NSW District Veterinarians are unique within Australia by being government veterinarians who have a large surveillance role performing diagnostic disease investigations on livestock enterprises. In some districts they provide the majority of livestock disease investigations. They are also unique in that they are funded by rates levied on regional landholders.
This paper will outline the livestock disease investigation principles the author has developed from 30 years as a government veterinarian. Its purpose is not to be prescriptive but rather to convey some principles and concepts that colleagues may consider in their own investigations.
Infertility investigations, while following similar principles, are very different in format and usually retrospective, and as such are not specifically covered by this paper.
The key principles the author applies in investigations
Meeting the needs of stakeholders in order of precedence
As government veterinarians funded by regional rates on livestock owners, District Veterinarians have a precedence for each of their several stakeholders' needs. This contrasts with the usual stakeholder needs of the private veterinary practitioner.
The livestock private veterinary practitioner, while working within the framework of professional standards, often has a precedence where the needs of the busines are met (a satisfied client who will happily pay, repeat business and recommend to others), before considering needs of their other clients ("is this something that will impact them and that I can pass on?""), before considering the possibility of state level conditions (e.g. lead poisonings). Often Emergency Animal Diseases (EADs) are not actively excluded but rather are only considered when a diagnosis has not been found (Hendra is a clear exception to this).
This often leads to symptomatic treatment as the first step (if an animal responds the client is satisfied regardless of the cause). Due to time cost pressures, comprehensive autopsies are often not viewed as cost effective, with a narrow range of samples (e.g. not removing the brain), paddock inspections rare and further research into the diagnosis often only occurs if the animal or group does not respond and has not died.
In contrast, the District Veterinarian has a clear precedence of first meeting the needs of the NSW government (who has national and industry agreements and program obligations), followed by the needs of their regional LLS (who has a role in supporting the landholders who fund the service by warning or advising on emerging animal biosecurity issues) and finally the needs of the animal owner (primarily at an enterprise than an individual animal level).
Surprisingly, the needs of these multiple stakeholders can be easily met by developing a range of diagnostic differentials from the first contact and then at each stage of the investigation focusing on excluding or confirming relevant conditions for each stakeholder in order.
The initial differentials list includes possible EADs, possible notifiable and regulatory conditions, conditions of regional significance, conditions that are a threat to the enterprise and finally conditions affecting the individual animal.
Then as the investigation progresses through each stage, there is a focus on excluding the stakeholder needs in order. That is, EADs are excluded as early as possible (or even if not excluded made improbable and sampling and testing undertaken to confirm the preliminary exclusion), next a similar approach is taken to possible regulatory conditions, conditions that are of regional significance (e.g. first Bovine Ephemeral Fever case of an epidemic, plant poisonings that historically occur in regional clusters), conditions that pose a risk to the enterprise (e.g. first case of bovine respiratory disease in a mob of weaners) and then conditions that are sporadic and treatable.
Conditions that are usually limited to single animals and untreatable such as actinomycosis are of the lowest priority. However, this does not mean likely diagnosis on clinical grounds, and advice should not be provided. Rather it means it's the last stage in the investigation and that minimal resources and monies should be expended on confirming the diagnosis.
Four recent commonplace investigations by the author illustrate this process.
In the first, a sheep producer was concerned because they had deaths in weaners, along with swollen faces and drenching had failed to stop the deaths. Current wet conditions and high midge numbers resulted in Bluetongue being included in the differentials, along with haemonchosis and anthelmintic resistance. A property visit found the weaners did not have a temperature (excluding Bluetongue), were anaemic and that the producer was unaware that in an haemonchosis outbreak deaths may continue for a dew days following drenching.
In the second, ewes two weeks off lambing in February showed lameness and deaths. Footrot is uncommon in the author's district and foot abscess leading to secondary pregnancy toxaemia a relatively common winter complaint, but rare in summer. Inspecting the paddock found large damp areas and the yarded ewes to have foot abscess, excluding virulent footrot.
In the third, a mob of maiden ewes had developed a distinct low body condition tail over the previous few months. This was despite excellent pasture conditions and drenching with known effective anthelmintics. While there have been no known clinical Ovine Johnes Disease (OJD) cases in the district, restocking with Western Australian sheep after the 2017-20 drought was widespread. Clinical examination confirmed a distinct ill thrifty tail but no dagging. Clinical examination found increased lung sounds over all affected sheep but not over normal ewes. An autopsy confirmed chronic respiratory disease, and found no gross pathology consistent with OJD. Histopathology confirmed the OJD exclusion, and a negative faecal egg count excluded internal parasites.
