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Histology

Lesion development in birds

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AVM is characterized by the presence of microscopic vacuoles in the white matter of the central nervous system (CNS), with rare involvement of the peripheral nervous system (Thomas et al., 1998; Augspurger et al., 2003). These lesions vary in severity, and have been documented in the brainstem, optic lobe, white matter tracts in the cerebrum, and the cerebellar folia of birds diagnosed with AVM (Thomas et al., 1998; Larsen et al., 2002; Augspurger et al., 2003). This ultrastructural change in brain tissue is very distinctive and is used to positively diagnose AVM. This particular type of lesion, called an intramyelinic edema, is caused by swelling and splitting of the lamellar myelin at the intraperiod line, which leads to vacuole formation and subsequent fluid accumulation (Duncan, 1985). The myelin sheath enables salutatory conduction of nerve impulses and myelinopathy can interrupt normal nerve impulse transmission (Duncan, 1985). Behavioral symptoms of AVM are consistent with damages to the nervous system, and include difficulty swimming or flying, limb paresis, decreased withdrawl reflexes, and loss of righting reflex (Thomas et al., 1998; Larsen et al., 2002). Lesions have persisted even after resolution of clinical symptoms (Larsen et al., 2002). Characteristic AVM lesions have been documented in birds not exhibiting clinical symptoms of the disease (Fischer et al., 2002). Partial to complete recovery has been documented in American coots after removing the birds from the affected habitat (Larsen et al., 2002). Bald eagles diagnosed with AVM have not survived longer than 24 hours after capture even with intervention and veterinary care (Thomas et al., 1998; Birrenkott et al., 2003).

Lesion development in fish

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Control carp brain (left panel) and AVM affected carp brain (right panel)

One mitigation strategy to protect birds from the toxin associated with AVM is to eliminate aquatic plants that provide substrate for the suspect cyanobacterium. Triploid Chinese grass carp (Ctenopharyngodon idella) have been used in successful, cost-effective, and long-term vegetation control in large, man-made reservoirs, similar to those affected by AVM (Haynie 2008) In order to answer the questions regarding the potential use of grass carp to control invasive aquatic plants in systems affected by AVM we conducted a series of field and laboratory experiments. The objectives of this study were: to determine if grass carp are affected when feeding on aquatic vegetation with the suspect cyanobacterium and if these fish will function as vectors of the AVM toxin if consumed by predatory birds. The initial trial indicated that grass carp could develop lesions similiar to AVM positive birds. Feeding trials with the fish, however, did not produce AVM lesions in laboratory chicken trials.