Written by Dr Devika Nair; Reviewed by Ms Savita Madhusudhan (Consultant Ophthalmologist)

Introduction

The eye comes into constant exposure with the external environment and is therefore susceptible to microbial invasion. The eye has various methods of defence, including antimicrobial components in the tear film, as well as tight junctions between cells of the outer epithelial layer of the cornea. These barriers can be disrupted by contact lens use, trauma, or surgery, which can result in intraocular infections. Other risk factors include immunosuppression and severe dry eyes.

Microbial eye disease may range from mild, self-limiting conditions such as viral conjunctivitis, to severe, sight-threatening diseases like microbial keratitis and endophthalmitis. This article provides an overview of the causative agents of microbial eye infections, in order to aid recognition of potentially sight-threatening infections.

Bacterial Eye Infections

Bacterial Conjunctivitis

Bacterial conjunctivitis is common, particularly in children. Patients may complain of red eyes with discharge, often starting unilaterally and subsequently becoming bilateral.

• Common pathogens: Haemophilus influenzae, Streptococcus pneumoniae, Staphylococcus aureus
• Clinical features: Redness and purulent discharge, often causing sticky eyelids, especially on waking. May be unilateral or bilateral.
• Diagnosis: Primarily clinical. Swabs can be sent for microscopy, culture, and sensitivity (MCS), and polymerase chain reaction (PCR) where indicated.
• Management: Topical broad-spectrum antibiotics (e.g., chloramphenicol)

Hyperacute conjunctivitis is a sight-threatening ophthalmic emergency caused by Neisseria gonorrhoeae. It presents with red eye and profuse purulent discharge. Without prompt treatment, severe cases may result in corneal perforation, hence same-day ophthalmology review is warranted. Neonates born to mothers with gonorrhoea are particularly at risk due to exposure during childbirth; this is termed ophthalmia neonatorum. Hyperacute conjunctivitis should also be considered in sexually active adults. Unlike common bacterial conjunctivitis, hyperacute conjunctivitis requires systemic antibiotics; typically, a stat dose of intramuscular ceftriaxone and oral azithromycin (1). Topical therapy alone will not eradicate the infection.

Chronic conjunctivitis is caused by Chlamydia trachomatis. This form presents more insidiously than hyperacute conjunctivitis and has the same modes of transmission. Diagnosis is made using PCR of conjunctival swabs. Systemic antibiotics are required; typically, a single dose of azithromycin or a seven-day course of doxycycline (1).

Bacterial Keratitis

Bacterial keratitis is an infection of the cornea, the round, transparent front layer of the eye. Keratitis is a serious eye infection as irreversible damage to the cornea can occur if not managed promptly. The most common risk factor for bacterial keratitis is contact lens use.

• Common pathogens: Pseudomonas aeruginosa (especially in contact lens wearers), Staphylococcus aureus, Streptococcus pneumoniae (2)
• Clinical features: pain, redness, photophobia, and blurred vision

On slit lamp examination, corneal ulcers or white infiltrates may be identified. Fluorescein staining should also be undertaken to identify if corneal epithelial defects are present; these will appear as bright green areas under cobalt blue light.

These patients require ophthalmology referral and may need corneal scraping for MCS. Patients should be advised to not wear contact lenses whilst the cornea heals. Typically, bacterial keratitis is treated with intensive topical antibiotics and close follow-up, but oral antibiotics can be considered in severe cases.

Endophthalmitis

Endophthalmitis is a severe infection of the fluid inside the eye, namely the aqueous humour and vitreous humour. It is often secondary to intraocular surgery or penetrating trauma, where exogenous bacteria is introduced into the eye. Rarely, it can be caused by endogenous spread of infection.

• Common pathogens: Staphylococcus epidermidis (postoperative), Bacillus cereus (post-traumatic), Cutibacterium/Propionibacterium acnes (chronic post-surgical) (3)
• Clinical features: Ocular pain, reduced vision, photophobia, hypopyon (refers to inflammatory/ infectious cell aggregate in the anterior chamber, behind the cornea), and vitritis (seen as a white pupillary reflex)
• Management: Intravitreal vancomycin and ceftazidime +/- systemic antibiotics

It is imperative to identify potential endophthalmitis early, as it can cause irreversible visual loss. Patients require urgent ophthalmology referral. Diagnosis is predominantly clinical, but specialists will tap the intraocular fluid for MCS to support management. Urgent ophthalmic intervention with intravitreal antibiotics is required, and surgical management (i.e. vitrectomy) may be considered in some patients (4).

