**2. New aspects of pathogenesis**

Pathogenesis of FK has not been fully elucidated. Recent studies and advances have contributed in better understanding of the complicated process and host immune response.

#### **2.1 Risk factors**

The common risk factors for fungal keratitis are trauma with vegetative matter or objects contaminated with soil, contact lenses, ocular surface disease, lacrimal duct occlusion, fungal skin infections, long-term use of antibiotics or steroids locally or systemically [2, 13–17]. Other relatively rare risk factors include history of eye surgery, herpes simplex virus keratitis, eyelid abnormalities, etc. [18, 19].

Still in developing countries, the most common risk factor for fungal keratitis is ocular trauma but in developed countries, contact lens emerged as more common risk factor. This change has been occurred due to industrialization of farming and increase use of contact lens in developed world. In a large case series of 695 cases with fungal keratitis reported from 10 tertiary eye care centres across the United States over a 7-year period, 283 (40.71%) cases involved the use of contact lens [1]. Similarly Keay et al. in a multicentre case series of 733 cases from 11 tertiary care centres across the United States reported that 37% cases were associated with refractive contact lens wear, 25% were associated with ocular trauma, and 29% were associated with ocular surface disease [20].

In a study, the storage of the anti-microbial agent alexidine in its plastic containers at higher than room temperatures was found as the reason for decreased effectiveness [21]. This temperature difference in the plastic containers led to decreased concentration of the agent in solution (2.8 times less) and a corresponding higher concentration in the walls of its plastic containers (3.1 times higher) [22]. The lens type and its soaking time significantly influences the fungicidal activity of cleaning and storage solutions and poor compliance significantly increase the risk of contamination [23, 24].

#### **2.2 Causative fungi**

A review article found about 144 species of fungi from 92 genera as causative agents in keratitis, showing largest diversity; whereas 77 species from 42 genera of bacteria, 12 species from 4 genera of protozoa and only 4 types of viruses were implicated in infectious keratitis. However, in the majority of cases of FK the causative organism belong to a few genera: Fusarium, Aspergillus, and Candida [25]. Other fungi implicated in mycotic keratitis are Curvularia, Alternaria, and Penicillium [2, 13, 14]. The rarely reported fungal pathogens include Lasiodiplodia theobromae, Cylindrocarpon species, Metarhizium anisopliae, Paecilomyces species, and Pythium insidiosum [15, 26–29].

#### **2.3 Host immune response**

Fungal infections initiates with adhesion of fungal cells with epithelial surfaces. Fungi produce various surface proteins to contribute to the adhesion to the corneal

epithelium, which has potential fungal binding sites such as laminins, fibronectins, and collagens [30, 31]. Alterations of the corneal surface due to trauma or other predisposing condition result in easy invasion of organisms deeper into underlying layers, which leads to an innate and adaptive immune-mediated inflammation, resulting in subsequent tissue necrosis of the surrounding area, consequently leads to further tissue damage, scarring, and opacification of the cornea.
