Fractal characterisation and computer modelling of herpes simplex virus spread in the human corneal epithelium

Herpes Simplex Virus (HSV) corneal ulcers have variable clinical behaviour. Classically their morphology has been (qualitatively) described as either dendritic or amoeboid. Using two techniques for fractal dimension estimation, a group of HSV corneal ulcers were analysed to determine the complexity of their shapes objectively. It was found that their outlines have a fractal dimension and its value decays with increasing ulcer size. This finding corresponds to the traditional (and qualitative) classification of the ulcers into 'dendritic' and 'amoeboid' types. An algorithmic model (cellular automaton) of viral spread was tested which assumed that variable susceptibility of the epithelial cells to viral infection played a fundamental role in infection dynamics. Varying the ratio of permissive/resistant cells to infection in the simulated epithelium and using specific automaton rules, the simulated ulcers closely resembled the characteristic form and growth of the clinical lesions. The model mimicked certain clinical features of the disease such as self-limitation and spontaneous resolution of most dendritic ulcers, large size amoeboid ulcers and the course of the disease following the misuse of corticosteroid therapy.