Keratitis: recognising a sight-threatening corneal infection

Keratitis — inflammation of the cornea — is one of the true sight-threatening emergencies in general practice. Unlike conjunctivitis, which is almost never dangerous, microbial keratitis can progress from early ulceration to corneal perforation within days if not identified and treated by an ophthalmologist. The clear cornea at the front of the eye lacks blood vessels, relying on tears and aqueous humour for immune defence; once bacteria or other pathogens breach the epithelium, they can destroy stromal collagen faster than the immune system can contain them. Approximately 3 million Australians wear contact lenses, and this population carries an estimated 10–20 times higher risk of keratitis than the non-wearing population, per RANZCO’s Microbial Keratitis guideline. Understanding the four main aetiologies — bacterial, herpes simplex virus, fungal, and acanthamoeba — and the specific red flags for each is core knowledge for any GP who sees patients with a red eye.

A. Core clinical — the AU general practice framework

The four keratitis types

RANZCO and eTG Eye outline four main causes of microbial keratitis in Australia:

Bacterial keratitis is the most common, accounting for roughly 50–70% of culture-positive cases. Organisms vary by context: Staphylococcus aureus and Streptococcus pneumoniae predominate in non-lens wearers; Pseudomonas aeruginosa dominates the contact-lens cohort (35–60% of culture-positive lens-related cases). Pseudomonas is the most feared — it produces proteases that digest stromal collagen, and an aggressive ring infiltrate with hypopyon can appear within 24–72 hours of symptom onset. The clinical picture is dense central suppurative infiltrate, pain, photophobia, and anterior chamber reaction. Hypopyon (pus settling at the bottom of the anterior chamber) is a serious sign requiring same-day review.

Herpes simplex keratitis (HSV) accounts for approximately 20–30% of all keratitis presentations in Australia. HSV-1 (the oral cold-sore virus) establishes latency in the trigeminal ganglion after primary infection; reactivation travels to the corneal surface, producing the classic branching dendritic ulcer with terminal bulbs visible on fluorescein staining. This epithelial pattern is pathognomonic. Progression to stromal keratitis — immune-complex-mediated — causes deep opacification and neovascularisation. The critical pitfall: topical corticosteroid prescribed alone for HSV epithelial keratitis accelerates progression to geographic ulcer and perforation. Corneal sensation is typically reduced — a useful bedside clue.

Fungal keratitis is uncommon in Australia overall (under 5% of cases) but disproportionately affects rural and tropical Queensland populations after corneal trauma with organic matter — twigs, hay, grass, agricultural debris. Filamentous fungi (Fusarium, Aspergillus) produce feathery-edged infiltrates with satellite lesions; yeast (Candida) is more common in immunocompromised patients. The course is indolent, and fungal keratitis is easily missed early, particularly if only antibiotic treatment is prescribed, per MUTT I (JAMA Ophthalmol 2013).

Acanthamoeba keratitis is rare but devastating. Acanthamoeba is a free-living amoeba present in tap water, swimming pools, and domestic water systems. It survives chlorination and is found in contact-lens cases rinsed with tap water. The Lim PLoS NTD 2021 Australian series confirmed that showering, swimming, or hot-tubbing while wearing contact lenses are the dominant risk factors. The hallmark is pain out of proportion to slit-lamp findings, with ring infiltrates and perineural infiltrates visible on confocal microscopy. Treatment is prolonged (months) and resistant to standard antibiotics.

History and examination in general practice

Every red eye in a contact-lens wearer should trigger a targeted history: type of lens (daily disposable carries lower risk than monthly reusable; extended overnight wear is highest risk), lens age, hygiene practices (showering, swimming, tap-water rinsing), solution type (hydrogen peroxide vs multipurpose), and how long the problem has been developing.

Ask about prior herpes labialis or recurrent unilateral red eye; prior corneal trauma with plant material; recent travel to tropical areas; systemic immunosuppression (diabetes, HIV, biologics, chemotherapy); and any prior prescription of topical steroid drops.

