A retrospective study was conducted to determine case histories, microbiological characteristics, and molecular subtypes associated with Listeria monocytogenes infections of the eye in large animals. For selected cases, environmental L. monocytogenes contamination patterns on case farms were also evaluated to probe for potential sources and spread of listerial eye infections. Records of 170 L. monocytogenes isolates from animal infections were reviewed to determine the fraction of isolates associated with eye infections (conjunctivitis, keratitis, and uveitis) of animals and to gather information on the clinical history of these cases. Overall, 4 of 170 Listeria monocytogenes isolates were associated with eye infections; 3 of these had occurred in cows and 1 in a horse. Molecular subtyping (by EcoRI ribotying) showed that 4 different L. monocytogenes subtypes were isolated from these 4 cases; the same ribotypes had previously been found among invasive animal listeriosis infections. Although a variety of L. monocytogenes subtypes were isolated from environmental sources, on 1 farm, the same ribotype associated with the eye infection was also isolated from a fecal sample of a healthy animal and from a soil sample. The data reported in this study further suggest that L. monocytogenes can be a cause of eye infections in several animal species. Listerial eye infections do not seem to require specific pathogen-related virulence characteristics but rather seem to be a function of environmental or host factors, such as direct exposure of the eyes of susceptible animals to high numbers of the pathogen. Although listerial eye infections are rarely diagnosed because of its ubiquitous nature, L. monocytogenes may have to be considered more commonly as a causative agent of eye infections in ruminants and horses.

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Virus clones from 2 Marek's disease virus (MDV) isolates from flocks with low natural incidences of Marek's disease (MD) were, at most, only mildly pathogenic compared to clones from the highly virulent JM isolate. Clones from a single donor bird appeared to vary in pathogenicity. The infectivity and pathogenicity of CU-2 (low virulence) and JM-10 (high virulence) were compared with age, genetic constitution and virus dosage as variables. In JM-10 infection, S-strain birds had only neural and gonadal involvement, while P-line birds exhibited extensive visceral lesions and commercial-strain birds had primarily neural involvement. In contrast, with CU-2 infection, commercial-strain birds were free of lesions but neural involvement alone was seen frequently in S-strain birds and occasionally in P-line birds. Microscopic neural lesions were seen in CU-2 infection at 8 d and 10 d after infection but not later. With JM-10 infection, they were not observed until 14 d but then progressively increased in intensity and distribution. Virus dosage did not affect the pathogenicity of either virus. Day-old chicks were more susceptible than 4-week-old birds to JM-10, but not to CU-2 infection. Viral antigen was detected in thymus, spleen, and bursa equally with both infections. Levels of cell-associated infectivity were either higher in early infection than in later infection with CU-2 or remained at a very low level, while infectivity levels increased more slowly but progressively in the case of JM-10 infection.