Nitrate Toxicity in Ruminants
Nitrates are relatively non-toxic; toxicity results when
they are reduced to nitrites in foodstuffs or by ruminal flora
in ruminants. The main hazard to ruminants is ingestion of
plants that have accumulated excessive amounts of nitrates
or nitrites. Drought conditions and the usage of manure as
fertilizer heighten nitrate accumulation in cereal grasses
(wheat, rye, oats), corn, sorghum, sugar beets, and many weeds,
including pigweed (Amaranthus retroflexus) and variegated
thistle (Silybun marianum), found in pasture. The
highest concentration of nitrates is in the stems, followed
by roots and leaves. Nitrate toxicity may also result from
ingestion of polluted water rich in nitrates and ingestion
of nitrate-based fertilizers (ammonium or potassium nitrate).
Nitrates can have an irritant effect on the gastrointestinal
tract and cause vasodilation. In addition, they can also
be irritants to the kidneys and urinary tract. However, in
ruminants in which reduction of nitrate to nitrite occurs,
the more toxic nitrites cause the iron component of blood
to become oxidized and thus unable to transport oxygen. This
may cause the blood to be discolored brown, often described
as "chocolate-colered." If the level of methemoglobineria
reaches 20-40%, the mucous membranes will start to appear
cyanotic and the animal may become dyspneic; tachycardia is
also often noted. In extreme cases, this may lead to coma
and death. Clinically, nitrate intoxication may also cause
weakness, ataxia and convulsions. Chronic sublethal nitrate
poisoning has also been reported. Clinical manifestations
include reproductive problems such as abortion, infertility
and lower birth rate, hypothyroidism, and depletion of dietary
vitamin A.
Gross lesions that may be used to diagnose nitrate intoxication
include brown discoloration of the blood, which is present
in approximately 64% of cases. The discoloration fades within
five hours after death so it may not be detected at postmortem
examination. Pale, blanched areas are sometimes noted on
the myocardium along with epicardial hemorrhages. The lungs
may show intense congestion and reddish-brown coloration.
Laboratory confirmation of nitrate poisoning can be achieved
by checking suspected plants, water, serum, ocular fluid,
and heparinized whole blood. Nitrate levels in forage should
not exceed 1% dry matter weight. A preliminary field test
using 1% diphenylamine blue in concentrated sulfuric acid
may be used. If the plant material changes to a dark color
quickly, the nitrate levels are excessively high. All field
tests are presumptive, however, and should be confirmed with
laboratory tests. Samples of grass or forage (about 1 kg)
should be collected from several locations of the pasture
or from several different bales of hay. At least 500 ml of
water should be collected and submitted in a sterile bottle.
Diagnosis of nitrate intoxication can also be based on finding
excessive amounts of methemoglobin in the whole blood. Heparinized
blood must be collected within two hours of death and promptly
assayed in order for this test to be valid; otherwise, blood
must be rapidly frozen to prevent the spontaneous reduction
of methemoglobin back to hemoglobin.
Serum and ocular fluid are the most commonly used samples
to assess nitrate levels (nitrites are typically not detected
due to their short in vivo half-life). Samples should
be sent to the laboratory immediately. Nitrate in serum is
stable for one week if refrigeration and one month if frozen.
The nitrate levels in ocular fluid remain stable for over
24 hours after death and diagnostically significant for up
to 60 hours after death. Ocular fluid is thus often the only
valid sample available. Nitrate crosses the placental barrier
in cattle and diffuses into the fetus. Amniotic fluid and
stomach contents from fetuses thus also represent valid diagnostic
samples.
Treatment in cattle consists of administering an 8.8 mg/kg
dose of a 1% isotonic saline solution of methylene blue IV;
sheep may need up to 20 mg/kg. Repetition of this regimen
may be necessary after an interval of 6-8 hours. It must
be noted, however, that methylene blue is not approved by
the FDA for use in food producing animals. Symptomatic treatment
involving cardiorespiratory stimulants, gastric antacids,
and oral antibiotics to reduce nitrite production by gastrointestinal
flora may be indicated. Ensiling high-nitrate forages with
other feeds and adding trace mineral supplements are useful
means to prevent nitrate intoxication.
-by Theresa Boulineau, Class of 2001
-edited by Melissa Popielarczyk,
VPB Graduate Student
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