Viral Eye Infections

Viral eye infections are a common cause of microbial eye disease, often presenting with redness, watering, and irritation. Management is usually supportive, although antiviral therapy can be indicated for certain infections. They can be highly contagious, hence patient education and counselling are essential components of care to prevent transmission and recurrence.

Viral Conjunctivitis

Adenovirus is the most common cause. Features include red and watery eyes. Viral conjunctivitis often develops following upper respiratory tract infections. On examination, there may be follicles when the lids are everted, and pre-auricular lymphadenopathy. Management is largely supportive, including lubricant eye drops and patient education to prevent spread; this includes advising patients to maintain strict hand hygiene and avoid sharing towels (5).

Herpes Simplex Keratitis

HSV-1 is a common cause of recurrent keratitis. HSV-1 is highly prevalent in the UK, with an estimated 60% of the population being carriers of the virus (6). The virus often lies dormant in the trigeminal ganglion and most individuals remain asymptomatic. But under certain conditions such as stress, systemic illness or UV light exposure, the virus can reactivate, potentially affecting the cornea and causing recurrent keratitis

• Pathogen: Herpes Simplex Virus type 1 (HSV-1)
• Clinical features: Pain, redness, tearing, photophobia, and decreased vision
• Management: Topical and/or oral acyclovir

On slit lamp examination, characteristic dendritic ulcers may be identified with fluorescein staining. These often appear as branching defects on the surface of the cornea under cobalt blue light. In some cases, there may be reduced sensation on the cornea.

Topical corticosteroids are strictly contraindicated in active HSV infections, as they can suppress the local immune response and facilitate viral replication. Careful examination and early antiviral therapy are critical to prevent corneal scarring.

Herpes Zoster Ophthalmicus (HZO)

HZO is shingles affecting the ophthalmic branch of the trigeminal nerve. Following an initial prodrome of burning pain along the dermatome, patients develop a unilateral rash on the forehead which can affect the eyelid, and sometimes the nose. Serious ocular complications can develop, such as keratitis, uveitis, or vision loss if the eye is involved.

• Pathogen: Varicella Zoster Virus (VZV)
• Clinical features: Painful vesicular rash in the V1 dermatome, dendritic corneal ulcer, intraocular inflammation (i.e. uveitis) 
•   
• Management: analgesia, oral antivirals (acyclovir, valacyclovir, or famciclovir) started within 72 hours of rash onset

As with many viral infections, patient education is important to prevent spread of VZV. Patients should especially be advised to avoid contact with individuals who are pregnant, elderly, or immunocompromised until the vesicles have crusted over.

HIV and the eye

Patients with Human Immunodeficiency Virus (HIV) may develop ocular manifestations involving any part of the eye (7). The most frequent finding is HIV retinopathy, characterized by cotton wool spots, microaneurysms, and retinal haemorrhages. As the CD4 count falls in patients with HIV, they become susceptible to opportunistic infections, including cytomegalovirus (CMV) retinitis as well as Toxoplasma gondii infection and HZO. Management involves treating the underlying infection and optimising antiretroviral therapy involving shared care between Ophthalmology and Infectious Diseases departments. Regular ophthalmic screening is recommended for patients with low CD4 counts.

Fungal Eye Infections

Fungal Keratitis

This is more common in tropical climates and predominantly in farming communities. The most common fungal forms of keratitis are Fusarium, Aspergillus and Candida albicans (8). Fungal spores damage the cornea and cause ulcers. Spores may penetrate the eye causing inflammation of the anterior chamber or even endophthalmitis if the posterior chamber (i.e. vitreous and retina) is affected.

Antifungals are only effective if initiated early, hence prompt recognition is imperative. Management of filamentous fungi such as Fusarium and Aspergills is with natamycin. For yeasts such as candida, amphotericin is utilised. Oral, intravenous, topical or intraocular injections of antifungal agents are all employed as necessary.

Parasitic Eye Infections

Acanthamoeba keratitis

Another form of microbial keratitis to be aware of is acanthamoeba keratitis. Acanthamoeba is a ubiquitous, free-living protozoa (parasite). It is often present in water supplies such as swimming pools, fresh water, and shower water. It is an important differential to consider in contact lens users and patients should be asked about recent exposure to water sources.