In the fourth, a small mob of 5-month-old dairy beef steers had one death and another ill over two weeks. Reported clinical signs were vague but mentioned ataxia. In addition, the calves had been grazing on small blocks, including some with abandoned buildings around a village. Lead poisoning was included in the differentials along with the usual range of conditions affecting early weaned calves run on small blocks. Clinical examination of the ill steer found CNS signs and failure to retract the tongue. An autopsy found material consistent with lead fragments in the reticulum and fresh kidney was submitted to confirm or exclude lead poisoning. The lack of any other gross pathology excluded respiratory and intestinal causes.
A planned staged investigation
As noted in the section above, at and during each stage of the investigation, differentials should be again developed, and consideration given to how to exclude the priority conditions. The stages include history gathering, clinical examination and sampling, autopsy and sampling, response to treatment, selection of laboratory tests, liaison with laboratory and where applicable, re-visit to repeat the previous steps.
Obtaining an accurate history
History taking is the most important stage in ensuring a planned investigation. The author's experience is that this is the most difficult stage to perform accurately and that errors made here will have major consequences for the investigation.
Questions during the history collection include those that may support or exclude EADs and other significant conditions that the history suggests (e.g. cattle neurological signs, any connection to Queensland or Far North Coast NSW for babesiosis?).
The questions include investigating what is different in this mob, season etc. to other mobs or past practices.
Obtaining a veterinary history is made difficult by several factors. Barriers include lack of a trusting relationship, history giver has already determined what the condition is (more so with internet searches), the history giver's version is second hand, with original observations taken by a family member or worker and finally reluctance to admit that they had not undertaken standard animal health preventative measures if they feel they it does not have any relationship to the problem.
Key steps in building trust are avoiding being perceived as judgemental (e.g. assurance that more bad luck than poor management, even if it is bad management!) and despite the previously mentioned stakeholder precedence, providing assurance that the investigation is primarily focussed on the animal owner or manager ("we just need to rule out this serious disease as it could be very costly to you").
The key strategy in avoiding history inaccuracy, is to obtain the history as many times as possible, from as many different people as possible. The author's general practice is to obtain a detailed history at the initial telephone call, again on arrival as part of start of the investigation and again at the end of the investigation, often using a notebook and a pen as a prop for the third history taking. On occasion with complex cases, where there are inconstancies in the histories and laboratory results, further discussions of the history after the property visit can assist. Opportunity should also be taken to gather the history from workers, other family members etc. That variations are usually found between histories from different people and at different times from the same person indicates that veterinarians should always have an open mind about the accuracy of the history. The difference between an in-person history and over the telephone, also points to the disadvantage in not being able to use body language in the history taking process.
An advantage of the District Veterinarian is that a property visit and paddock inspection are direct cost free to the various stakeholders. Visiting the property has many advantages over animals presented at the office. An in-person history is superior, being on property as a guest creates a better veterinarian to animal owner relationship, there is opportunity to question others, the risks with the owner choosing the animals is reduced, the remainder of the mob can be inspected, and the paddock examined for risks unknown to the lay person.
In the author's experience animals brought to town for examination or autopsy, especially by others than the direct animal manager, create major risks for the investigation.
Clinical examination and sampling
When performing the clinical examination, the author's practice is to first check the key clinical signs that may exclude EADs and conditions of significance (e.g. sheep deaths and lethargy in periods of high Cullicoides presence - rectal temperature for fever to exclude Bluetongue, drooling, ill cattle - presence of mouth lesions to exclude vesicular diseases) and then progress through a systematic tail to head examination.
Wherever possible, several affected animals are examined, up to five, plus a few unaffected animals.
A common pitfall is that owners do not present animals with the typical presentation but rather ones that stand out to them for some reason (e.g. the longest and worse affected). This raises the risk that an animal presented is not affected with the condition affecting the other animals, or at the very least has a misleading presentation. This risk can be managed by awareness, requesting the 'typical' case be presented, asking why each animal is presented, examining multiple animals and observing the affected mob as a whole.
An examination of the unaffected animals in the mob for mild clinical signs (e.g. coughing, diarrhoea) is worthwhile. This can be successfully performed in the yards or in the paddock. Paddock inspections require turning the vehicle off and allowing time to view the mob both settled and moving. This also assists in assessing level of nutritional and other stresses on the mob and whether the affected animals have significant differences to others in the mob (e.g. Body Condition Score, breed).
Sampling during the clinical stage depends on the findings from the examination. Often it includes a prudent range of samples (clotted, EDTA and smear, faecal, lesion) based on the principle that samples can be disposed of if not required, but testing cannot be performed on samples not collected. Where a specific EAD requires exclusion that sample is taken. See comments below on resources to assist in determining which samples are required to confirm or exclude various differentials. When sampling to confirm or exclude mob level conditions (e.g. leptospirosis, Mycoplasma ovis) up to ten samples are collected (see comments below re testing).
