Pathomorphology of Acute Duck Cholera Infection in Meat Ducks

Ducks constitute a major component of the poultry industry throughout the world. In India, the 20^th Livestock Census identified 33.51 million ducks, which was 23.53 million during the 19^th Census (DAHD, 2019). The substantial increase (42.3%) in duck population indicates high demand for duck farming. Like chicken, ducks are reared for eggs and meat. Vigova Super M (Super Meat) is a broiler strain of duck which is a cross of White Pekin and Aylesbury and has its origin in Vietnam. They have high disease resistance, faster growth rate and good feed conversion efficiency. These are ideally suited for the Indian climate and are gaining popularity in India. Information on the prevalence of duck disease especially in meat ducks is very scanty (Bhattacharya, 2005) even though it often tends to bring great economic loss to the farmers.
      
Infectious diseases are considered as one of the most important causes of economic loss in duck farming. Duck plague (Duck Viral Enteritis), Duck Cholera (Pasteurellosis) and Duck Viral Hepatitis are commonly encountered infectious diseases in duck flocks (Panickan et al.,  2021). Duck cholera (FC) is a highly contagious bacterial diseases caused by Pasturella multocida (P. multocida) and remains as constant threat to duck industry because of the prevalence of P. multocida carriers in healthy duck flocks (as high as 63 per cent), mortality (may reach 50 per cent) and low productivity of egg and meat both in domestic and wild water fowl (Baki et al., 1991; Kumar et al.,  2004). Hence it is essential to have a proper surveillance and monitoring of this disease to take appropriate precautionary measures such as vaccination, better health management practices and also other farm related biosecurity procedures to avoid infections. In ducks age susceptibility to P. multocida infection peaks between 4 and 11 weeks whereas older birds as well as meat type ducks are less vulnerable (Hunter and Wobeser, 1980), hence, the occurrence of duck cholera in adult and meat type ducks has been reported rarely. The purpose of the present communication is to document the pathomorphological features of naturally occurring duck cholera in Vigova Super M meat breeder flock.

Vigova Super M meat duck flock of 1000 birds was maintained for breeding in commercial farm at Karur District of Tamil Nadu. The flock age was 25 week old, fed with commercially prepared feed and maintained under intensive rearing system. The birds were recently shifted from grower shed to breeder shed which were located 75 km apart. After a week of shifting, the flock experienced persistent mortality of 5 to 7 birds per day with a cumulative mortality of 18 per cent inspite of antibiotic treatment as per the advice of Veterinarian. Birds were previously vaccinated against duck viral enteritis and Duck viral hepatitis.
 
Necropsy and histopathological examination
 
Dead birds presented for postmortem examination to Poultry disease diagnosis and Surveillance Laboratory, Veterinary College and Research Institute, Namakkal during the month of July 2023. Necropsies were performed as per approved procedure.  After external and internal examination of tissues and organs individually for gross lesions, materials for histopathology were collected from heart, lung, liver and spleen and fixed in 10% neutral buffered formalin. After fixation, samples were embedded in paraffin, sectioned at 5 μm thickness and stained with hematoxylin and eosin for histopathological examination.
 
Isolation of causative agent
 
Impression smears of spleen, liver and lung were collected from dead birds and stained with Leishman stain. Heart blood swab and tissue pieces from lung, liver and spleen collected from dead birds were subjected to bacterial isolation and identification. The heart blood swab and tissue samples were inoculated into Brain Heart Infusion (BHI) broth and incubated at 37°C overnight and the broth culture was streaked onto blood agar and Mac Conkey agar. The colonies suggestive of P. multocida were subjected to biochemical tests for identification. The biochemical tests included IMVIC, sugar fermentation test and catalase and oxidase test
(Quinn et al., 1994).
 

In the present study, mortality of Vigova Super M meat ducks within 24 to 48 hrs after exhibiting clinical signs of dullness, depression, mild conjunctivities and mucus nasal discharge were suggestive of duck cholera infection (Islam et al., 2003). Death of ducks in acute fowl cholera might be caused by endotoxic shock (Park, 1982; Lee et al., 1992). However, Hossain et al., (1999) suggested that the neural degeneration, cardiomyopathy, severe pneumonia, extensive lesions in liver, blood loss due to haemorrhage, dehydration and inanition alone or in combination may have also contributed  to the clinical manifestations and death of birds.  The infection is characterized by rapid onset and high mortality rate, as observed in the present study. Field outbreaks of avian cholera in waterfowl are characterized by acute to peracute mortality and chronic disease has not been a reported feature of epornitics. In the present study cumulative mortality of 18 per cent was concurrence with Das et al., (1991) who recorded 19.5 per cent mortality during an outbreak of duck cholera in an organized farm. Increased mortality in duck flock was noticed after shifting of birds from grower to layer shed which is corroborated with the findings of Christensen and Bisgaard (2000) who reported that the clinical disease is precipitated by stress such as overcrowding, laying and severe climatic change.
       
