I was asked this question recently. There are many other tools out there including virology, bacteriology, molecular diagnostics (DNA testing), serology (looking for antibodies or pathogen antigen), epidemiology and so on and so forth. And when I was asked this question, I naturally answered, “Pathology is the most useful and versatile tool for disease investigations”. The discipline of pathology covers necropsy/autopsy and microscopy (histopathology). Why did I say this?
For my Masters project, I utilised the discipline of pathology to provide a case definition for Devil Facial Tumour Disease that was affecting Tasmanian devils. There was a lot of talk about it being caused by toxins, by a virus and a combination of other things. From the work my team and I did, we found no such thing. To see what we found, see the following articles:
The Pathology of Devil Facial Tumor Disease (DFTD) in Tasmanian Devils (Sarcophilus harrisii).
In my thesis, I entered a quote from John Hughlings Jackson which states, “The study of the causes of things must be preceded by the study of things caused”. And from the recent movie World War Z, “Mother nature leaves clues.” Why do these quotes and pathology go hand in hand? It is because when you are investigating a disease outbreak, you won’t necessarily know what you’re dealing with. And so, it’s impossible to study what has caused the disease without first trying to identify what has caused it. But to try to identify what’s caused it, you need to find the clues.
Quite often, we can see the pathogen (the ‘bug’) at necropsy or on microscopy and we have our diagnosis. But what happens if the pathogen is too small or invisible? What then? When the host animal is under attack by a bug, the host animal will leave us with clues. They do this by reacting to the bug in a certain pattern. The clues we get from looking at the reaction of the host to the bug can be very revealing. For example, if we’re dealing with a bacterial disease, special white blood cells called neutrophils will be recruited to fight off the bug; in the case of fungi, you may get macrophages; parasites, eosinophils; viruses, lymphocytes and so on. We are able to identify diseases that are not only infectious, but also, those associated with nutrition, water quality, toxins, trauma and cancers. We may also be able provide an estimate on how long they’ve had the disease. It is only by examining the host through the discipline of pathology that we can gauge what’s happening. It draws heavily on being observative, this is the discipline of pathology. And when dealing with different animal species, you need to be able to compare what’s known in one species, with another. This is the discipline of veterinary pathology.
The great thing about pathology is that it can draw upon the other disciplines where necessary. So once you have found the clues, you can then utilise other disciplines from molecular diagnostics, bacteriology or virology. If you were to use any other tools first, you would be wasting valuable time and resources. There is no point chasing viruses using molecular (PCR) techniques when what you’re dealing with is a bacterial, parasitic or water quality issue. You need to take a broader approach to things.
