Infectious Agents

Chapter 63
Infectious Agents: Mycotic Abortion


Frank W. Austin


Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Starkville, Mississippi, USA


Introduction


Fungi are a diverse group of eukaryotes that includes yeasts, molds, mushrooms, rusts, smuts, and the recently reclassified microsporidia. They constitute one of the five kingdoms of life (Prokaryotae, Fungi, Protista, Plantae, and Animalia) in the current classification system. Like other eukaryotic organisms, fungi have membrane-bound nuclei, membrane-bound cytoplasmic organelles, and 80S ribosomes contained within a plasma membrane. Their membranes contain sterols, namely ergosterol, which determines cell membrane fluidity and acts as boundary lipid. Fungi lack chloroplasts, found in plants and some protists, and have cell walls that contain chitin, unlike other eukaryotes. Many can reproduce both sexually (teleomorphic state) and asexually (anamorphic state). There are several valid and recognized classification schemes for the kingdom Fungi. Recent phylogenetic analysis, based on ribosomal RNA and associated genes, divides the kingdom Fungi into one subkingdom, the Dikarya including the phyla Ascomycota and Basidiomycota, and five additional phyla composed of Chytridiomycota, Neocallimastigomycota, Blastocladiomycota, Glomeromycota, and the Microsporidia.1,2 Taxonomy based on molecular detection of genetic sequences has had little effect on the commonly known genus and species names of fungi, but did substantially change their higher taxonomic relationships. From a classical medical perspective, pathogenic fungi can also be classified into the dermatophytes, yeasts, dimorphic fungi (having two anamorphic forms), hyaline hyphomycetes (e.g., Aspergillis spp.), dematiaceous hyphomycetes (a disparate group of melanin-pigmented fungi), coelomycetes (anamorphic fungi that form conidia in a cavity called pycnidia), zygomycetes (aseptate fungi), and the basidiomycetes (a large group of fungi including puffballs, shelf fungi, rusts, smuts, and mushrooms that bear sexually produced spores on specialized cells called basidia).3 Additionally, based on morphology and reproductive features fungi have been historically classified into the Ascomycota (sac fungi), Basidiomycota (club fungi), Zygomycota (conjugation fungi, aseptate), Deuteromycota (fungi imperfecti, with no known sexual state), and the Mycophycophyta (lichens and symbiotic fungi).4 At a very basic level, fungi are easily separated into two major morphologic categories: the filamentous or true fungi and yeasts. Most fungi are ubiquitous saprophytes in the environment, which feed on and recycle decaying organic matter. They prefer dark moist environments in which to grow. Filamentous saprophytic fungi, represented by the hyaline hyphomycetes, dematiaceous hyphomycetes and the zygomycetes, are by far the most commonly encountered agents producing bovine mycotic abortion.


Mycotic agents of abortion


A wide variety of filamentous fungi and several yeasts have been reported to cause bovine abortion.5 Mycotic abortion is a sporadic event. The incidence of mycotic abortion varies from 2 to 20% depending on the environment, location, and time of the year and generally less than 10% of the herd is affected.6 The most common cause of bovine mycotic abortion is Aspergillus spp., responsible for approximately 84% of the cases reported. Of these, A. fumigatus was cited as the cause in 64% of 369 cases in which only one fungal isolate was obtained in culture.7 In mixed fungal infections, occurring in about 11% of the cases, A. fumigatus was found in association with other Zygomycetes in 87% of cases examined. In approximately 14% of the 369 cases examined, Zygomycetes were confirmed as the sole causal agents, constituting the second most common cause of bovine mycotic abortion. Zygomycetes were responsible for 21% of the abortions when considered with mixed infections. Dematiaceous ascomycetes as a group were associated with 7.5% of these cases. Yeasts of the genus Candida caused 2.4% of bovine abortions7 (Table 63.1).


Table 63.1 Fungi isolated in culture from 369 North American cases in which a single isolate was obtained.


