Basic Field Epi: Manual/id

Basic Field Epidemiology Course Resource Manual

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In addition, para-veterinarians are often employed by local government as district animal health officers to help with activities such as disease investigations, control and vaccination programs, collecting census data and provision of breeding services. Some also receive additional fee-for-service payments from farmers. Good relationships are very important to the success of a para-vet’s work.

iSIKHNAS, Indonesia’s animal health information system, depends on the vital work of para-vets and their contribution of valuable data is crucial to the success and strength of the system. Using simple SMS messages para-vets can enter animal health and disease information quickly and efficiently from the field. The system has been designed to improve the quality and efficiency of data collection and to make the data available quickly to those who may need it for making good evidence-based decisions. iSIKHNAS allows all animal health related staff including para-veterinarians to provide better services to livestock owners and bring better outcomes to their communities.

This training course is designed to support their work and increase the job satisfaction of para-vets.

Mengapa

"Pelatihan epidemiologi lapangan akan membantu paravet untuk:"

• understand causes of disease at the population level to explain why diseases are occurring, even when you are not sure of the exact cause
• provide better advice to farmers on disease treatment and prevention

Para-veterinarians use clinical veterinary skills all the time to diagnose, treat and prevent disease in individual animals or small numbers of animals. An understanding of field epidemiology will provide additional skills to build on these clinical skills and give you a deeper understanding of the problem and how to respond to it.

Field epidemiology skills will also help you to provide good data to iSIKHNAS and to use iSIKHNAS information to help monitor, prevent, and treat disease within your area.

Improving your skills in field epidemiology will help your local community and also increase the importance of your role within that community. The benefits will include;

• Better disease prevention and animal management will lead to healthier animals which are more productive. Farmers will get better outcomes and improve their overall well-being and financial security.
• Improved appreciation for and trust in para-veterinary services will lead to farmers asking their local para-vets to help more quickly and more often when they have an animal health problem. Para-veterinarians will have more opportunity to treat animals and increase their income if farmers tell them about the sick animals
• The village/community will be more productive and healthier (animals and people) through having healthier animals, satisfied farmers and less zoonotic disease.
• The use of better skills in disease investigation, disease control, and reporting at local, District and Province levels will improve:

• • identification of diseases
  • surveillance for local disease needs with some information available to the national level
  • information to government to help share resources to reduce disease (i.e. vaccination programs) and the effects of disease within a District or Province
  • community economic benefits through having healthier animals

For example field epidemiology skills will help in the following situations:

• Better explain why a known disease has occurred at a particular time and place

Q: Why has anthrax occurred at this place and time in these animals?

• Identify treatments and preventions

Q: Anthrax - How can I stop animals dying and more becoming sick?

• Investigate and prevent disease whose cause(s) may be unknown or not well understood.

Q: I don’t know what the disease is, but how can I stop animals dying and more becoming sick?

• Explain how and why disease occurs through understanding the interaction between multiple causes of disease

Q: Why during the rainy season do my cows always get diarrhoea 3 weeks after my neighbour’s cattle are sick?

Field Epidemiology

Epidemiology is the study of the patterns and causes of disease in groups of animals or populations.

Field epidemiology refers to applying epidemiology skills in the field - on farms and in day-to-day work to address real problems for livestock owners.

Field epidemiology helps para-vets to look beyond the individual animal but rather at patterns and causes of disease within the wider population as well as other sources of information. This accumulated information helps a para-vet to then go back to the individual cases and treat more effectively, control the spread of disease, prevent further deaths or sickness, and reduce the presence of chronic problems in the livestock.

Field epidemiology also allows para-vets to assist farmers in matters to do with livestock production (things like weight gain, milk production or fertility) and factors that may affect production.

The clinical approach and the epidemiological approach

Clinical skills and laboratory tests are used to gather information from a single sick animal to diagnose the cause of the disease.

Epidemiology gathers information from a group of animals (sick and healthy) to describe patterns which help us determine the possible causes of disease.

At the individual animal level, veterinary clinical skills are used to:

• Examine a sick animal
• Identify the condition or disease causing the animal to be sick
• Apply a treatment to help that animal recover

On Budi’s farm, a single calf develops an abscess at the navel (where the umbilical cord attached to the calf during pregnancy). The calf gets sick and stops drinking. Pak Paimin, the para-veterinarian examines the calf, finds the abscess at the navel, cuts it open to drain the pus and treats the calf with antibiotics. The calf then makes a complete recovery.

