INTRA AND INTERSPECIFIC VARIATION OF DIFFERENT ISOLATES OF HALICEPHALOBUS GINGIVALIS (NEMATODA: PANAGROLAIMIDAE) IN A PHYLOGENETICAL CONTEXT

Halicephalobus gingivalis is a small (235 - 460 µm) free-living bacteriovorous nematode (Panagrolaimidae) which is a known facultative parasite of horses and zebras. Also three human cases have been described, all with a fatal outcome. After an extensive sampling of horse manure heaps and a compost heap in Flanders, 9 different isolates were obtained. Together with the 4 isolates that were already in culture, this resulted in 13 isolates which allowed a thorough combined morphometrical, morphological and molecular species analysis. Cultures were maintained under different food conditions and temperatures to analyze intraspecific morphometrical variation in H. gingivalis. Specimens of different stages were fixed and different parameters were measured and analyzed statistically. These results form the basis of a well thought-out interpretation of interspecific variation and associated species delimitation. In the future, all available data, including scanning electron microscopy (SEM) and molecular data will be combined for all the isolates

Unusual intestinal microvilli in the facultative parasitic nematode Halicephalobus gingivalis (Panagrolaimidae)

Halicephalobus gingivalis (Panagrolaimidae), formerly known as Micronema deletrix, is a free-living, bacteriophagous soil nematode which is also known to be a facultative parasite of horses and zebras. The first report of an infection of a horse with H. gingivalis dates from 1954. Since then, there have been approximately one hundred reports of an infection of which only two cases were not fatal. There are also three infections known in humans, fatal in all cases within 8 to 10 days after the first symptoms of disease. Little is known about the ultra structure of this nematode. Does it have all the characteristics of a free-living nematode or does H. gingivalis have specifications that are typical for parasitic species? We took a closer look at the ultra structure by means of transmission electron microscopy and found branched microvilli in the intestine. Furthermore, H. gingivalis seems to have all the characteristics of a free-living nematode

Different levels of anthelmintic resistance in the facultative parasitic nematode Halicephalobus gingivalis

In recent years many studies have proven that most equine parasites have some form of resistance to all main classes of anthelmintic drugs. However, studies on anthelmintic resistance in equine parasites only include obligatory parasites, not facultative parasites. Halicephalobus gingivalis is a free-living bacteriovorous nematode and a known facultative parasite of horses. In all reported infections the horses were regularly treated with common anthelmintics, strongly indicating the possibility that H. gingivalis has some form of resistance to these anthelmintic drugs. We present the results of an in vitro study on the resistance of several isolates of H. gingivalis, both free-living and parasitic. All were tested in vitro for the extent of their resistance to two different anthelmintic drugs, ivermectine and thiabendazole, using an adaptation of the micro-agar larval development test and focusing on egg hatching and larval development. Main conclusion is that the anthelmintic resistance of H. gingivalis was considerably higher than that of closely related panagrolaimid nematode species, with the parasitic isolates having a higher resistance than the free-living ones

Experimental induction of intraspecific morphometric variability in a single population of Halicephalobus cf. gingivalis may surpass total interspecific variability

Although molecular techniques are revolutionising nematode taxonomy, morphological data still form the basis of nematode species descriptions. However, morphological characters show a natural variability that should be taken into account before describing new species. The current study presents the results of an elaborate morphometric study of Halicephalobus cf. gingivalis, including 15 measurements and 13 indices of 540 specimens, the progeny of a single parthenogenetic female and cultured under different temperature and food conditions and measured in different adult age groups, i.e., young adults with a developed vulva but before the onset of oviposition, adults laying eggs, and old, post-reproductive adults near the end of their life cycle. The morphometric characters were analysed using both univariate (analysis of variance) and multivariate (principal components and canonical discriminant analysis) techniques. The main results reveal that the morphometric characters most used in Halicephalobus identification keys have a huge variability within a single progeny, e.g., body length 1.9 times longer than the shortest or ratio VA/tail length 3.9 times larger than the smallest. This variability has a magnitude that has not been observed in nematodes before. Further, by changing the environmental factors, the morphometric characters are influenced to an extent that one could assign - with seemingly 'statistical support' - different 'species' of the genus to different subpopulations. With this experimental study we provide convincing elements to advocate an integrative taxonomic approach and to discourage the description of new species based only on morphometric differences

