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Extension Toxicology Network

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University, and University of California at Davis. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.


Publication Date: 5/94


Agridip, Asunthol, Meldane, Muscatox, Umbethion, Co-Ral, Asuntol, Bay 21, Baymix, Dilice, Resistox, Suntol, Negashunt.


The U.S. Environmental Protection Agency (EPA) classifies most formulations of coumaphos as general use pesticides. Certain formulations have been classified as Restricted Use Pesticides (RUP) because they pose a hazard of acute poisoning from ingestion. These RUPs are 11.6% EC and 42% flowable concentrate end-use products (20). RUPs may be purchased and used only by certified applicators. Check with specific state regulations for local restrictions which may apply.


Coumaphos is an insecticide used for control of a wide variety of livestock insects, including cattle grubs, screw-worms, lice, scabies, flies, and ticks. It is used against ectoparasites, which are insects that live on the outside of host animals such as sheep, goats, horses, pigs, and poultry (15). It is added to cattle and poultry feed to control the development of fly larvae that breed in manure. It is also used as a dust, dip, or spray to control mange, horn flies, and face flies of cattle (11). Coumaphos is considered a selective insecticide because it kills specific insect species while sparing other nontarget organisms.

Coumaphos is one of a class of pesticides referred to as organophosphates. These chemicals act by interfering with the activity of naturally-occurring enzymes called cholinesterases. Cholinesterases are essential for the proper working of the nervous systems in the bodies of both humans and insects. Please refer to the Toxicology Information Brief on cholinesterase-inhibition for a more detailed discussion of this enzyme.



Coumaphos is highly toxic by inhalation and ingestion, and moderately toxic by dermal absorption (19). As with all organophosphates, coumaphos is readily absorbed through the skin. Skin which has come in contact with this material should be washed immediately with soap and water and all contaminated clothing should be removed. Skin and eye contact with this insecticide may cause mild irritation, as well as cholinesterase-inhibition. Coumaphos does not cause skin sensitization allergies (20). Toxic symptoms in humans are largely caused by the inhibition of cholinesterase. Individuals with respiratory ailments, impaired cholinesterase production, or with liver malfunction may be at increased risk from exposure to coumaphos. High ambient temperatures or exposure to UV light may increase the toxicity of coumaphos (19). Signs of poisoning include diarrhea, drooling, difficulty in breathing, leg and neck stiffness (3). Some of the symptoms of acute inhalation of coumaphos begin immediately, or within four to 12 hours, of exposure. These include headaches, dizziness and incoordination. Moderate poisoning is characterized by muscle twitching and vomiting. Severe poisoning is indicated by diarrhea, fever, toxic psychosis, fluid retention (edema) of the lungs, and high blood pressure. Symptoms of sublethal poisoning may continue for 2 to 6 weeks (8).

The organophosphate insecticides are cholinesterase inhibitors. They are highly toxic by all routes of exposure. When inhaled, the first effects are usually respiratory and may include bloody or runny nose, coughing, chest discomfort, difficult or short breath, and wheezing due to constriction or excess fluid in the bronchial tubes. Skin contact with organophosphates may cause localized sweating and involuntary muscle contractions. Eye contact will cause pain, bleeding, tears, pupil constriction, and blurred vision. Following exposure by any route, other systemic effects may begin within a few minutes or be delayed for up to 12 hours. These may include pallor, nausea, vomiting, diarrhea, abdominal cramps, headache, dizziness, eye pain, blurred vision, constriction or dilation of the eye pupils, tears, salivation, sweating, and confusion. Severe poisoning will affect the central nervous system, producing incoordination, slurred speech, loss of reflexes, weakness, fatigue, involuntary muscle contractions, twitching, tremors of the tongue or eyelids, and eventually paralysis of the body extremities and the respiratory muscles. In severe cases there may also be involuntary defecation or urination, psychosis, irregular heart beats, unconsciousness, convulsions and coma. Death may be caused by respiratory failure or cardiac arrest (19).