Epidemiology applies a structured approach to investigate disease and causes of disease in groups of animals (populations). The most common approach is to collect information on animals affected with a disease and similar animals that are not affected (sick animals and healthy animals). Information on these two groups is then compared looking for differences that explain why some animals are getting sick and others are not.

On Soleh’s farm, there are many cows and a large number of calves are born in one season. This year most of the calves develop abscesses on the navel. Soleh wants his animals treated so they can recover and wants to know why so many of this animals developed the problem this year. He also wants to know what he can do to prevent it from happening again next year.

Para-vet Ibu Putri, visited Pak Soleh’s farm. She found that all the calves that had abscesses on the navel were born in a small yard that was very dirty. All the calves that were born out in paddocks on the grass were healthy.

She concluded that calves born in the dirty yard were getting sick because the dirty environment was exposing them to bacterial infection soon after birth. She advisors Pak Soleh that cleaning the calving yard or calving cows in clean pasture will help to reduce the risk of abscess in the future.

The reason a disease occurs at a particular time, place, and in only some animals is because the causes for that disease are present for some animals and are not present for others. If we can understand the causes then we may be able to change management practices in order to prevent disease.

Good veterinarians and para-veterinarians need both individual clinical skills and field epidemiology skills to provide the best services to livestock owners. In some cases where a particular disease is very well known and easily diagnosed, epidemiology skills may add little to the veterinary clinical skills in diagnosing the disease and providing effective advice for treatment and prevention. Epidemiology skills are especially useful when a new or unknown disease occurs or where the causes of disease are not known.

Epidemiological skills can help prevent zoonoses

Zoonoses are animal diseases that are able to infect humans.

Epidemiology skills help you to understand how zoonotic diseases occur and how to prevent both animals and humans from exposure.

There are a number of diseases of animals in Indonesia that are capable of causing disease in humans (zoonotic diseases):

Rabies, Brucellosis, Q-fever, Leptospirosis, Psittacosis, Trichinosis, Echinococcus, Japanese Encephalitis, Toxoplasmosis, Salmonellosis, Scabies, Ringworm, Nipah virus, and others.

Below is a little more detail about some common zoonoses.

• Rabies - Rabies is a viral disease of the central nervous system (brain and spinal cord). It is most often transmitted through a bite or scratch from a rabid animal. Humans that contract rabies almost always die.
• Brucellosis - Brucellosis is a bacterial disease. These bacteria can cause disease in many different animals. Humans become infected by coming in contact with animals or animal products that are contaminated with Brucella bacteria.
• Q fever - Q fever is a bacterial disease caused by Coxiella burnetii. Q fever mainly affects cattle, sheep and goats. It is most commonly passed to humans by inhalation of these organisms from airborne dust that is contaminated by dried placental material, birth fluids, and excreta from infected animals.
• Leptospirosis - Leptospirosis is a bacterial disease that affects animals and humans. It can cause a wide range of symptoms in humans.
• Psittacosis – also known as parrot fever and ornithosis — is a bacterial disease caused by Chlamydia psittaci, many different bird species can be infected and spread the disease. In humans it can cause severe pneumonia and other serious health problems.
• Trichinosis - also called Trichinellosis is passed to humans by eating raw or undercooked meat of animals that are infected with the Trichinella worm larvae. Infection of Trichinella occurs commonly in wild carnivorous (meat-eating) animals but can also occur in pigs.

Animal Health and Disease

The effect of disease on animal health and production

Diseases in animals will often result in reduced health and production and may result in death.

Livestock are generally raised for production of meat, milk, hides, eggs, manure, and offspring (calves, chickens, etc.). Healthy livestock are more productive than livestock that are not healthy.

Some diseases cause animals to change their behaviour and look sick. Some of the sick animals may die and others may recover. It is easy to pick animals that are very sick with a disease. Sometimes it can be hard to pick whether or not an animals is sick. They may not show obvious signs of a problem. That is why it is important for farmers to watch (monitor) their animals frequently.

Sick animals may stop eating for a while and lose weight. Sometimes animals may look normal but suffer from reduced production (weight loss, infertility, pregnancy loss, reduced egg production). Many diseases cause a decrease in animal production.

There are many things other than infectious diseases that cause poor production in animals. Examples include animals that are offered poor quality feed, not enough feed, or feed and water that may be spoiled.

