Study: Ocular Fluid Nitrate Concentrations in Foals

One dilemma in diagnosing nitrate/nitrite-induced abortions in foals is the lack of well-established normal reference ranges for ocular fluid (eyeball fluid) nitrate/nitrite concentrations. The purpose of the study reported here was to establish a normal reference range for these concentrations in aborted, stillborn, and neonatal foals that died in Central Kentucky.

In cattle, excessive nitrate or nitrite exposure can lead to poisoning and subsequent abortions in pregnant animals. Nitrate/nitrite poisoning is caused by ingesting large amounts of nitrate or nitrite in heavily fertilized forages or nitrate-accumulating weeds, consuming nitrate-containing fertilizers, or ingesting nitrate/nitrite contaminated water.

Nitrate/nitrite poisoning is fairly common in cattle and is always a consideration when abortions occur. To determine if a bovine abortion is the result of excessive nitrate/nitrite exposure, a veterinarian can test the fetal ocular fluid for nitrate/nitrite concentrations. Some nitrate is present naturally in cattle’s ocular fluid, as nitrate is a natural component of the plants they eat. Normal fetal and neonatal ocular fluid nitrate concentrations in cattle can extend up to 25 ppm (parts per million); concentrations greater than this can be associated with nitrate-induced abortions. Nitrite is normally not present in ocular fluid unless bacterial conversion of nitrate to nitrite has occurred postmortem.

In horses, nitrate/nitrite poisoning is very rare and related abortions are even less frequently reported. Some studies indicate that pregnant mares can safely ingest forages with at least twice the amount of nitrate that can cause acute poisoning and death in cattle, with no adverse effects or abortions. However, exposure to extremely high concentrations of nitrate or nitrite in forages or contaminated water could potentially cause abortion.

In this study researchers collected ocular fluid from 61 aborted or neonatal foals submitted to the University of Kentucky Veterinary Diagnostic Laboratory for postmortem examinations in the spring of 2011. Nitrate and nitrite concentrations were determined by ion chromatography (a process that separates ions based on their charge).

Results revealed that the ocular fluid nitrate concentrations ranged from less than 5 ppm up to 12.8 ppm, with a mean of 7.8 ppm and a median of 7.4 ppm. Nitrite was not detected in any of the samples (minimum level of quantitation: 1 ppm). Thoroughbreds were overrepresented (48 foals). Causes of death included various types of infectious placentitis (inflammation of the placenta), dystocia (difficult birth)-related conditions, congenital deformities, equine herpesvirus type 1 infection, premature placental separation, uterine torsion, as well as four cases with no identifiable diagnosis. Ultimately, none of the diagnoses were associated with excessive nitrate/nitrite exposure. Factors such as age, breed, sex, gestation length, month of abortion, or diagnosis were not correlated with ocular fluid nitrate concentration.

In conclusion, these results indicate that fetal, stillborn, and neonatal foals dying from causes other than nitrate poisoning normally have ocular fluid nitrate concentrations that extend up to 12.8 pm. Additional investigations are under way, with analyses of ocular fluid from additional foals over a two-year period of time planned.

This information was obtained from the research abstract "Ocular fluid nitrate and nitrite concentrations in aborted, stillborn, and newborn equines," by Cynthia L. Gaskill, DVM, PhD, and Lori L. Smith, PhD; University of Kentucky Veterinary Diagnostic Laboratory; Proceedings of the American Association of Veterinary Laboratory Diagnosticians Annual Conference, Buffalo, 2011; p. 56.

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