The genus Salmonella comprises nearly 2500 serovars, traditionally based on Kauffman-White scheme in which H (flagelar) and O (somatic) antigens determine the serovar. Recent molecular techniques like multilocus enzyme electrophoresis and DNA-DNA hybridization analysis revealed that the genus can be divided into two species S. Bongori (20 serovars) and S. Enterica (2443 serovars). Serovars of S. Enterica are commonly responsible for causing the disease. Most of the salmonellae species are pathogenic to man and animals, and therefore there is risk of human infection arising out of animal sources and less frequently of animal infection derived from human beings. The presence of organisms in milk or meat derived from infected animals is of great significance in infection of human beings.Salmonella organisms are intestinal pathogens. The organisms are short rods, mostly motile and are gram-negative. They have little resistance to sunlight, heat and drying. Some serovars of the organisms are host specific, e.g. S. Abortus-ovis in sheep, S. Abortusequi in horses and S. Gallinarum and S. Pullorum in fowl. The other serotypes are predominantly of one host but may be found in others. S. Typhimurium is the most common one, found in man, cattle, sheep, pigs, horses, fowl and rodents.
Stress is a pre-disposing factor in most instances of salmonellosis, especially in adult animals, but in young animals, particularly calves, explosive outbreaks occur even without stress. The stress may be due to intercurrent disease, starvation, calving, vaccination or administration of anthelmintics. Sometimes, administration of antibiotics may lead to salmonellosis caused by the antibiotic resistant strains.
Transmission: Transmission takes place through contamination of water or pasture or feed by faeces of carrier or infected animals. Ingestion is the usual mode of entry. Sheep are frequently affected under drought conditions. Carrier animals bring the infection to the farm. Rodents, wild birds, lizards, turtles and snakes are also responsible for the spread of infection at the farm.
Symptoms: Calves 1 to 2 weeks of age usually develop enteric symptoms when infected with S. Typhimurium and septicaemia with S. Dublin. The septicaemic syndrome occurs in young animals 10-14 days old. There is dullness, high fever, in co-ordination of gait and recumbency. The faeces are blood streaked. Death takes places within 1-2 days or may be delayed to 5-7 days. In calves the carpal and tarsal joints become enlarged. Occasionally pneumonia and enteric symptoms occur. In pigs there is, in addition, a tendency to remain standing or wander; nervous symptoms like tremors, paralysis and convulsions may be present.The enteric syndrome is commonly seen in adult animals. There is high fever followed by severe diarrhoea in which there are clots. The temperature becomes normal or subnormal. Death takes place in about 5 days time. More prolonged cases show dehydration; polyarthritis may follow. Abortions may occur in pregnant animals.In pigs, in addition to enteric syndrome, there may be pueumonic syndrome. When the reproductive system is affected as in sheep and horses, abortions result in pregnant animals.
Lesions: In young animals there may be haemorrhages in myocardium and peritoneum, and acute inflammation of the intestine and bladder. Spleen is enlarged, and mesenteric lymph glands are oedematous and haemorrhagic. In pigs there may be discolouration of skin and haemorrhages throughout the body. In adults there is diffuse haemorrhagic enteritis. The spleen is enlarged and congested. The caecum may show zebra markings. In pigs caecum and colon are mainly affected.
Diagnosis: The symptoms and lesions in dead animals are only suggestive of salmonellosis. The final diagnosis depends upon bacteriological examination.
Treatment: Sulphonamides, antibiotics and nitrofurazones are effective against this organism but treated animals remain as carriers.
Control: It is very important to remove all possible sources of infection, carrier animals and wild rodents contaminating food or water supplied. Killed and attenuated vaccines are being used especially against Salmonella Abortus equi in horses with reasonable success. In developed countries, salmonellosis is becoming an increasing menace