Epidemiology and clinical skills need to be used when investigating poor performance in livestock. A good investigator will be able to tell whether poor production or poor health is due to an infectious disease or due to some other cause such as poor feed.

Yesterday Budi reported to his local Pelsa that one of his cows was lame. The Pelsa sent a General Signs report (Tanda Umum) to iSIKHNAS.

Pak Paimin (the para-vet) received the notification from iSIKHNAS and talks to Budi. Budi tells Pak Paimin that there is one cow that is lame and she has been lame for several weeks now. She is skinny and her calf is weak. There are also 2 cows with diarrhoea.

Pak Paimin talks to Pak Budi and comes up with a list of possible causes (differential diagnoses) for this situation. They include: injury, abscess, infectious diseases (brucellosis, black leg, etc.), or trauma (broken leg).

The 2 cows that have diarrhoea could be (differential diagnoses) sick because of parasites, grain overload, bacterial infection, or liver disease from poisons.

Pak Paimin decides to conduct a farm visit to get more information to narrow down the list.

Signs of disease

Signs are changes in an animal that are caused by disease and that people can detect.

Many diseases make animals feel sick, develop a fever and they might stop eating or drinking for a period of time. Many signs of disease are easily observed by people. Farmers that know their animals well will be able to detect small changes in behaviour that indicate an animal may be getting sick. This will help them to get assistance from their para-vet early, before the case becomes very serious.

It can be easy to see the signs of some diseases but more difficult to see the signs of other diseases.

• Signs of disease that are easy to see include lameness, coughing, diarrhoea, severe weight loss and death
• Signs of disease that may be harder to see include infertility and reduced weight gain or milk production

Most signs provide an indication of which parts of the body and body systems are being changed by the disease

For example:

• Diarrhoea indicates diseases that affect gut motility or absorption (digestive system). There are many diseases that can cause diarrhoea.
• Coughing and difficulty breathing indicates disease of the lungs or airways (respiratory system)
• Saliva drooling from the mouth indicates inability to swallow either due to nervous system disease, physical obstruction of the throat, or increased production of saliva
• Lameness or altered gait may indicate disease or injury affecting the leg(s), spine or brain

Technical staff are trained to conduct clinical examinations of sick animals, to look for signs that indicate the animal may be sick. These include changes in: * rectal temperature

• heart rate
• pulse rate
• respiratory rate
• respiratory sounds
• gut sounds
• mucous membrane colour

The time it takes for signs of disease to develop can vary depending on the infectious disease. Therefore, the time it takes you to identify if an animal is sick can also vary.

For example:

• Anthrax and haemorrhagic septicaemia are diseases that develop very rapidly and produce severe clinical signs or death.
• Papilloma virus (causing warts on the skin of cattle) is a disease that produces mild signs in many animals with little impact on health or productivity.
• Bovine Johnes disease (BJD) is a disease which develops very slowly, it causes chronic diarrhoea and wasting in older cattle. It takes a long time for BJD to these produce serious effects, the cattle are infected when they are very young but do not show signs till they are older. BJD produces the same signs as a cow with an intestinal worm infection.

Sometimes animals show signs such as weight loss that may be due to things other than an infectious disease, such as an injury (broken jaw), problems with teeth or even just poor feed.

iSIKHNAS has developed lists of common signs and codes to make reporting these signs easier and more consistent. Village reporters (pelsa), para-vets and veterinarians play a vital role in the identification, reporting, and treatment of these signs.

Syndromes

Syndrome refers to a particular sign or a group of signs that can be easily recognised and which may indicate a particular important disease.

For example:

• A respiratory syndrome may be defined as including any animal that shows one or more of the following clinical signs: coughing, difficulty breathing, nasal discharge, elevated respiratory rate, and so on.

Syndromes are used to identify animals that may be suffering from a specific disease of importance.

For example:

Rabies is an important disease. It is not possible to diagnose rabies for certain in a live dog (or any animal). Definitive diagnosis of rabies can only occur when brain or other tissue is examined by a pathologist after the animal has died or has been killed.

However, many dogs infected with rabies will show changes in behaviour – becoming more aggressive, showing drooling of saliva from the mouth and more likely to attack and bite other animals and people. Some dogs show these signs even though they are not infected with rabies – just because they are aggressive dogs or drooling because they have something stuck in their mouth or throat.

