Nocardioform placentitis was first diagnosed in central Kentucky in 1986, with surges in case numbers occurring in 1998, 1999, and 2011.
Cases of nocardioform placentitis have also been reported sporadically in Florida, South Africa, Italy, and most recently in New Zealand.
Nocardioform placentitis is characterized by late-term abortions, premature foals, neonatal deaths, and weak foals born at term, writes Barry Ball, an equine reproduction specialist at the Gluck Equine Research Center, in the latest issue of Equine Disease Quarterly.
Nocardioform placentitis may involve large regions of the chorion (the outermost membrane surrounding an embryo) that can result in fetal growth retardation due to placental insufficiency.
The distribution of the lesion in the placenta is distinct from ascending bacterial placentitis with lesions of nocardioform placentitis commonly distributed in the cranial, ventral portion of the placenta near the junction of the uterine horns and body.
The lesion is often sharply demarcated from the surrounding normal placenta, and the affected placenta is frequently covered with thick, tan mucoid material.
Nocardioform placentitis is associated with Gram-positive, branching actinomycetes, including multiple Amycolatopsis spp., Crossiella equi, and, more recently, Streptomyces atriruber and Streptomyces silaceus.
Characterization of actinomycetes associated with abortions during the 2011 outbreak of nocardioform placentitis in central Kentucky revealed that Amycolatopsis spp. (49% of cases) were most commonly identified followed by Crossiella equi (29% of cases).
Nocardioform placentitis results in episodic abortions. A major epizootic of nocardioform placentitis was recorded in 2010-11, consisting of 390 confirmed cases of placentitis with 76 confirmed abortions.
These abortions occurred primarily between December 2010 and April 2011 and were diagnosed almost exclusively during the last trimester of pregnancy.
Nocardioform placentitis is primarily characterized as a mucoid placentitis in which the bacterial infection is limited to the chorionic surface of the ventral placenta without infection of the fetus.
To date, the pathogenesis remains poorly understood. Attempts to induce infection in mares by intrauterine inoculation of Crossiella equi at breeding and in pregnant mares via oral, intravenous, and intranasal routes with this organism have been unsuccessful.
Likewise, the ecology and biology of the causative agents, Crossiella equi and Amycolatopsis spp. remains unknown as these causative organisms have only been isolated from affected placentae in mares.
Nocardioform placentitis abortions occur from November to June, with a peak incidence in January and February. The majority of aborted fetuses are in the last trimester of pregnancy, and the identification of nocardioform lesions on the placenta of term pregnancies is a common presentation.
A retrospective epidemiologic study of on-farm risk factors associated with the 2010-11 epizootic of nocardioform placentitis identified several associations with nocardioform placentitis.
Larger farms, higher mare numbers and higher animal density on farms were positively associated with the incidence of nocardioform placentitis. Longer grazing times during late winter, pre breeding administration of progesterone to mares, human chorionic gonadotropin (hCG) administration post-breeding and nonsteroidal anti-inflammatory drugs (NSAIDs) were negatively associated with the incidence of nocardioform placentitis.
Retrospective evaluation of monthly climate data (precipitation and average temperature) for central Kentucky and the number of nocardioform placentitis cases seen by the University of Kentucky Veterinary Diagnostic Laboratory for the period of 1990-2018 was conducted to evaluate climatologic associations with nocardioform placentitis.
This analysis demonstrated a moderately strong negative association between August and September rainfall and the number of nocardioform placentitis cases submitted for the subsequent foaling season.
Likewise, there was a moderately strong positive association between mean temperatures in August and September and the number of nocardioform placentitis cases submitted the next foaling season.
Because average temperatures in August and September 2019 were 5.3% above the long-term average and average rainfall was only 52% of the long-term average, the 2020 foaling season seems likely to have a higher incidence of nocardioform placentitis.
Although there does seem to be an association between a hot and dry August and September preceding an increase in nocardioform cases, the causal relationship is less clear.
Other members of the actinomycetes, such as Rhodococcus equi, are soil-dwelling pathogens that are associated with hot, dry, dusty environments.
It is worth considering that the actinomycetes responsible for nocardioform placentitis may also originate as soil-borne organisms, despite the failure thus far to isolate these pathogens from environmental sources.
Related members of Amycolatopsis and Streptomyces are well characterized as soil-associated microorganisms, and a likely environmental association (hot, dry periods in late summer) may be associated with the disease.
It appears likely that the pathogenesis of nocardioform placentitis is multifactorial and may involve environmental conditions (hot, dry periods in late summer) and possibly effects related to host susceptibility.
Pregnancy in many species, including the mare, involves some degree of immunosuppression, and many actinomycetes are more pathogenic in immunocompromised hosts.
More research is needed to better unravel this complex disease process in the mare.
Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.