CUTTING EDGE, NEW TOPICS, AND PRACTICAL INFORMATION IN THERIOGENOLOGY


placentitis through controlled and field studies car- ried out in different parts of the world. Reviews covering equine placentitis have been published,11,12 with one recent review emphasizing the immuno- logic aspects of the preterm delivery in mares.13 The following review aims to present a comprehen- sive review of classical and recent findings regarding equine placentitis.


2. Etiopathogenesis and Epidemiology


Several infectious agents have been recovered from placentitis cases; bacteria are responsible for the large majority of cases and fungi for the minority (Table 1). Bacterial agents commonly associated with placentitis include Streptococcus equi subspe- cies zooepidemicus, Escherichia coli, Streptococcus equisimilis, Klebsiella pneumoniae, Pseudomonas aeruginosa, Leptospira sp., Crossiella equi, and Amycolatopsis species (Amycolatopsis kentuckyen- sis, Amycolatopsis lexingtonensis, and Amycolatop- sis pretoriensis)2,4,8,14 (Table 1). Fungi associated with equine placentitis primarily include Aspergil- lus ssp. and Candida albicans.4 Mixed bacterial and fungal infections have been reported in ascend- ing, however, less commonly. Anecdotally, it seems that some cases start with bacterial placentitis and then fungal secondary infections follow the primary bacterial infection. The incidence of leptospiral infections causing abortions is highly variable across the years in cen- tral Kentucky.4,5,15 In North America, the serovar Pomona type Kennewicki is the most important type associated with Leptospiral abortions in mares16; however, in other parts of the world, the serovars Bratislava, Grippotyphosa, Copenhageni, Autumna- lis, Hebdomadis, Arborea, and Icterohaemorrhagiae are more commonly associated with abortion in mares.17 Environmental conditions such as precip- itation and flooding are alleged to be important in the dissemination of the bacteria from wildlife to horses, which are considered incidental hosts. Wild animals including raccoons, white-tailed deer, striped skunks, opossums, and foxes are known to harbor the serovar Pomona and are thought to serve as a source of infec- tion for pregnant mares.5,18 Based upon morphologic lesions and suggested pathogenesis, there are four types of equine placenti- tis: ascending, focal mucoid (nocardioform), diffuse (hematogenous), and multifocal5 (Fig. 1). Ascending placentitis, the most frequent type of placentitis, is commonly associated with beta hemolytic streptococci and coliforms5 (Table 1). Nocardioform placentitis is commonly associated with Gram-positive branching bacilli, Crossiella equi and Amycolatopsis species (Fig. 2).14,19 Recently, two new species of Streptomyces (Streptomyces atriruber and Streptomyces silaceus) have been isolated in nocardioform placentitis cases.20 A multifocal distribution pattern is rare but is sporad- ically seen in isolated cases in association with fungi or bacteria.4,5 Diffuse placentitis is diagnosed in iso- lated cases in association with bacteria or fungi, or


Table 1. Ten Most Commonly Isolated Microorganisms in 236 Equine Placentitis Cases in Central Kentucky


Frequency Microorganisms


Streptococcus equi subspecies zooepidemicus Leptospira ssp. Escherichia coli Fungi


Pseudomonas aeruginosa Streptococcus equisimilis Enterobacter agglomerans Klebsiella pneumonia


-hemolytic Streptococcus Staphylococcus aureus


Adapted from Hong et al.4 No. of Cases %


39 37 33 16 11 11 5 5 5 2


16.5 15.7 14.0 6.7 4.7 4.7 2.1 2.1 2.1 0.9


during outbreaks in association with Leptospirosis (Fig. 1D).


Nocardioform placentitis has periodically been the predominant type of equine placentitis in central Kentucky4,21 with the 2010–2011 foal crop season in central Kentucky experiencing an exceptionally high number of nocardioform cases (Fig. 3). Con- junctly, Crossiella equi and Amycolatopsis ssp. were responsible for 85% of the cases submitted during that period,14 with lesions compatible with nocardio- form placentitis present without an identified iso- late in the remainder of cases. Based on these cases it seems that Crossiella equi infections may be more likely to result in abortion, whereas infections with other type actinomycetes tend to result in live but premature foals.19 It is worth noting that, to date, nocardioform organisms have only been iso- lated from clinical placentitis cases and not from the environment.19 Bacteria causing nocardioform placentitis and


other actinomycetes that affect horses differ in their propensity to infect not only the fetal membranes but also the offspring in utero. For instance, mares affected by Cellulosimicrobium cellulans in Ken- tucky present mucoid placentitis associated with se- vere fetal lesions.22 Similarly, the actinomycetes Cellulosimicrobium sp and Cellulomonas sp, along with other nonspecific bacterial species, have been associated with mucoid placentitis and fetal lesions in equine amnionitis and fetal loss, reported to occur with ingestion of processionary caterpillars in Aus- tralia.23 Conversely, actinomycetes causing nocar- dioform placentitis do not infect the fetus in utero as reported in other types of mucoid placentitis; the bacterial infection in nocardioform placentitis is re- stricted to the chorioallantois.19 Infectious agents associated with ascending pla- centitis seems to enter the uterus via the vagina/ cervix and then colonize the caudal pole of the chorioallantois (Fig. 4).11 Once infection is estab- lished, clinical signs such as premature udder devel-


AAEP PROCEEDINGS  Vol. 61  2015 491


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