Fowl typhoid is caused by Salmonella Gallinarum, a Gram negative bacillus in the family enterobacteriaceae (serogroup D). Chickens are the natural hosts for S. Gallinarum however; the disease can also affect turkeys, ducks, quail, guinea fowl, pheasants and pigeons. Outbreaks have also been described in parrots, sparrows, ostriches etc. This disease is commonly seen in growers and adult birds.
Transmission may be transovarian or horizontal by fecal-oral contamination. Hens may become chronic carriers and pass the disease to their embryos by egg transmission. This type of vertical transmission is not commonly seen in fowl typhoid in contrast to pullorum disease but when occurs it may lead to an increase in dead-in-shell embryos and small, weaklings or dead chicks in the hatching trays are seen. Mortality is increased in stressed or immuno-compromised flocks and may be up to 100%.
Symptoms and lesions: In growing and adult birds, the disease may be inapparent but when appears as clinical infection, the symptoms may include decreased appetite, depression, dehydration, weight loss, ruffled feathers, pale and shrunken combs and diarrhea. There may also be a decrease in egg production or fertility. When chicks are affected it is difficult to differentiate from pullorum disease. The affected bird shows ruffled feathers, reluctance to move, drop in egg production, poor feed intake, watery to mucoid yellowish diarrhea, and purple discolouration of the comb and wattles. Birds that survive may be underweight and poorly feathered, and may not mature into productive adults.
In young birds, the PM lesions include enteritis, dehydration and anemia. The liver may be swollen, friable, bile-stained and may contain white necrotic foci. The spleen is enlarged and mottled and the kidneys may be enlarged. Petechial hemorrhages can sometimes be seen in the fat and musculature surrounding the internal organs and the peritoneum, pericardium and capsule of the liver may contain a fibrinous exudate. In some birds, there are white nodules in the myocardium, lung, gizzard, and sometimes the cecum. Some of these nodules may resemble tumors. The joints may be swollen and contain a viscous creamy fluid. In turkeys, a characteristic sign is the appearance of small, white plaques visible through the wall of the intestine.
Diagnosis: Isolation and identification of the pathogen is essential for the diagnosis. In clinical cases direct plating on Brilliant Green, McConkey and non-selective agar is advisable. Enrichment procedures usually rely on selenite broth followed by plating on selective media. Fowl typhoid can also be diagnosed by serology. Agglutinating antibodies appear 3 to 10 or more days after infection. The rapid whole blood agglutination test has been widely used to identify reactors in the field flocks, but is not reliable in turkeys. Other serologic tests include tube agglutination and enzyme- linked immunosorbent assays (ELISA). Cross-reactions with other species of Salmonella, particularly S. Enteritidis, may occur. Testing for reactors should be repeated at three weeks interval, as single test may not detect all carrier birds.
Prevention and control: Procurement of clean chicks from salmonella-free hatcheries and biosecurity are important aspects of control. As with other salmonellae, recovered birds are resistant to re-infection but may remain carriers. S. Gallinarum is highly host-adapted and is not considered to be a serious public health concern.