The names of many sheep diseases reflect the symptoms that sheep show when affected. Names such as pulpy kidney, staggers , pink eye and knob rot are all to some extent an historical carry over from a time when the true nature of many diseases was not known or understood. Much of the understanding and many of the diagnostic tests now used were not available in the not too distant past and to reach a diagnosis was as much an art form as a science. These days however not only is the knowledge available to veterinarians truly vast but diagnostic testing, that at one time was involved and complicated, has become steam-lined, accessible, accurate and commonly used. With accessability and common use the costs involved have also reduced.
It is important not to confuse a symptom with a diagnosis. Symptoms may help localise a problem and shorten a diagnostic list but some quite diverse problems can display similar symptoms. It is good to know the list of potential causes but farmers need to know which problem on that list is actually their problem. Getting an accurate diagnosis has many advantages . An accurate diagnosis allows for a correct targeted treatment. Once a diagnosis is made we then know what caused the problem and with this information we have the knowledge of how to prevent it happening again. Also, although, some health problems will actually kill animals many just lead to a failure to thrive. Correcting these leads to improved management decisions and better use of financial and other resources. With advances in diagnostic capabilities have come improved treatments and with this, an increased likelihood of positive treatment outcomes. There is more value in diagnosing a problem accurately if it can then be effectively treated. In these times this is usually the case.
Some sheep farmers , particularly those with smaller numbers of animals, can be reluctant to contact a vet. It is good however to get to know your local vet and work with him/her to solve any health issues that may occur. So just how does a vet actually figure out what is the problem? How is a diagnosis reached? Understanding the process will enable you to work better with your vet.
Diagnosis is based on a combination of four things namely the history, the clinical signs ( ie the symptoms displayed by the sheep when sick),laboratory testing and sometimes autopsy. 1/History The number of animals affected, their age, breed, sex, diet, worming and vaccination status, access to shade and shelter, reproductive status , time of year , soil type, as well as the type of pasture available are all important. Taking a good history enables the clinician to develop an understanding of the nature of the problem.
2/Clinical Signs Stance , respiratory pattern, temperature, general behaviour, mucous membrane colour, heart rate, recent droppings, body score and level of awareness are all relevant. This is one of the main diagnostic areas where the clinicians experience is important. Abnormalities become obvious to someone who has spent a lot of time around sheep . These may not be readily apparent to someone less experienced .
3/Laboratory results Everything costs money. It is important that the client’s money is spent wisely. So, it is important that the clinician diagnose the problem not only expediently but also as economically as possible. The history and clinical signs will enable the experienced clinician to develop a list of likely potential causes and guide his selection of tests to reach a diagnosis.
Commonly used tests include a/ blood tests – blood can be collected into a variety of tubes. A range of biochemistry ( organ and metabolite ) tests and haematology ( red and white cell counts ) tests can be done.. These tests give valuable information about nutrition, the level of care and overall health, hydration, organ function, the immune system and possible infection. In addition the levels of important trace elements such as selenium and copper can be measured in blood .Blood smears are also usually collected. Smears help to confirm the accuracy of the haematology. There are also tests for specific diseases, such as Johne’s disease, that can be done on a blood sample. Blood is usually collected with the animal standing from a jugular vein. This is usually done with the animal conscious but if the sheep is to be autopsied some vets will collect blood after captive bolt stunning as the heart keeps beating but the animal can’t feel the blood sampling .
b/the eye, this may sound unusual but if the animal is euthanised or has already died often a sample of liquid (the aqueous humour ) from inside the eye is collected– calcium levels ( low in milk fever) , magnesium levels ( low with grass tetany),urea ( elevates with kidney disease), AST and GGT ( elevate with liver disease), BHB,( a ketone associated with acidosis ),glucose, nitrate ( poisoning is common when sheep graze lush pastures containing plants high in this substance eg capeweed, oats and canola, this irritates the bowel causing scouring) , nitrite ( a metabolite of nitrate, nitrate is converted to nitrite which is in turn is converted to ammonia which is then “burped” away during rumination, if too much gets into the blood then poisoning occurs) and other values can all be measured. Collecting aqueous humour has the advantage that a diagnostic sample can be collected from a recently dead animal. This cannot be done with blood. Care however must be taken with interpretation as some values change with time.
c/ Faeces – worm egg counts, worm identification, coccidia oocyst (egg) counts, faecal bacterial culture and some specific tests for disease (eg Johne’s disease) can all be done
d/ Other samples—stomach contents, plant material, fresh and frozen tissues, urine and sometimes other samples are all occasionally used for diagnosis
4/ Autopsy It is important to match the degree of diagnostic effort with the severity of the problem. It is not reasonable to kill an animal to get a diagnosis for an apparently straightforward problem affecting low numbers of animals. However, if a single animal has recently died then an autopsy may present itself as the logical diagnostic intervention. Also sometimes if there are ongoing problems in a flock and a diagnosis cannot be made from samples collected from live animals then an individual displaying symptoms typical of the problem is selected for euthanasia and autopsy. Most veterinarians open the abdomen on the left hand side so that gravity moves the rumen out of the way. It is important that the autopsy is carried out in a methodical way , so that nothing is overlooked,with every system being examined. Most vets have their own routine. The aim of the autopsy is to not only visibly examine the internal tissues but to collect a full range of tissues for subsequent testing. Most tissues are preserved (“fixed”) into formalin for subsequent microscopic examination. Often if the animal has died after only a short illness there are no visible changes apparent but the cause of the problem will be apparent microscopically. Fresh tissues can also be collected .Swabs for bacterial culture are also collected from any obviously infected area. Swabs are often also collected from other areas such as the bowel, particularly if the sheep has diarrhoea, and also from the liver or bile duct. Most value can be gained by autopsy if it is done immediately after death. As the hours go by, autolysis ( decomposition ) and invasion by bacteria begins making interpretation of any pathology increasingly difficult. Autolysis occurs more quickly in some tissues such as the gut, liver and brain. Usually an autopsy will not be attempted if the animal has been dead for 24 hours. If more than 4 hours have passed then usually all relevant samples are still collected but it is anticipated that some will no longer be diagnostic. Samples that have been frozen are no longer suitable for histopathology ( microscopic examination ) or bacterial culture but are useful, and are in fact a preferred sample for viral testing. If a veterinarian is unsure what sample(s) may be best for a suspected diagnosis he can contact his local testing laboratory. The pathologists there are experts and can assist clinicians in the diagnostic process.
