ETIOLOGIC AGENTS

Various molds and yeasts have been implicated in mycotic bovine abortion. Most of these agents are saprophytic fungi found in moist organic environments such as soil, hay, and poor-quality silage.

Aspergillus fumigatus is the most commonly isolated fungus, accounting for 60% to 80% of mycotic abortions.¹ Other Aspergillus species, including Aspergillus flavus, Aspergillus terreus, and Aspergillus nidulans, are encountered less frequently. The Zygomycetes compose the second most commonly encountered group; these include Absidia corymbifera, Absidia ramosa, Mortierella wolfii, Rhizomucor pusillus, and Rhizopus arrhizus.¹ Pseudallescheria boydii is another prevalent agent, albeit less commonly encountered. Species of Penicillium; dematiaceous fungi such as Curvularia geniculata, Exophiala jeanselmei, and Wangiella dermatitidis; and yeasts, usually of the genus Candida or Torulopsis, account for most of the remaining mycotic organisms encountered.¹ The relative frequency with which different fungi occur varies according to the geographic region. In northern New Zealand, for example, M. wolfii rather than A. fumigatus is the most common cause of mycotic abortion.²

EPIDEMIOLOGY

The incidence of bovine mycotic abortion varies widely, ranging from 2% to 20%.^3,4 Some surveys found mycotic agents to be the most commonly diagnosed cause of bovine abortion.⁵ Mycotic abortions occur sporadically. Rarely are more than 10% of pregnant cows affected in a herd.⁶ The age of the cow does not appear to be a factor predisposing to mycotic abortion.⁷

In the Northern Hemisphere, most mycotic abortions occur in the winter and spring months. Increased confinement of cattle during this period and feeding of hay or poor-quality silage are risk factors.⁴ Confined housing in conjunction with feeding hay poses the greatest risk.⁴ Some evidence suggests that abortions associated with M. wolfii are linked to nonstandard methods of silage preparation, particularly inadequate wilt-time before storage or inadequate silage compaction or storage due to incomplete sealing of the container.⁵

The relationship between frequency and amount of rainfall during the previous hay or silage-making season and the annual incidence of mycotic abortion remains unresolved. Some studies demonstrate a positive correlation; others have found no significant relationship.^4,8,9

The suggestion that mycotic abortion is associated with the use of artificial insemination has not been substantiated.⁵

CLINICAL FINDINGS

Generally, mycotic abortions occur late in pregnancy, most often between 6 and 8 months of gestation, although they can occur as early as 2 months of gestation. Prodromal signs usually are not noted. The fetus is expelled soon after death in abortions caused by Aspergillus spp; however, it may be retained for up to 24 hours when a zygomycetous fungus is the etiologic agent.¹ The placenta frequently is retained and remains firmly attached. A secondary infection also may ensue.¹⁰ Maternal caruncles may rupture at the peduncle and be expelled still attached to the cotyledons. This is most often observed when abortions are caused by zygomycetes.⁶ Unless severe endometrial damage occurs, most cows recover sufficiently to have normal subsequent pregnancies.^4,10

In approximately 25% of cows with M. wolfii–associated abortions, a fulminating, postabortion pneumonia occurs.¹ Cows exhibit rapid, shallow respiration with forced expiration.¹¹ Death usually occurs within 72 hours of onset of clinical signs.² Infrequently, signs of pneumonia are observed before the abortion.

PATHOGENESIS

The portal of entry of fungal agents is unknown; however, the respiratory and gastrointestinal tracts are the most likely routes of exposure. Although most fungal conidia that reach the lower respiratory tract remain there or are eliminated, it is speculated that some may enter blood vessels in alveolar septa and reach the placenta through the systemic circulation. Rumen infections, omasal ulcers, and intestinal lesions also may be factors predisposing to hematogenous spread to the gravid uterus, perhaps by facilitating penetration of mucosal barriers.¹⁰ The hematogenous route is believed to be the primary route of infection because the lesion develops initially in the placentomes.¹⁰ Once established, the infection spreads from the initial focus to the arcade zone of the placentome and advances laterally. Infection subsequently spreads to the interplacental space between cotyledons. Fetal tissues may be involved, primarily skin and lungs. Brain or liver involvement occurs occasionally.¹⁰

Mycotic abortions have been induced experimentally by intravenous administration of fungal conidia; however, intratracheal and oral administration have not successfully reproduced disease. Ascending infections in the genital tract are not thought to be a common route for infection. Attempts to experimentally induce abortion with intrauterine inoculation of Aspergillus conidia have been unsuccessful.¹²

The pneumonia that develops in cows after M. wolfii–associated abortions is thought to result from a lung-uterus-lung cycle. Although the initial site of entry is unknown, spores most likely enter through the alimentary tract and pass by the venous or lymphatic system through the pulmonary vasculature, with subsequent localization in caruncles of the placenta. After abortion, a large number of fungal elements are absorbed from the uterus, causing an acute, fulminating, embolic pneumonia in the cow. This typically occurs 2 to 4 days after abortion.¹³