Autopsy and sampling
In contrast to clinical examination, when performing autopsies, the same systematic method is followed for each autopsy. An exception to this is when performing multiple autopsies and a pathognomonic lesion is observed (e.g. ulceration pillars of the omasum with cardiac glycosides) on an earlier autopsy. In this situation, subsequent autopsies proceed as quickly as possible to this point.
When multiple carcasses are present, general practice is to autopsy three for sheep and two for cattle. This is increased if the gross pathology is inconsistent between autopsies.
Again, during the autopsy care is taken to check for gross pathology that excludes or confirms the ever-developing differentials list (e.g. haemorrhages at the base of the pulmonary artery and oral lesions for Bluetongue, thickening of the ileocecal valve for Johnes).
There is always a risk that the animal presented for autopsy is not typical of the problem condition. This is especially so when a live animal is presented for possible euthanasia and autopsy. These may be the worst animal and least valuable in the mob, and so suffering from a variety of other conditions.
Carcase side tests are a necessary part of autopsies. Where sudden death is part of the presentation Anthrax ICT tests on up to two carcasses are conducted prior to beginning an autopsy. This is done regardless of the likelihood of other causes. If during an autopsy the gross pathology is still consistent with Anthrax, another ICT test performed on a swab of the cut spleen. Where history suggests Nitrate / Nitrite poisoning aqueous humour is tested. Similarly, enterotoxaemia can be confirmed by urine test strips finding a glycosuria.
Generally, fixed samples are taken from liver, kidney, myocardium, lung and brain and from any affected organs. Fresh samples are taken from affected organs. Again, sampling is based on the principle that samples can be disposed of if not required but testing cannot be performed on samples not collected.
Specific tissues are taken when required to exclude significant conditions (e.g. fresh kidney lead poisoning nervous cases, ileocaecal valve for Johnes, tonsils and lymph nodes for African Swine Fever). See comments below on resources to assist in determining which samples are required to confirm or exclude various differentials.
Response to treatment
This can be quicker than laboratory tests in confirming and excluding the current differentials. Common examples include response to Calcium/Magnesium/Glucose injections in metabolic cases, thiamine for thiaminase caused polioencephalomalacia and anthelmintics for internal parasite caused diarrhoea.
Selection and interpretations of laboratory tests and lab liaison
A laboratory testing plan is developed that is staged and addresses the potential differentials.
Laboratory testing is directed initially to exclude significant conditions and confirm likely diagnoses (e.g. nervous case PEM history fresh kidney to exclude lead and brain histopathology to confirm polioencephalomalacia).
All samples collected are submitted to the laboratory to allow additional tests should the initial tests be negative (e.g. lambs acute diarrhoea - faecal culture only performed if worm egg count and coccidia negative).
Sometimes samples are submitted with a request that the duty pathologist telephone to discuss the best laboratory testing plan.
Laboratory testing is targeted avoiding 'fish expeditions' or 'shotgun testing'. Not only are these wasteful of available laboratory testing funding or owner's monies, but they often raise risks with misleading results. See comments below on red herrings.
Where the testing is at the mob level and ten animals were sampled (e.g. leptospirosis, Mycoplasma ovis) five animals are initially tested, with the potential to test another five if the results are equivocal.
Laboratory results are not viewed in isolation from the history, clinical findings and history. If the history, clinical finding and autopsy all point to one condition, laboratory results that suggest otherwise are considered with scientific scepticism, and an awareness of the test's sensitivity, specificity and predictive value.
The District Veterinarian has a very considerable advantage in having the opportunity and time to develop a relationship with the laboratory, individual pathologists and undertake repeated consultation on individual cases.
Often laboratory pathologists have superior knowledge of available testing techniques (especially more recently developed tests), the sensitivity and specificity of tests and of other cases submitted to the laboratory for similar syndromes and conditions.
In contrast the District Veterinarian, has knowledge of current and historical conditions common in their district, usually a greater understanding of the history and clinical presentation and thus the predictive value of the test. From experience they may also be aware of the applied sensitivity and specificity of some older tests, especially histopathology.
Advice to owner based on risk assessment
Advice to the animal manager is provided within a risk assessment and management framework that considers the likelihood of the possible differentials at this point of the investigation, the impact of each differential and whether there are practical measures to manage the impact of these risks.