We use the syndrome of changes in behaviour (aggression, biting, salivation, depression) as a way of identifying dogs that may have rabies. Dogs that show these signs can be isolated and watched to see if they continue to develop signs that are suggestive of rabies and they may be killed or sent for post mortem to test for rabies.

Some syndromes are strongly related to one disease so that when an animal shows these particular signs it is more likely that it has the disease. Examples include rabies (as described above) or sudden death with blood from the orifices (likely to be anthrax).

Syndromes are often used in disease control programs for important diseases to make sure that these diseases are identified when they occur. Livestock owners and animal health staff are encouraged to look for animals with defined syndromes. Animals that show those signs can then be examined more carefully or sampled for laboratory testing to try and diagnose the disease. Some animals will be found not to have the disease of interest. If the disease of interest is confirmed then other disease control activities may need to be done.

iSIKHNAS uses broad syndromes, this is because the diseases of interest are very important and we don’t want to miss any cases. Village reporters (pelsa) and para-vets should report every suspected priority syndrome case. The important thing is that we all remain alert to the threat of these diseases. Most priority reports will probably end in a diagnosis which is not the priority disease. It is still very important that you report a priority syndrome (using a P SMS reporting message) and let the vet carry out a more thorough investigation. iSIKHNAS uses the following priority syndromes for disease reporting:

• MMU - Sudden increase in mortality in chickens and other poultry
  • this syndrome is trying to identify cases of Avian Influenza

• • Other infectious diseases that could produce this syndrome include: Newcastle disease, infectious laryngotracheitis, and duck plague.
  • Other non-infectious causes include: acute poisoning.

• KGS - Abortion in third trimester or swollen joints in cattle
  • this syndrome is trying to identify cases of Brucellosis

• • Other infectious diseases that could produce this syndrome include: many bacterial and viral infections can cause abortion and swollen joints.
  • Other non-infectious causes of this syndrome include: genetic conditions, exposure to poisons, and administration of some drugs.

• MTD - Sudden death with blood from the orifices in cattle
  • this syndrome is trying to identify cases of Anthrax

• • Other infectious diseases that could produce this syndrome include: blackleg and leptospirosis.
  • Non-infectious causes include lightning strikes, lead poisoning, hypo-magnesia and bloat.

• PLL - Limping, excessive salivation, and vesicles on the mouth / foot / teat in cattle
  • this syndrome is trying to identify cases of Foot and Mouth disease

• Other infectious diseases that could produce this syndrome include: vesicular stomatitis, bluetongue, bovine herpesvirus, malignant catarrhal fever, pestivirus, mycotic stomatitis, and rinderpest.

• GGA - Changes in behaviour, increased aggression or depression, hyper-salivation, and biting in dogs
  • this syndrome is trying to identify cases of Rabies

• Other infectious diseases that could produce this syndrome include: canine distemper, Aujesky’s disease, and any infectious disease involving the brain.
• Non-infectious causes include - neoplasia, trauma, oral foreign bodies and poisoning.

• DMB - High fever, conjunctivitis, and increased mortality in pigs
  • this syndrome is trying to identify cases of Classical Swine Fever

• Other infectious diseases that could produce this syndrome include: African swine fever virus, and many other bacterial and viral infections.
• Non-infectious causes include exposure to some poisons such as anticoagulant.

Differential diagnoses

A differential diagnosis is a disease that could cause the clinical signs that have been observed. Often there is more than one disease that can cause the same signs and when sick animals are examined. It is common to produce a list of multiple differential diagnoses as possible diseases that could be affecting the animals.

It is common practice to list diseases in order from most likely to least likely.

At Budi’s farm there were 2 cows with diarrhoea. Immediately Pak Paimin can create a list of common diseases that could cause the clinical signs.* Parasites (worms, coccidia, liver fluke)

• Grain overload
• Poisoning
• Salmonella infection in the gut (bacteria)
• Johne’s disease infection (bacteria)
• Bovine virus diarrhoea infection (virus)
• Very rich, fresh pasture

• The disease investigation is then used to try and determine which diseases are unlikely to be causing the signs and which ones might be more likely. This process leads to a short list and sometimes a single differential diagnosis.

Definitive diagnosis

A definitive diagnosis is reached when the vet is confident there is one disease that is most likely to be affecting the sick animal(s). The vet uses all the diagnostic information available (including history, clinical, environmental, laboratory and epidemiological).

