Causes and Prevention Measures of Nitrite Poisoning in Beef Cattle

I. Analysis of Causes

The source of nitrite is nitrate, which itself is basically non-toxic, but if it is reduced to nitrite, it becomes toxic. Various cereal crops and vegetables in feed contain some nitrate. Among them, various wild vegetables, forage grasses, crop seedlings and straws (especially oat straw), tuber crops such as potato, radish and sugar beet, and leafy vegetables such as spinach, rapeseed and Chinese cabbage are all rich in nitrate. In addition, for the same plant, the nitrate content can change significantly in different regions and different years, mainly determined by the balance among the processes of nitrate formation, absorption, decomposition and utilization within the plant. As long as factors that promote nitrate formation and absorption exist, such as fertile soil and application of large amounts of nitrogen fertilizer; and as long as factors that inhibit nitrate utilization and protein assimilation exist, such as lack of light, insufficient trace elements including iron, copper, manganese and aluminum, and use of herbicides, the nitrate level contained in the plant will increase. In addition, harvesting plants at different stages or treating them by different methods will also lead to different nitrate levels. For example, the nitrate content of crops decreases at the heading stage; after ensiling treatment, the content also decreases somewhat, but silage seepage and water sources adjacent to garbage piles often contain high levels of nitrate. Cattle are prone to nitrite poisoning after eating large amounts of nitrate.

Forage grasses, vegetables and agricultural by-product leftovers left piled for a long time, or simmered over a slow fire and then left stewing in the pot for a long time after gradually cooking, will form a suitable temperature for the reproduction of nitrifying bacteria, and with sufficient time, can cause nitrate to undergo a reduction reaction to generate toxic nitrite. Cattle will suffer poisoning after eating it. In addition, in hotter summer temperatures, especially when reaching 20-40°C, it is favorable for the reproduction of nitrifying bacteria, and poisoning is likely to occur at this time. There is another cause that is easily overlooked, namely gastrointestinal dysfunction in the body. At this time, eating forage grasses, vegetables and other feeds rich in nitrate can also cause large numbers of nitrate-reducing bacteria in the gastrointestinal tract to reproduce, prompting nitrate to undergo a reduction reaction to generate nitrite, thereby causing nitrite poisoning, also called enterogenous nitrite poisoning.

II. Clinical Manifestations and Necropsy Changes

Clinically, the disease can be divided into acute and chronic types. In the former, affected cattle mainly develop methemoglobinemia after exceeding the standard, usually showing onset within 15 min to 3 h. The main symptom is the formation of cyanosis, notably in the skin and mucous membranes, eyelids, lips and tongue, and it is also accompanied by restlessness, abdominal pain, vomiting, diarrhea and increased heart rate. When symptoms are severe, the body temperature of affected cattle is basically normal or slightly decreased, the body extremities are cold, the heart rhythm is abnormal, responses are dull, and convulsions or coma occur, often resulting in death due to respiratory failure. In the latter, affected cattle do not show obvious clinical symptoms, are easily misdiagnosed, and their feed conversion rate and productivity are affected, bringing relatively large economic losses.

Necropsy of dead cattle shows that the blood is soy-sauce colored and cannot coagulate completely, and quickly turns bright red after exposure to air; the oral mucosa is grayish red; the tracheal mucosa and gastrointestinal mucosa show hemorrhage; the myocardium shows hemorrhage; the liver is somewhat enlarged; the lungs show congestion, emphysema and hemorrhage; and the kidneys also show congestion and hemorrhage.

III. Diagnostic Methods

Based on the main clinical symptoms of affected beef cattle such as mucosal cyanosis, brown blood and difficulty breathing, especially the very short and acute course of the disease, as well as the sudden onset and herd nature, and the correlation with the type of feed consumed and improper feed preparation, a preliminary diagnosis can be made, and relevant veterinary staff should be organized immediately for rescue. Methylene blue has a particularly good detoxifying effect on nitrite poisoning, and this treatment result further verifies the accuracy of the preliminary diagnosis.

In order to establish the diagnosis, methemoglobin testing and a simple nitrite test are carried out at the site where the beef cattle became ill. The method for testing methemoglobin is to take a small amount of blood in a small test tube, expose it to air and shake it, and within a very short time it will turn bright red and become reduced hemoglobin; if it remains brown after shaking in air, it is methemoglobin. Simple nitrite test method: take 1 drop of rumen contents from the beef cattle or the liquid from leftover feed and place it on filter paper, add 1-2 drops of 10% benzidine solution, then add 1-2 drops of 10% acetic acid solution. If nitrite is present, the filter paper will turn brown; otherwise the color will not change.

IV. Prevention and Control Measures

1. Drug treatment. After beef cattle suffer nitrate poisoning, make sure they are moved to a quiet environment, and use drugs that raise blood pressure. Methylene blue is a specific antidote for treating this disease. Affected cattle are given 20 mg/kg by body weight, prepared as a 1% solution for intravenous injection. Affected cattle can also be given 5 mg of toluidine blue per kilogram of body weight and 10-100 mL of 25% vitamin C. If the cattle farm has neither methylene blue nor toluidine blue, affected cattle can immediately be drenched with 2000-3000 mL of 0.05%-0.1% potassium permanganate solution, combined with intravenous injection of sodium thiosulfate as well as large doses of vitamin C and hypertonic glucose. If affected cattle have severe difficulty breathing, a mixed solution composed of 10% glucose and 3% hydrogen peroxide can be given by intravenous injection. After the severe poisoning symptoms of affected cattle disappear, an appropriate amount of a laxative (such as sunflower oil) or a mucosal protectant (such as bran decoction, linseed and starch paste) can be given orally.

2. Strengthen feeding management. This disease is mainly prevented, and the fundamental measure is to strengthen feeding management. Establish a complete feed hygiene system, and use simple methods to check the nitrite content in feed to ensure safety. In daily management, cattle herds must not be fed moldy or spoiled feed; when feeding green fodder, the amount fed must be strictly controlled, and sufficient sugar-containing feed should be supplied together, while adding appropriate amounts of iodized salt, vitamin A and vitamin D; leftover green forage after cattle eat should be spread out for storage to prevent fermentation from piling up. Chemical fertilizers must be strictly used on pastures to avoid contaminating drinking water and feed, and cattle must not be grazed there during fertilization, so as to prevent them from accidentally eating fertilizer or contaminated drinking water and feed. Workers should be careful to avoid using nitrate fertilizer, nitrite and drugs as feed salt. For lactating cows and calves, an appropriate amount of 1% methylene blue solution can be given before suckling, or 30-10 mg of tetracycline can be added to each kilogram of feed, which can effectively prevent this disease.

The above is an introduction to nitrite poisoning in beef cattle and the diagnostic and preventive measures, in the hope of providing some reference value for farmers.