This is often provided after the field investigation but before laboratory tests have confirmed the diagnosis. For example, a couple of ewes in a large mob near lambing showing circling and head tilt. Listeriosis, polioencephalomalacia and pregnancy toxaemia are all differentials. All appear unlikely as there is no supplemanatry feeding, no known polioencephalomalacia plants and sufficient pasture. If either listeriosis or polioencephalomalacia is the cause, there will be a limited number of cases. In contrast, an unmanaged pregnancy toxaemia outbreak could lead to major losses. After the risk assessment, the advice at the time of the property visit is to provide extra energy by feeding cottonseed, as well as treating any cases with thiamine and antibiotics. A paddock shift was considered to manage the polioencephalomalacia risk, but all other paddocks had insufficient feed for lambing ewes.
Red herrings in the history, clinical signs, gross pathology or laboratory results
Red herrings have the potential to derail an investigation, delaying correct diagnosis or causing the diagnosis to be missed.
Red herrings the author has fallen for include: assuming that the current case is just the latest in a recent cluster of cases (e.g. assuming death with subcutaneous haemorrhages is the latest of a series of local Cheilanthes seiberi poisonings, not Anthrax), when deaths recommence in an outbreak that the cause is the same (e.g. bovine respiratory disease in paddock steers, outbreak recommences when shifted to another paddock for easy yarding for treatment, with a lead source causing poisoning), a strong epidemiological feature of the affected mob that is actually not related to the condition (e.g. the condition only occurring in introduced mob and not home mobs but it was the hay they were fed), and assuming that laboratory tests have a 100% sensitivity and specificity (e.g. sheep dying of hypocalcaemia with normal range blood calcium). Laboratory results red herrings are more likely with the greater the number of tests requested, again stressing the need for targeted testing.
Gross pathology red herrings the author has chased include: mistaking yellow tissues due to carotene feed for jaundice (e.g. canola hay feeding, rumen impaction not Theileriosis), assuming swollen livers are primary not secondary to cardiac and circulatory problems, mistaking blood pooling in loops of intestine that causes mucosal congestion for a disease process, assuming gravel and other material in reticulum of animal with nervous signs is lead poisoning rather than aberrant pica following CNS disease of other causes, agonal haemorrhages of the myocardium and repeatedly assuming coloration of lung lobes and foam in the trachea must be respiratory disease.
Resources they can utilise to assist with decision making and advice during the investigation
District Veterinarians have several resources they can use to assist with investigations.
District Veterinarians have the advantage of being able consult with District Veterinarian colleagues who have more experience with specific conditions that occur commonly in their district. This is especially useful when considering conditions that while common in other parts of NSW are rarely seen in your district. Besides direct contact, the District Veterinarian email list is often elucidating.
Australia Veterinary Association Sheep, Camelid and Goat Veterinarians and Cattle email lists are also useful in consulting with other livestock veterinarians.
As noted above, the relationship with laboratory pathologists and other staff is a key resource in any investigation.
The NSW DPI veterinary laboratory website is also a valuable resource outlining required samples and available tests for various conditions. Consulting these after the initial telephone history before the property visit and when considering the testing program is often rewarding.
Similarly, consulting the relevant DPI procedure for a regulated disease may assist.
While the quality and volume of web accessible resources is constantly increasing, outside of www.flockandherd.net.au there is dearth of information on the likely presentation of diseases of extensively grazed livestock in NSW.
At the time of writing, the exclusion of EADs is well served by Emergency animal diseases, A field guide for Australian veterinarians (www.outbreak.gov.au) as well as current NSW DPI advice for veterinarians for specific current risks (e.g. African Swine Fever).
Useful texts covering beef cattle in Australia include recent Diseases of Cattle in Australasia Parkinson et al., and Veterinary Medicine, A textbook of the diseases of cattle, horses, sheep, pigs and goats (10th or 11th edition).
In contrast, sheep medicine in Australia has few recent texts. The Practice of Sheep veterinary Medicine by Kym Abbot covers most conditions and contains references to articles for more detail.
Diagnosis of diseases of sheep by John Plant (University of Sydney. Post-Graduate Foundation in Veterinary Science) is an excellent text for developing differentials for various syndromes.
Chris Bourke's Australian Veterinary Journal article, The clinical differentiation of nervous and muscular locomotor disorders of sheep in Australia, doi.org is of great assistance in developing and excluding differentials for ovine nervous conditions seen in NSW.
Toxicology for Australian Veterinarians by Ross McKenzie is an incredibly comprehensive ebook based on Australian reports of poisoning.
Finally, the Australian Bureau of Animal Health book series from the early 1980s is often again the best guide to how conditions present in Australia and are regularly consulted by the author. These include Viral diseases of farm livestock by W. Beveridge, Chemical and plant poisons by A. Seawright, Viral, bacterial and fungal diseases of poultry by Beveridge & Hart and Bacterial diseases of cattle, sheep and goats by W. Beveridge