Raul Manjavacas Pintado defended this Degree Final Project

July 16^th, 2021

Myxomatosis is a characteristic viral disease of rabbits caused by classical Myxoma virus (MYXV), a leporipoxvirus introduced in Europe in the 1950s classically affecting European rabbits (Oryctolagus cuniculus) and causing high lethality. However, in 2018 Iberian hares (Lepus granatensis) from Spain exhibited symptoms compatible with myxomatosis. In the wake of this outbreak, a new recombinant strain of Myxoma virus (MYXV-Tol) was identified. In order to study the distribution of these two variants and their involvement in pathology, 14 animals (8 rabbits and 6 hares) from Madrid and Castilla-La Mancha were collected and necropsied at VISAVET-UCM. Samples were taken for PCR analysis against both MYXV and MYXV-Tol. Moreover, the macroscopic and histopathological study of lesions was carried out. The results obtained from the molecular study showed 7 rabbits and 1 hare positive to MYXV, 1 rabbit and 1 hare coinfected with both MYXV and MYXV-Tol strains, and 4 hares negative to both variants. Macroscopically, classical lesions of myxomatosis were observed (blepharoconjunctivitis, subcutaneous edema of the head and presence of myxomas) in both MYXV and MYXV-Tol positive cases, with greater intensity of the lesions in coinfected animals (78.91 points), followed by those infected with MYXV (61.36 points). Negative animals displayed less severe gross lesions (9.46 points). Histologically, the main skin lesions observed in infected animals were chronic hyperplastic dermatitis with intracytoplasmic eosinophilic inclusion bodies with myxedema in the lamina propria, and the presence of modified fibroblasts ("star cells"). Despite there were no differences between groups at the microscopic level, an identical high score (54.69 points) was observed in the MYXV positive hare, and MYXV and MYXV-Tol coinfected hare. In coinfected animals and the MYXV positive hare, cutaneous vascular lesions in myxomas (such as lymphocytic vasculitis, mural hyaline degeneration and recruitment of perivascular pericytes), lungs and spleen differentially stood out. To our knowledge, this is the first time that a MYXV and MYXV-Tol coinfection in rabbits and hares has been described in the literature, as well as the first case of an Iberian hare infected by the MYXV. These findings highlight the loss of host specificity of MYXV to indistingly infect rabbits and hares, and a greater sensitivity of hares to develop lesions after infection with any Myxoma virus strain