The study of diseases affecting invertebrate animals, including etiology, pathogenesis, symptomatology, pathology, histopathology, physiopathology, and epizootiology. Interactions between invertebrates and their diverse pathogens, such as bacteria, viruses, and protozoa, range from obligate parasitism to various associations that may result in disease. Disease is usually described as a disturbance of the equilibrium between the invertebrate animal and the environment, and it should be understood as a process and not a thing. The disease process represents the response of an invertebrate organism to injury. Its occurrence is a normal biological phenomenon, and it has been recognized as a balancing factor in nature. Understanding invertebrate pathology is essential to understanding invertebrate life and behavior.
Infectious diseases of invertebrates have been thoroughly studied, while less attention has been devoted to noninfectious diseases. The latter involve mechanical and physiological injuries caused by chemicals, nutritional disturbances, inherited abnormalities (genetic diseases), tumors, predation, and the actions of other invertebrates.
Numerous diseases and pathogens of invertebrate animals have been described throughout the world. By 1999, in Mollusca as many as 46 diseases of oysters, 20 of clams and cockles, 19 of scallops, and 9 of abalone had been described. Diseases of Crustacea included 17 lobster diseases, 35 diseases of shrimp and prawns, 17 of crabs, and 13 of crayfish.
Immune reactions of invertebrates can be cellular or humoral. In cellular immunity, several types of hemocytes have been demonstrated. Three defense reactions have been recognized: phagocytosis, encapsulation, and hemostasis (coagulation and wound healing). Phagocytosis is localized in the plasma membrane and the cytoskeleton. Encapsulation occurs when bacteria, fungi, nematodes, or protozoa, as well as nonliving objects that are too large to be phagocytyzed, are encapsulated by plasmatocytes or granulocytes. Wound healing and coagulation in invertebrates differs from hemostasis in vertebrate animals.
All types of hemocytes involved in invertebrate immunity are called immunocytes. The recognition of foreign antigens is affected by surface receptors and molecules located on the plasma membrane of immunocytes. Parasitoids and parasites of invertebrates often possess defensive mechanisms that can overcome immunological reactions of their hosts. Humoral immunity of invertebrates provides the second line of defense against massive invasion, when immunocytes become depleted.
The immune system of invertebrate animals produces antimicrobial proteins in immunocytes or in the fat body. Arthropods do not possess antibodies, but they synthesize lectins that can agglutinate microorganisms. Cytokine-like molecules have been found in mollusks, worms, echinoderms, and arthropods. In many invertebrate animals, a prophenoloxidase-activating system can contribute to humoral immunity. Neuropeptides and opiates mediate phagocytosis in the earthworm, mollusks, arthropods, starfishes, and sea urchins. Complement-like molecules have been detected in all invertebrates, and in some the C-reactive protein has been reported. Obviously the immune system of invertebrates is very sophisticated, sharing many fundamental mechanisms with vertebrates.
Invertebrate pathology has applications in agriculture, medicine, general biology, and biotechnology. In agriculture the prevention of diseases affecting honeybees and silkworm, and the use of bacterial toxins and baculoviruses for microbial control of insect pests, constitute the most important applications of invertebrate pathology. See Annelida, Antibody, Arthropoda, Echinodermata, Immunity, Insecta, Mollusca, Nemata, Porifera