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STAKEHOLDERS GOVERNMENT Department of Agriculture, Food and the Marine PROCESSING AND MARKETING ABP Arrabawn Co-op Bord Bia Carbery Group Connacht Gold Dairygold Dawn Meats Glanbia Kepak Group Kerry Agribusiness Lakeland Dairies Slaney Foods Tipperary Co-op Wexford Milk Producers Understanding infectious disease For Irish Farmers, Advisors and Vets FARMERS’ ASSOCIATIONS AND LIVESTOCK MARTS Cork Marts Ltd Irish Cattle and Sheep Farmers’ Association (ICSA) Irish Co-Operative Organisation Society (ICOS) Irish Creamery Milk Suppliers’ Association (ICMSA) Irish Farmers’ Association (IFA) Irish Milk Quality Cooperative Society (IMQCS) Macra na Feirme BREED SOCIETIES Irish Charolais Cattle Society Irish Holstein Friesian Association Pedigree Cattle Breeders’ Council of Ireland SERVICE PROVIDERS Irish Cattle Breeding Federation (ICBF) Teagasc University College Dublin Veterinary Ireland Biosecurity leaflet series Vol 1, Ver 1, Sept 2012 One of a series of leaflets produced by the Biosecurity TWG of Animal Health Ireland Page 2 Understanding infectious disease Introduction Infectious agents include many different types of bacteria, viruses, fungi and parasites that can cause a wide range of clinical diseases. Individual infectious diseases often have quite different treatment and control strategies. Trying to put a plan in place for each one can be a difficult task. However, there are many similarities between infectious diseases that can help our understanding of these diseases and their control. Figure 1 illustrates the interactions common to infectious diseases. This highlights that there are three different components which all interact to influence the occurrence and severity of infectious disease. Thinking about each component when we see an animal affected by infectious disease is a good starting point to understanding infectious disease and preventing infections in the future. The infectious agent The animal The farm environment Figure 1 Interactions common to infectious diseases. The infectious agent Infectious agents are the small organisms (mainly bacteria, viruses and parasites) that are capable of causing an animal to become sick. However, infectious agents don’t necessarily cause an animal to become obviously ill, and they can often be found in and around healthy animals. Environmental organisms These infectious agents are called ‘environmental’ because they originate from the environment. These infectious agents are always present in and around cattle on every farm and do not normally cause any disease. However, they are opportunists and can cause disease when they are allowed to increase in the farm environment, or when an animal has low immunity to them. As environmental organisms are everywhere, they can never be truly eliminated. This means the management of the animal and environment is critical in the control of the diseases caused by these agents. Common examples include E.Coli and streptococcus bacteria. Page 3 Understanding infectious disease Primary pathogens Other infectious agents are more readily able to cause disease and may only be present in some herds. These are called primary pathogens. However, they may still be found in apparently healthy herds and animals. Animals with little or no immunity to these pathogens will often show severe signs of disease. This happens most often when a disease enters the herd for the first time. When the pathogen has been circulating in the herd for some time, many of the animals will develop immunity. However, the disease may remain present in the herd by existing in carrier animals, spreading to animals with low immunity, such as younger stock, stock previously unexposed to the pathogen and animals whose immunity is suppressed. These animals are at high risk of developing disease. When infection in the herd reaches this state it is called an ‘endemic infection’ or ‘endemic disease’. Common examples of primary pathogens that can cause endemic disease in a herd include bacteria (salmonellae, leptospirae, Johne’s disease bacteria) and viruses (IBR and BVD). Several endemic diseases can be present in a herd at the same time. Some endemic infections (e.g. Johne’s disease) can be present in a herd for a long time (years) before any animal becomes sick. In some cases, endemic infectious agents can be eliminated from a herd if proper controls are put in place. These controls could include vaccination, test and removal, or screening of purchases. The animal Not all animals will get sick when they contact an infectious agent. Whether an animal gets sick or not will often depend on the status of its immune system, which is the main defence mechanism against an infectious agent. Young animals An animal is born with very little immunity to any infectious agents (including environmental and primary pathogens). Colostrum contains antibodies that will give a new-born calf increased protection. Without adequate intake of colostrum quickly after birth, the animal will be very susceptible to infectious disease. For certain diseases, vaccination of the dam pre-calving can boost the quality of the colostrum and hence improve protection of the newborn calf if colostrum intake is adequate and timely. Case example E.Coli septicaemia is a common cause of new-born calf death. The bacteria are environmental (always present, on every farm). A frequent contributing factor is that the calf has not received sufficient colostrum and therefore has only a very poor immunity to the bacteria. Calves with a low immunity are very susceptible to picking up infections directly from their environments Page 4 Understanding infectious disease Naive animals An animal must come in contact with an infectious agent before the immune system can respond. Until the animal meets the