Biomedical and Biotechnology Research Journal (BBRJ)

: 2021  |  Volume : 5  |  Issue : 1  |  Page : 16--20

Nitrate-nitrite toxicosis associated with duckweed (Portulaca Oleracea L.) (Portulacaceae) consumption in a herd of sheep in Kenya

Joseph Mwanzia Nguta 
 Department of Public Health, Pharmacology and Toxicology, Faculty of Veterinary Medicine, University of Nairobi, Nairobi, Kenya

Correspondence Address:
Dr. Joseph Mwanzia Nguta
Department of Public Health, Pharmacology and Toxicology, Faculty of Veterinary Medicine, University of Nairobi, Nairobi


Background: Nitrate-nitrite toxicosis associated with the consumption of duckweed (Portulaca oleracea L.) (Portulacaceae) was diagnosed in a small herd comprising of 11 sheep in Korayo Sub Location, Rangwe Sub County of Homabay County in Kenya. Methods: The clinical signs were dyspnea, froth from the mouth, labored breathing, incoordination, tachycardia, increased urination, neck, fore and hind limb distension, aggressive movements, bloat, convulsions, and coma, before death. Results: All the 11 sheep died of intoxication before institution of therapy. Brownish mucous membranes and chocolate colored and poorly clotted blood were the significant necropsy observations. Mild pulmonary edema was characterized by the presence of limited froth in the trachea and bronchioles. At postmortem, the liver of the sheep was grossly congested, and the cardiac pericardium was inflamed. There was gastritis, enlarged, and congested kidneys with pin-point hemorrhages. Diphenylamine tests carried out on the fed duckweed and on ruminal duckweed contents were positive for nitrates. Duckweed (Portulaca oleracea) fed on sheep had 7.4% nitrate on dry matter basis (DM), whereas ruminal contents had 5.38% nitrate on DM basis. Chocolate-colored blood collected during postmortem had a methemoglobin fraction of 86%. Nitrate levels in pastures of 0.5% and above have been shown to be potentially dangerous, with acute poisoning likely to occur at times when the levels are above 1%. Death in sheep can occur when blood methemoglobin levels are in the range of 67%–90%. Conclusion: The observations from the present study are indicative of death in a herd of sheep due to nitrate-nitrite poisoning associated with the consumption of duckweed (Portulaca oleracea).

How to cite this article:
Nguta JM. Nitrate-nitrite toxicosis associated with duckweed (Portulaca Oleracea L.) (Portulacaceae) consumption in a herd of sheep in Kenya.Biomed Biotechnol Res J 2021;5:16-20

How to cite this URL:
Nguta JM. Nitrate-nitrite toxicosis associated with duckweed (Portulaca Oleracea L.) (Portulacaceae) consumption in a herd of sheep in Kenya. Biomed Biotechnol Res J [serial online] 2021 [cited 2021 Apr 11 ];5:16-20
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Portulaca oleracea L. commonly known as duckweed, little hogweed, or purslane is an annual succulent in the family Portulacaceae, which may reach 40 cm (16 in) in height. Duckweed is considered to be a nitrate accumulating plant under the ideal conditions.[1],[2] Oral administration of duckweed to goats at a dosage rate of 5 mg/kg of body weight or exposure ad libitum for up to 40 days has been associated with ruminal tympany, in appetence, weight loss, increased urination, incoordination, tremors, sternal recumbency, coma, and death.[3] Postmortem findings have revealed gastritis, hepatitis, and kidney lesions.[4] Duckweed has been reported to be a major cause of bloat and other digestive disorders in cattle in Brazil.[5]