Later sections of this manual, as well as other training courses in disease investigation, will provide more detail on how to conduct a disease investigation.

When Pak Paimin completes his visit to Budi’s farm, has taken a detailed history, conducted a clinical exam, received results from the laboratory samples he sent for testing, followed up on the response to treatment, and analysed the results of each part of the investigation process it is likely that he along with the Dinas vet will be able to reach a definitive diagnosis.

Causes of disease

A cause is anything that can influence whether or not a disease occurs in one or more animals.

When diseases occur in animals there are nearly always some animals that develop disease and others that do not. There are a very small number of diseases that are so infectious that when they occur nearly every animal gets infected and develops disease. These are rare.

Causes of disease include many different things. Infectious diseases will have an infectious agent as one of the causes. For example, Rabies virus is the infectious agent that causes rabies. For diseases that have only one infectious cause, if the infectious agent is not present we can be confident that the disease will not occur. BUT, exposure of an animal to an infectious agent does not mean that the disease will occur.

Often many different causes have to occur together before an animal will develop disease.

For example:

When one dog bites another dog there is a chance that the second dog might develop rabies. The likelihood of rabies occurring is much higher if the first dog:

• is infected with rabies
• is shedding rabies virus in its saliva
• the bite causes a break in the skin and the rabies virus can get into the tissues of the second dog

If the first dog is not shedding virus in its saliva then the bite may not cause rabies. If the bite does not break the skin then the virus may not get into the tissues of the second dog and it may not develop rabies. If the second dog is vaccinated against rabies then it may not develop rabies even if bitten by a rabid dog.

Knowledge of the causes of a disease and how they act to cause disease is important. This knowledge can be used in disease control to choose prevention measures to reduce the risk of disease occurring.

To investigate disease in populations, we need to understand how different possible causes of disease might act together to influence if disease will occur.

'Disease'Figure Disease can occur when the causes in the environment, host and agent are present

Host refers to the animal which can get the disease. The host (animal) has a variety of characteristics that influence if disease will occur. Some diseases occur only in young animals while others only in older animals, for example. Pregnancy and abortion can only occur in females. The vaccination status of animal influences the risk of getting the disease.

Pinkeye is more common in younger cattle because they generally have not been exposed to Moraxella bovis, the bacteria that is involved in pinkeye. Lack of exposure in younger cattle mean they have a lower immunity to this bacteria and therefore a higher risk than older cattle of getting the disease.

Pinkeye will be more common in animals with physical characteristics where the eyes stick out (protrude) more than in others animals. This characteristic may be inherited and associated with a certain sire or breed.

Agent refers to the particular infectious agent that is a cause of the disease: virus, bacteria, fungus, parasite, or other microbe. Different strains of the same bacteria or virus may either produce no disease or more severe disease. There are many different infectious agents and different sub-types or strains, these may all have different abilities to cause disease. Sometimes non-infectious causes are classified as agents for example lead as the agent that can cause lead poisoning or a plant toxin that can cause a type of plant poisoning.

Some strains of avian influenza do not cause a high mortality rate, whereas other strains do.

Some strains of Moraxella bovis (the infectious agent causing pinkeye in cattle) produce a toxin that is more powerful and causes more severe eye disease than other strains.

Environment refers to external things that affect the host and agent and influence if disease will occur. Environmental characteristics include: land type, humidity, rainfall, insects that transmit the agent, crowding, sanitation, etc.

Ultraviolet light damages the eyes (cornea) of cattle which is a cause of pinkeye. Pinkeye is more common in summer when ultraviolet radiation is high.

Pinkeye is more common where a lot of cattle are kept very close together and less common where cattle have a lot of space.

Pinkeye is more common when there are lots of flies.

Pinkeye is more common when cattle are eating long grass which is hard. Hard grass can cause small injuries to the cornea which allows the bacteria to get in.

Simple exposure to the agent does not mean disease will occur. There are many causes and many combinations of these causes required for disease to occur. The causes may relate to the host, agent, or environment.

The relationships among causes of disease can be represented in diagrams. These often show the direct and indirect associations among the different causes.

The bacteria Pasturella multocida is the infectious agent involved in Haemorrhagic septicaemia. The diagram below shows some of the causes of Haemorrhagic septicaemia and how these causes influence the occurrence of disease in an individual animal. You can see that this diagram is trying to show a complex interaction between the causes.

