Vertical migration by the infective larvae of three species of parasitic nematodes: is the behaviour really a response to gravity?

Monitoring environmental conditions on the speed of development and larval migration of gastrointestinal nematodes in Urochloa decumbens in northeastern Brazil

The study aimed to evaluate the speed of development of gastrointestinal nematodes (GIN) eggs to infective larvae (L3) and its migration under effect of meteorological variables: temperature, rainfall, relative humidity, solar radiation to Urochloa (=Brachiaria) decumbens grass pasture during rainy season, from June to September 2019, and dry season, October to March 2020, in Recôncavo baiano region of Bahia state, Brazil. Monthly, fecal samples obtained from goats with recurrent GIN infection were deposited in six plots of one square meter. In +7, +14 and + 21-days post-deposition, lower and upper strata grass (0-15 and > 15 cm, respectively) and remaining feces were collected and submitted to Baermann's technique modified to perform larval count and identification. Meteorological data were obtained from a local weather station database. The log-transformed larval count results were analyzed regarding the collection day effect, stratum effect, comparing the means by Tukey's test (p < 0.05). Multivariate regression analysis and correlation of meteorological variables with larval counts was performed. In the rainy season, the largest proportion of recovered L3 was concentrated in +7-days post-deposition in the months of June and July, both in the remaining feces and herbage samples, while in August development took place more slowly, after +14-days post-deposition. During the dry season, L3 development only occurred after the first collection in January and February. Lower strata had higher proportion of recovery than in upper strata. Negative correlation was found for solar radiation. Greater rainfall in the rainy season compared to dry season was a favorable condition for high contamination of grass samples. However, reduced vertical migration to upper strata may have been influenced by low temperatures during the rainy season. Regarding the genera present in the fecal samples used in the experiment were found Haemonchus sp., Trichostrongylus sp. and Oesophagostomum sp. Thus, it is possible to conclude that in the region of the study, during the rainy season, there is a rapid larval development and the L3 are able to migrate more effectively to the grass. On the other hand, in the dry season, larval development tends to be slower, with less migration to the grass. Solar radiation can be useful for predicting months with the highest risk of infection. Therefore, prophylactic measures should be employed in goat herds during the rainy season whilst natural reduced pasture contamination in the dry season may favor animal maintenance for a longer time.

Pasture rewetting in the context of nature conservation shows no long-term impact on endoparasite infections in sheep and cattle

BACKGROUND

Nature conservation with reduced drainage of pastures has been increasingly promoted in agriculture in recent years. However, moisture on pastures is a crucial factor for the development of free-living stages of many parasite species in ruminants. Hence, for the first time, we conducted a field study between 2015 and 2017 at the German North Sea coast to investigate the long-term effect of pasture rewetting (since 2004) on endoparasite infections in sheep and cattle.

METHODS

We examined faecal samples of 474 sheep and 646 cattle from five farms in spring, summer and autumn each year for the presence of endoparasite infections. Animals were kept on conventionally drained, undrained and rewetted pastures. The association between pasture rewetting and endoparasite infection probability was analysed in generalized linear mixed models and including further potential confounders.

RESULTS

Infection frequencies for gastrointestinal strongyles, Eimeria spp. and Strongyloides papillosus were significantly higher in sheep (62.9%, 31.7% and 16.7%) than in cattle (39.0%, 19.7% and 2.6%). Fasciola hepatica was detected with a frequency of 13.3% in sheep and 9.8% in cattle, while rumen fluke frequency was significantly higher in cattle (12.7%) than in sheep (3.8%). Nematodirus spp., lungworms (protostrongylids, Dictyocaulus viviparus), Moniezia spp., Trichuris spp. and Dicrocoelium dendriticum were identified in less than 7% of samples. Co-infection with more than three endoparasite taxa was present significantly more often in sheep than in cattle. We identified significant positive correlations above 0.2 for excretion intensities between S. papillosus with strongyles, Eimeria spp. and Nematodirus spp. in sheep and between strongyles and Nematodirus spp. in cattle. Pasture rewetting had no long-term effect on endoparasite infections, neither in sheep nor in cattle. Interestingly, F. hepatica infections decreased significantly in sheep and cattle from 2015 (10.9% and 13.9%) to 2017 (1.4% and 2.1%).

CONCLUSIONS

Pasture rewetting for nature conservation did not increase endoparasite infection probability in ruminants in the long term. This finding should be confirmed in ongoing studies aimed at further animal welfare parameters. The rapid decrease in F. hepatica infections over 3 years may suggest climatic impact or competition with rumen flukes in addition to potential anthelmintic treatment after feedback of the results to the farmers.