In farming systems, nitrate exposure in sheep through the ingestion of duckweed and other plants accumulating nitrates to toxic levels such as pigweed can be a major cause of mortalities and morbidities.[6],[7] Following nitrate exposure to cattle through different pastures, the median lethal dose (LD50) has been shown to be approximately 0.5 g/kg of body weight.[3] The route of exposure and nitrate formulation have been shown to play a key role in the mediation of nitrate-nitrite toxicity, since low LD50 values were observed following nitrate administration as a drench.[8] Livestock diets rich in carbohydrates have been associated with decreased nitrate-nitrite toxicity, suggesting that acute nitrate poisoning is a function of livestock dietary composition.[9] Acute nitrate poisoning in livestock has been reported with forage nitrate levels above 1%, with nitrate levels of 0.5% and above considered to be potentially dangerous.[10] Researchers have reported pastures with nitrate level in the range of 0.34% to 0.45% nitrate-nitrogen (1.5%–2.0% nitrate) to be potentially toxic,[11] while other scientists[12] have reported mortality in cattle either under poor diets or fed carbohydrate deficient diets from a nitrate level of 0.7%.[12] Acute toxicity and death can result following livestock exposure to forages with nitrate levels of 1% on the dry matter basis (DM).[13] Following ingestion of nitrates by livestock at a rate that exceeds the rate of ruminal microbial conversion into proteins and later ammonia, nitrates enter into the bloodstream as nitrites, combining with hemoglobin in erythrocytes to form methemoglobin, which cannot transport oxygen. When consumed more rapidly than they can be converted in the rumen to protein, nitrates enter the bloodstream as nitrites, which combine with hemoglobin in red blood cells to form methemoglobin. Death in livestock results from the cellular and tissue deprivation of oxygen as methemoglobin levels approach 80%.[14]

The nitrate reducing potential of ruminal bacteria to potentially toxic nitrites has been shown to increase the vulnerability of ruminants to nitrate poisoning.[3] Nitrates (NO-3) and nitrite (NO-2), ions which occur freely in nature are formed following nitrogen oxidation by microbes in water, soil, plants, and to a lesser extent by lightening.[3] Even though nitrate is the more stable form of oxidized nitrogen than nitrite, it can easily be reduced by bacterial action to nitrite.[15] The intermediate step in the microbial biochemical process in the formation of fully reduced ammonia (NH3) used to form microbial proteins is the reduction of nitrate to more toxic nitrite.[3] Since the amount of ammonia produced is limited, ammonia toxicosis does not occur in ruminants exposed to feeds containing nitrates.[16],[17] Nitrite accumulates in the rumen following rapid ingestion of forages-containing high levels of nitrates. Absorption of nitrites takes place at the ruminal epithelium into the blood steam, where passive diffusion facilitates their entry into the red blood cells (erythrocytes) in exchange for chloride ions. The nitrite ion in the erythrocytes converts hemoglobin into methemoglobin through oxidation. Methemoglobin is not capable of binding and subsequently transporting oxygen into body cells and tissues, thus resulting in cellular and tissue hypoxia. Clinical signs are observed in animals when 30%–40% of hemoglobin is oxidized into methemoglobin. Death in animals occurs when the methemoglobin fraction in blood exceeds 80%.[18],[19] Abortion in ruminants occurs 3 to 7 days after exposure of pregnant dams to feed containing toxic nitrate levels due to fetal hypoxia sequel to nitrate toxidrome.[18],[19]

The most common cause of nitrate-nitrite poisoning in livestock is nitrate accumulating plants.[3] A number of plants contain nitrates at concentrations that are high enough to be converted by the ruminal microorganisms into nitrite levels enough to cause nitrate-nitrite toxicosis. Nitrate accumulating plants that can easily poison livestock populations include potatoes, lettuce, turnips, cabbages, sugar beets, and carrots.[3] These plants concentrate nitrate ions more so when grown in soils where nitrogen containing fertilizers have been used; or in soils where organic manure has been applied or are grown during drought periods.[3] Consumption of large quantities of uncooked cabbages has also been associated with goitrogenic effects, in addition to nitrate-nitrite toxicosis.[20] Toxic nitrate concentrations can also be accumulated by farmed crops such as Sorghum (Sorghum bicolor [L.] Moench [Poaceae]), Sudan grass (Sorghum bicolor subsp. drummondii [Nees] de Wet [Poaceae]), small grains, especially pearl millet (Pennisetum glaucum [L.] R. Br. [Poaceae]), and corn (Zea mays L. [Poaceae]). These crops have the potential to cause nitrate poisoning when fed in large quantities during the dry season.[3],[10] Weeds that concentrate nitrates into toxic amounts include duckweed (Portulaca oleracea L. [Portulacaceae]), bindweed (Convolvulus arvensis L. [Convolvulaceae]), Sunflower (Helianthus annuus L. [Asteraceae]), Variegated thistle (Silybum marianum [L.] Gaertn. [Asteraceae]), Jimson weed (Datura stramonium L. [Solanaceae]), Lambs quarter (Chenopodium album L. [Amaranthaceae]), and Pigweed red root (Amaranthus retroflexus L. [Amaranthaceae]). These weeds can accumulate nitrates into toxic concentrations under adverse weather conditions, and hence, a potential cause of nitrate-nitrite poisoning in livestock.[3],[10],[21] Unripe plants and plant parts that are usually regrown after harvesting such as sprouting potatoes afford the greatest potential for toxic nitrate concentrations.[3] Development of nitrate-nitrite toxicosis in the animal husbandry is attributed to animal and plant related factors. For example, herbicidal application, especially the phenoxy derivatives of fatty acids, have been shown to increase nitrate concentration by plants.[18],[22] The levels of nitrates in duckweed can be as much as 1.2 g/kg fresh weight.[23] In nitrate poisoning, the formed nitrites by ruminal microorganisms cause poisoning due to fatal anoxia arising when methemoglobin, which is incapable of binding and transporting oxygen to body cells and tissues is formed.[24] A suspected case of nitrate-nitrite poisoning in a small herd of eleven sheep fed on duckweed is reported. The case was reported by the owner to the local Veterinary officer approximately 3 h after feeding the whole herd duckweed collected from the farm.


