• Different infectious agents may get entrance to the eye either through the external or endogenous source.
• Superficial structures like the conjunctiva and the cornea are affected during external infections.
• Microorganisms present in the blood (e.g., endocarditis )may cause infection endogenously
• Endogeneous infection may be caused by the reactivation of latent viruses or parasites (e.g., cytomegalovirus or toxoplasmosis).

Different types of eye infections

1. Blepharitis:

• It is the inflammation of the margins(edges) of the eyelids; (eyelids, eye lashes, or associated pilosebaceous glands or meibomian glands).
• Symptoms include irritation, redness, burning sensation, and occasional itching. Condition is typically bilateral.
• Causative agents:
  • Bacteria: Staphylococcus aureus
  • Virus: Herpes simplex virus
  • Fungi; Malassezia furfur
  • Parasites: Phthirus pulis

2. Conjunctivitis:

• Inflammation (conjunctivitis) produces redness (pink eye), itching, and a discharge, which may be mucous or purulent.
• In this case, eyelids may stick together because of the exudation in bacterial infections which are thick, sticky, and encrusted.
• In patients having seasonal allergies, acute noninfectious inflammation may also occur.
• Conjunctivitis is highly contagious and can be transferred easily to the other eye or other individuals by contact (e.g., rubbing the infected eye and then the normal eye).
• Causative agents:
• Bacteria
  • Streptococcus pneumoniae
  • Haemophilus influenzae
  • Staphylococcus aureus
  • Haemophilus spp.
  • Pseudomonas aeruginosa
  • Chlamydia trachomatis
  • Neisseria gonorrhoeae
  • Streptococcus pyogenes
  • Moraxella spp.
  • Corynebacterium spp.

• Viruses
  • Adenoviruses
  • Herpes simplex (HSV)
  • Varicella-zoster
  • Epstein-Barr virus (EBV)
  • Influenza virus
  • Paramyxovirus
  • Rubella
  • HIV
  • Enterovirus
  • Coxsackie A

3.Keratitis:

• Keratitis, inflammation of the cornea, is a much more serious infection than conjunctivitis.
• Although there are no specific clinical signs to confirm infection, most patients complain of pain.
• Usually decrease in vision may occur, with or without discharge from the eye.
• Keratitis can result in scarring and blindness.
• Causative agents:
• Bacteria
  • S. aureus
  • S. pneumoniae,
  • Pseudomonas, aeruginosa
  • Moraxella lacunata
  • Bacillus spp.
• Virus
  • Herpes Simplex Virus
  • adenoviruses,
  • varicella-zoster

• Fungi
  • Fusarium solani,
  • Aspergillus spp.
  • Candida spp.
  • Acremonium,
  • Curvularia

• Parasites
  • Acanthamoeba spp
• A different non-infectious injury like trauma and ultraviolet radiation can cause keratitis.

4. Keratoconjunctivitis:

• It is an infection that involves both the conjunctiva and cornea.
• Ophthalmia neonatorum is acute conjunctivitis or keratoconjunctivitis of the newborn which is caused by either gonorrhoeae or C. trachomatis.
• Causative agents:
• Bacteria
• It includes the agents for keratitis/ conjunctivitis.
  • Streptococcus pneumoniae,
  • Haemophilus influenzae,
  • Staphylococcus aureus,
  • Haemophilus spp.
  • Pseudomonas aeruginosa
  • Chlamydia trachomatis,
  •  Neisseria gonorrhoeae,
  • Streptococcus pyogenes,
  • Moraxella spp.,
  • Corynebacterium spp.
  • Bacillus spp
• Virus
  • It includes the agents for keratitis/ conjunctivitis
  • Adenoviruses,
  • Herpes simplex (HSV),
  • Varicella-zoster.
  • Epstein-Barr virus (EBV)
  • Influenza virus,
  • Paramyxovirus,
  • Rubella,
  • HIV
  • Enterovirus,
  • Coxsackie A

• Fungi
• It includes the agents for Keratitis
  • Fusarium solani,
  • Aspergillus spp.,
  • Candida spp.,
  • Acremonium,
  • Curvularia
• Parasites
  • Toxoplasma gondii,
  • Toxocara

5. Chorioretinitis and uveitis:

• It is the inflammation of the retina and underlying choroid or the uvea.
• The infection can result in loss of vision.
• Causatiive agents:
•  Bacteria:
  • Mycobacterium tuberculosis
  • Treponema pallidum,
  • Borrelia burgdorferi
• Virus
  • Cytomegalovirus
  • HSV

• Fungi
  • Candida spp.

• Parasites
  • Toxoplasma gondii,
  • Toxocara
  • Treponema pallidum

6. Endophthalmitis

• It is the infection of the aqueous or vitreous humor.
• This infection is usually caused by bacteria or fungi. It is rare, develops suddenly, and progresses rapidly, often leading to blindness.
• During the movement of the eye, there is pain. Vision is decreased.
• Causative agent:
• Bacteria:
  • S. aureus,
  • S. epidermidis,
  • pneumoniae,
  • Streptococcal spp.
  • P. aeruginosa,
  • Gram-negative organisms,
  • Nocardia spp

• Virus
  • HSV
  • Varicella zoster

• Fungi
  • Candida spp.,
  • Aspergillus spp.,
  • Volutella spp.,
  • Acremonium spp

• Parasite
  • Toxocara
  • Onchocerca volvulus

7. Lacrimal infections, canaliculitis:

• It is a rare, chronic inflammation of the lacrimal canals in which the eyelid swells and there is a thick, mucopurulent discharge.
• Causative agent:
• Bacteria:
  • Actinomyces,
  • Propionibacterium
  • Propionicum

8. Dacryocystis:

• It is the inflammation of the lacrimal sac that is accompanied by pain, swelling, and tenderness of the soft tissue in the medial canthal region.
• Causative agents:
• Bacteria:
  • S. pneumoniae
  • S. aureus,
  • S. pyogenes,
  • Haemophilus influenza

• Fungi:
  • C. albicans,
  • Aspergillus spp.