Predatory activity of nematophagus fungus Duddingtonia flagrans in infective larvae after gastrointestinal transit: biological control in pasture areas and in vitro

Biological control is a strategy to decrease parasitic populations, and the action takes place through natural antagonists in the environment. We studied the predatory activity of the fungus Duddingtonia flagrans in infective larvae (L3) of gastrointestinal nematodes after gastrointestinal transit. Ten heifers were divided into two groups: treated (animals received pellets containing fungus) and control (animals received pellets without fungus). Twelve hours after administration, faeces samples were collected for in vitro efficacy tests. The animals then remained for 7 h in the experimental pasture area. At the end of this period, 20 faecal pads (ten treated and ten control) were selected at random. Pasture, faecal pad and soil collections occurred with an interval of 7 days, totalling four assessments. In vitro activity demonstrated that fungi effectively preyed on L3, achieving a reduction percentage of 88%. In the faecal pad of the pasture area, there was a difference (P < 0.05) between collections 3 and 4 for both groups; in the treated group a reduction of 65% was obtained, while in the control group there was an increase of 217% in the number of L3. The recovery of L3 in the soil and in the pasture was similar in both groups. There was no influence (P = 0.87) of the passage time on the fungus predatory activity. Duddingtonia flagrans demonstrated the ability to survive gastrointestinal transit in the animals, reducing the number of L3 in the faeces, indicating that this biological control has great potential in the control of worm infections.

Effects of vertical position on trematode parasitism in larval anurans

Spatial distributions of animals can affect interactions with their natural enemies, such as parasites, and thus have important implications for host–parasite dynamics. While spatial variation in infection risk has been explored in many systems at the landscape scale, less attention has been paid to spatial structure at smaller scales. Here, we explore a hypothesized relationship between a common spatial variable, vertical position, and risk of parasite infection in a model aquatic system, larval frogs (Rana) and trematode (Digenea) parasites. Vertical position is relevant to this system given evidence that the densities of snail first intermediate hosts, tadpole second intermediate hosts, and trematode infective stages can vary with depth. To test the effects of depth on infection risk of larval frogs by trematodes, we performed two enclosure experiments, one in the laboratory and one in the field, in which larval frogs in cages just below the water surface or near the bottom of the water column were exposed to parasites. Compared with near-surface cages, mean infection load (number of cysts) in tadpoles in near-bottom cages was 83% higher after 48-h exposures in the laboratory and 730% higher after 10-day exposures in the field. Our findings thus indicate that infection risk depends on depth, which may have adaptive significance, as tadpoles have previously been shown to change vertical position in response to parasite presence. These results motivate future work examining vertical variation in infection risk and may have broader implications for host–parasite dynamics and evolution of host and parasite behavior.

Signaling in Parasitic Nematodes: Physicochemical Communication Between Host and Parasite and Endogenous Molecular Transduction Pathways Governing Worm Development and Survival

Signaling or communication between host and parasite may occur over relatively long ranges to enable host finding and acquisition by infective parasitic nematode larvae. Innate behaviors in infective larvae transmitted from the soil that enhance the likelihood of host contact, such as negative geotaxis and hypermotility, are likely mediated by mechanoreception and neuromuscular signaling. Host cues such as vibration of the substratum, elevated temperature, exhaled CO2, and other volatile odorants are perceived by mechanosensory and chemosensory neurons of the amphidial complex. Beyond this, the molecular systems that transduce these external cues within the worm are unknown at this time. Overall, the signal transduction mechanisms that regulate switching between dauer and continuous reproductive development in Caenorhabditis elegans, and doubtless other free-living nematodes, have provided a useful framework for testing hypotheses about how the morphogenesis and development of infective parasitic nematode larvae and the lifespan of adult parasites are regulated. In C. elegans, four major signal transduction pathways, G protein-coupled receptor signaling, insulin/insulin-like growth factor signaling, TGFβ-like signaling and steroid-nuclear hormone receptor signaling govern the switch between dauer and continuous development and regulate adult lifespan. Parasitic nematodes appear to have conserved the functions of G-protein-coupled signaling, insulin-like signaling and steroid-nuclear hormone receptor signaling to regulate larval development before and during the infective process. By contrast, TGFβ-like signaling appears to have been adapted for some other function, perhaps modulation of the host immune response. Of the three signal transduction pathways that appear to regulate development in parasitic nematodes, steroid-nuclear hormone signaling is the most straightforward to manipulate with administered small molecules and may form the basis of new chemotherapeutic strategies. Signaling between parasites and their hosts' immune systems also occurs and serves to modulate these responses to allow chronic infection and down regulate acute inflammatory responses. Knowledge of the precise nature of this signaling may form the basis of immunological interventions to protect against parasitism or related lesions and to alleviate inflammatory diseases of various etiologies.

