Hematological, coprological and tracheoscopy results in pheasants (Phasianus colchicus) experimentally infected with Syngamus trachea

Syngamosis is a severe, pathogenic parasitic infection caused by the nematode Syngamus trachea (Ruff, 1999) and belongs to the Family Syngamidae and the Superfamily Strongylidae. It has been recovered from many gallinaceous and passerine species (Morgan & Clapham, 1934; Campbell, 1935; Goble & Kutz, 1945; Bandelj et al., 2015) with successful experimental transfer between species (Clapham, 1938). Among the parasites that affect pheasants, Syngamus trachea (Montagu, 1811), as cited by Chapin (1925), is one of the most pathogenic and economically significant, which results in high morbidity and mortality. In particular, it can cause significant production losses, poor weight gain, and even mortality in heavily infected birds (Ruff, 1999; Krone et al., 2007; Atkinson et al., 2008). However, very little is known about the parasite’s biological dynamics, transmission, and direct health effects on pheasants (Phasianus colchicus). Particularly, there is a lack of information about the dynamic changes and hematological profile of pheasant blood during parasitic infestation with S. trachea.

Syngamosis is most frequently observed in breeding and rearing establishments that use outdoor pens for breeding pheasants (Akand et al., 2020). The life cycle of the gapeworm S. trachea is peculiar in that transmission from bird to bird may be accomplished either directly (by ingesting embryonated eggs with third-stage larvae (L3)) or infective larvae L3 or indirectly (by ingestion of earthworms containing free or encysted gapeworm larvae they had obtained by feeding on contaminated soil). Artificially reared game birds, especially pheasants, predominantly suffer from gape worms (Fernando et al., 1971), and parasite pressure, influenced by various factors encountered, creates an annual challenge for pheasant keepers to rear newly hatched pheasants in their rearing aviaries successfully. Young pheasants, despite having a full-balanced feed mixture presented, are highly omnivorous and look for invertebrates and insects as a source of natural protein, as they do in their natural habitats. The release of several-week-old (6 – 8 weeks) young, immunologically naive pheasants from indoor aviaries into outdoor pens is associated with massive picking of earthworms with encysted Syngamus trachea L3 larvae in their bodies from the pen soil contaminated with fecal eggs shed by the previous year’s reared pheasants. Breeding aviary soil is constantly contaminated by earthworms, which become prey for young pheasants that feed on invertebrates and insects. Earthworms serve as transport (paratenic) hosts. Larvae have been shown to remain viable for more than 3 years encapsulated in earthworm muscles. However, in natural conditions, it is unclear as to what proportion of earthworms within a release pen are indeed infected, and even what proportion of those infected earthworms are ingested by pheasants (Gethings, 2018). The fact that the eggs hatch inside the earthworms and that the hatched larvae migrate through the intestinal mucosa and encyst in the muscle tissue is noteworthy. If the earthworm were not an important factor, then there would be no further hatching inside a non-avian host. It is possible that, either in the past or in the future, natural selection may favor earthworms as the primary method of transmission. (Gethings, 2018). As a result, the modes of Syngamus trachea eggs can affect the manifestation of clinical symptoms. There is marked pathology associated with infection by S. trachea (Fernando et al., 1971; Nevarez et al., 2002; Atkinson et al., 2008), and mortality rates of affected birds can be as high as 80 % (Wójcik et al., 1999). It is estimated that the average mortality rate of birds infected with S. trachea is approximately 25 % (Andreopoulou et al., 2012). The developmental stages of S. trachea damage the integrity of the intestinal wall, liver, and trachea, which can cause local inflammatory responses that affect hematological parameters in infected birds.

Despite the large number of publications that study hematological and biochemical parameters in poultry, there is limited knowledge about these parameters in feathered game, including the common pheasant. Moreover, the role of paratenic hosts is often underestimated in many studies, and direct methods of infecting pheasants are preferred.

The primary purpose of this study was to investigate the changes in values of hematological parameters in comparison with the intensity of S. trachea infection in two different age groups of ringnecked pheasants experimentally infected, utilizing two different methods: either with inoculated S. trachea eggs or a third-stage S. trachea larvae encysted in the earthworm’s body.

This paper also aimed to highlight the significant role of paratenic hosts in the transmission of syngamosis in naive pheasants in rearing pens, where regular rotations are not possible.

The Ethics Committee of the Institute of Parasitology, Slovak Academy of Sciences, approved the use of animals for this study. All procedures performed in this study were conducted in accordance with the ethical standards of the Ethics Committee, as outlined in the Animal Welfare Act No. 23/2009, and met the requirements of the Ethics Committee of the Institute of Parasitology of the Slovak Academy of Sciences, as established on 17 November 2021. The study was approved on 1 January 2022.

Syngamosis, a disease caused by the parasite Syngamus trachea, poses a significant health issue in poultry farms and wild birds, particularly in areas where the density of susceptible animals is high. The present work presents the impacts of experimental infection in adult (10 months old) and young pheasants (8 weeks old) with 3 and 5 infected earthworms and 5 mL of inoculum containing 200 embryonated eggs of S. trachea, as evaluated through complete hematological, coprological, and tracheoscopic examinations.

