## 2 Materials and Methods  
This study was carried out on clinical samples taken from two flocks of 4-week-old Pekin ducks from Polish farms (Western and Central Poland)--one with a recognised SBDS and a second clinically healthy one. The farms belonged to the same owner and had similar environmental conditions and the same feed supplier. Each flock consisted of approximately 6000 birds.  
For this study, the veterinarian cooperating with those farms randomly chose 20 birds from each flock for sampling. The measurements of the beaks (from the anterior edge of the nostril) and metatarasi (from regio tarsi to regio metatarsophalangea) of the chosen live birds were conducted on the farm during flock monitoring. Blood samples were collected from the wing vein into lithium heparin tubes for morphological analyses and into clot-activator tubes for biochemical assays.  
The haematological parameters were assessed in the analytical laboratory of the Department of Immunology, Pathophysiology and Veterinary Prevention. The count of red blood cells (RBCs) and white blood cells (WBCs) was determined manually using a haemocytometer (Burker chamber, Sigma, St. Louis, MO, USA), while Natt and Herrick's solution was used as a diluting fluid. Haematocrit (Ht) was measured in heparinised capillary tubes after centrifugation. The haemoglobin concentration (HGB) was determined using the cyanohaemoglobin method with [PERSON]'s solution. Blood smears stained according to May-Grunwald-Giemsa were used for leukocyte differential counts. The evaluation was performed using an optical microscope Primo Star (Carl Zeiss Microscopy GmbH, Gottingen, Germany). The percentages of particular leukocyte forms were determined by counting 200 subsequently encountered cells and differentiating into lymphocytes, heterophils, eosinophils, basophils, and monocytes. Based on the above, the heterophils-lymphocytes ratio (H/L) was estimated.  
The biochemical parameters were assessed in serum after centrifugation using the ThermoScientific Konelab Prime 30 ISE (Thermo Scientific, Waltham, MA, USA) biochemical analyser in the analytical laboratory of the Department of Internal Diseases of Horses, Dogs and Cats. The activity concentration of aspartate transaminase (AspAT), alanine aminotransferase (AIAT), alkaline phosphatase, gamma-glutamyltransferase (GGT), creatinine, amylase, lipase (DGGR), total protein, albumin, total bilirubin, glucose, total calcium, inorganic phosphorus, sodium, potassium, chlorides, cholesterol, and urine acid was determined.  
Histopathological examination of the tongue, heart, lungs, liver, kidney, ileum, and bursa of [PERSON] was performed. The tissues were fixed in 4% formalin at room temperature for 48 h, embedded in paraffin, and cut into 5 um thick sections. After deparaffinisation, the sections were stained with haematoxylin and eosin (H & E). Pathological changes were observed using an Olympus microscope (Olympus, Tokyo, Japan).  
To confirm that clinically sick ducks were infected by parvovirus, genomic DNA was extracted from ducks' livers using the Genomic Mini Kit (A & A Biotechnology, Gdynia, Poland) according to the manufacturer's instructions. PCR amplification was conducted with primers P2 nF 5'-GTTTAGTTCATTGCTACTC-3' and P2 nR 5'-CTCATTAGTCCAGTTAACG-3' in conditions described previously [4]. The PCR products were visualised on a 1.5% agarose gel. To confirm the accuracy of the PCR results, the representative three products were selected for sequencing (Macrogen, Amsterdam, The Netherlands). The sequences were analysed using Mega X software ver. 10.2.5 and were compared to sequences from the National Center for Biotechnology Information (NCBI) GenBank database.  
Statistical analyses were performed using StatSoft Statistica PL 12.0 Software. Data were expressed as the mean and standard deviation (\(\pm\)SD) or median (dependent on the distribution of variables). All the results obtained were statistically evaluated by Student's \(t\)-test. A \(p\) value < 0.05 was considered significant.