Infectious diseases a major driver of abortions in horses, review shows

Causes of horse abortions internationally are examined in a just-published scoping review in Australia.
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Infectious diseases are the main causes of concern for pregnancy loss in horses, the authors of a just-published scoping review have found.

Equine pregnancy loss is frustrating and costly for horse breeders, Claudia Macleay and her fellow researchers wrote in the journal Veterinary Sciences.

The reproductive efficiency of mares has major implications for a breeding operation’s economic success, and widespread losses can have a trickle-down effect on the communities that rely on these businesses, they said.

“Understanding the causes and risks of equine pregnancy loss is essential for developing prevention and management strategies to reduce the occurrence and impact on the horse breeding industry.”

The authors conducted a scoping review in which 514 published records on equine pregnancy loss from six scientific databases met the criteria for full analysis. Their initial searches yielded 8854 records on their key search terms.

The researchers sought to categorise the location and timings of pregnancy losses, the reported causes, and syndromes considered responsible between 1960 and 2020.

The main reported cause of pregnancy loss was equine herpesvirus-1 (EHV-1), which was documented in 223 records (43%). Other common reported causes were placentitis, reported in 91 records (17.7%), followed by leptospirosis at 14% (72 records), twinning at 9.7% (50 records), congenital abnormalities at 7.2% (37 records), EHV-4 at 6.2% (32 records), umbilical cord torsion at 6.2% (32 records), and equine amnionitis and foetal loss/mare reproductive loss syndrome at 5.4% (28 records).

Ninety-six publications recorded no specific cause of pregnancy loss and were classified as “unspecified cause” (18%).

Miscellaneous pregnancy loss causes accounted for 29% of reported causes. The most commonly reported cause in this category was abortion through infections from Streptococcus equi subsp. zooepidemicus (28 records), Escherichia coli (18), and equine viral arteritis (20).

There were 18 records of mares losing a pregnancy because of hydroallantois or hydrops, both involving allantoic fluid issues.

Premature placental separation was reported as a cause of pregnancy loss in 14 records.

Other miscellaneous causes were rare diseases (for example, Bunyamwera virus), uncommon genetic abnormalities (for example, Lymphosarcoma) and unusual bacterial infections (for example, Hafnia alvei).

When examining records for trends in reported causes over time, the earliest reported causes in the 1930s were twinning, umbilical cord torsion and unspecified causes.

EHV-1 appears in records in the 1940s, congenital abnormality appears in the publications in the 1950s, and the first descriptions of placentitis and leptospirosis appear in the 1960s.

Ascending placentitis and nocardioform placentitis both entered into the literature in the 1990s.

Records on twinning, leptospirosis, EHV-4 and congenital abnormalities peaked in the 1990s.

Chlamydia, equine amnionitis and foetal loss/mare reproductive loss syndrome entered the literature in the 2000s, with records on EHV-1, unspecified causes and other miscellaneous causes peaking in the 2000s.

The stage of gestation at which the abortion or stillbirth occurred was reported in 301 records (59%). Early losses were noted in 66 of the records (22%), mid-term losses in 181 records (60%) and late-gestation losses were reported in 206 records (68%).

Embryonic pregnancy loss was reported in 25 records (8%).

During early and mid-gestation, the most frequently reported cause of pregnancy loss were miscellaneous causes. The next most frequent cause reported during mid foetal gestation was EHV-1, followed by placentitis and leptospirosis.

In late gestation, EHV-1 was the most frequently reported cause, followed by miscellaneous causes, placentitis and leptospirosis.

Discussing their findings, the authors noted an increase in reporting of equine pregnancy loss since 1960 in the literature.

“The causes of equine pregnancy loss documented in this review can be broadly divided into two categories: Infectious and non-infectious causes,” they said.

“Infectious disease accounted for a large proportion of the records, demonstrating that these are the main causes of concern for equine pregnancy loss.”

EHV-1 was the predominant reported cause of equine pregnancy loss throughout the study period for all stages of pregnancy. It is a highly infectious virus in horses, and abortion is a serious consequence of infection.

