It can be quite diffi cult to determine the cause of a disease. Factors that are thought to be causative factors of disease are called putative factors because their occurrence is often correlated with presence of the disease, but eventually evidence of their association is necessary before a cause and effect relationship is confirmed. The cause of a disease is often called the etiologic agent . It is also common to discuss the sign or clinical sign to indicate objective evidence that a disease is present, such as hair loss. It is unusual to use the term symptom with respect to diseases of animals, as this is a subjective assessment such as pain or lack of energy, and is something that cannot be communicated to humans by animals. It is interesting to note that entomologists have developed slightly different terminology than have wildlife biologists when describing diseases; entomologists use ‘ sign ’ to indicate physical manifestations of disease, and ‘ symptom ’ to indicate changes in behavior or function attributed to disease.
In some cases, criteria have been established to defi ne whether or not a cause and effect relationship exists between a disease and a putative cause. Often, healthy animals are exposed to a putative cause; if the disease develops, causation is considered to be proved. For infectious disease agents, it is necessary to prove Koch ’ s Postulates to establish the relationship between a disease and its putative cause. The rules of Koch ’ s postulates are:
• the putative agent must be found in every case of the occurrence of disease;
• the putative agent must not be associated with the absence of disease;
• the putative agent must multiply in the host when provided the opportunity;
• the putative agent must be capable of being re - isolated from experimental inoculated individuals.
Though Koch ’ s postulates are generally acceptable in ascribing cause and effect relationships, in many cases disease results from many stressors, including some abiotic factors. Thus, disease may be expressed only when a combination of events or factors occurs. Commonly, disease has primary and secondary causes, or situations that predispose an animal to disease. For example, it is easy to imagine how poor diet or adverse weather could interact with microbial pathogens by weakening the host ’ s ability to fend off infection. Some diseases are latent , which means that the potential for expression of the disease is present, but expression is suppressed and the animal is not contagious until triggered by something in the animal ’ s biology or environment. Indeed, it is not uncommon for an animal to be infected (to have the disease - causing organism within its body), but for there to be no apparent dysfunction (lacking disease). The lack of detectable dysfunction can be due to inherent resistance (the same pathogen in another population or species induces expression of disease) or by the condition or vigor of the animal, which suppresses disease expression.
For noninfectious diseases (those lacking an infectious agent that can be spread from host to host) such as exposure to pesticides, it is not possible to apply Koch ’ s postulates. In these cases, we must be content with exposing healthy animals to the stressor and observing the animal ’ s response, or by searching for chemical residues in animals suffering from disease. This is not always satisfactory, as it is difficult to know the appropriate dose to test. Also, though it is easy to assess acute toxicity caused by high doses because the animal ’ s response is usually rapid, it can be diffi cult to assess the effects of chronic toxicity caused by low doses applied over a long period. Low doses can interact with host metabolism, such as the hormonal system of the animal, or with other agents such as microbial pathogens, and it can be difficult to identify the true factor responsible for poor animal performance.
