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Understanding
infectious disease
For Irish Farmers, Advisors and Vets
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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
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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.
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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
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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.
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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.
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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