are the four main staphylococcal eye infections?
- Dacryocystitis - inflammation of the lacrimal sac
- Blepharitis - infection of eyelid or sebacious gland
- Conjunctivitis - can be keratoconjuctivitis if corneal involvement
- Endophthalmitis - infection of aqueous or vitreous humor
What are required for keratitis or endophthalmitis?
Some trauma to cornea for keratitis
Ulceration or penetrating injury of cornea or sclera for endophthalmitis
What are three major virulence factors of S. pneumoniae?
- Polysaccharide capsule - primary, avoids complement-mediated phagocytosis and lysis
- Pneumolysin, a membrane-damaging cytolysin related to SLO of S. pyogenes
- Cell wall teichoic acid / peptidoglycan promote inflammation
What is the major way in which S. pneumoniae infects the eyes? What does it cause?
Autoinoculation from upper respiratory tract (touch nose then eyes)
Causes Dacryocystitis, conjunctivitis, and keratoconjunctivitis
What are two other names for S. pneumoniae?
Pneumococcus or diplococcus (gram positive diplococci)
What Lancefield group is S. pneumoniae in?
It is not part of the Lancefield grouping scheme
What biochemical tests are used on S. pneumoniae?
- Capsular serotyping
- Quellung reaction -> capsular swelling induced via mixing with anti-capsule antibodies
- P disk susceptibility testing
What is the primary virulence factor of H. influenzae?
Polysaccharide capsule, which is antiphagocytic and subject to antigenic variation. Has serotypes a-f.
What is the most virulent H. influenzae capsule comprised of? How is the vaccine made for it?
b capsule: contains polyribitol phosphate (PRP)
Hib vaccine made by giving PRP conjugated with diphtheria toxoid or Neisseriae outer membrane protein
Where is H. influenzae found and what does it cause??
Found exclusively in humans, with high carriage rate in upper respiratory tract, most of which lack capsule
Causes same things as S. pneumoniae (just not endopthalmitis or blepharitis)
What is the morphology of H. influenzae?
Gram negative coccobacillus
What special factors are required for the growth of H. influenzae? Why is this relevant?
X factor and V factor (cytochromes), which are blood products. Will grow on chocolate agar but not blood agar (need lysed erythrocytes)
What is the satellite growth test?
Put cytochrome factors X and V on discs which diffuse out into blood agar, H. influenzae will only grow around discs (diagnostic of this pathogen)
What are the three primary virulence factors of P. aeruginosa?
- Exotoxin A
- Elastase
- Adhesin
What does Exotoxin A do and what other toxin does it relate to?
It is an AB cytotoxin, with receptor part and enzymatic part - ADP-ribosylating enzyme which attaches ADP-ribose to host elongation factor 2, killing the cell. Promotes tissue invasion.
Same mechanism as diphtheria toxin
What does elastase do? How does it relate to eye pathology?
Cleaves elastin and human Igs, collagen, and complement
It is the primary cause of corneal perforation in eye infection
What does the P. aeruginosa adhesin require?
Requires trauma (scratch) to adhere to corneal exposed receptors
What is one major risk factor for P. aeruginosa to colonize?
Extended contact lens use (can live in contact lens solution, opportunistic pathogen)
In what ways is P. aeruginosa eye infection similar to staph?
Most host damage is mediated by inflammatory response via neutrophils
P. aeruginosa can also cause endophthalmitis like S. aureus, requires corneal perforation or ulceration
What biochemical test is diagnosis of P. aeruginosa?
High levels of cytochrome oxidase -> positive oxidase test
Is P. aeruginosa aerobic?
Believed to never ferment, but is called facultative anaerobe because it can use NO3- as a terminal electron acceptor
What are the two stages of Chlamydia trachomatis?
- Elementary body - infectious, expresses adhesins
2. Reticulate body - replicative, intracellular
How does the elementary body get into host cells?
Expresses adhesins, then does not stimulate lysosomal fusion. Organism replicates as reticulate body.
What is inclusion conjunctivitis?
Chlamydia eye infection of the neonate from direct contact with cervical secretions during delivery.
