Environmental relationships and anthrax epidemiology: field experiences of host resistance as opposed to dose-dependent experiments

Abstract

Even though anthrax is a disease of antiquity that has been studied for centuries, serious concerns have been raised about our understanding of its epidemiology. Since the 1960s, we have based the epidemiology of anthrax on the results of dose-dependent experiments, especially those involving cattle at that time. In this species the experiments demonstrated that the severity of infection was dependent upon the numbers of Bacillus anthracis spores ingested. The opinion was that ingesting only a few spores would be insufficient to cause an apparent infection; any infection that resulted would be latent (i.e., unrecognized). Based on the results of these experiments, it was accepted that the ingestion of large numbers of spores was the source of infection for hundreds of anthrax outbreaks. However, many investigations of both human and animal anthrax outbreaks have failed to identify sources of large numbers of spores, suggesting that these outbreaks are only rarely a consequence of ingestion or inhalation of large quantities of spores. This opinion piece builds upon the indirect evidence previously presented in an article focused on the existence of latent infections. Much of the evidence for the existence of latent infections was predicated upon a reduction of host resistance, which revealed how latent infections could be a source of more severe forms of the infection. That is, a latent infection can be the source of a severe infection, but the cause of the severe infection is the reduced host resistance. That first article concentrated on the arguments for latent infections, while this article concentrates on the arguments for host resistance. Host resistance is virtually impossible to measure objectively in the field. To provide a subjective measure of host resistance during anthrax outbreaks, we suggest the use of the opinions of livestock owners and or their veterinary practitioners and or field workers during investigations of anthrax outbreaks. When veterinary personal work in the field they are much like field biologists. In some ways field biologists better appreciate environmental factors, population ecology and other perspectives that are of use to epidemiologists. The more diverse the information the better the epidemiology is understood. To this effect we present our personal anecdotal and theoretical ideas from our experiences as well as a collection of bibliographic observations from others'. Our conclusions are that a combination of latent infections and reduced host resistance based on the host's relationship with its environment would better explain the epidemiology of severe infections in anthrax outbreaks for which large quantities of spores have not been located. This applies especially if the area has a history of the disease and/or if necropsies have shown the presence of latent infections in otherwise normal animals in the area and/or if environmental conditions are considered stressful and include intense insect activity.

Keywords: Anthrax; Dose-dependent model; Epidemiology; Host resistance; Latent infections; Peracute infections.