9. Dacryoadenitis

• It is an acute infection of the lacrimal gland.
• These infections are rare and can be accompanied by pain, redness, and swelling of the upper eyelid, conjunctiva discharge.
• Causative agents:
• Bacteria:
• S. pneumoniae,
• S. aureus
• S. pyogenes

Laboratory Diagnosis of eye infection:

Specimen Collection and Transport

• A sterile swab should be taken for sample collection.
• From the lower conjunctiva sac and inner canthus (angle) of the eye, purulent material is collected on the sterile swab.
• Both eyes need to be cultured separately.
• For the Chlamydial culture, a dry calcium alginate swab should be taken.
• Then it should be placed in a 2-SP (2-sucrose phosphate) transport medium.
• If for the detection, Direct Fluorescent antibody (DFA) are to be used, then in such case additional slide also should be prepared.
• In that slide, the swab should be rolled across its surface which needs to be fixed with methanol.
• In the case of keratitis, scrapings of the cornea should be taken with a heat-sterilized platinum spatula.
• Multiple inoculations with the spatula are made to blood agar, chocolate agar, an agar for the isolation of fungi, thioglycollate broth, and an anaerobic blood agar plate.
• Other special media may be used if indicated.
• Corneal specimens for the detection of HSV and adenovirus should be cultured. They should be placed in viral transport media.
• Recently, the collection of two corneal scrapes (one used for Gram stain and the other transported in brain heart infusion medium and used for culture) was determined to provide a simple method for diagnosis of bacterial keratitis.
• From the anterior and posterior chambers of the eye, wound abscesses, and wound dehiscence (splitting open) specimens are collected for the culture of endophthalmitis.
• Lid infection material is collected on a swab conventionally.
• Under anaerobic conditions, transportation of the material should be done from the lacrimal canal in the case of canaliculitis.
• Aspiration of fluid from the orbit is contraindicated in patients with orbital cellulitis.

Direct Visual Examination:

• The smear should be prepared and a Gram stain should be performed.
• If there are other appropriate microscopic techniques, it should be performed.
• In bacterial conjunctivitis, polymorphonuclear leukocytes predominate; in viral infection, the host cells are primarily lymphocytes and monocytes.
• For the detection of Chlamydia, elementary body or inclusions should be checked.
• For this, it should be stained immediately with a monoclonal antibody conjugated to fluorescein.
• Using histologic stains, basophilic intracytoplasmic inclusion bodies are seen in epithelial cells.
• To detect herpes group infection in the conjunctivitis specimens, a Tzanck smear can be made. It shows the multinucleated epithelial cells.
• For the rapid diagnosis of the virus infection, DFA stains available for both HSV and VZV
• For the keratitis, the examination can be done using:
  • Gram stain
  • Giemsa stain
  • periodic acid-Schiff (PAS)
  • methenamine silver stains.
• Motile trophozoites should be observed by using the direct wet preparation in case of Acanthamoeba or other amebae and a trichrome stain should be added to the regimen.
• Culture is the most sensitive detection method for the diagnosis.
• In the case of endophthalmitis, the specimen needs to be examined by:
  • Gram
  • Giemsa
  • Periodic Acid-schiff (PAS)
  • Methenamine silver stains.
• Centrifugation should be done if the specimen is fluid and is in large volume.

Culture for eye infection:

• The number of organisms recovered from culture is low due to the washing action of tears.
• If the specimen is not purulent, large inoculums in a variety of media should be used to find out the etiological agent.
• The best result can be obtained when the conjunctival scrapings are placed directly onto the media.
• At a minimum, blood and chocolate agar plates should be inoculated and incubated under increased carbon dioxide tension (5% to 10% CO2).
• Sample from Both eyes should be cultured.
• Potential pathogens also may be present in an eye without causing infection.
• If the organism is isolated from both the infected and non-infected eye, it may not responsible for causing infection.
• If an organism only grows in culture from an infected eye, it might be the causative agent.
• Loeffler’s medium can be used when Moraxella lacunata is suspected.
• In this case, the growth of the medium causes the proteolysis and pitting of the medium.
• Loeffler’s or cystine-tellurite medium should be used if diphtheritic conjunctivitis is suspected.
• For the isolation of the organism from the keratitis, endophthalmitis, and orbital cellulitis, a reduced anaerobic blood agar plate, a medium for the isolation of fungi, and a liquid medium such as thioglycolate broth should be used.
• A reduced anaerobic blood agar plate should be used for the more serious eye infections.
• Blood culture also should be done in severe infections.
• From the transport broth, Chlamydia and virus should be inoculated inappropriate media.
• Cycloheximide-treated McCoy cells should be used for the Chlamydia
• For viral isolation, human embryonic kidney, primary monkey kidney, and Hep-2 cell lines can be used.

Molecular diagnosis for eye infection:

• Enzyme-linked immunosorbent assay (ELISA) tests and DFA staining are available for the detection of Chlamydia trachomatis.
• An ELISA test of aqueous humor is available for the diagnosis of Toxocara
• Single and multiplex polymerase chain reaction (PCR) assays

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