Environmental factors influencing the transmission of Haemonchus contortus

Infection with the gastrointestinal nematode Haemonchus contortus causes considerable losses in the sheep industry. In this study, we evaluated the effect that climate has on third-stage larvae (L3) of H. contortus in terms of their migration from sheep feces to Brachiaria decumbens grass, as well as their distribution among the forage plants. Fecal samples containing H. contortus L3 was deposited on the soil among the herbage at an initial height of 30 cm. Sample collection began 24h after contamination and was performed on alternate days over 13 days. The L3 were recovered and quantified in three strata (heights) of grass (0-10 cm, 10-20 cm and >20 cm) as well as in the remaining feces and a superficial layer of soil, collected from beneath the feces. In order to obtain results under different environmental conditions, fecal samples containing H. contortus L3 were deposited on pasture in January (summer), in April (autumn), and July (winter). In all of the periods, the L3 were able to migrate from the feces to the herbage. However, rains, accompanied by high relative humidity and high temperatures, apparently favored migration. The highest L3 recovery rate in the pasture was in the summer observation period, which had the highest number of days with measurable precipitation, high relative humidity (>68.2%), and the highest temperatures at the soil level (minimum and maximum means of 19°C and 42°C, respectively). Under those conditions, larvae began to reach the upper stratum of the grass (>20 cm) by 24h after the deposition of fecal matter, the number of larvae having reached that stratum peaking at seven days after deposition. In the autumn observation period, there was no rainfall in the first five days post-contamination. During that period, high numbers of larvae were found in the fecal samples demonstrating that feces can act as a reservoir of larvae in the absence of rain. Except for two days in the summer observation period, when most of the L3 were recovered from the tops of blades of grass, L3 where located predominantly at the base of the herbage. In conclusion, rainfall favors the migration of L3 from feces to herbage. In addition, larval migration up and along blades of grass can occur relatively rapidly when the temperature is high.

Application of in vitro anthelmintic sensitivity assays to canine parasitology: detecting resistance to pyrantel in Ancylostoma caninum

Resistance of the canine hookworm Ancylostoma caninum to anthelmintic therapy with pyrantel is an emerging problem in canine veterinary practice. Detecting anthelmintic resistance in parasites of pets is problematic because traditional resistance-monitoring techniques used with livestock parasites, such as the faecal egg count reduction test, are often impractical for use in small animals. We used two field-collected isolates of A. caninum in an abbreviated critical trial to test their pyrantel resistance status. The strains showed high-level and low-level resistance, with in vivo pyrantel efficacies of 28% and 71%, respectively. We noted a distinct worm density dependence effect on faecal egg count during the critical trial; egg counts in the dogs containing the low-level resistant isolate were 41% higher 6 days after drug treatment, despite the removal of 71% of the adult worms by the drug treatment. We then assessed four candidate in vitro assays for their ability to detect pyrantel resistance in A. caninum larvae, using these two isolates. The assays included a new format termed the larval arrested morphology assay (LAMA), based on observation of the effects of pyrantel on the body shape adopted by infective stage A. caninum larvae in vitro. Our data suggests that three of these assays, the LAMA, the larval motility assay (LMA), and larval feeding inhibition assay (LFIA) show promise with regards to detection of pyrantel resistance in A. caninum, but the complexity of the LFIA would likely limit its suitability for field studies. In vivo pyrantel efficacies of 28% and 71% in the two A. caninum isolates were associated with a 17-fold shift in the in vitro IC(50) values measured using the LAMA. Further testing with isolates of varying degrees of resistance is required to determine which of these assays is suitable as a rapid in vitro laboratory test for pyrantel resistance in A. caninum. The present study also indicates that potential exists for the novel LAMA or the LMA to be of use in detecting pyrantel resistance in the human hookworms, Necator americanus and Ancylostoma duodenale.

Infective larvae of the human hookworms Necator americanus and Ancylostoma duodenale differ in their orientation behaviour when crawling on surfaces

The infective third stage larvae of hookworms infect their hosts by active skin invasion, and they find and recognize their hosts by the behavioural phases of activation, directed crawling, and penetration. Here we analyse the orientation of the infective larvae of the human hookworms Necator americanus and Ancylostoma duodenale when crawling on surfaces. Their behaviour differed from that of the larvae of the dog hookworm Ancylostoma caninum, but the two species also differed from each other. N. americanus crawled towards light, but A. duodenale did not. Both species migrated towards the warm ends of thermal gradients, and this response was more sensitive than in other skin-invading helminths (threshold 0.09 degrees C/cm). However, A. duodenale turned back and accumulated at higher temperatures than N. americanus [turn-back 45.7 (44.5-49.9) vs 41.5 (38.5-43.9) degrees C; accumulation 43.6 (41.6-46.0) vs 39.5 (37.9-43.0) degrees C]. In contrast to other skin-invading helminths, both species showed no chemo-orientation towards skin compounds when crawling on surfaces. This behaviour may reflect adaptations for reaching the skin surface from hairs or adhering material, but the differences in the orientation of the two species could not be attributed to differing transmission strategies.

Vertical migratory behavior of the infective third-stage larvae of Oesophagostomum dentatum

The vertical migratory behavior of third-stage infective larvae (L3i) of Oesophagostomum dentatum was investigated using upright truncated agarose cones and equivalent conical depressions in agarose. Geotactic response varied with the age of the infective larvae. Four-day-old L3i showed no preference for the sloping surfaces of either indented or upright cones, while the 8-day-old L3i showed a positive geotactic reaction, migrating down the sloping surface of the depressions.