Veterinary Microbiology and Microbial Disease by P.J. Quinn & B.K. Markey & F.C. Leonard & E.S. FitzPatrick & S. Fanning & P.J. Hartigan

Author:P.J. Quinn & B.K. Markey & F.C. Leonard & E.S. FitzPatrick & S. Fanning & P.J. Hartigan
Language: eng
Format: epub
Publisher: John Wiley & Sons
Published: 2011-06-12T16:00:00+00:00

Further reading

Davidson, W.R. and Nettles, V.F. (1977). Rhinosporidiosis in a wood duck. Journal of the American Veterinary Medical Association, 171, 989–990.

Meyers,D.D.,Simon,J.andCase,M.T.(1964).Rhinosporidiosis in a horse. Journal of the American Veterinary Medical Association, 145, 345–347.

Chapter 49

Pneumocystis carinii

Pneumocystis carinii is a unicellular organism with a life cycle resembling that of a protozoal parasite. On the basis of significant genetic evidence it is classified in the family Pneumocystidaceae, phylum Ascomycota, in the kingdom Fungi (Pixley et al., 1991). Although the life cycle of P. carinii has not yet been fully determined, intrapulmonary infection probably involves asexual and sexual phases. The life cycle involves a thin-walled haploid trophic or vegetative form which can undergo binary fission or can enter into sexual reproduction through conjugation. Mating results in production of a diploid zygote and the initiation of sporogenesis. This is followed by transitional changes involving three consecutive sporocytic stages into a thick-walled cyst containing eight spores. Following their release, these ascospores enter a vegetative stage, completing the life cycle. A cell wall is present only in the cyst form of the organism. Because the main sterol in this thin cell wall is cholesterol rather than ergos-terol, this form of the organism is refractory to standard antifungal drugs. Recent studies suggest that benign colonization by Pneumocystis carinii in the lungs of immunocompetent mammals is relatively common, resulting in carriage and airborne transmission to susceptible in-contact animals of the same species (Chab é et al., 2004). In immunocompromized individuals, infection may result in pneumonia. Pneumocystosis in humans is often associated with immunosuppression following HIV infection. The organism has been found in a wide range of domestic, wild and captive mammalian species. Formerly, the genus was described as having a single species. Current molecular studies suggest that P. carinii is made up of a heterogeneous group of genetically isolated strains that have undergone functional and genetic adaptation to their respective host species over millions of years. Strains of P. carinii from different animal species exhibit a narrow host range and have distinct genetic and antigenic profiles (Peters et al., 1994 ). A trinomial nomenclature has been proposed whereby the name of a particular strain or ‘special form’ (forma specialis ) is derived from the host species from which it originated. As an example, the name P. carinii f.sp. equi would apply to horse strains. In recent years additional species have been formally described and proposed: P. carinii and P. wakefieldiae isolated from rats, P. jirovecii from humans, P. murina from laboratory mice and P. oryctolagi from Old World rabbits. The organism is difficult to culture in vitro.

Key points

Member of the kingdom Fungi

Distinct strains appear to be associated with particular animal species

Difficult to culture in vitro

Trophic, cyst and spore forms may be found in lungs of affected animals

Pneumonia, which occurs only in immunosuppressed animals, occasionally affects horses and dogs

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