Special focus issue on invertebrate pathology: A growing discipline requiring veterinary diagnosticians

Comparison of disease mechanisms and morphology across species is central to the discipline of veterinary pathology. Origins rooted in public health and food safety have led to a historical focus on vertebrate species. Yet, there are over 30 times more invertebrate species on Earth, requiring the same expertise for their welfare and survival. While investigations of invertebrate pathology arguably predate the veterinary profession, in premodern times invertebrates were primarily studied as biological curiosities, production animals (silkworms and honeybees), and agricultural pests, as outlined in this issue’s commentary on the history of invertebrate pathology. ⁶ The last century continued on this theme but with greater attention to their role as intermediate hosts or vectors for zoonotic infectious diseases, or as enigmas to comparative oncology (eg, disseminated neoplasia of clams). ¹⁷ More recently, invertebrate pathology has expanded in the context of food production (eg, amoebiasis in farmed honeybees ¹³ and viral diseases in spiny lobsters ³ ) and species conservation. It is in these niches where the greatest engagement of veterinary pathology slowly emerges, but with great promise and need.

Many invertebrates are keystone species that provide critically important ecosystem services. These include habitat production or stabilization, herbivory, waste removal, nutrient cycling and food provision to higher trophic levels, purification of water and air, and pollination. One recent feature of the Anthropocene epoch is the large number of invertebrate species that are now imperiled, affected by poorly understood declines, diseases, or mortality events. Their loss profoundly deteriorates ecosystems. Hope relies on identifying aspects of disease occurrence that can be mitigated. Responses to their loss often focus on propagation and restoration efforts; however, the success of these endeavors simultaneously depends on improved understanding of the diseases causing their decline, and the impending diseases that inevitably emerge when animals are “farmed”; for example, a densovirus outbreak in commercially farmed mealworms. ² In any case, disease specialists are needed.

Veterinary diagnosticians may be reluctant to engage with invertebrate species as residency training programs and qualification examinations traditionally focus on domestic mammals. However, those who blaze the trail will find exciting discoveries and opportunities. Basic medical principles are largely applicable, as demonstrated as early as the 19th century by Pasteur’s documentation of silkworm microsporidiosis. ⁶ Moreover, invertebrate disease investigations are increasingly supported by a rapidly growing body of literature, including texts which summarize their clinical management and normal histology.^15,16 Of course, several challenges also await, but these will best be confronted by a One Health approach, involving interdisciplinary groups that include veterinary diagnosticians in addition to experts to the relevant taxa and paraclinical scientists already on the forefront of invertebrate health issues.

The articles in this series demonstrate several themes that test the traditional vertebrate-focused medical approach when applied to invertebrates. As we journey beyond the veritable dark ages of invertebrate medicine, where diagnostic investigations have comparatively seldom been undertaken, it is no surprise that there is limited knowledge regarding taxon-specific responses to injury. There is a tendency for invertebrate disease literature to focus on the presence or absence of micro-organisms without much detail to host responses. This approach, which lacked morphologic characterization, confounded initial investigations of aspergillosis in sea fans. ⁸ Extrapolations from vertebrates regarding lesion pathogenesis or morphology may not readily apply in some scenarios. Lesions may be observed that are poorly described or do not integrate well into nomenclature developed in mammals. Primitive species may have minimal responses or produce nonspecific lesions representing stereotypical responses that do not correspond to a distinct cause. Etiologies may be completely unfamiliar, undetected by conventional methods, or complicatedly multifactorial, such as in mass mortality events of the noble pen shell. ⁵ Interpretations of diagnostic investigations may easily be thwarted by a void of data addressing “background” lesions, normal microbiota, or even the ability to accurately classify invertebrates as healthy or sick prior to death. Experimental trials are often not possible due to inabilities to cultivate an etiologic agent or re-create a reasonably representative environment in the laboratory setting. These challenges emphasize the need for: (1) baseline health studies which help define “normal,” such as the study of commensal organisms in Madagascar hissing cockroaches, ¹⁸ or background lesions of freshwater mussels; ⁴ (2) basic research addressing morphological responses to injury, like defining ocular reactions in cephalopods; ¹⁰ (3) augmented efforts in describing and categorizing gross and microscopic lesions, as demonstrated by the large-scale retrospective study detailing pathology in nearly 600 cephalopods; ¹⁹ (4) development of standardized nomenclature, as provided in a guide to scleractinian coral pathology; ¹² (5) integration of cutting-edge technologies with traditional morphologic assessments, like the integration of quantitative polymerase chain reaction, transmission electron microscopy, and histology to diagnose viral infection in spiny lobsters; ⁹ and (6) veterinary engagement with other disciplines, as demonstrated by the collaboration among plant pathologists, entomologists, and veterinary pathologists to diagnose fungal disease in cicadas. ¹ As with other animals, veterinary pathologists can direct the stepwise diagnostic process that makes sense of findings and can meaningfully contribute to disease surveillance and appropriate response.

It is hoped that this special focus issue will encourage veterinary pathologists to engage with the discipline of invertebrate pathology. Its articles were selected to familiarize the readership with emerging issues and pertinent questions, including how to perform mortality investigations in invertebrates.^14,21 It includes several case studies,^{1,2,3,9,11,13,17} which demonstrate how traditional investigative and descriptive pathology methods can effectively improve our understanding of invertebrate diseases, even in the face of the many challenges posed by invertebrates. Articles, including studies on arachnids and giant African millipedes,^7,20 bring together current knowledge so as to enable diagnosticians wishing to engage in important frontiers. These are fundamental steps in a rapidly evolving and improving field. As one reviewer for this special focus issue astutely pointed out, the challenge is to build the frameworks and then refine them with new information, and disease specialists are crucial to this process.