What is yellow fat disease in pigs? A detailed explanation of causes, diagnosis, and prevention and control methods

1. The concept of yellow fat disease in pigs

Yellow fat disease in pigs is a pigmentation disorder characterized by a yellow appearance of the pig's adipose tissue, commonly known as "yellow fat," and sometimes also called yellow fat disease or nutritional panniculitis. In pork whose subcutaneous fat turns yellow after slaughter, if it is caused by feed it is called "yellow fat pork," and if it is caused by disease it is called "jaundice pork."

2. Causes of yellow fat disease in pigs

There are mainly 2 causes that induce yellow fat disease in pigs: one is pathological, called jaundice, which includes parenchymal jaundice, obstructive jaundice, and hemolytic jaundice. Its cause is that Trypanosoma suis, piroplasms, or Leptospira invade the body, causing massive hemolysis in the body, poisoning and systemic infection, and impaired bile excretion, so that a large amount of bilirubin enters the blood and stains all tissues of the body yellow, resulting in jaundice pork. Here we mainly discuss the causes of yellow fat formed by feed factors.

2.1 The content of unsaturated fatty acids in feed is too high

If the feed consists wholly or partly of fish or its by-products (fish liver oil scraps, with flounder and salmon by-products being the most dangerous), fish meal, silkworm pupa meal and oil residue, oil dregs, rice bran, corn, soybean cake, linseed cake, fly feed, and other high-fat, easily rancid raw materials, when the feeding amount exceeds 20% of the daily ration and the feed has a high content of unsaturated fatty acids or insufficient tocopherol, the body's consumption of vitamin E increases greatly, causing a relative deficiency of vitamin E in the body. Together with the combined effect of insufficient other antioxidants, this leads to the deposition of anti-acid pigments in adipose tissue and causes adipose tissue to form a brown or yellow amorphous unsaturated polymer body, promoting the production of yellow fat.

2.2 The feed contains raw materials with high pigment content

Such as Chinese milk vetch (grass seed), turnip, carrot, and pumpkin, etc. These raw materials have a relatively high content of carotene and phytochrome, and incomplete metabolism in the body causes yellow staining. In addition, if the raw materials sold by the raw material supplier are themselves dyed, for example dyed and adulterated cottonseed meal, citric acid residue, fake DDGS (a soybean meal substitute made from corn husk, urea, and yellow dye), etc., and pigs eat feed made from these raw materials, the dye will deposit on the fat and become yellow fat.

2.3 Drugs that cause yellow fat disease in pigs were added to the feed

Such as sulfonamides and certain colored Chinese herbal medicines. If used for a relatively long time or if slaughter occurs without a sufficiently long withdrawal period, it will cause local or systemic yellowing of the pig carcass fat.

2.4 Feed mildew

When pigs are fed feed infected with Aspergillus flavus, such as corn and peanuts, necropsy after death will reveal that the systemic fat is light yellow. "Mutated" premix will harm the pig's body during the piglet stage, and in addition, mildew in raw materials will cause oil oxidation, and factors such as rising temperature and lengthened feeding period cause some pigs to "be affected" and develop jaundice.

2.5 Unreasonable formula or production process of feed additives

A high-copper formula can cause oxidative rancidity of oils and fats in feed, leading to yellow fat. Feed in which yellow fat occurs almost always uses high copper! In fact, high copper itself does not cause yellow fat; rather, it lies in the catalytic oxidation effect of high copper itself. The use of copper is mainly related to antibiotic-like effects. When the added amount of vitamin E is at a critical state where it may or may not be present, high copper accelerates feed oxidation and increases the requirement for vitamin E, especially under hot and humid conditions. Under general conditions, at 30°C, vitamin E mixed with feed copper sulfate remains for about 3d, with more than half lost; under humid conditions, this loss is faster and more obvious. This is the main reason why feed subjected to conditioning (a processing step of hydrothermal treatment of powdered materials before pellet feed granulation) and granulation is more likely to cause yellow fat.

If the feed production line has poor ventilation (especially the corn crushing system), the large amount of heat and water vapor generated during corn crushing will condense on the surface of the crushed corn, causing peroxidation of unsaturated fatty acids in corn, or a long interval from production to use of the compound feed will cause peroxidation of unsaturated fatty acids in the feed. In seasons of high temperature and high humidity, the unsaturated fatty acids in complete feed are more likely to become rancid, and rancid fat can form yellow fat; in addition, deteriorated starch causes bile leakage, forming yellow fat, which is actually similar to jaundice; during conditioning and granulation, when high temperature and high humidity are encountered, and with the participation of copper, this yellow fat change will be even more rapid.

3. Pathogenesis

The main feature of the lesions of yellow fat disease in pigs is obvious inflammation of adipose tissue, with "waxy" pigment deposited into fat cells. The unsaturated fatty acids in adipose tissue are easily oxidized to form ceroid. According to reports: ceroid is brown or yellow droplets or amorphous bodies of 2-40μm, insoluble in fat solvents, and appears deep reddish in acid-fast staining. It is precisely this acid-fast pigment that is the fundamental cause of yellowing of adipose tissue. Vitamin E, as an antioxidant, can only prevent or delay the auto-oxidation of unsaturated fatty acids and promote fat cells to convert unsaturated fatty acids into stored fat. When both feeding excessive unsaturated fatty acids and vitamin E deficiency exist at the same time, the oxidation of unsaturated fatty acids is enhanced, ceroid deposition in adipose tissue accelerates, and inflammatory reactions occur in adipose tissue, causing fat to turn yellow and form yellow fat pork.

Jaundice, however, is the result of massive hemolytic disease, certain poisonings, and infectious diseases in the animal body, leading to impaired bile excretion, causing a large amount of bilirubin to enter the blood and tissue fluids, and staining all tissues of the body yellow.