Horses that are infected become life-long carriers due to the ability of EHV-1 to remain latent, providing a viral reservoir that can infect susceptible populations.

In unvaccinated herds with inadequate biosecurity, up to 75 to 80% of mares may abort from EHV-1 infection, the authors noted.

“In major horse breeding areas, including the United States and the United Kingdom, cases of EHV-1 have decreased over time, which was reflected in the literature captured in this review.”

This, they said, could be attributed to a combination of vaccination and improved disease risk reduction, such as implementing farm quarantine and separating vulnerable animals from those that could be infectious.

The authors noted that placentitis is often described as the most common cause of infectious abortion. However, it accounted for only 16% of reported pregnancy loss causes in the review.

There may be under-reporting of placentitis cases, they said, because it is frequently diagnosed following gross placental examination, and further cause-specific investigations requiring laboratory tests and histology are declined.

“An area identified as a gap in the literature from this review was a lack of reporting on equine abortion associated with the zoonotic bacterias Chlamydia psittaci and Coxiella burnetii,” the review team noted. “Both can cause serious illness in people and both can infect people following exposure to infected placental membranes and aborted material.”

However, only a handful of records relating to abortions associated with these infections were identified.

Abortions from C. psittaci have been reported in Australia and Europe (France, Germany, Hungary and Switzerland), and abortion associated with C. burnetii has been reported in Australia and France. Both bacteria are globally distributed.

C. burnetii should be a pathogen considered in equine abortion investigation, they said. It requires further research and disease surveillance to understand its incidence, determinants and risks to both horses and people.

The two main non-infectious causes of equine pregnancy loss identified in the review were twinning and umbilical cord torsion. Twinning clustered most strongly with Thoroughbred horses.

If left untreated, twinning usually results in mid to late-term abortion or stillbirth because gestation proceeds normally until the twin foetuses begin to compete for placental and uterine space.

Foals are affected by growth retardation and inadequate nutrition because of placental insufficiency leading to abortion, premature birth and stillbirth.

Twinning had the largest decline as a reported cause of loss over the publication period. It accounted for 35% of reported losses in the 1960s but decreased to 3% in the 2000s.

Umbilical cord torsion as a cause of pregnancy loss in domestic livestock is almost unique to the equine, they noted. In the current review, it was reported mainly from Europe and North America, occurring in both mid and late gestation.

“The length of the umbilical cords has been cited as a risk factor for umbilical cord torsion,” they noted. Further investigation of the potential risk factors for umbilical cord torsion would be worthwhile, they said.

The authors said further investigation in several areas may yet yield unknown risk factors associated with equine pregnancy loss.

The review team also highlighted knowledge gaps among some infectious agents capable of infecting both horses and people.

“People working in the horse breeding industry come into close contact with equine bodily fluids and infectious materials, placing them at an increased risk of infection with zoonoses.

“Identifying whether these zoonoses are underdiagnosed or emerging in equine populations, as well as their risk factors, should be a priority for researchers because of the potential for severe level of illness in people,” they said.

The authors said their findings can be used to help in developing surveillance systems for equine abortion and important potential gaps in reporting.

The study team comprised Macleay, Patrick Shearer, Jane Heller and Victoria Brookes, all with Charles Sturt University in Wagga Wagga; Joan Carrick, with Equine Specialist Consulting in Scone; Angela Begg, with Diagnostic Laboratories Pty Ltd in New Lambton; Melinda Stewart, with Starling Scientific in Pearl Beach; and Catherine Chicken, with Scone Equine Hospital.

Macleay, C.M.; Carrick, J.; Shearer, P.; Begg, A.; Stewart, M.; Heller, J.; Chicken, C.; Brookes, V.J. A Scoping Review of the Global Distribution of Causes and Syndromes Associated with Mid- to Late-Term Pregnancy Loss in Horses between 1960 and 2020. Vet. Sci. 2022, 9, 186.

The scoping review, published under a Creative Commons License, can be read here

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