Isolation of a vahlkampfiid amoeba from a contact lens: Tetramitus ovis (Schmidt, 1913) n. comb.

Abstract

A vahlkampfiid amoeba has been isolated from a contact lens. Based on 5.8S rDNA sequence analysis the strain belongs to the genus Tetramitus. The internal transcribed spacer (ITS1 and ITS2) sequences differ by 3 and 9 bp, respectively, from T. lobospinosus. The morphology of the cyst does not correspond to T. lobospinosus, but is identical to that published for Vahlkampfia ovis at the beginning of the last century. There is no reference strain of V. ovis to investigate using molecular techniques. Therefore, we propose that the strain under study should be considered to represent the neotype strain of V. ovis, now classified as T. ovis (Schmidt, 1913) n. comb.

Introduction

During recent years there have been several investigations of human keratitis cases in which vahlkampfiids have been isolated (Aitken et al. 1996; Alexandrakis et al. 1998; Dua et al. 1998). In none of these keratitis cases was there sufficient evidence to conclude that these vahlkampfiids actually caused the infection (De Jonckheere 2003). Walochnik et al. (2000) also concluded that three clinical vahlkampfiid isolates in Austria had no clinical relevance to keratitis. Although a Vahlkampfia strain and a Hartmannella strain showed similar cytopathogenicity to keratocytes as an Acanthamoeba strain (Kinnear 2003), this is still no proof that any species of the former two genera can cause keratitis.

In most cases these vahlkampfiid strains were not identified to the level of genus or species, even in the case of the isolate that was used in the cytopathogenicity tests of Kinnear (2003). This is because identifying amoebae is notoriously difficult and time consuming. Furthermore, a few years ago it was shown that genus identification based on morphology is unreliable in the family Vahlkampfiidae (Brown and De Jonckheere 1999), and rearrangements in genus assignments were proposed based on small subunit ribosomal DNA (SSU rDNA) sequences.

We have demonstrated that the ribosomal internal transcribed spacer (ITS) region (including the 5.8S rDNA) can also be used to differentiate between vahlkampfiid species (De Jonckheere 1998; De Jonckheere and Brown 2005) and that phylogenetic trees derived from 5.8S and SSU rDNA sequence data have very similar topology. The ITS region has the advantage that it is much shorter than the SSU rDNA (<0.6 kbp in vahlkampfiid amoebae) and thus easier and quicker to obtain. In addition the ITS is more variable and, therefore, allows for more precise differentiation of closely related strains.

This paper presents the description of a vahlkampfiid strain isolated during an investigation of amoebic keratitis in Belgium (De Jonckheere 2003). The strain was not involved in the cornea infection but was isolated from the contact lens of the patient. The 5.8S rDNA sequence showed the strain belongs to the genus Tetramitus, and the morphological features of the cysts seem to correspond to Vahlkampfia ovis (Schmidt 1913).