|
|
|
|
SALMONELLOSIS Infection with a number of serotypes of Salmonella spp. may give rise to a syndrome in which fever, diarrhoea (frequently with dysentery) and deaths occur in both calves and adults but, on occasions, in pregnant cows abortion may be the only overt clinical sign. The disease in cattle is a source of infection for man. AETIOLOGY S.dublin and S.typhimurium are the most common causes of bovine salmonellosis in Britain. Many other serotypes can cause disease. It should be remembered that all salmonellae may become pathogenic under the right conditions. INCIDENCE Salmonellosis is an important disease of cattle in Britain and throughout the world. The cost to the agricultural industry is not known but it has been claimed to be of major importance to calf rearers and is currently the second most commonly diagnosed cause of bovine abortion. CLINICAL SIGNS Salmonellosis can affect cattle of all ages but it is most frequently seen in adult cows and calves. The clinical signs for all serotypes are similar. Adult cattle: Initially, in acute salmonellosis, there is rapid onset dullness and marked fever (40-41.50C) with inappetance and a dramatic drop in the milk yield of affected (dairy) cows. A severe diarrhoea ensues with foetid watery faeces which may contain blood, mucus and shreds or casts of necrotic bowel epithelium. Presumably as a result of this latter feature, abdominal pain is often noticed. Some Calves: Calves of any age can be affected but it most frequently occurs between 2-8 weeks of age. The median age for S.dublin outbreaks is 4 weeks and for S.typhimurium it is 3 weeks.Clinically the affected calves are very dull with marked lethargy. Unlike calves affected by colibacillosis which may continue to consume milk even when recumbent, calves with salmonellosis are usually completely inappetent. There is a marked fever, often as high as 41.50C with a profuse, evil-smelling diarrhoea. There may or may not be dysentery. Affected calves rapidly lose condition, become very weak, unable to stand and emaciated. Death occurs within 2-3 days of the development of clinical signs and the mortality can be as high as 30% and occasionally higher. In non-treated survivors the diarrhoea may persist for up to 2 weeks. Following infection with S.dublin the development of gangrene-like lesions and osteitis of the extremities has been described with sloughing of the ear-edges, the tip of the tail and even the distal extremities of the limbs. PATHOGENESIS Infection usually occurs by the oral route. The source of organism is usually the faeces of an infected animal, whether clinically affected or a faecal carrier, although infected milk, uterine discharges and products of abortion may be important where infection occurs in adult cattle. Following ingestion, the organism may multiply within the gut. The organisms multiply within the gut wall and from there disseminate to the systemic circulation. Multiplication in the liver and blood stream gives rise to a septicaemia. The ability of Salmonella spp. to produce diarrhoea is largely associated with the invasion of the gut wall and the inflammatory reaction. In recovery the organism may become localised in joints, the foetus to give abortion, in the gall bladder or in other organs. Adult animals which have recovered from S.dublin infection may continue to excrete the organism in the faeces for several months (perhaps for life). Those which have recovered from S. typhinurium infection cease excreting organism after 6-14 weeks. Calves which have recovered normally cease to excrete the organism after a few weeks. It is suggested that intercurrent disease such as fascioliasis (liver fluke) may predispose to infection. EPIDEMIOLOGY Host range. S.dublin infection does occur in species other than cattle but is primarily an infection of cattle. S.typhimurium can infect a wide variety of species and infections may originate from any of these including wild life. SOURCES OF INFECTION Although there are many possible ways by which susceptible cattle may acquire salmonella infection, the overwhelming evidence suggests that infection is most often acquired from other cattle which are excreting the organisms. For clinical disease to be established in cattle it would seem that challenge has to be relatively high.1) Infection from other cattle. As stated above this is the most important source of infection with S.dublin and S.typhimurium. Infection may be introduced into a population of cattle either by the purchase of an adult carrier or by mixing, in market or in transit, with clinical or pre-clinical cases. Infection may pass from adults in a herd down to the calves or vice versa. Congenital infection of calves born at full-term does not appear to occur, but those calves born to cows which are faecal excretors almost invariably become infected either at parturition or soon after birth. Intensive husbandry systems, especially loose housing, have facilitated the spread of salmonella infection. 