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Strategies for Avoiding Health Problems of Farmed Animals

 

 

Prof. Martin Wierup,
Head of Swedish Animal Health Service,
12186 Johanneshov, Sweden,
www.svdhv.org

Introduction

Sick animals should be given the best possible treatment in order to prevent suffering, death and economic losses. However, in the animal production it is too late to undertake actions first when clinical signs of disease have developed. Therefore, a health control should be established that continuously focus on disease prevention in order to avoid health problems. This summary describes elements involved in the prevention and control of infectious diseases and the strategy for their use.

Diagnose

All actions for the control of infectious diseases should be based on a proper diagnose. Access to qualified diagnostic laboratory is therefore essential. As disease preventing health control is a specialised work deep insight in the epidemiology, pathogenesis and possible ways to control occurring diseases is also needed.

Recording of disease occurrence

Control and prevention of animal diseases also require insight in disease occurrence. Suitable ways of recording occurrence of diseases should therefore be established. Especially for the health control at herd level, many valuable disease- recording systems can be based on observational results like the recording of lesions at slaughter or production data such as weight gain, pregnancy and farrowing rate. These data are often of non- etiological type but are good tools for further evaluation of possible involvement of specific infectious diseases.

Strategies for control

The causative agent to be controlled should be focused .The following model for factors influencing the occurence of infections can be used as a base for an analysis of the concepts used for prevention of infectious diseases :

  MICROBIAL EXPOSURE/ INFECTIVE DOSE ------HOST DEFENCE / IMMUNITY

MICROBIAL EXPOSURE
Prevention of infections can simply be done by the exclusion of the target animal from exposure to infectious doses of the pathogenic microbe. This can be done by:
1. Total exclusion
The most extreme and safest way to prevent an infectious disease is by eradication of the pathogenic microbe. The concept is to reach a status where exposure of the microbe can be totally excluded. An eradication procedure needs the access to an organisational capacity and technical resources necessary to stop the spread of the infection and to combat its source. This method is best exemplified by the control of the epizootic diseases (separately discussed at this work shop). From scientific and economic points of view it is also possible and justified to control also other diseases with the aim of eradication. Examples of such diseases are Aujeszky´s disease in swine and IBR in cattle and also diseases with a more complicated epidemiology like BVDV in cattle. In addition today we have the knowledge and it is economically justified to maintain an already disease- free status in an area or a herd also for other than the epizootic diseases. This can be done by preventing the introduction e.g. by control of imported animals or animal products.

If an eradication status cannot be achived on a country basis it can be achieved on a herd basis. As part of an organized health control scheme or quality control programme, individual and specified types of herds can reach and maintain a disease- free status for certain diseases. This is usually also officially regulated for herds of special importance like boar - and bull stations as well as for hatcheries.The SPF production in swine is a special form of production where total freedom from certain infections exists.

Measures aiming at controlling and eradicating diseases are in conflict with the idea of free international trade as being continuously discussed within the EU and WTO. If these problems are not solved, the free international trade tends to result in a situation where ambitions to improve animal health are discouraged and instead we may end up in a situation where the lowest disease status of a participating country will be considered as the standard.

2. Partial exclusion
The methods used for total prevention of microbial exposure are needed also for actions aiming at decreasing the intensity of animal exposure to pathogenic microbes. In the latter case, additional health supporting actions are also needed. All the measures, as exemplified below, needed to control infections by microbes being prevalent in animal production also have to be continuosly implemented and improved in contrast to measures in an eradication programme. The latter can largely be withdrawn when the eradication status has been achieved.
The theoretical concept for the disease preventing method is to obtain a microbial exposure that is below the infective dose, or decreased to such an extent that immunity is induced but no clinical disease occurs in exposed animals. Doing so, a possible further spread of the infectious agent from the primarily exposed animals is limited to such an extent that secondary clinical outbreaks in contact animals are prevented. Some methods empirically and/ or scientifically found to work alongthis concept are:
optimize hygiene, isolation of sick animals, do not introduce sick animals in a herd and the replacement of live breeding animals by semen and embryos.

ANTIBIOTICS
Antibimicrobials can decrease or eliminate pathogens, so far primarily only bacteria and parasites. Due to the emergence of antibiotic- resistant strains control of bacterial infections can not uncritically be based only upon the use of antibiotics. The use of antibiotics in animals is also a real and potential threat to human health. The implementation of disease prevention measures in animal production as a means to decrease the need for the use of antimicrobials is therefore strongly recommended. This recommendation is further supported because of the limited acess or possible ban to use antibiotics for growth promoting purposes. The use of antibiotics should thus be considered as an integrated and usually the last part of the other disease- preventive methods, and should be used until the others have failed, and not as a replacement for them.

HOST RESISTANCE
An optimal physiological general resistance of healthy animals should be full filled by correct feeding housing and other environmental factors. In addition natural defence mechanisms are enhanced by the use of vaccination. Immunodeficient diseases like Bovine Virus Diarrhoea, Bovine leucosis in bovines as Aujeszky´s disease in pigs decrease the resistance. Control and erradication of such diseases thus have a significant health supporting effects beyond the direct losses due the absence of clinical disease caused by those diseases.

COMBINATIONS
The production systems of animal husbandry usually combine several of the above principles in order to optimize the disease- preventing effect. Examples are the all- in all- out system is to prevent the spread of infections between consecutive groups of animals raised in the same unit. In contrast to the continuous production the all in all out system facilitates the cleaning and disinfection between batches, which is a further step to minimize the spread of pathogens from older growing animals to new ones.
A further development of this concept is age- segregated batch production In the multi- site production of swine the concepts of age- segregated production and all- in all- out system are combined, and when the piglets being weaned at 2-3 weeks, segregated early weaning (SEW), also with the use of the protection given by the maternal immunity. As movement of pigs (and most likely also of other animal species) has a negative impact on growth rate the FTS (farrow- to finish) - production offers a stress reduced alternative system to multi- site production. In the FTS- system the pigs are placed in the same pen from birth to slaughter.

The significance of the measures exemplified above can be better understood when considering that most infectious diseases caused by one specific microbe usually primarily have a multifactorial course. All factors thus decreasing the risk of an infection becoming established will contribute to an improved health situation in individual animal and on a herd basis.

Implementation

The intelligent use of the wide panorama of available disease preventive measures, as exemplified above, easily contributes to a good health situation and improved economy of animal production. The implementation on both herd and national level needs close co-operation between producer, stakeholders, vets and responsible authorities and sometimes also consumers. Economic incitements are usually a driving force. Ideally a disease preventing health control should be driven from the producer. At herd level basic hygienic routines should first be implemented and an initial focus on prevention of one disease complex usually has a positive impact also on the general health situation and leads to increased ambitions. Disease preventing health controls is a continuously ongoing process with the object to maintain and improve animal health.

Reference

1. EU/SSC ( European Comission / Scientific Steering Committee,DG XXIV) 1999Opinion of the Scientific Steering Committee on Antimicrobial Resistance, 28 May, 1999

2. Wierup M.: The control of microbial disease in animals: alternatives to the use of antibiotics. International J. of Antimicrobial Agents , 2000, 14, 315- 319

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