Microsporidia-host interactions in beneficial arthropods: Factors that influence chronic disease

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Microsporidia are spore forming, eukaryotic pathogens that cause chronic infections in a wide range of invertebrate and vertebrate hosts. Depending on the host infected, these pathogens cause economic loss and are of veterinary and medical importance. Microsporidia live inside host cells and are fully dependent on their hosts for survival. These pathogens lack functional mitochondria and their reduced genome has resulted in the loss of many metabolic pathways. As a result, microsporidia require host energy for their reproduction and survival. Microsporidia are being described with increased frequency from beneficial arthropods, both those that provide pest control in nature and those that are mass-reared for use as natural enemies in biological control programs. Although microsporidia are defined as pathogens (organisms that grow and develop at the detriment of the host), recent studies have shown that microsporidia do not always cause negative effects with respect to host fitness. In some cases, microsporidia cause chronic debilitating disease, whereas other related hosts remain seemingly unaffected even though the pathogen is able to exploit the latter for its development and proliferation. The characteristics/factors that permit microsporidia to be successful pathogens in some hosts, while remaining unsuccessful in other related hosts, remains poorly understood. The purpose of this proposed study is to evaluate some of the factors that may be associated with pathogen success and could ultimately influence the impact of chronic disease. Our past efforts have focused on the identification and interaction of microsporidia in beneficial insects used for biological pest control, recently including microsporidian pathogens among several lady beetle species. Our more current studies have provided some insight on host specificity of microsporidia and host-pathogen effects. Microsporidia seem to have a relatively broad host range in lady beetles, oftentimes having pronounced effects on larval development while having no observable, detrimental effect on adults. Furthermore, microsporidia affect some lady beetle species profoundly whereas others closely-related species seem to remain unaffected. In some cases, the latter become filled with microsporidian spores (evidence that they serve as a suitable host for the pathogen) and even transmit the pathogen while remaining unaffected. Because microsporidia are designated as pathogens in the strict sense, the question remains as to what role these pathogens play in the many hosts that they infect and what enables microsporidia to become successful pathogens in some hosts while remaining rather benign in others. The proposed study will focus on factors that may influence the success of microsporidia as pathogens that cause chronic disease. These include (1) the impact of stress on the pathogen and disease progression, (2) the effects of multiple infections (more than one pathogen per host) on disease development and host fitness, and (3) the role secondary defense chemicals (alkaloids) in lady beetles on pathogen development and host fitness. Information gained during this study will help contribute toward our current understanding of disease epizootics and the role that chronic disease plays in the regulation of beneficial arthropod populations. Our increased understanding of chronic disease is important to help ensure that beneficial arthropods remain a viable alternative to chemical pest control in agriculture. Furthermore, microsporidia infections are becoming more common in humans, primarily those with a compromised immune system. As a result, microsporidia are emerging pathogens of increasing medical importance.