key: cord-008147-lyfh0ixi
authors: Hutber, Marcus
title: The use of vaccines to control disease is not a simple matter
date: 2006-09-07
journal: Vet J
DOI: 10.1016/j.tvjl.2006.07.010
sha: 
doc_id: 8147
cord_uid: lyfh0ixi

nan

The use of vaccines to control disease is not a simple matter The paper by Hägglund et al. (2007) , published in this issue of The Veterinary Journal, is interesting for two reasons. Firstly, it presents empirical data, with rationales that are subsequently open to useful debate; secondly, the authors raise some valuable generic questions concerning the effectiveness of vaccination against a variety of infectious diseases.

A recent letter in the Veterinary Record (Leslie, 2006 ) asked why vaccination is available and yet (for some animal diseases) has not been used as an implemented control measure. Infectious diseases that exhibit high morbidity, heightened mortality and high transmission rates, can significantly diminish farming productivity and it has been argued that all modes of control (including vaccination) should be used to abate disease.

Of course, the usefulness of a vaccine is largely dependent upon its efficacy or efficiency. Some vaccines are very effective (such as rinderpest), whereas others may only be partially effective (e.g. foot-and-mouth disease or avian influenza). The efficacy can also be significantly dependent upon age, reproductive status, species, mode of the transmission or aetiology, and, interestingly, the degree of the disease challenge. Disease challenge can, in turn, be affected by climate (temperature and humidity), spatial and geographical barriers between affected and susceptible animals, farm management techniques on infected premises (stocking densities, the number of farm segregations, the spatial extent of segregations, etc.), and the innate immunity of individual animals and species. The use of vaccination is not simplistic.

As the time period from vaccine administration to a protective boost of antibody levels can take several days, culling can be more effective as a control when disease transmission is rapid. An alternative approach is to vaccinate more regularly, but regular, prophylactic vaccination can lead to the development of subclinical disease such that occult transmission occurs between animals without the manifestation of clinical signs. This in turn can lead to endemic disease, where the infection persists and control becomes even more difficult. Within regions of endemic disease, regular vaccination becomes more worthwhile, however, since the disease challenge is persistent, and the likelihood of disease-free status diminished. Conversely, for non-endemic regions, a trading ban can be levied (at least for Notifiable Diseases) until the disease has been eradicated, or sometimes for several months afterwards. Moreover, the period of the ban can be greater when vaccination has been implemented instead of culling control. The administration of vaccines is further complicated by the need to vaccinate concurrently for different diseases strains.

There are also practical difficulties with vaccinal control. Some vaccines remain effective for only weeks or months, so that repeated vaccine administrations could be required during any lengthy epidemics. Regular surveillance of herd immunity levels become an integral requirement for vaccination programmes that deliver partial immunity in order both to monitor the efficacy of the vaccinal protection and to permit concurrent testing for subclinical disease. Whilst surveillance remains practicable for intensively farmed livestock, it becomes less viable for extensively managed animals.

The theoretical benefits of using vaccination remain significant. However, in practical terms, the use of vaccines is complex and they can be less effective for disease control than other measures. For all of the reasons mentioned above, it is important to be aware of research that examines the disease control measures for any given disease. Hägglund et al. (2007) reported a reduced morbidity of bovine respiratory disease (BRD) amongst calves using management techniques without vaccination. The paper further outlines that the breed of cattle may affect disease incidence and differential levels were indicated for Charolais, Simmental, Blonde d'Aquitaine < Hereford, Aberdeen Angus. Concurrent disease was also recognised as a significant factor in BRD, and the respective prevalence of BRD components was found to be parainflueneza virus (PiV) > bovine coronavirus (BCoV) > bovine respiratory syncytial virus (BRSV). These findings should be useful in examining how to better control BRD. 

With an ''all-in-all-out'' management system, Hägglund et al. (2007) reported that free mixing (or unrestricted spatial cohabitation) of livestock did not increase disease incidence but the finding was dependent upon prior quarantine impositions being applied to any incoming livestock, combined with disease screening (for BVD), which can be compared to the livestock movement restrictions that are usually imposed for a highly contagious disease. The article by Hägglund et al. (2006) indirectly highlights both quarantine and the restriction of livestock movements as effective measures for disease control and questions whether such biosecurity could enhance the impact of subsequent vaccinations presumably by maintaining and maximizing a clean susceptible pool. It is a good question.

EpiVet Limited, Chesil House, Shakespeare Road, Boyatt Wood, Hampshire, SO50 4SY, UK E-mail address: Hutber@epivet.fsbusiness.co.uk

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