Source: adapted from Knudtson WU, Kirkbride CA. Fungi associated with bovine abortion in the northern plains states (USA). J Vet Diagn Invest 1992;4:181–185.












































































































Fungus Classification/features Prevalence (%) in 369 cases
Filamentous fungi
Aspergillus fumigatus Ascomycota, hyaline hyphomycetes 64
Aspergillus terreus Ascomycota, hyaline hyphomycetes 7.3
Aspergillus flavus Ascomycota, hyaline hyphomycetes 2.7
Aspergillus nidulans Ascomycota, hyaline hyphomycetes 3.8
Aspergillus rugulosus Ascomycota, hyaline hyphomycetes <1.0
Penicillium thermophilus Ascomycota, hyaline hyphomycetes <1.0
Penicillium vermiculatus Ascomycota, hyaline hyphomycetes <1.0
Penicillium flavus var. flavus Ascomycota, hyaline hyphomycetes <1.0
Scedosporium boydii Ascomycota, hyaline hyphomycetes 2.4
Phialophora mutabilis Ascomycota, dematiaceous hyphomycetes <1.0
Curvularia geniculata Ascomycota, dematiaceous hyphomycetes <1.0
Exophilia jeanselmei Ascomycota, dematiaceous hyphomycetes <1.0
Scytalidium dimidiatum Ascomycota, dematiaceous coelomycete <1.0
Exophiala dermatitidis Ascomycota, dematiaceous hyphomycetes <1.0
Absidia corymbifera Zygomycetes, aseptate 6.5
Rhizomucor pusillus Zygomycetes, aseptate 3.0
Rhizopus arrhizus Zygomycetes, aseptate 3.8
Rhizopus rhizopodoformis Zygomycetes, aseptate <1.0
Mortierella wolffi Zygomycetes, aseptate <1.0
Yeasts <1.0
Candida krusei Ascomycota, yeast 1.0
Candida pseudotropicalis Ascomycota, yeast <1.0
Candida tropicalis Ascomycota, yeast <1.0
Candida lusitaniae Ascomycota, yeast <1.0
Candida glabrata Ascomycota, yeast <1.0

Pathogenesis and pathology


The most likely routes for exposure to fungi causing mycotic placentitis and abortion are the respiratory and gastrointestinal tracts.8 Intrauterine inoculation of Aspergillus conidia has been unsuccessful in experimental induction of mycotic abortion and ascending reproductive tract infections therefore seem unlikely.9 Moreover, mycotic abortions have been experimentally induced by intravenous administration of Aspergillus microconidia and the hematogenous route is thought to be the path because lesions develop initially in the placentomes.8 Fungi may enter the circulation through the alveolar septa or through gastrointestinal lesions resulting from penetration of mucosal barriers by rumen, reticular, or omasal infections or ulcers. Once the infection is established in the placentomes, it advances laterally to the intercotyledonary spaces. The placenta becomes grossly thickened, with a leather-like appearance of the intercotyledonary spaces. There is necrosis of the cotyledons and caruncular tissue and a thickening at the margins of the caruncle, imparting a concave or dished appearance. The placenta may have dry, thick, yellow plaques covering the intercotyledonary areas and most cotyledons when infected by Aspergillus spp.10 (Figure 63.1). A severe placental necrotizing vasculitis with thrombosis is a hallmark lesion associated with mycotic abortions. However, placental lesions with similar appearance can be produced by brucellosis and genital campylobacteriosis. Subsequently, fetal infection may occur involving the skin, lungs, brain, or liver. Fetal death is usually attributed to placental insufficiency. Skin lesions typically appear as dry, wrinkled, raised plaques with Aspergillus spp.; contrastingly, the fetus may appear wet and moist when infected by the Zygomycetes. The fetus is aborted soon after death, which occurs most commonly between 6 and 8 months of gestation. Frequently the placenta is retained and the cotyledons remain firmly attached to the maternal caruncles, which may rupture at the peduncle. These latter events are most commonly observed with zygomycotic abortions and may lead to subsequent secondary infections.5 Without severe uterine damage, most cows recover and have normal pregnancies later in life.

c63-fig-0001

Figure 63.1 Bovine mycotic placentitis produced by Aspergillus fumigatus. The cotyledons are necrotic and there are thick yellow plaques covering the intercotyledonary areas.



Photograph courtesy of Dr J. Cooley.

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Aug 24, 2017 | Posted by in GENERAL | Comments Off on Infectious Agents
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