The presented patients were adult Maasai sheep, comprising of eight females and three males and were localized in Rangwe sub County, Homabay County. The sheep were reported to graze on their own in the field. The owner of the herd of sheep indicated that the whole herd was treated against ectoparasites every month using Bayticol. The sheep were dewormed after every 3 months with 10% albendazole. The local veterinarian used to visit the herd once every month. The sheep were reported to have been healthy, with no disease history before they were fed on duckweed that had earlier been collected from the farm. The sheep had not been previously fed on duckweed. In mid-June 2020, a small herd of 11 adult Maasai sheep in Korayo sub location, Rangwe sub County, Homabay County were fed with duckweed that had been collected from the farm. Organic livestock manure was reported to be regularly applied on the farm from where the duckweed was collected from. In addition, potassium nitrate and urea fertilizers were reported to be regularly used in the farm to increase crop production. About 3 hours postduckweed feeding, all the 11 sheep exhibited signs and symptoms of poisoning including increased urination, ruminal tympany, unsteady gait and incoordination, weakness, inappetance, head pressing, difficulties in breathing, dyspnea, aggressive movements, foam in the mouth, muscle tremors, sternal and later, lateral recumbency, neck, fore and hind limb extension, neck distension, coma, and death. All the 11 sheep died shortly after the arrival of the local veterinarian, approximately 4 h after the onset of clinical signs. Thus, no clinical intervention was carried out. Postmortem examination was carried out, soon after the arrival of the local veterinarian. There was limited froth on the tracea and also in the bronchioles. Mucous membranes were pale to purple; poorly clotted venous blood was chocolate/dark brown in color. The liver and the kidneys were congested [Figures 1-3; supplementary materials]. Ruminal contents, urine, kidney and liver sample, and venous blood were all collected for further analysis. Duckweed from the feeding tray was also collected for taxonomic identification and further analysis.

Presumptive diagnosis of nitrate-nitrite toxicosis was made based on the acute nature, clinical signs and symptoms observed, and history of feeding sheep with duckweed that had grown on a farm that had been applied nitrate-containing fertilizers. Chocolate/dark brown-colored venous blood samples were randomly collected from the dead sheep. Diphenylamine blue test confirmed the presence of nitrates in duckweed, kidney, liver, ruminal contents, and also in whole blood. Urine samples were positive for nitrites when tested by urine test strip (Combi-Screen, Germany). CO-oximetry performed on chocolate/dark brown blood on a radiometer 625 ABL showed a methemoglobin fraction of 86%. Duckweed samples and ruminal contents were taken to the Department of Public Health, Pharmacology and Toxicology, toxicology laboratory for the analysis of nitrate content. Analysis of duckweed and ruminal content samples by radiopotentiometry revealed 7.4% and 5.38% nitrate in dry mater (DM) basis, respectively, confirming the diagnosis of nitrate-nitrite toxicosis in the affected herd of sheep.


