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Bovine Prototheca mastitis is an infection of the udder caused by a type of green alga that lacks chlorophyll. The disease occurs worldwide and cannot be cured. Up to this time the species Prototheca (P.) zopfii was said to be the causative agent of this infectious disease. However, the classification of the genus Prothotheca has undergone considerable changes in the last couple of years. In 1985 P. zopfii was subclassified into three so called variants. By the use of genetic differentiation these variants were reclassified as P. zopfii genotype 1, P. zopfii genotype 2 and a new species P. blaschkeae.
The aim of this study was the identification of the actual etiological agent of Prototheca mastitis and to increase the level of knowledge concerning the ethiopathogenesis of this disease. Thus a mastitis infection model in cattle was developed with the aim of monitoring the host-pathogen interaction during a period of three weeks after experimental infection. The tests and assays which were performed daily included physical examination of the seven test animals, determination of shedding of the agent in milk and faeces and several milk parameters were investigated as well. In addition blood samples were taken regularly for analysis. At the end of the experimental period the animals were dissected and the collected tissue samples analyzed microbiologically and histopathologically.
The results obtained in this study showed that only those quarters of an udder which were infected with P. zopfii genotype 2 developed signs of clinical mastitis. Affected quarters were hard, presented diffused swelling and the quarter milk yield was reduced by more than half. Shedding of the agent in milk was also strikingly higher in quarters infected with P. zopfii genotype 2 compared to those infected with P. zopfii genotype 1 and P. blaschkeae. At no time had there been any shedding of Prototheca spp. in faeces and changes in blood count were marginal in all cows. By using an indirect ELISA on whey it was demonstrated that antibodies against P. zopfii genotype 2 were produced at a much higher number than antibodies against P. zopfii genotype 1 and P. blaschkeae. The results of the examination of the tissue samples also revealed a major difference between the three infectious agents investigated in this study: Only P. zopfii genotype 2 could be found outside the udder. Histopathological findings in tissue sections obtained from quarters that had been infected with P. zopfii genotype 2 always included inflammatory changes with a pyogranulomatous character, whereas inoculation with P. zopfii genotype 1 did not cause any signs of inflammation. Experimental infection with P. blaschkeae lead not only to different types but also to varying intensities of inflammation.
The results of the present study prove P. zopfii genotype 2 to be the causative agent of clinical Prototheca mastitis in cattle. P. zopfii genotype 1 seems to be an ubiquitous and non-pathogenic agent, whereas P. blaschkeae seems to have the ability to cause subclinical mastitis.