Examination should include visual acuity in each eye (document it), external inspection for periorbital vesicles (herpes zoster V1 distribution), eyelid position, and, where a slit lamp is available, fluorescein staining to identify the ulcer. Pattern recognition cues, per AAO Bacterial Keratitis PPP 2024:

• Branching dendrite with terminal bulbs + reduced corneal sensation = HSV until proved otherwise
• Ring infiltrate + severe pain + contact-lens history = acanthamoeba until proved otherwise
• Feathery infiltrate + satellite lesions + organic trauma = fungal until proved otherwise
• Dense central infiltrate + hypopyon + contact-lens wearer + 24–72 h onset = Pseudomonas until proved otherwise

B. Evidence on management — what ophthalmology does and what GPs need to know

The Constantinou Ophthalmology 2007 RCT established that topical fluoroquinolone monotherapy (moxifloxacin 0.5% or ciprofloxacin 0.3%) is non-inferior to fortified antibiotic combinations for most community-acquired bacterial keratitis, with lower corneal toxicity. The loading protocol is intensive: one drop every five minutes for five doses, then one drop every hour day and night for the first 24–48 hours, tapering over 7–10 days.

For HSV epithelial keratitis, eTG and the Herpetic Eye Disease Study Group (HEDS 1996) support topical ganciclovir 0.15% gel five times daily or aciclovir 3% eye ointment five times daily, with adjunctive oral aciclovir 400 mg five times daily for stromal involvement. The HEDS Acyclovir Prevention Trial (NEJM 1998) showed that long-term oral aciclovir 400 mg twice daily reduces recurrence by approximately 45%, offered after the second recurrence or following any stromal episode.

For filamentous fungal keratitis, the MUTT I NEJM 2013 trial showed natamycin 5% is superior to voriconazole for Fusarium. Natamycin is only available via TGA Special Access Scheme in Australia — compounded by specialist pharmacy. Acanthamoeba is treated with polyhexamethylene biguanide (PHMB) 0.02% plus propamidine 0.1% (Brolene), both compounded; treatment continues for months under tertiary care.

C. The GP action plan — identify, cease, refer

The GP role in keratitis is clearly defined by RANZCO and eTG:

1. Identify — take the history, examine the eye (slit lamp + fluorescein where available), document visual acuity.
2. Cease contact-lens wear immediately — both eyes; the patient brings all lenses, cases, and solutions to the ophthalmology appointment for culture.
3. Same-day ophthalmology referral for every suspected microbial keratitis — no exceptions; after hours, direct to the emergency department.
4. Do not prescribe topical corticosteroid without ophthalmology consultation — this is the single most dangerous error in keratitis management.
5. Do not start antibiotic drops before the ophthalmologist scrapes the ulcer unless transfer will be delayed more than 2–4 hours and the specialist advises.

If the patient’s contact lens is still in the eye, advise them not to reinsert it. Discard the current lens, case, and solution after culture. Future lens hygiene counselling is essential — no showering, swimming, or sleeping in lenses; daily-disposable lenses carry substantially lower infection risk.

D. Australian operations

MBS items: Standard GP consultations 23/36/44 apply. Specialist attendance: 104/105. Flexible nasendoscopy / videolaryngoscopy for anterior segment photography is specialist-billed. Corneal scrapings and culture are performed under hospital pathology at specialist initiation. GP-initiated pathology can include HSV/VZV PCR (general serology items) and HbA1c 66551 or HIV serology 69384 for risk stratification.

PBS: Ciprofloxacin 0.3% (Ciloxan) and moxifloxacin 0.5% (Vigamox) are PBS Authority Required for bacterial keratitis. Aciclovir 3% eye ointment is General Schedule. Ganciclovir 0.15% gel (Virgan) is PBS Authority Required. Natamycin 5% is TGA SAS only — not on PBS; specialist-compounded. Oral aciclovir and valaciclovir are General Schedule for systemic antiviral use. PHMB + propamidine (acanthamoeba) is compounded, not PBS listed.