The reported acute toxic signs, necropsy findings, and laboratory observations demonstrated that duckweed (Portulaca oleracea) was responsible for nitrate-nitrite poisoning in the small herd of sheep. In Kenya, nitrate-nitrite poisoning has been earlier diagnosed in a small herd of goats fed on cabbages grown using manure and top dressed with nitrogen-containing fertilizers during drought. Earlier findings have reported acute toxicity and death of sheep following exposure to Portulaca oleracea at the dosage rate of 80 g/kg of bodyweight, suggesting that, as it has been observed in cases of poisoning following ingestion of other nitrate containing plants, exposure of sheep to large amounts of Portulaca oleracea within a short period of time plays an important role in the occurrence of nitrate-nitrite toxicity.[25]

Nitrate-nitrite poisoning is also associated with feeding animals with nitrate concentrating plants grown during the periods of drought in soils where manure has been applied and the plants top dressed with nitrate-containing fertilizers.[26] Both drivers of nitrate-nitrite poisoning described above were observed in the poisoning reported in the current study, where sheep were fed with duckweed collected from a farm where nitrate-containing fertilizers had been used and livestock manure had been applied. Nitrate-nitrite poisoning in livestock has been shown to correlate well with the nutritional status of animals, with poor nutrition favoring the occurrence of poisoning.[27] This observation is in agreement with the findings from the current study, where the reported herd of sheep was in poor nutrition due to the dry spell and shortage of pastures that was prevailing at the time of poisoning. Clinical signs and symptoms observed in the current study such as distended neck, limbs, dyspnea, labored breathing, frequent urination, difficulties in breathing, cyanotic mucous membranes associated with methemoglobinemia are suggestive of nitrite toxicosis.[28] The gastrointestinal symptoms reported in the current study, such as ruminal tympany, bloat, and decreased ruminal movements are mostly caused by decreased cholinergic activity and subsequent relaxation of gastrointestinal tract smooth muscles.[28] Earlier reports have indicated that nitrites can decrease the spontaneous motility of the gastrointestinal tract of ruminants, and also the esophagus,[29] a finding that was consistent with observations from the current study.

Nitrates following ingestion of nitrate accumulating plants have been associated with irritation of the smooth muscles of the gastrointestinal tract, in addition to urinary disorders that cause frequent urination.[4] This observation is in agreement with the findings from the current study, where sheep that were fed with duckweed containing high nitrate levels exhibited frequent urination and gastrointestinal disturbances before death.

The clinical signs and symptoms and the macroscopic changes reported in the current study are similar to those earlier described in nitrate-nitrite poisoning in cattle,[30],[31] goats,[32] and in sheep.[33] Chocolate/brownish dark blood and muscles along with cyanosis are the observations suggestive of nitrate-nitrite poisoning following postmortem examination shortly after death.[28] This observation is in agreement with the findings from the current study in which necropsy findings immediately after the death of sheep indicated cyanotic mucous membranes and chocolate colored blood that was poorly clotted. However, it has to be noted that, a few hours following the death of animals following ingestion of large amounts of nitrate containing plants, the hemoglobin bound nitrites dissociates, with the formed methemoglobin changing into hemoglobin, and subsequent disappearance of the chocolate color of the blood, muscles, and the carcass.[34] This observation is in agreement with the findings from the current study since the dark brown color of the blood, muscles, and carcass had disappeared when necropsy was conducted in one sheep approximately 6 h after death. In sheep poisoned with nitrites following exposure to plants containing high concentrations of nitrates, microscopic lesions have also been reported to be absent.[30],[33],[34],[35] However, the current study did not investigate the histopathological changes associated with nitrate-nitrate poisoning in the reported herd of sheep. Nitrate-nitrite poisoning in sheep has not been reported previously in Kenya. This could be due to the fact that sheep are more efficient in nitrite conversion into ammonia, and thus less susceptible to nitrite intoxication than cattle, possibly explaining the low prevalence of nitrate-nitrite poisoning among sheep.[27] Duckweed (Portulaca oleracea) is a poisonous plant characterized by high nitrate accumulation, especially if grown in nitrate rich soils at times of drought and can potentially cause nitrate-nitrite poisoning in sheep.

In conclusion, in case of acute death in sheep following ingestion of large amounts of duckweed (Portulaca oleracea), nitrate-nitrite poisoning should always be considered, especially if the duckweed is growing under drought conditions in a well manured farm or in a locality where nitrogen containing fertilizers have been used. Nitrate levels in the duckweed forage and also in ruminal contents (in case of death) should be analyzed, so as to support any suspicion and also to facilitate follow-up. In addition, nitrate dip stick tests and diphenylamine blue test should be carried out on both urine and forage, respectively. Serum from surviving animals should be considered for the examination of nitrite levels. Methemoglobin levels also need to be analyzed in chocolate-colored blood collected immediately after death. Sheep owners should avoid feeding their animals with duckweed (Portulaca oloracea), so as to reduce the cases of nitrate toxicosis.

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