How to prevent and treat fowl typhoid? A detailed guide to practical prevention and control measures
Fowl typhoid is a septicemic infectious disease of poultry caused by Salmonella gallinarum, occurring mainly in chickens and turkeys. The course of the disease is acute or chronic. The mortality rate is related to the virulence of the pathogen.
Fowl typhoid is distributed worldwide, and the losses caused by the disease are severe, with mortality rates reaching 10%-50% or higher. Salmonella gallinarum can infect chickens of any age. According to a report by Hall et al., on farms where fowl typhoid occurred, the mortality rates of chickens aged 1-6 months were 25.6%, 13.5%, 24.9%, 19.2%, 13.8% and 2.7% respectively. Eggs laid by infected breeding chickens carry Salmonella gallinarum, so the disease can be transmitted vertically. After culturing 1000 eggs laid by two flocks of naturally infected and artificially infected chickens, Hall et al. found that 50% of the eggs laid by fowl typhoid-positive chickens were contaminated. Of the 906 chicks hatched from eggs laid by these positive chickens, 296 (32.6%) died of the disease within the first 6 months, with the most severe deaths occurring within the first month.
The pathogen
The pathogen of fowl typhoid is Salmonella gallinarum, a short, thick rod-shaped bacterium measuring 1.0-2.0×1.5μm, usually occurring singly and scattered, occasionally in pairs. It is Gram-negative, does not form spores, has no capsule and no flagella.
Salmonella gallinarum readily grows on beef extract agar, beef infusion agar and other nutrient media at pH 7.2. It is aerobic and facultatively anaerobic, growing best at 37℃. This bacterium can grow on selective media such as selenite and tetrathionate broth, and can grow on differential media such as MacConkey, bismuth sulfite, SS, desoxycholate, desoxycholate citrate lactose sucrose and brilliant green agar.
This bacterium forms small, moist, round, blue-gray colonies with entire edges on nutrient agar. In broth it grows to form flocculent sediment.
The biochemical characteristics of Salmonella gallinarum are described under “pullorum disease”.
The pathogen is killed by heating at 60℃ for 10 minutes or by a few minutes of direct sunlight; 0.1% carbolic acid and 1% potassium permanganate can kill it within 3 minutes; a 2% formaldehyde solution can kill it within 1 minute. Under certain conditions the bacterium can survive for a longer time, for example, it can survive for 20 days in water in the dark; in chickens that died of fowl typhoid, highly virulent Salmonella gallinarum can still be isolated from the bone marrow 3 months later.
The O antigens of Salmonella gallinarum are 1, 9, 12, and the antigenicity of its O12 does not vary, which differs from Salmonella pullorum.
Epidemiology
Besides chickens and turkeys, fowl typhoid can also occur in ducks, guinea fowl, peafowl, quail, grouse, pheasants and others. Chickens of all ages can be infected.
Sick chickens and carrier chickens are the main sources of infection. There are many modes of transmission. The keepers' clothing, shoes and hats, transport vehicles, equipment, wild birds, flies and others can all mechanically transmit the pathogen. The digestive tract is the main route of infection.
This disease can also be transmitted vertically. For example, Rao et al. cultured 36 eggs laid by positive chickens, and Salmonella gallinarum was isolated from 13 of them (36%). Hall et al. hatched eggs laid by positive chickens, and of the 906 chicks hatched, 296 (32.6%) died of typhoid within the first 6 months.
Clinical symptoms
The symptoms in chicks and turkey poults are similar to those of pullorum disease. Contaminated breeding eggs can hatch weak chicks and dead chicks. For infection after hatching, the incubation period is 4-5 days, and the signs after onset are the same as those of pullorum disease.
After young chickens or adult chickens develop the disease, the initial signs are listlessness, ruffled feathers, a pale head, shrinkage of the comb, and a sharp drop in feed consumption. Within 2-3 days after infection, the body temperature rises by 1-3℃ and remains elevated until a few hours before death.
When fowl typhoid breaks out in a turkey flock, the signs are increased thirst, poor appetite, listlessness, separation from the flock, and discharge of green or yellow watery droppings. The body temperature rises, reaching 44-45℃.
Mechanism and pathology
In acute fowl typhoid, there is severe hemolytic anemia. This is because the red blood cells are altered by the action of endotoxin and are removed by the reticuloendothelial system.
In the most acute cases, the tissue lesions are not obvious; in cases with a longer course, red swelling of the liver, spleen and kidneys appears, and these lesions are commonly seen in young chickens. In subacute and chronic cases, enlargement of the liver is commonly seen, appearing greenish-brown or bronze-colored. In addition, there are miliary grayish-white lesions in the liver and myocardium, pericarditis, peritonitis caused by rupture of ova; hemorrhage, deformation and color change of ova; and catarrhal enteritis. After chicks are infected, grayish-white necrotic foci are sometimes visible in the lungs, heart and gizzard. The lesions in turkeys are similar to those in chickens.
Prevention and control measures
When fowl typhoid occurs, promptly isolate the sick chickens, bury deeply or incinerate the moribund and dead chickens as well as the excreta of the flock, and give drug treatment to the sick chickens and the whole flock. Sulfonamides are selected for treatment. In addition, the sanitation and disinfection work for breeding eggs, hatching and brooding should be strengthened, and the routine work of feeding management and the cleaning and disinfection of equipment should be strengthened.