Anthelmintic resistance in facultative parasitic nematodes: different levels of resistance in free-living and parasitic isolates of Halicephalobus gingivalis (Panagrolaimidae)

In recent years many studies have proved that most equine parasites have some form of resistance to all main classes of anthelmintic drugs. However, studies on anthelmintic resistance in equine parasites only include obligatory parasites, not facultative parasites. Halicephalobus gingivalis is a free-living bacteriovorous nematode and a known facultative parasite of horses. In all reported infections the horses were regularly treated with common anthelmintics, strongly indicating the possibility that H. gingivalis has some form of resistance to these anthelmintic drugs. We present the results of an in vitro study on the resistance of several isolates of H. gingivalis, both free-living and parasitic. All were tested in vitro for the extent of their resistance to two different anthelmintic drugs, ivermectine and thiabendazole, using an adaptation of the micro-agar larval development test and focusing on egg hatching and larval development. Main conclusion is that the anthelmintic resistance of H. gingivalis was considerably higher than that of closely related panagrolaimid nematode species, with the parasitic isolates having a higher resistance than the free-living ones

Nematode succession during composting and the potential of the nematode community as an indicator of compost maturity

One of the key issues in compost research is to assess when the compost has reached a mature stage. The maturity status of the compost determines the quality of the final soil amendment product. The nematode community occurring in a Controlled Microbial Composting (CMC) process was analyzed with the objective of assessing whether the species composition could be used as a bio-indicator of the compost maturity status. The results obtained here describe the major shifts in species composition that occur during the composting process. Compared to terrestrial ecosystems, nematode succession in compost differs mainly in the absence of K-strategists and numerical importance of diplogastrids. At the beginning of the composting process (thermophilic phase), immediately after the heat peak, the nematode population is primarily built by bacterial feeding enrichment opportunists (cp-1) (Rhabditidae, Panagrolaimidae, Diplogastridae) followed by the bacterial-feeding general opportunists (cp-2) (Cephalobidae) and the fungal-feeding general opportunists (Aphelenchoididae). Thereafter, during the cooling and maturation stage, the bacterial-feeding-predator opportunistic nematodes (Mononchoides sp.) became dominant. Finally, at the most mature stage, the fungal-feeding Anguinidae (mainly Ditylenchus filimus) were most present. Both, the Maturity Index (MI) and the fungivorous/bacterivorous ratio (f/b ratio), increase as the compost becomes more mature (ranging, respectively, from 1 to 1.86 and from 0 to 11.90). Based on these results, both indices are suggested as potential suitable tools to assess compost maturit

Intestine ultrastructure of the facultative parasite Halicephalobus gingivalis (Nematoda: Panagrolaimidae)

Halicephalobus gingivalis, classified as a free-living nematode, is a known facultative parasite of horses, zebras and humans. However, detailed information concerning its internal morphology is lacking, especially in relation to possible adaptations to its lifestyle as a facultative parasite. The research presented here uses TEM and PI staining to analyse the morphology of the intestine of H. gingivalis. Specimens cultured under different conditions were included to determine if differences in ultrastructure are induced by culturing method. TEM analysis revealed that the intestinal tract comprises a single layer of cells in which nine pairs of nuclei can be distinguished. Further, unusual dichotomously and trichotomously branched microvilli were observed next to finger-like cylindrical microvilli, the latter being the most commonly described form in nematodes. Finally, three different types of secretion vesicles, i.e., spherical (type I), thread-like (type II) and enlarged globular (type III), occurred independently from each other along the intestinal tract. The relationship of morphological adaptations of the microvilli to parasitism in nematodes is discussed