Do you ever focus on gall bladder while performing a necropsy of a poultry bird? I am quite sure that very few of us would know the importance of gall bladder in poultry. It is a matter of great relevance that in our B.V.Sc curriculum we hardly taught the physiology of birds and especially of commercial chicken. The gall bladder is one such important organ which must be known to a vet. When you perform a postmortem of a poultry bird you may see a greenish yellow organ lying behind the liver . This greenish organ is a sac-like structure and called as the gallbladder. Usually, many of the poultry doctors only knows a single fact associated with a gallbladder that gall bladder helps in fat digestion. After this statement, almost nobody knows its pathophysiological significance in disease diagnosis. Anatomically, gallbladder fits in a shallow groove-like space in the posterior part of the right lobe. When you open up the bird ventrally you will see a liver and other organs as shown in the picture. When you remove liver you will see mainly spleen, gizzard and gall bladder. You can see a greenish structure pointed by an arrow.
What is gallbladder’s significance? We know from litirature that there are many infectious diseases which can infect gallbladder. These diseases include Chicken Infectious Anemia, Salmonellosis, Campylobacteriosis, E.coli infection, Clostridium perfringens, these viruses and microbes bring inflammatory changes in the wall of the gallbladder. These microbes affect the normal functioning of the gall bladder.
The main physiological function of the avian gallbladder is to store and concentrate bile juice secreted by hepatocytes. Bile juice contains bile salts, electrolytes, phospholipids, cholestrol and pigment biliverdin. Here we focus only on biliverdin because due to this pigment gallbladder appearance varies which have some secondary diagnostic value, which we are going to investigate.
Liver and spleen of bird normally work as a scavenger of old and dead RBCs. RBC life in chicken is just 20 to 30 days which is very less as compared to mammals. So, RBCs turnover in chickens is quite high that means erythrocytes ages rapidly and undergoes destruction in the liver and spleen. Total RBC count is 3 million and haemoglobin concentration is also very low that is about 9g/dL, however, metabolic requirements of oxygen is quite high. Due to these factors, erythrocyte turnover is higher.
As erythrocytes age, several metabolic changes occur: the membrane becomes more rigid and fragile, and the discocyte converts to a poorly deformable spherocyte. They start expressing some receptors which give signals to macrophages (kupffer cells) present in spleen and liver to destroy these erythrocytes. Different components of haemoglobin get recycled, like protein of globin comes back to the amino acid pool, iron from heme goes for the formation of new haemoglobin or storage. Heme without iron is porphyrin ring which is destined to remove from the body and this is converted into biliverdin by the heme oxygenase enzyme. In mammals, this biliverdin is further reduced by biliverdin reductase to bilirubin. In birds biliverdin from liver and spleen goes to gall bladder through hepatic and bile ducts. So, RBCs turnover and availability of heme decides the colour intensity of gallbladder.
As I said above that various diseases infect gall bladder among which Clostridium, Salmonella and E.coli are prominent which causes cholangiohepatitis and gallbladder wall’s inflammation and responsible for its discolouration. Some times gall bladder is seen as dark yellow or mustard colour, in such cases bacterial reduction of biliverdin into bilirubin occurs, which is yellow in colour.
These infections also make RBCs very weak and fragile and its destruction increases. Further, intravascular destruction of erythrocytes increases. Intravascular hemolysis follows substantial damage to the RBC membrane. An important feature of this process is the fate of the RBC contents, particularly the heme moiety of haemoglobin (Hb) and its iron. Iron is an essential nutrient for pathogen as well as for host (poultry bird), and access to iron within the body is the focus of an intense evolutionary battle. All bacteria and particularly the E.coli required iron for growth so their proliferation (during bacteremia) increases RBC destruction. Ultimately, sepsis-induced hemolysis makes excess haemoglobin available for destruction in the liver and spleen which produces a high amount of biliverdin and gives more and more intense tinge to the gall bladder. Excess biliverdin in distended gallbladders comes into the intestine and gives frank green colour to the droppings.
Here I discuss pathophysiology of the gallbladder in poultry which should be used just as secondary diagnostic feature in cases of infectious diseases. It requires lots of practice and experience to differentiate between different green and yellow colours seen during different cases. Kindly correlate above information with all other signs and symptoms present with the case while investigating any infectious disease in poultry.