Demodex, y Ahora Que?

The aim of the present conference is to sow inquisitiveness in the dermatology community about the possibility that Demodex cornei is not a different species but a stage of Demodex canis.

Three species of Demodex mites have been described in dogs with different morphological characteristics: Demodex canis, Demodex cornei (short-body mite) and Demodex injai (long-body mite). The classification of these species of Demodex was based on morphology, location in the skin, and clinical characteristics. Recently, results obtained by several investigators using PCR for phylogenetic taxonomy, suggest that Demodex cornei is actually a morphological variant of Demodex canis.

Our goal was to observe alive Demodex canis mites up to their status postmortem and record their morphological changes. We hoped to show that at least some mites of Demodex canis, when they die or shortly after death, adopt morphology similar to Demodex cornei.

Introduction

Canine demodicosis is a non-contagious, inflammatory, common parasitic dermatosis, characterized by an overgrowth of the commensal mite Demodex spp., within hair follicles and sebaceous glands. Demodex canis (250–300 μ) was first described in 1859 by Leydig. Firstly, it was believed to be the only species of Demodex in canines until recently two new species (Demodex cornei and Demodex injai) were reported.

Demodex injai, long follicular mite, measures from 334 to 368 μ of length and it was first reported in 1999. It is often associated with pruritus and oily skin in the dorsal trunk of terriers. A short-body Demodex mite, tentatively called Demodex cornei for his presumed location in the stratum corneum, was first reported in 1993. This mite measurement varies from 90 to 164 μ of length and has a rounded opisthosoma at its terminal end (tail). Specific clinical features still have not been associated to Demodex cornei. In all cases reported Demodex cornei mites were always found in association with Demodex canis mites, usually in a much smaller number unless the sample was collected on the surface of the skin or with acetate tape preparation.

Recently, mitochondrial DNA molecular markers were used by three research teams to establish phylogenetic relationships in several canine Demodex mites in an attempt to resolve questions of taxonomy. Results reported by Rojas suggest that Demodex canis, Demodex injai and Demodex cornei are polymorphisms from the same species. Conversely, results reported by Sastre suggest that Demodex cornei is a morphological variant of Demodex canis while Demodex injai and Demodex canis are different species. Another study reported by Milosevic described that Demodex injai is morphologically and genetically different from Demodex canis, supporting the theory that Demodex injai is a separate species.

Studies of our group have identified dogs with Demodex cornei mites but always in conjunction with Demodex canis. The Demodex cornei mites observed were usually dead, ill-defined and more translucent than Demodex canis mites. These observations led one of the authors (GF) to hypothesize that some Demodex canis mites could change their morphology, adopting the appearance of Demodex cornei when they die.

The purpose of this prospective observational study was to document the morphological transformations that occur over time with Demodex canis adult mites.

Results

As expected, the number of living Demodex canis mites decreased over time, although the survival rate of mites was very variable. All mites were found dead within two to four days after sample collection in 17 preparations with cover slips. However, in one preparation, a few mites of Demodex canis were still alive 7 days after sample collection. The most interesting observations were that Demodex cornei mites began to be identified as early as 18 to 24 hours after sample collection and their numbers typically increased over time.

In 14 of the 18 preparation samples, it was possible to individually observe Demodex canis mites in a few hours to several days and their morphological changes were registered. Nine of those 14 Demodex canis mites showed some antemortem or postmortem transformations. The first portion of the body that experienced alteration was the head portion of opisthosoma, which begins to fold and retract. [Figure 2 (a), (b)]

En algunos ácaros, la retracción de la porción craneal del opistosoma forma una estructura como un cinturón, simulando una división entre el podosoma y el opistosoma. En muchos casos, el extremo distal del opistosoma se tornó más romo. No se observaron transformaciones significativas en las regiones del gnatosoma (cabeza) y podosoma (patas). [Figura+ 3 (a), (b)]

Most of these transformations happened antemortem but sometimes continue in the early postmortem stage. The reduction in the length of the opisthosoma and rounding of its distal end generally occur simultaneously, but the reduction in the length of the opisthosoma is more significant.

These morphological changes were not observed in all Demodex canis mites. In fact, a large number of mites (approximately 50–70% of the mites in the preparations with cover slips and 30–50% of mites in preparations without cover slips) retained their normal morphology until death, while others maintained a normal body length, but showed a reduction in the diameter (possibly due to drying).

Discussion

Canine demodicosis is a frequent parasitic skin inflammatory disease associated with an increase in the number of Demodex canis mites. Traditionally, it was thought that canine demodicosis was a disease caused by an excessive proliferation of Demodex canis mites. However, recently, two new species of Demodex mites (Demodex injai and Demodex cornei) have been identified in dogs with demodicosis.

Demodex cornei, short-body mite, was first reported in a dog by Mason. Flakes obtained in strips of tape revealed a short-body Demodex mite and its supposed eggs and profound skin scrapings revealed all the stages of the life cycle of the typical Demodex canis mites. The findings of other authors also support the theory that Demodex cornei would reside in the stratum corneum.

Some studies describing the morphometric data of Demodex cornei reported that the lengths of the mites were quiet variable. Chesney reported a total body length ranging from 90 to 148 μ (average length 122±12 μ) while Sivajothi described a range from 96 to 164 μ (average length 137±37 μ) and Saridomichelakis a range from 145 to 200 μ (average length 165±19 μ). Furthermore, Chen defined Demodex cornei mite as a short-body mite with a rounded tip opisthosoma exhibiting three bilateral lines of abdominal folds. Moreover, Demodex canis was described as a slim and stylish opisthosomal terminal end without bilateral lines of abdominal folds. Chen concluded that the total body length and width are not easily distinguishable between Demodex canis and Demodex cornei, while the relationship of the body and the morphological characteristics of opisthosoma with a rounded end were more useful to identify Demodex cornei. Additionally, Bordeau reported that Demodex mite length varies widely in each dog, increasing continuously from the more superficial strip of tape to the deeper skin scrapings. Indeed, it is possible that variation in body size and morphological characteristics of Demodex cornei reported in the literature may simply reflect the various stages of transformation of Demodex canis mites pre- and post-mortem.

In our study, at first examination, Demodex cornei mites were found in 22% of dogs (5/23) and always in conjunction with Demodex canis. These findings were consistent with Kuznetsova who reported short-body mites in 25% of the dogs (15/58) with Demodex canis. Kuznetsova also reported that short-body mites were found dead, motionless and ghostly with a faint body outline.

Retraction of the cranial part of opisthosoma creates a structure like a belt that was observed in some mites simulating a division between the podosome and the opisthosoma. These observations are in accordance with that previously described by Rejas Lopez. Tamura also describes a segmental plate strip-shaped or belt-shaped. Both reports would describe the retraction in the cranial area of the opisthosoma observed in our study.

Although it was not the objective of our study, it is interesting to mention that in the samples obtained after initiation of the acaricide therapy Demodex cornei mites were identified in a greater number in the 18 dogs that initially did not show Demodex cornei. In some dogs, Demodex cornei became more numerous than Demodex canis. These findings suggest that during the course of a successful acaricide treatment a higher proportion of weakened or killed mites are more likely to become "transformed." If Demodex canis simply represents moribund or dead Demodex cornei, it is possible that these mites are most likely found on the surface of the skin than their alive and healthy counterparts.

In conclusion, our observations support the theory that at least some short-body Demodex mites (currently referred to as Demodex cornei) actually represent a moribund or dead morphological variant of Demodex canis instead of a separate species. Further studies are needed to confirm these findings.

VIN editor: Figures were not provided at the time of publication.