Marked by acute, mucopurulent discharge ~7 days post-partum
What is chronic follicular conjunctivitis? What complicates it?
Also called trachoma, it is a chronic inflammation of the eyelids which leads to vascularization and scarring of the cornea.
Often made worse by trichiasis which is inward growth of eyelids which continually scrape the cornea, causing visual impairment
Infections are recurrent due to poor hygiene in developing countries, leading to reinfection via hands and fomites
What is the major morphological characteristic of chlamydia?
Gram negative, but lacks peptidoglycan (no cell wall)
Can be diagnosed in conjunctival scrapings, then cultured for 3-7 days, immunostaining outer membrane proteins
What is seen on retinoscopy for Candida enophthalmitis? What is required for this to occur?
White cotton ball expanding on retina
Requires injury to occur
What condition causes blindness in the immunocompromised, and can be a result of systemic Candida infection?
Chorioretinitis
What type of yeast is Histoplasma capsulatum and how does it evade immune response?
Dimorphic fungi, with pathogenic yeast form occurring at 37 degrees C
Evades host immune response by entering dormant stage in host, and growing inside phagocytes despite oxidative burst
How are humans typically infected via histoplasma?
Inhalation of infectious conidia while handling bird (i.e.. chicken) or bat droppings
How can histoplasmosis have ocular manifestations?
Typically a granulomatous disease of the lungs, but can disseminate via reticuloendothelial system (macrophages) and cause chorioretinitis
What is presumed ocular histoplasmosis syndrome (POHS)?
When fungus causes small areas of inflammation / scarring on the retina.
Peripheral scarring is unlikely to cause issues
What is Coccidioides immitis infection also called? Why?
Valley fever, occurs in hot, dry regions of southwest U.S.
What type of fungi is Coccidioides and what are its growth stages?
Dimorphic
Mold form - in the environment which releases spores which are inhaled (primarily a pulmonary infection).
Spherule - invasive tissue form which contains endospores as reproductive unit, engulfed by T-cell activated macrophages
When does Coccidioides cause chorioretinitis?
Primarily in immunocompromised (happens in <1% of pulmonary infections)
Similar pathogenesis to histoplasmosis
What amoeba causes amoebic meningoencephalitis?
Naegleria fowleri - very rare
What are the two forms of Acanthamoeba species?
- Trophozoites - free-living and tissue-invading
2. Cysts - infectious stage, often get in following mild corneal trauma and improper contact lens sterilization
How do amoeba cause ulcerative keratitis?
Invade corneal epithelium, and penetrate corneal stroma causing granulomatous inflammation. May lead to blindness if clinician continues failed anti-bacterials, anti-fungals, and antivirals
How are amoeba cultured? Biopsied?
On media containing gram-negative bacteria (their food).
Can be seen on light microscopy / immunofluorescence on corneal biopsy
How is T. gondii contracted in humans?
Eating oocyst from cat litter (sexual stage) or tissue cyst from infected meet
What type of pathogen is T. gondii and what is its tissue stage?
Obligate, intracellular protozoan with tachyzoite stage for cell invasion, which resists phagocyte killing
What is uveitis?
Inflammation of the vascular layer of the eye: iris, ciliary body, choroid (layer between brunch’s membrane and sclera
What does T. gondii account for 1/4 of all? Who does it effect?
1/4 of all granulomatous uveitis. Can be due to reactivation of dormant tissue cysts in 20-40 year olds (sequelae of congenital toxoplasmosis), especially in immunocompromised
This is a type of chorioretinitis
What is the definitive host of Toxocara canis and how do they grow in humans?
Dogs, eggs are found in dog poop. This is the dog round-worm
Humans, especially kids, ingest the steadfast eggs and larvae pass through pulmonary capillaries and reach systemic circulation, they penetrate the wall and enter tissue
What condition does T. canis cause in humans?
Visceral larva migrans, especially to ocular tissues but can hit liver, lung, heart
Who does T. canis typically infect?
Children 1-6 years, especially those who like to eat dirt (pica)
What is chorioretinitis caused by T. canis called? How is it detected?
Ocular larva migrans
Detected via retinoscopy or via elevated antibody titers to Toxocara antigens
