Transformación de la práctica docente

The production (growth and reproduction) increase through vaccination in deer shown from this study suggested a likely economical effect on most farms over a period of time. It is estimated that about 67% of deer farms could achieve economic response (Wilson et al. 2009) in any one year, and since the status of farms is not static, it is likely that almost all farms will experience infection and therefore risk negative effects of leptospirosis on a long-term basis. In the absence of public subsidies for the control of human leptospirosis, vaccination against leptospirosis in livestock for the purpose of protection of people needs to be economically attractive for farmers to motivate the investment.

There are three commercial vaccines labelled for use in deer in New Zealand, namely Leptavoid-2, Leptavoid-3 (Intervet/Schering-Plough Animal Health Limited, NZ) and Leptoshield (Pfizer Animal Health Limited, NZ). However, only Leptavoid-2 and Leptavoid-3 have been tested for vaccine efficacy on farmed deer. While there is no evidence that other vaccines are not efficacious it should not be assumed that all vaccines will have the same effect until supported by scientifically sound evidence (Wilson et al. 2009).

It is likely that continued whole herd vaccination would reduce the incidence of disease and infection, as shown in dairy cattle. Evidence from a longitudinal study of cattle in Luing Island, Scotland demonstrated that when whole herd immunity was sustained by vaccination for at least five years, Hardjobovis infection could be eliminated (Little et al. 1992). However, elimination could be challenged by potential sources of re-infection such as introduction of infected and shedding stock, and contaminated waterways from neighbouring properties and wildlife vectors. The principles of risk evaluation in relation to animal health decision-making, including vaccination should be considered (Wilson et al. 2009).

Prior to consideration of vaccination, serological testing can determine the leptospiral infection status of a herd. Animals to target for sampling are those 1-2 years of age since the seroprevalence in that group has been shown generally to be higher than in other age classes (Ayanegui-Alcérreca 2006). A minimum of 20 samples should be collected to enable the presence or absence of infection to be evaluated with reasonable confidence as determined in Chapter 4. However, if a herd is believed on

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the basis of this testing to be uninfected it therefore lacks herd-level immunity. It may be that if the organism is introduced, the morbidity and mortality rate may be higher than in an endemically infected herd where natural immunity exists. For a vaccination regime, Ayanegui-Alcérreca (2006) recommended that young deer should be sensitised at 3-months old followed by a booster a month later. All classes of hinds should be boosted one month prior to calving to enhance maternal derived antibody. Stags should be boosted annually at a convenient time that fits normal management practice. While this proposed programme is based on a range of epidemiological and vaccine studies, it is recommended that it should be tested in longitudinal studies on a number of deer farms for its effectiveness.

Maintaining farmed deer in a closed herd would reduce the risk of introducing infection. A quarantine program is an option when introducing new stock to the herd in order to minimise potential transmission. This applies particularly to amalgamation of young weaner deer from different sources when they are most susceptible. Wilson and McGhee (1993) noted that transport and amalgamation of deer from several sources, particularly of young deer in the autumn would appear to be the most significant risk factor for disease outbreaks in that age group. If replacement deer are introduced to the herd, their serological status could be tested. If seropositive, infection could be reduced by treatment with Streptomycin at 25 mg/kg (Mackintosh 1993) immediately prior to shipping and quarantined when arriving at farms. Deer introduced to the herd should be vaccinated and boosted prior to introduction. Optimum nutrition and animal health (e.g. trace elements and internal parasites) could improve resilience to clinical disease, but would unlikely affect infection per se (Wilson et al. 2009). Transmission risks such as waterways on farm could be fenced out from grazing. Drainage, reducing the risk of surface water contaminated with urine would reduce risk of transmission.

Recent data from 237 deer herds surveyed in 2008-09 revealed that 11 herds (4.6%) had clinical leptospirosis confirmed or suspected in the past three years (Verdugo and Heuer 2009). This figure could well be an under-estimate due to unawareness of the disease by farming and veterinary sectors. The risk of clinical disease depends on management, environment, climate, immuno-competence, and exposure to various sources of infection, thus making it impossible to predict even with reasonable confidence the magnitude of herd, mob or individual animal level losses. A clinical outbreak can be controlled by the use of an antibiotic and vaccination, the former providing short-term effect and the latter providing longer term protection. However, this would rely on early detection since some outbreaks have been explosive, with large losses before detection and diagnosis (Wilson et al. 2009).

Cessation of a vaccination programme after a period of whole herd vaccination will result in an increased in risk of clinical disease unless a total prevention of future exposure can be assured. This is because the unvaccinated generation will be

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immuno-naïve and therefore at higher risk of disease, should they be exposed to the organism, than a naturally infected herd that has developed herd-level immunity by continuous natural challenge, although disease can still occur in this circumstance. There is no information on the efficacy or duration that vaccine protects leptospiral infection and/or disease in farmed deer. However, studies in cattle and dogs (Hancock et al. 1984; Klaasen et al. 2003) suggested that the immunity could persist for one year then an annual booster is needed. Thus, a decision to vaccinate has long-term implications (Wilson et al. 2009).

Since most deer farming properties are mixed-species, vaccination control programmes need to consider all species on-farm. However, the need to vaccinate all species will depend on establishment of whether sheep are maintenance hosts for Hardjobovis or not. For example, if sheep are not maintenance hosts, vaccination of cattle and deer may indirectly reduce or eliminate the risk of infection in sheep. Research into this question needs to be commenced as soon as possible in order to provide evidence for appropriate control measures for leptospirosis on mixed-species farms.