When monitoring and assessing hematological indicators, it is essential to consider the disease’s pathogenesis. After the consumption of embryonated infectious eggs or a paratenic host, the infectious L3 larva is released into the intestine of the definitive host, which subsequently migrates through the intestinal mucosa into the blood and is carried to the lungs and then to its definitive site - the trachea (Winward & Russell, 1976). Already, during the penetration of the intestinal wall and the passage of the larva through the bloodstream, an inflammatory reaction of the infested organism can occur, which is also reflected in the hematological status by an increase in the number of leukocytes (Gethings et al., 2015a,b) what was also observed in our study, where all three experimental groups showed an increase in the number of white blood cells, primarily eosinophils. Eosinophilia in the experimental groups increased by 90 – 150 % compared to the control group. Eosinophilia has been described as one of the fundamental changes in the hematological picture during parasitic infections in vertebrates. The principle and mechanism of this phenomenon can be described from several perspectives (Rosenberg et al., 2013). In parasitic infections of animals, a strong Th2-induced immune response occurs, which is accompanied by metabolic and physical interactions between the host and the worm. The effector mechanisms that eliminate the worms from the body vary depending on the habitat and niche of the parasite. Best understood in the intestine, in some cases, goblet cells are key; in others, mast cells or antibodies play an essential role in host defense. Immune mediators can cause crowding, deprive the worm of energy or nutrients, or disrupt its habitat (Huang & Appleton, 2016). In particular, IL-5 plays a central role in inducing eosinophil activation and recruitment to sites of infection, while eotaxin also promotes eosinophilia (Pelaia et al., 2019). This phenomenon is also supported by earlier studies that demonstrated the ability of eosinophils to kill nematode larvae in vitro (Capron et al., 1979). Changes in the hematological picture during helminth infection are also notable, particularly in the red blood cell count, as evidenced by the values of erythrocytes, hemoglobin, and hematocrit (Varga, 1971). The same was also observed in our study. However, changes in the blood count concerning the red blood cell count are not as striking as those in the case of eosinophils, which are related to the organism’s immune response. The very principle of changes concerning the hematocrit, the number of erythrocytes and hemoglobin values is primarily due to the parasitic Syngamus trachea way of life, when blood-sucking by the parasite occurs and also the disruption of the integrity of the tracheal wall and the associated formation of hemorrhagic lesions (Meister et al., 2022). When assessing hematological indicators, it is essential to note that changes in the hemogram in birds and poultry occur even under physiological conditions throughout the year and are related to the season, training, and nutrition. It is also necessary to note the differences between red and white blood cells, which vary depending on the individual’s age. In young individuals, we observe lower values of PCV, Hb, and RBC than in adult individuals of the same species. However, the numbers of polychromatophils and normoblasts were higher in young animals (Hrabčáková et al., 2014).

The pathology of Syngamus trachea also consists of the obstruction of the respiratory tract by adults (Martin et al., 2020). In our study, we determined the number of adults by endoscopic examination. The primary clinical significance of McMaster EPG is its ability to provide a numerical estimate of the number of parasite eggs in a given amount of feces. This allows for a more objective assessment of infection intensity than a simple qualitative fecal flotation test (Bondarenko et al., 2009). Utilizing quantitative methods such as McMaster, it is possible to assess the accuracy and effectiveness of anthelmintic treatment, which is also closely related to the potential for anthelmintic resistance (Tsukahara et al., 2017). On the other hand, in livestock, regular EPG testing helps manage parasite control programs, allowing producers to identify high-shedding individuals or groups of animals and implement targeted treatments or management strategies to reduce the overall parasitic load (McEvoy et al., 2024). Egg production by adult worms also plays a vital role in the epidemiology of the disease. We determined the number of eggs produced by coprological examination of feces by the quantitative McMaster method, where the highest EPG values, the average for the group of 750 and the average number of adults (2.1) present in the trachea, were found in group B (young pheasants).

The EPG values in the group of young pheasants were, on average, 250, with six values being negative. In the group of adults, the endoscopy was negative in 8 individuals, and in two cases, one adult was found to have a tracheal lesion. EPG values of 0 were found in 7 individuals. Groups D (young and adult) were negative tracheoscopically for the presence of S. trachea adults in the trachea of the observed animals, and EPG findings were also 0, except for the adult group, where EPG values of 50 were found in two individuals. Very similar results were obtained in a study focused on the prevalence of syngamosis in European starlings. Where 1.82 adults were found in the tracheae, and the prevalence reached 42 % (Chen & Sukhdeo, 2023). In domestic poultry, they may be found in high numbers, as recent studies have reported the presence of 12 – 48 adults in the trachea of hens (Gurumyen et al., 2020). The spread and intensity of S. trachea infection in pheasant farming is also influenced by climatic conditions. In general, it can be stated that high relative humidity and environmental temperature increase the risk of infection, as they positively affect the development cycle of the parasite. On the contrary, dry periods, without precipitation and with sufficient sunlight, suppress the development of infection, as the parasite eggs produced in the feces dry out (Gethings et al., 2015).

Several compounds have been shown effective against S. trachea under experimental conditions. Flubendazole is the only licensed anthelmintic for use in poultry and game birds. There are some more compounds proven to be effective such as thiabendazole, mebendazole, fenbendazole and cambendazole. Levamisole has proven effective in game birds. Even ivermectin injections may be effective in treating resistant strains but ivermectin will not kill adult gapeworms (Akand et al., 2020).

The above findings confirm that several factors contribute to the epidemiology of Syngamus trachea, including stocking density of susceptible individuals, the presence of paratenic hosts, season, breeding hygiene, general health status, and individual immunity levels (Gethings et al., 2015a,b; Meister et al., 2022).

The results of this study may be helpful as a complementary diagnostic tool in pheasant clinical evaluations. They could contribute to the early detection of gapeworm infestations in pheasant farming, even when the birds are still asymptomatic. The tracheoscopic examination of the trachea may be considered a rapid intravital diagnostic method in cases involving smaller numbers of game birds, as well as at small breeding farms. The use of hematological parameters in relation to parasite burden may also be considered.