Some organophosphates may cause delayed symptoms beginning 1 to 4 weeks after an acute exposure which may or may not have produced immediate symptoms. In such cases, numbness, tingling, weakness and cramping may appear in the lower limbs and progress to incoordination and paralysis. Improvement may occur over months or years, and in some cases residual impairment will remain (19).

The amount of a chemical that is lethal to one-half (50%) of experimental animals fed the material is referred to as its acute oral lethal dose fifty, or LD50. The oral LD50 for coumaphos in rats is 13-41 mg/kg, 28 to 55 mg/kg in mice, 58 mg/kg in guinea pigs, and 80 mg/kg in rabbits (8, 10, 2, 18, 19). An oral dose of 8 mg/kg of the insecticide killed sheep. Other sheep survived a 20 mg/kg dose, but were severely affected for several days (11). The dermal LD50 in rats is 860 mg/kg, and 500 to 2400 mg/kg in rabbits (19, 20). The eye, or ocular, LD50 in rabbits is 2.6 mg/kg (8).

The lethal concentration fifty, or LC50, is that concentration of a chemical in air or water that kills half of the experimental animals exposed to it for a set time period. The 1-hour inhalation LC50 for coumaphos in rats is 341 mg/m3 for females and 1080 for males mg/m3 (20).


Repeated or prolonged exposure to organophosphates may result in the same effects as acute exposure including the delayed symptoms. Other effects reported in workers repeatedly exposed include impaired memory and concentration, disorientation, severe depressions, irritability, confusion, headache, speech difficulties, delayed reaction times, nightmares, sleepwalking and drowsiness or insomnia. An influenza-like condition with headache, nausea, weakness, loss of appetite, and malaise has also been reported (8, 19).

Coumaphos has a residual period of two to three weeks on livestock. During this time, traces of the chemical may still be active (4). No adverse effects were seen in cattle that were sprayed weekly with a concentration of 200-400 parts per million (ppm) of coumaphos, or dipped over a two-year period in a solution containing 200 ppm (11).

Rats tolerated a two-year diet with 5 mg/kg/day of coumaphos. Rats that were given daily doses of 1.25 or 5 mg/kg in a two-year chronic feeding study had shortened life spans by 10% and 25%, respectively. A dose-related inhibition of cholinesterase was observed at 0.5 mg/kg or higher. In other words, as the dosage of coumaphos increased, cholinesterase inhibition also increased. No pathological changes were seen that could be attributed to coumaphos (13).

Reproductive Effects

Once in the bloodstream, coumaphos may cross the placenta. Mice fed coumaphos at a dietary level of 100 ppm exhibited a decrease in the number of pregnancies, litter size, and surviving offspring. No reproductive effects were observed in 3 generations of mice fed a dietary doses of 1.25 mg/kg/day (19).

Teratogenic Effects

Based on studies with rats and rabbits, EPA has determined that coumaphos is not teratogenic (20). When rats were given doses of 0, 1, 5 and 25 mg/kg on days 6 through 15 of pregnancy, the mothers given 25 mg/kg exhibited tremors, but no developmental effects occurred in offspring at any dose. Similar results were observed when pregnant rabbits were given doses of up to 18 mg/kg on days 7 through 19 of pregnancy. The mothers exhibited signs of cholinesterase inhibition at the highest dose tested (18 mg/kg), but no evidence of developmental effects on the offspring were seen at any dose (21). No increase in embryonic deaths or teratogenesis was observed in heifers given dermal application of coumaphos at a rate of 14.2 grams (g) or 28.5 g/kg during various stages of gestation (11).

Mutagenic Effects

Gene mutation and DNA damage studies performed on bacterial cultures showed no evidence of mutagenicity (20, 21).

Carcinogenic Effects

Coumaphos was not found to be cancer-causing, or carcinogenic, in tests done on mice and rats. There was no increase in the number of tumors reported in rats given doses of 1.25 or 5 mg/kg/day of coumaphos in a 2-year chronic feeding study (13).