infectious agent it is said to be naive. Naive animals are more susceptible to disease from primary pathogens. Once the animal has been in contact with the infectious agent, and recovers from any illness and with an immune response, it will be much better at defending against this disease in the future. Case example If a closed herd (one that a farmer does not add any animals) is naïve to an infectious disease such as IBR (the animals do not have protective antibodies to the IBR virus), the risk of an IBR outbreak is high if infected cattle are then introduced. Immuno-compromised animals Even after an animal has contacted all of the environmental and primary pathogens in a herd, its immune system can become compromised leaving it at risk of disease again. Common factors which can affect an animals immune system are: • Other disease • Poor nutrition • Calving • Stress (e.g. from lameness, overcrowding, castration, dehorning, etc) • Immuno-suppressive drugs. Animals affected by one or more of these factors will be at an increased risk of getting sick. Case example If suckler calves are dehorned or castrated close to weaning these multiple stressors increase the risk of weanling pneumonia. See the AHI leaflet ‘Management of the Suckler Calf at Weaning to Prevent Pneumonia’ for more details on planning for stress free weaning on www.animalhealthireland.ie. The farm environment In some situations, infectious agents can build up in the farm environment to such high levels that the immune system can be overwhelmed (even when it is not compromised). There are commonly three reservoirs for infectious agents to build up in the farm environment. • Clinically sick animals: When an animal is already ill because of an infection, it produces millions of the infectious agent and can shed them into its immediate environment; for example, in dung, urine, nasal secretions, abortion and calving fluids. • Sub-clinically affected animals: Animals can be infected without appearing very ill (though they may be suffering reduced production). These are called ‘sub-clinical cases’ and they can also shed infectious agents into the farm environment and directly to nearby animals. The number of sub-clinical cases may be far greater than the number of clinically ill animals. Thus, once you detect a sick animal in a group look further for the sub-clinical cases present also. • Carrier animals: Animals can carry and shed infectious agents without any impact on their own health or production. Environmental agents are carried by most animals on the farm; primary pathogens are usually only carried by a smaller number. Page 5 Understanding infectious disease Case example Coccidia are very commonly found in the farm environment. In most cases, growing animals are able to mount a good immune response quickly after first exposure, so that they show no signs of disease and shed only moderate numbers of coccidia into the environment. If conditions favouring a large increase of coccidia are allowed to occur (e.g. in an overcrowded pen), the immune system can be overwhelmed and animals can develop severe scour and become very sick. The sick animals will shed many more coccidia, resulting in many more clinical and sub-clinical cases. Therefore, the poor management of the farm and the infectious agent both contribute to the outbreak. Clean housing is essential to stop disease spread Preventing infectious disease All outbreaks of infectious disease involve an interaction between the infectious agent, the animals’ immune system and the farm environment. Preventing infectious disease requires paying attention to all three factors. Page 6 Understanding infectious disease Whether an animal gets sick or not depends on its immunity, its environment and the infectious agent involved The farm environment is normally contaminated with environmental infections (e.g. E.Coli) Some farms and animals are also sources of primary pathogens such as salmonella Animals with poor immunity are more likely to get sick A build up of environmental infections can cause disease Sick, sub-clinically infected and carrier animals are major sources of infection In controlling infections we need to focus on the animals’ immunity, their environment and the infectious agent, not just on one or two of these factors Technical Working Group: John Mee (Chair) - Teagasc Moorepark, Stephen Conroy - National Bull Performance Centre Tully, Bosco Cowley MSD Animal Health, Bernard Eivers - National Cattle Breeding Centre, Tim Geraghty - University of Glasgow, David Graham - Programme Manager AHI, Richard Fallon, Pat Kirwan - Veterinary Practitioner, John Moriarty - CVRL, DAFM, Luke O’Grady - UCD, Ronan O’Neill - CVRL, DAFM, Michael Sexton Veterinary Practitioner. TWG Rapporteur: Fionnuala Malone, AHI. IMPORTANT NOTICE — DISCLAIMER While every reasonable effort has been made to ensure the accuracy of the contents of this leaflet at the time of printing, no representation or guarantee is given, whether by AHI, its employees, subcontractors, agents, distributors or any other person, that the contents of this information leaflet are comprehensive, up to date, or free from error or omissions, nor that the advice provided is appropriate in every particular circumstance. The contents of this information leaflet are not intended to be a substitute for appropriate direct advice from a veterinary practitioner. Appropriate veterinary advice should be taken before taking or refraining from taking action in relation to the animal disease dealt within this information leaflet. If reusing material in this leaflet, please attribute AHI as the source of the information. Animal Health Ireland, Main Street, Carrick-on-Shannon, Co. Leitrim 071 9671928 www.animalhealthireland.ie email: [email protected] Biosecurity leaflet Series Vol 1, Ver 1, September 2012