Shared care after discharge: GPs support adherence with intensive drop schedules, address mental health impact (vision-threatening illness is highly distressing), and review discharge letters for ongoing management. Post-keratitis corneal scarring with visual impairment may require referral to Vision Australia or NDIS for functional assessment.

Mandatory reporting / public health: Clusters of acanthamoeba keratitis tied to a specific lens solution or hygiene practice should be reported to the TGA via the TGA Adverse Event Reporting portal, which may trigger product recall investigations, as occurred in the 2021 Australian outbreak documented by Lim et al..

E. Special populations

Tropical Queensland / Top End: Fungal keratitis risk is higher due to climate and agriculture. Any corneal trauma with organic matter in these regions warrants explicit fungal workup by the ophthalmologist, with early natamycin empirical cover pending culture.

Immunocompromised patients: Diabetes, HIV, organ transplant, biologic therapy (anti-TNF, JAK inhibitors), and high-dose corticosteroids all increase susceptibility to atypical organisms — CMV endotheliitis, filamentous fungi, unusual bacteria. Lower threshold for same-day referral; ensure immunosuppression review at follow-up.

Children: Keratitis in children is uncommon but can occur with contact-lens wear (adolescents), neonatal HSV exposure, or corneal abrasion leading to secondary infection. Viral conjunctivitis does not produce a stromal infiltrate; any infiltrate in a child requires urgent ophthalmology.

Older adults: Neurotrophic keratopathy from prior HSV, herpes zoster ophthalmicus, or trigeminal nerve damage creates persistent epithelial defects that secondarily infect. Reduced corneal sensation in diabetic neuropathy can mask pain, delaying presentation. Annual ocular review under a GP Health Assessment is appropriate for high-risk older patients.

ATSI populations in remote areas: RFDS referral pathways should be activated early. Telemedicine for ophthalmology is available through RANZCO and some state health services, but physical corneal scraping and intensive drop regimens require in-person specialist care. MBS item 715 (ATSI Health Assessment) includes an ocular review component.

When to escalate

• Same-day ophthalmology / ED: every suspected microbial keratitis without exception — there is no clinical scenario where “watch and wait” is appropriate for a corneal ulcer with infiltrate.
• Urgent re-referral if pain worsens, vision drops further, hypopyon enlarges, or the patient fails to improve after 48 hours of prescribed drops.
• Tertiary corneal unit (Centre for Eye Research Australia, state corneal services): fungal keratitis, acanthamoeba, atypical mycobacterial, refractory bacterial, impending or actual corneal perforation, corneal transplant candidates.

What this article is and is not

This article provides general health information drawn from RANZCO’s Microbial Keratitis guideline, eTG Eye, AMH, and major keratitis trials. It is not personal medical advice and does not create a doctor–patient relationship. Individual management decisions are made with an ophthalmologist following specialist examination and investigation.

For Australian consumer information: HealthDirect — Eye infections, Better Health Channel, Vision Australia.

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Sources cited

1. RANZCO — Microbial Keratitis Clinical Practice Guideline
2. Therapeutic Guidelines (eTG) — Eye: Microbial keratitis
3. Australian Medicines Handbook
4. AAO — Bacterial Keratitis Preferred Practice Pattern 2024
5. Constantinou M et al. — Fluoroquinolones vs fortified antibiotics for bacterial keratitis. Ophthalmology 2007
6. HEDS Group — Aciclovir prevention of recurrent HSV eye disease. NEJM 1998
7. Prajna NV et al. — MUTT I — Natamycin vs voriconazole for fungal keratitis. JAMA Ophthalmol 2013
8. Lim CH et al. — Acanthamoeba keratitis cluster in Australia. PLoS NTD 2021
9. HealthDirect — Eye infections
10. Better Health Channel — Eye infections
11. TGA — Special Access Scheme