There are many combinations of causes that lead to disease, so it is possible for a disease to occur on separate farms or at different times due to a different combination of causes. Epidemiological investigations help identify the important causes of disease.

Knowledge of the causes of disease is used to determine how to prevent disease occurring in populations.

We may find in a research study that cattle farms feeding dry grass and hay are at five times the risk of having pinkeye outbreaks in weaner cattle compared with those using silage or fresh grass.

We might expect from this finding that a farmer who changed from feeding dry grass and hay to fresh grass and silage would dramatically reduce the risk of having pinkeye in the weaner cattle.

A disease may be prevented by doing something that breaks an important relationship between different causes.

For example:

• using better hygiene measures and managing the property to avoid environmental contamination with the infectious agent(s)
• vaccinating animals to increase host immunity and help prevent infection with the infectious agent

Understanding the different causes can help find cost effective ways to prevent or reduce the effects of the disease.

If shade is provided for cattle, the damage caused by UV light to the eyes may be less and the amount of pinkeye disease reduced.

If we vaccinate all the cattle for haemorrhagic septicaemia and avoid crowding during the wet season we would reduce the number of disease events.

Below are a few examples of causal webs. These are simple ways of describing the interaction between causal factors in the environment, the agent and the host and help in determining the necessary causes – the causes which must be present in order for the disease to be present – and the factors which animal health staff are able to influence through various activities, changes to management practices or husbandry.

CAUSAL WEBSFigure Causal web for Anthrax

An animal infected with Anthrax sheds the disease on the ground. These spores can stay alive in the soil for a very long time if the spores are not properly killed through burning and decontamination. Unvaccinated animals coming into contact with the spores can become infected.

Figure Key component causes of Anthrax

By understanding the interaction between causes in the environment, the way the agent behaves and the susceptibility of the host you can do things which will lower the risk of infection to other animals. Burning any carcases to ash, properly disposing of soil contaminated with body fluids from the infected animals and vaccinating the entire herd will lower the risk of ongoing infection from spores in the environment. There are many other management and husbandry activities which can also be considered.

Lantana camara is an invasive species of flowering plant. The leaves on the plant are toxic to animals and cause liver damage. Animals usually avoid the plant but may eat it when there is no other food due to reasons such as drought or flooding, or poor management. Poor herding may allow access to the plant. In addition, animals will always seek access to green fodder (grass, reeds, leaves etc). This causes an accumulation of chlorophyll in the skin and when exposed to strong sunlight the pale skin of animals quickly becomes badly damaged by sunburn.

With a better understanding of the causes in the environment, host and the agent, we are able to significantly influence the outcome of the disease. Careful herding to avoid areas with lantana, and ensuring sufficient feed of adequate quality can lower the likelihood of animals eating lantana. Providing cattle who have had access to lantana (particularly pale skinned animals) with good access to shade can provide immediate protection against photosensitisation. Providing them with dry fodder (hay etc) instead of green (chlorophyll filled) feed will also greatly diminish the sunburn. It is also important to remember that lantana camara poisoning is not the only cause of liver damage however the combination of chlorophyll and sun for a pale animal with liver damage without shade will result in similar signs.

The progression of infectious disease

Progression of disease in an individual animal

First we need to learn and understand some definitions before we learn about the progression of disease in individual animals.

• Infectious agent: living organisms that are capable of causing disease in susceptible animals. Infectious agents include: bacteria, viruses, parasites, protozoa, and fungi.
• Infectious disease: A disease due to a specific infectious agent that occurs due to transmission of the agent from an infected host to a new host, either directly or indirectly through intermediate hosts, vectors, or the environmental.
• Contagious disease: A contagious disease is an infectious disease that can spread directly from animal to animal. All contagious diseases are infectious but not all infectious diseases are contagious.

• Examples of contagious diseases include Foot and Mouth Disease (FMD) and Highly Pathogenic Avian Influenza (HPAI). These diseases can pass from one animal to another directly.
• Examples of infectious diseases that are not contagious include Tetanus, Anthrax, and Liver fluke infection. These diseases cannot be passed directly from one animal directly to another. With these diseases, infected animals contaminate the environment, and other animals get the disease from this environmental contamination.

Please note: the terms infectious disease and contagious disease are sometimes incorrectly used interchangeably and this can create confusion sometimes.

• Susceptibility: An animal must be susceptible to the infection in order to develop the disease. Animals that are not susceptible may be exposed to causes of disease including an infectious agent and they will not develop disease.

