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
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.
Bacteria: Staphylococcus aureus
Virus: Herpes simplex virus
Fungi; Malassezia furfur
Parasites: Phthirus pulis
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).
Herpes simplex (HSV)
Epstein-Barr virus (EBV)
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.
Herpes Simplex Virus
A different non-infectious injury like trauma and ultraviolet radiation can cause keratitis.
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.
It includes the agents for keratitis/ conjunctivitis.
It includes the agents for keratitis/ conjunctivitis
Herpes simplex (HSV),
Epstein-Barr virus (EBV)
It includes the agents for Keratitis
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.
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.
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.
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.
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.
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:
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:
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