• Clinical features: pain out of proportion to clinical findings. Ring shaped corneal infiltrates.
• Diagnosis: corneal scraping for microbial culture.
• Management: topical treatment with cytotoxic drugs including biguanides such as chlorhexidine +/- diamidine

Patients should be followed up frequently to monitor and ensure improvement in condition.

Toxocariasis

• Pathogen: Toxocara, a helminth (roundworm)
• Transmission: ingestion of contaminated soil or animal faeces
• Diagnosis: ELISA for Toxocara excretory-secretory antigen
• Management: Topical corticosteroids for inflammation and systemic albendazole in selected cases

Toxocara eggs can be found in the faeces of cats and dogs, or in contaminated soil. This makes playgrounds and sandboxes potential places for accidental ingestion of eggs. Once ingested, larvae cause local inflammatory reactions leading the formation of granulomas to contain the parasite (9). If larva migrate to the eye, patients may experience unilateral vision loss accompanied by pain or photophobia. Initial assessment may reveal leukocoria, an abnormal white pupillary reflex where normally a red reflex is expected on shining light into the eye. This clinical finding differentiates it from other parasitic infections of the eye.

Toxoplasmosis

Toxoplasmosis is a very common cause of posterior uveitis.

• Pathogen: Toxoplasma gondii (an obligate intracellular protozoa)
• Definitive host: cats
• Transmission: faeco-oral, uncooked food/meat, contaminated water
• Diagnosis: PCR of aqueous humour

Patients may complain of floaters and blurred vision. ‘Headlight in the fog’ appearance is a sign of necrotising retinitis on fundal examination. Presentation in HIV patients may be indicative of progressions to AIDS (10). Once identified, management is via triple oral therapy comprised of: Pyrimethamine, Sulfadiazine, and Corticosteroids.

Congenital Infections (TORCH)

These are infections that occur whilst the baby is still in utero. Congenital eye infections are grouped together as TORCH infections. This stands for:

• Toxoplasmosis
• Other (e.g. syphilis)
• Rubella
• Cytomegalovirus
• Herpes Simplex Virus

Early detection in pregnancy and neonatal screening is key to preventing vision loss.

Conclusion

The eye is susceptible to infection by a wide range of bacterial, viral, fungal, and parasitic pathogens, each presenting with distinct clinical features. Accurate identification of the causative organism is essential for targeted management and for preventing potentially sight-threatening complications. A vital component of management is patient education, particularly regarding appropriate contact lens hygiene and personal hygiene, to minimise the risk of transmission and recurrence.

References

1. Feldman BH et al.. Bacterial Conjunctivitis. EyeWiki. American Academy of Ophthalmology. Available from: https://eyewiki.org/Bacterial_Conjunctivitis.
2. Österhed E, Oldberg K, Gustafsson I. Aetiology and antibiotic susceptibility of bacterial keratitis at a referral centre in southern Sweden. Sci Rep. 2025;15:20123. doi:10.1038/s41598-025-04404-7.
3. Durand ML. Endophthalmitis. Clin Microbiol Infect. 2013;19(3):227‑234. doi:10.1111/1469‑0691.12118.
4. Feldman BH et al. Endophthalmitis. EyeWiki. American Academy of Ophthalmology. Available from: https://eyewiki.org/Endophthalmitis.
5. National Health Service. Conjunctivitis. Available from: https://www.nhs.uk/conditions/conjunctivitis/.
6. Herpes Viruses Association. Frequently asked questions – Herpes simplex virus. Available from: https://herpes.org.uk/frequently-asked-questions/herpes-simplex-virus/.
7. Davis JL, Moshirfar M. Ocular Involvement in HIV/AIDS. EyeWiki. American Academy of Ophthalmology. Available from: https://eyewiki.org/Ocular_Involvement_in_HIV/AIDS.
8. Brown L, Leck AK, Gichangi M, Burton MJ, Denning DW. The global incidence and diagnosis of fungal keratitis. Lancet Infect Dis. 2021;21:e49‑e57. doi:10.1016/S1473‑3099(20)30448‑5.
9. CDC. Toxocariasis. DPDx. Available from: https://www.cdc.gov/dpdx/toxocariasis/.
10. de‑la‑Torre A, Gómez‑Marín J. Disease of the year 2019: ocular toxoplasmosis in HIV‑infected patients. Ocul Immunol Inflamm. 2020;28(7):1031‑1039. doi:10.1080/09273948.2020.1735450.