Getting the right diagnosis

In last month’s article, I introduced the concept that preventative herd health management is about assessing the risks, determining their likelihood and consequences, and then putting control measures in place to manage, mitigate or eliminate those risks.

Building from this idea, this month I want to talk about the importance of making a diagnosis when disease is occurring on your farm.

Knowing the organism that is causing disease is a critical part of developing a preventative strategy, not just because we can use specific treatments or prevention measures for that organism, but by determining what organism has caused a current outbreak of disease, we are able to understand what the underlying issues might be that allowed that bug to cause the disease in this case.

Most farmers I work with are very good at identifying the victim(s) and can usually identify the syndrome that is causing the illness.

An example would be a group of sick calves with pneumonia.

Often, based on previous experience or a treatment protocol, the pneumonia cases are treated with a particular medication and if the outcome seems good, no further investigation is done.

Historically, as a veterinarian, I would see my job was to determine what organism was causing the calf to have pneumonia and I would perform postmortems and collect various samples to identify what bacteria or virus is causing the pneumonia.

This ‘work up’ may occur because a farm has not previously experienced this issue, because the cases are failing to respond to treatment, because the incidence is very high, or the losses are significant.

Once the lab results are back, we arrive at our ‘diagnosis’ when we identify the organism causing the syndrome.

Armed with the knowledge we have gained from our diagnostic work, we might choose a different treatment or decide to introduce a specific vaccine to prevent the specific disease we have identified.

If we just stop there, I am now convinced that only half the job is done.

Achieving a diagnosis should be seen as the beginning not the end when deciding on what is the cause of a disease outbreak.

Once we understand there are risk factors that have allowed one particular bug to cause disease, failing to fix those underlying problems means that any other control measures that specifically target just the one cause of the current outbreak will probably fail to reduce the overall incidence of the syndrome because as we prevent one potential cause, there is always another waiting in the wings ready to ‘step up’.

A proactive approach to animal health considers and then seeks to manage the risk factors which have resulted in the particular bug that caused the disease we have diagnosed to flourish.

By putting in place steps to manage or eliminate those risk factors, we could reduce the future likelihood of the disease from even occurring, which is clearly a better option than reaching for any treatment.

Using the example of calves with pneumonia, as a farmer I might reach for a trusty bottle of antibiotics which seems to cure the infection in most cases (but not all) and the new cases will likely continue and losses will accumulate.

As a vet, I might diagnose the pneumonia is caused by Mannheimia after a tracheal wash and some postmortem samples and advise a preventative vaccine and a ‘better’ antibiotic which may reduce the current cases.

But in this example, knowing that Mannheimia is a bacteria that we find in the respiratory tract of healthy animals, it is more likely that the underlying real cause of pneumonia is that multiple stressors are causing the calf’s normal defence to be compromised, and the Mannheimia bacteria that were already there waiting, gained access to the compromised lungs.

The risk factors in this case we must consider include overcrowding, inadequate nutrition, poor ventilation and shed design, immune suppression caused by heat stress, cold stress, high humidity or concurrent disease, water quality, failure of passive transfer and mixing of age groups.

If we simply decided just to start vaccinating for Mannheimia without addressing the underlying risk factors, it is likely that at some later stage another of the common respiratory bacteria or viruses would just step in and cause disease and the farmer would see no real overall measurable improvement in the syndrome we know as pneumonia.

Dr Rob Bonanno is a Prodairy regional veterinary lead based in Gippsland, Victoria.