On necropsy examination, all the dead birds showed similar type of lesions with varied severity. Carcasses were good in body condition with generalized congestion. Heart showed petechial to ecchymotic (Fig 1) and even suffusive haemorrhages (Fig 4) on epicardial and endocardial surfaces. In few birds pericardial sac contained serous fluid. Lungs were edematous, congested with petechial heamorrhages (Fig 4). Tracheal lumen contained mucus exudate with mild congestion. Spleen was enlarged with multiple greyish white foci (Fig 2). Petechia to ecchymotic haemorrhages were observed on abdominal fat and intestinal serosa. The small intestine was found hyperemic and congested with excess mucus exudate, whereas large intestine was devoid of these changes. Liver was enlarged with multiple necrotic foci measuring 0.5 to 1 mm in size (Fig 3 and 4). Ovarian follicles and testicles were congested. Kidneys were enlarged, congested with petechial haemorrhages. Gross lesions observed during the necropsy of birds might be due to vascular damage of capillaries and post capillary venules induced by circulating endotoxin produced by P. multocida alone or in combination with products of inflammatory cells (Srinivasan et al., 2011; Goswami et al., 2022; Kannaki et al., 2022).








       
Leishman’s staining of the liver and spleen impression smears revealed the characteristic bipolar cocco-bacillary organisms suggestive of Pasteurella species. Heart blood swab and tissue samples produced small, glistening, mucoid, dew drop like colonies in nutrient agar plates after overnight incubation at 37º​C. The bacteria were found to be non-hemolytic on blood agar and no growth on McConkey’s agar, Eosine methylene blue agar and Salmonella-shigella agar. Gram staining of the individual colonies revealed Gram negative coccobacillary rods. Biochemically the isolates were positive for indole, nitrate reduction, oxidase and catalase and negative for methyl red and Voges-Proskauer tests which are specific characters of P. multocida. The carbohydrates fermented were glucose, mannose, galactose, fructose, sucrose and manitol and the carbohydrates that were not fermented included rhamnose, cellobiose, raffinose, inulin, erythritol, adonitol, m-inositoland salicin. The results of morphological, cultural and biochemical tests confirmed the causative agent as P.multocida. These findings were in agreement with several other studies (Shivachandra et al., 2005; Ievy et al., 2013; Panna et al., 2015; Borah et al., 2023).
       
Histopathological examination of heart exhibited epicardial haemorrhage and myocardial congestion and hemorrhage with moderate degeneration of myocardial fibers (Fig 5). Mutifocal thrombi of the small vessels were found in the lungs, with homogenous fibrinous exudates in the parabronchial lumens (Fig 6). Liver revealed diffuse vacuolar changes of hepatocytes, multifocal coagulative necrosis with bacteria and heterophilic infiltrations (Fig 7). Congestion and hemorrhages were noticed in the surrounding hepatic sinusoids, which were filled with many erythrocytes and mononuclear cells. Depletion of lymphocytes in lymphoid follicles and moderately increased reticulocytes, necrosis and karyorhexis of leukocytes with accumulation of heterophilic exudate was noticed in red pulp (Fig 8). Focal fibrin deposition was also noticed in red pulp. This could possibly signify that the birds were responding to the inciting bacterial agent which is a common characteristic in the acute form of fowl cholera. The microscopic lesions are in consensus to the findings reported by Kwon and Kang, (2003) in fowl cholera outbreaks in waterfowl.

Pasteurella multocida is an important pathogen of the duck industry and cause acute to chronic infection. In the present study, the cause of persistent mortality in adult breeder meat ducks was confirmed as P. multocida by their staining, morphological and biochemical characteristics. The clinical symptoms in affected birds and pathological changes observed in dead birds were consistent with duck cholera. Further studies on the molecular characteristics of pathogen and various predisposing factors are essential in order to contain the disease.

The authors are highly thankful to Tamil Nadu Veterinary and Animal Sciences University for providing necessary facilities to conduct the research.

All authors declared that there is no conflict of interest.