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LEPTOSPIROSIS DEFINITION Infection with Leptospira spp. can cause clinical signs which include fever, jaundice, anaemia, kidney infection and abortion. The majority of infections are subclinical. It is also a ZOONOSIS, and can infect man. INCIDENCE Infection is widespread in cattle in Britain. Serological evidence suggests that at least 70% of British cattle have been infected with one or more serotypes of Leptospira spp. Death from leptospirosis is rare in cattle in Britain. The occurrence of haemolytic anaemia due to Leptospira spp. is unknown but is probably rare. CLINICAL SIGNS Calves less than two months old, meningitis is the main clinical syndrome. Affected animals suffer severe depression with a greatly reduced appetite. There is marked fever. They exhibit neurological signs such as, muscle tremors and paddling when in lateral recumbency. Occasionally, they will exhibit extensor rigidity. Some calves may be blind. In slightly older calves, around six months of age, the clinical signs are due to a septicaemia with anorexia, dullness and fever. Bleeding of the mucous membranes occurs and there is also an anaemia, with pallor and jaundice and blood in urine. Infection of the adult milking cow in early lactation results in an acute syndrome, the milk drop syndrome in which there is a sudden drop in milk yield with a fever. Surprisingly, affected cows often continue to eat normally with no obvious abnormality in demeanour. The small amount of milk which is produced has an orange-yellow colour with a thick, sticky consistency which resembles colostrum. All four quarters are affected, but the udder itself is soft and flabby with no evidence of inflammation or hardness. The milk yield slowly returns to normal, with or without treatment. Very occasionally abortion will occur at this stage.With chronic infection, following localisation in the kidneys and uterus, abortion occurs about 6-12 weeks after infection. Abortion can occur at any stage of gestation between four and nine months. Abortion tends to occur in the younger cows during their second or third pregnancy but also occur in the older cows. The incidence of abortion on individual farms, but in endemically infected herds an annual abortion rate of between 4-7% due to leptospirosis can occur. Calves born alive in late gestation may be very weak, lethargic and even non viable. It is also likely that the foetal membranes will be retained. EPIDEMIOLOGY Transmission occurs when the urine, aborted foetuses, or uterine discharges of an infected animal containing viable leptospirae reach a susceptible animal. Venereal and congenital transmission occurs but is less important. The source of infected urine may be a rodent, a dog or a cow. Cattle remain carriers of L.hardjo for up to 18 months. The organisms remain viable in damp soil or free surface water for periods of up to 6 months. The most common method of transmission is in splashes of freshly-voided urine from other cattle. L.hardjo infection may be enzootic in a herd and transmission may occur between carrier and susceptible animals, particularly in yarded cattle and in cool damp conditions in late summer and autumn. Rodents may form a reservoir of infection. Clinical signs such as abortion storms and fever or mastitis occur when a new serotype is introduced to a herd (by the purchase of a carrier cow, for instance) or when susceptible animals (e.g. in calf heifers) are introduced to an infected environment (e.g. a carrier herd or pasture infested with infected rodents). Serum antibody levels are used to determine the extent of leptospiral infection. In one survey in Scotland 54% of 3,000 cows were found to have serum antibody to L.hardjo. DIAGNOSIS Leptospirosis may be suspected as a result of the clinical signs or post-mortem findings but as the clinical signs (other than the mastitis which is fairly characteristic) are non-specific, laboratory findings are required to confirm a diagnosis of leptospirosis. TREATMENT A number of antibiotics can be used to treat leptospiral infections. The elimination of the organism from the kidneys can be achieved using 3-5 daily injections of streptomycin 10 mg/kg or 25 mg/kg in a single injection. Tetracyclines and ampicillin may also be satisfactory. Penicillin will prevent death from septicaemia but will not eliminate leptospira. CONTROL Carrier animals should be eliminated by treatment or blood testing and removal. The problem of the reservoir of infection in wild life means that, with many serotypes, the source of infection cannot be removed although rat infestations in buildings can be controlled. Vaccination of herds where leptospirosis has been identified with vaccines containing inactivated cultures of L.hardjo is now widely practiced in Britain. Vaccination does not eliminate renal or uterine carriage and should take place before first exposure to infection. In open herds, the treatment of incoming animals for possible leptospirosis may be considered worthwhile to prevent the introduction of new serotypes. Hygiene, the provision of clean water supplies and the fencing-off of stagnant surface water may also reduce the transmission of the disease. |
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