• Only the horse family (Equidae) are susceptible to equine infectious anaemia
• Younger cattle are more susceptible to pinkeye because they have a lower immunity to Moraxella bovis compared to older cattle who have been exposed previously.

• Exposure: The interaction between an animal and an infectious agent. Animals that are not exposed to an infectious agent will not develop the disease. Not every animal that is exposed will get infected.

• • • When an influenza virus affects a crowded flock of chickens, every chicken will be exposed but not every chicken will become sick.
• Incubation period: the period of time from infection until the animal develops clinical signs of disease.

Within the host (animal) there are a number of steps that determine if the animal develops disease after being exposed to an infectious agent for that disease.

Figure Progression of an infectious disease in individual animals

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RecoveredImmuneSusceptible

Firstly an animal must be susceptible to a disease to become infected. Once an animal is infected there is usually a period of time before the animal develops any clinical signs of disease. This is called the incubation period.

During this period the infectious agent is multiplying; as the amount of agent increases within the animal the greater the effect on the animal (host). The incubation period starts with infection of the animal and ends with the onset of clinical signs of disease. In some cases infected animals may not show any signs of disease.

Figure The incubation period of an infectious disease

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Each infectious disease has a characteristic incubation period. The length of this period is dependent on:

• the way the infectious agent entered the host
• the amount of infectious agent that entered the host
• how quickly the infectious agent multiplies within the host
• the ability of the agent to cause disease
• the immune response of the host

This interplay causes the incubation period to vary among individual animals.

Soleh has a lot of chickens. All his chickens have got sick before with influenza. He noticed that from the day he put he put a sick bird in with the others it took between 3 – 5days before all his other chickens became sick.

Soleh has measured the incubation time of this influenza virus to be between 3 and 5 days.

Infected animals may recover, become a carrier, or die depending on the disease. In some diseases, infected animals may never develop clinical signs of disease. In other diseases almost all infected animals may develop signs of disease.

Figure The potential outcomes of an infectious disease for an individual animal

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Soleh has some cows as well. One year some of his cows had bad diarrhoea. Pak Paimin, the para-vet, investigated the problem. As part of a university study he took faecal samples from all the cows. He found all Soleh’s cows were infected with high worm numbers yet only a few had diarrhoea. All the cows recovered and became healthy again once they were wormed.

The year before Soleh had anthrax all his cows were infected and they all died.

Some diseases are capable of producing persistently infected animals, often termed carriers. These animals may show little or no signs of disease but may be capable of shedding the infectious agent. These carrier animals pose a risk to other susceptible animals in the population.

For example:

Bovine viral diarrhoea virus can produce a carrier where the animal never grows very well and continually sheds the virus. This acts as a reservoir and infects other animals.

Some animals that clinically recover from pinkeye will remain carriers of Moraxella bovis. The bacteria can live in the eyes, nose, and vagina of these carrier animals. Flies carry the disease from these animals to other non-carrier animals, sometimes this causes disease if the other host, agent, and environmental characteristics are right.

Recovered animals may develop immunity to the infectious agent so that if exposed again they do not become infected. Immunity may last a lifetime for some diseases while for other diseases it may be shorter. In these situations as the animals immunity declines the may become susceptible to infection again.

Figure The potential outcomes of a recovered animal from an infectious disease

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Soleh noticed that once all his chickens got better from the influenza they didn’t get sick again. Even the new healthy chickens didn’t get sick when he put some sick chickens in with them. He didn’t get any influenza in his chickens for about a year.

He asked Ibu Putri, the para-veterinarian, why this might have happened. She explained that his chickens had immunity for the most common influenza virus. It must have taken a year for a different or new influenza virus to come along or the current population of chickens consists of very few of the original chickens due to them being sold or dying.

Progression of disease in a population

Imagine a population of animals that has never been exposed to a particular disease agent before. This population is likely to be highly susceptible to an infectious disease. Introduction of infection to the population is likely to result in rapid spread of disease (an outbreak or epidemic).

Some diseases spread quickly through a population whereas other disease spread slowly.

For example:

Newcastle disease spreads rapidly between birds housed together whereas Bovine Diarrhoea virus spreads slowly within a population of cattle.

For diseases that are already present in a population (endemic diseases), the population will usually be made up of a mixture of animals that are:* Susceptible

• Infected
• Diseased
• Recovered
• Immune