Background
The major risk factors for the entry of Leptospira infection into a cattle herd are river access, sheep grazed with cows (as sheep can act as non-symptomatic carriers), hired bulls and purchased animals (Egan and O’Reilly, 1988; Bennett, 1991; Kamp et al., 1990). In a UK survey, river access was shown to be a significant risk factor (Bennett, 1991).
Control of the disease is often complicated by the fact that farmers do not usually know whether their farms are infected or not. Control measures are usually first undertaken when there has been an outbreak and significant losses have been suffered. At this point, the control measures are directed towards minimising the effects of the infection and usually include combined vaccination and treatment with antibiotics (Ellis, 1999).
In beef herds, both vaccination and non-vaccination policies, combined with the separation of younger age groups of cows from the infected older cows, have been shown to eradicate the disease over a long period of time (i.e. six years). Whilst vaccination and treatment alone did not eradicate the disease, combined treatment and vaccination were more efficient than vaccination alone in preventing persistent infections(Kingscote and Proulx, 1986; Kingscote, 1986). A similar result was produced in a closed beef herd by vaccination and testing over a five-year period (Little et al., 1992).
A Dutch model of leptospirosis control in dairy herds suggests that in open herd situations only vaccination and antibiotic treatment after an outbreak is economically attractive. Studies also show that vaccination in infected herds limits the effects of the infection on fertility and abortion rates (Dhaliwal et al., 1996). In closed herds, however, control by vaccination or by preventing the entrance of infection into the herd by improving biosecurity has been shown to be economically feasible (Bennett, 1991).
Whilst vaccines are considered a valuable tool against leptospirosis, they do not offer full protection against herd infection in herds that are considered free of infection, and there is significant doubt about the length of protection they offer (Ellis, 1999). The use of vaccines is further complicated by the fact that the antibody tests for Leptospira do not differentiate between real infection and antibody reaction to vaccine.
Practical approaches
When decisions are made concerning prevention and control of leptospiral infection in a herd, it is important to find out what the present health status of the herd is. In dairy herds, this can be done simply and cheaply by testing for L. hardjo infection in the bulk tank milk. This test is accurate enough to tell whether the herd has been exposed to the disease and approximately how many animals might be infected (Pritchard, 1999). Individual blood sampling of suspect animals can then be used to clarify the picture. In beef herds, the herd history may give an indication of the herd status. Otherwise, a representative sample of different parity cows need to be blood-sampled and tested for antibodies. Once the herd’s infection status has been investigated, approaches to control and eradication can be decided upon.
When a herd is known to be free of disease, the main aim should be to keep it that way. As the main route of entrance for Leptospira infection into a new herd is via a purchased animal or contaminated water source, these entrance routes should be controlled. Preferably, the herd should be kept genuinely closed, and no purchased animals should be brought in, as testing for individual infected animals is not totally reliable. However, if animals are purchased, all bought-in cattle should be bought from a Leptospira-free, accredited herd, or must be quarantined for 4 weeks before inclusion in the main herd and tested for Leptospira antibodies. The animals should not have access to any surface water that has passed through another farm or contact with animals of a neighbouring farm, unless these are accredited free of leptospirosis. Furthermore, cows should not be grazed together with sheep, unless the health status of the sheep is known and monitored. If these guidelines are carefully adhered to, vaccination against leptospirosis should not be necessary, as the wildlife reservoirs have been shown to be insignificant in the maintenance of infection (Little et al., 1992). With monitoring and the necessary testing, such a herd can be accredited free of leptospirosis (see Premium Cattle Health Scheme and Herdcare).
If herd testing (20% of breeding cows) or bulk tank milk testing shows that the herd is infected with leptospirosis, it is important to assess the level of infection (Murray, 1999). If the herd has a very low level of infection, the options are eradication and monitoring or monitoring alone to ensure that no changes in this low-level status occur. If a higher level of infection is detected, a vaccination programme is likely to be the best option (Ellis, 1999).
Organic herds
In organic herds that have a high level of infection, vaccination combined with the introduction of a closed herd policy and improved biosecurity may be the preferred option, in order to work towards a situation where vaccination can be discontinued (i.e. closed herd with high biosecurity and no mixing with sheep). If the farm is not willing or able to establish good biosecurity, and there are many risk factors present (contact with animals from neighbouring farms, shared watercourses, sheep), continuous vaccination may be considered, particularly if raw milk from the farm is used for food processing or direct sales.
In organic herds where no infection can be demonstrated or a very low level of infection is detected by the bulk milk testing, accreditation for disease free status may be considered, particularly if the farm intends to sell organically certified breeding animals or deals with raw milk for human consumption. As accreditation is costly, an alternative is to monitor the situation continuously with bulk milk sampling to ensure a continued status of no infection or low level residual infection. It is important to note, however, that the only way to ensure the maintenance of an existing status of no infection or low-level infection is to maintain a closed herd policy and high biosecurity as described above, if it is decided that vaccine should not be used.