The Victorian government is here to help sheep owners. I have personally been impressed by the help that I have received by Agriculture Victoria in several health issues that I have had with my own sheep. Sheep owners should to be aware of the Agriculture Victoria Significant Disease investigation program . For a disease to be considered “significant” one or more of the following criteria must be met by the disease event. 1. An unusual or atypical manifestation of disease, including high morbidity ( ie number of affected animals), mortality and/or rate of spread. 2. An initial investigation fails to establish a diagnosis, including when veterinary treatment does not produce the expected response. 3. There are findings suggesting a possible effect on trade, public health, biodiversity or the viability of a farm, industry or region. 4. Where there is a genuine suspicion of an exotic or emergency animal disease. If one or more of these criteria are met then Agriculture Victoria will lead the disease investigation and cover the cost of the investigation. This really is a great system and is a good example of our taxes at work. The government is interested in determining the cause of such a disease event. Government vets can collect an entire complement of samples and provide a full history to the lab. The experts at the lab can then decide which tests are appropriate to reach a diagnosis but at the same time avoid the costs associated with unnecessary testing.
If a health problem does not meet one of these criteria then a sheep farmer can contact his local sheep vet. His/her experience should, in the same way guide the investigative process and a diagnosis be reached. Although “common things happen commonly”, this is not always the case and sometimes testing can be involved, lengthy and costly.
Other states have similar programs to Victoria. Sheep farmers can contact their local authorities to see what diagnostic assistance is available
Getting a diagnosis – many problems have similar symptoms. “Staggers” is a good example of how a wide range of health issues can display similar symptoms. Getting an accurate diagnosis has many advantages. Without an accurate diagnosis time, money and sheep lives can be wasted on inappropriate treatments. As I mention above it is important not to confuse a symptom with a diagnosis. With an outbreak of “staggers” the primary symptom is that some sheep appear unsteady on their feet. To an experienced veterinary clinician or farmer this symptom may suggest several potential causes. It is then a matter of following an appropriate diagnostic pathway to figure out which of these is the actual problem in that particular case. It is best not to think of a potential cause and then just test for that. One can end up doing one test after another and not advancing the diagnostic process. It is always best to initially “cast the diagnostic net” fairly widely. More specific tests can be done later if required. There are at least seven common possible causes of “staggers”. These include :- 1/ grass tetany -- this is caused by magnesium deficiency and usually occurs in ewes within 6 weeks of lambing, it is diagnosed by measuring magnesium levels in eye fluid and also by a rapid response to treatment, affected ewes injected with magnesium walk normally in minutes, 2/polioencephalomalacia -- a brain problem caused by a deficiency of vitamin B most commonly associated with feeding high grain diets or plants that break down vitamin B such as bracken, diagnosed by examining the brain at autopsy and also by a response to giving vitamin B in 6 -8 hours, 3/ annual ryegrass staggers -- occurs in Spring when sheep ingest a toxin produced by a bacteria that infects a worm that infects annual ryegrass, there is no definitive test but is diagnosed by the circumstance, it is managed by cutting hay or heavily grazing before the grass becomes toxic, 4/white muscle disease --due to selenium deficiency this most commonly occurs in weaning lambs. As lambs wean they no longer source this nutrient from their mother. If the soil is low in selenium then the grass that grows from it will also be low in selenium. As lambs become deficient they develop symptoms, diagnosed at autopsy where visibly pale muscles are apparent and a blood test to measure selenium, 5/milk fever -- low calcium in lactating ewes, diagnosed by circumstance, response to treatment and if required blood estimation of calcium levels, 6/pregnancy toxaemia --seen in the last 6 weeks of gestation, with inadequate energy intake the ewe breaks down her own tissues, if this happens too rapidly ketones are released leading to death, diagnosed by circumstance and post mortem changes ( yellow soft liver with plenty of fat on the body), 7/perennial ryegrass staggers -- seen in summer and autumn, due to a toxin on fungus that infects, there is no diagnostic test, diagnosis is based on circumstance ie grazing perennial ryegrass dominant pasture and signs, managed by monitoring the sheep , moving them if signs develop and using resistant cultivars.
So even though the symptoms may look superficially the same, the potential causes may be as diverse as eating toxic grass, eating not enough grass, eating nutrient deficient grass, a major metabolic disorder associated with a management practice or a simple nutrient deficiency. Often the solution to a problem and subsequent prevention is straightforward once that problem has been identified . Having an understanding of the diagnostic process will help you work with your local vet to find those solutions