2) Infection from contaminated foodstuffs and water. Although the infection rate of cattle food constituents (e.g. bone, meat and blood meals and milk powders) can be high, surveys of the end-products (cattle and calf cakes or pellets) have shown a low incidence of infection and low counts of organisms.The change to loose-housing has been associated with an increase in the volume of slurry for disposal. S.dublin may survive for up to 30 weeks in winter slurry, but the survival time on grass after being spread is much shorter. Survival in soil cores may be as long as 24 weeks. The recommendation to reduce the risk of spread of infection via slurry is: (1) storage of slurry for a minimum of 4 weeks; (2) pasture should not be grazed until 4 weeks after slurry spreading. There is at least one recorded outbreak of salmonellosis due to S.typhimurium occurring after slurry was spread on fields at a much higher rate than normal. Outbreaks of salmonellosis have been attributed to cattle drinking contaminated water, effluent from knackeries, contamination of fields due to sewage overflow, or sewage sludge. 3) Infection from other domestic animals (and man). Few instances are recorded and often the primary source of infection is unknown. However, it is recorded that on a few occasions, Salmonella typhimurium infection has passed to cattle from chickens, ducks, geese, pigs and humans. 4) Infection from wild animals. Rats may allow a farm infection of S.typhimurium to persist. Only low infection rates in British farm rats have been recorded although in one case infected rat droppings in old hay were found to be the source of infection for beef cattle. Sparrows, starlings and seabirds have also been found to S.typhimurium rarely remain infected for any length of time and excretion of the organism usually only occurs for a few weeks.Some animals which have recovered from S.dublin infection may become intermittent excretors. Animals which merely ingest the organism and excrete it in the faeces without becoming infected are referred to as passive carriers.DIAGNOSIS In calves, time of onset (3-4 weeks of age), the clinical signs of the disease - fever, depression, diarrhoea, sometimes with the presence of blood, the post-mortem findings and a history of recent purchase through markets may suggest a diagnosis of salmonellosis. In adults the clinical signs are also suggestive. Diagnosis must, however, be confirmed by the isolation of the organism.TREATMENT Affected animals should be treated by both the parenteral and oral routes with an antimicrobial drug. This is possible in calves but in adult ruminants many such drugs may disturb rumen flora if given orally. The same drug or class of drug e.g. an aminoglycoside should be given parenterally and by mouth as far as possible. Ampicillin, trimethoprim/ sulphonamide combination, neomycin, spectinomycin, enrofloxacin and sulphonamides are all available in injectable and oral forms. With drugs which can be absorbed from the gut, injections need only be given initially. Compounds available for oral dosing also include furazolidone and may be given as boluses, from oral dosers, or as soluble products for administration in drinking water or milk. The drug of choice may be ampicillin, amoxycillin, enrofloxacin, trimethoprim/ sulphonamide (destroyed in functioning rumens) neomycin or furazolidone in that order. Antibiotic sensitivities of the salmonellae involved should be taken into account if treatment is unsuccessful or in continued outbreaks. Calves do not usually become carriers following treatment but adults may, and disposal of adult cases to a secure rendering plant should be considered. Careful nursing and management of individual calves is of great importance and the animals should be kept under lamps, turned regularly and fed several times daily until they improve. It is important to impress upon the person in charge of infected animals, adults or calves, the possibility of spread occurring to himself and his family and strict hygiene measures must be observed. Prophylactic (oral) use of antibiotics should be discouraged as in most cases low levels of antibiotics have no effect on the excretion pattern of salmonellae. Indeed it has been shown in man that with non-invasive salmonellosis, antibiotics significantly increase the excretion period of the organism in the faeces. CONTROL Vaccines Currently only multicomponent dead vaccines are available in the U.K. e.g. Bovivac (Hoechst). High serum concentrations of absorbed colostral immunoglobulins aid survival and reduce the severity of alimentary disturbance in calves with salmonellosis but, unless specific antibody to Salmonella is present cannot prevent infection. Control measures which should be instituted during an outbreak of salmonellosis
|
|