Organ Toxicity

Coumaphos primarily affects the nervous system through cholinesterase inhibition, by which there is a deactivation of cholinesterase, an enzyme required for proper nerve functioning. No organ effects were seen in acute or chronic studies of coumaphos. The toxic effects of coumaphos are limited to those related to cholinesterase inhibition.

Fate in Humans and Animals

Following its oral administration to mammals, coumaphos was rapidly broken down into nontoxic products which were eliminated in urine and feces with no evidence of bioaccumulation (20). Seventy percent of an oral dose given to rats was eliminated in 7 days. With dermal doses, 5% was eliminated. Single oral doses produced no changes in metabolism and no evidence of bioaccumulation in rats (21).

Significant amounts of organo-soluble substances were found in the livers and kidneys of cattle that had coumaphos applied to the skin. Coumaphos was also found in the milk of dermally treated cows (5).

Unchanged coumaphos and other breakdown products were found in the excreta of hens that were dusted with the insecticide. Similar results and substances were found after oral treatment of hens coumaphos (5).


Effects on Birds

Coumaphos is highly toxic to birds (20). The symptoms of acute toxicity in mallards given dietary concentrations of 29.8 mg/kg include spraddle-legged walking, wing twitching, wing drop, tearing of the eyes, and spread wings. These symptoms persisted in some survivors for up to 13 days, accompanied by weight loss. Death usually occurred between 2 and 12 hours after treatment. (12). Severe acute toxicity, and eventual death, was caused in hens after they were given daily oral doses of 10 mg for 1 to 8 days. Hens given single oral doses of 50 mg/kg recovered from the initial effects of cholinesterase- inhibition and developed signs of delayed nerve poisoning, or neurotoxicity (11). The oral LD50 for coumaphos in wild birds is 3 mg/kg, 29.4 mg/kg in mallard ducks, and 7.94 mg/kg in pheasants, and 14 mg/kg in chickens (7, 9, 20). When they were fed the insecticide at 100 mg/kg, chickens developed leg weakness (9).

Effects on Aquatic Organisms

Coumaphos is moderately toxic to fish and highly toxic to aquatic invertebrates (20). The 96-hour LC50 for coumaphos (95-97% technical) in channel catfish is 0.8 ppm at 18 degrees C, in largemouth bass is 1.1 ppm, and in walleyes is 0.8 ppm (11). The 96-hour LC50 in rainbow trout is 5.9 ppm, in bluegill sunfish is 5 ppm, and in freshwater invertebrates (amphipods) is 0.15 parts per billion (ppb) (20).

Effects on Other Animals (Nontarget Species)

Coumaphos poses a moderate hazard to honey bees and a slight hazard to other beneficial insects (4).


Breakdown of Chemical in Soil and Groundwater

Coumaphos was relatively immobile in a sandy loam soil and is unlikely to contaminate groundwater. However, because coumaphos use patterns create potential concentrated or point sources of pollution at animal treatment sites, EPA has called for additional data related to coumaphos's potential for groundwater contamination (20).

A general characteristic of organo-phosphates such as coumaphos is that they adsorb, or bind, fairly well to soil particles. Chemicals that have a high tendency to bind to soil do not readily move (leach) with water percolating through the soil (17).

Breakdown of Chemical in Water

Coumaphos is relatively resistant to breakdown in water (hydrolysis). It is nearly insoluble in water, and is relatively stable over a wide pH range in water (18, 23).

Breakdown of Chemical in Vegetation

No information was found on the breakdown of coumaphos in vegetation.


Technical coumaphos is a tan crystalline solid with a slight sulfur odor (15, 5). Although it is stable in water, coumaphos hydrolyzes slowly in alkaline conditions. Alkaline and strong oxidizing materials should be avoided when using this material (11). It is also incompatible with pyrethroids and piperonyl butoxide (4).

Coumaphos is stable under normal temperatures and pressures (19). It may burn, but does not ignite readily. When heated to decomposition, coumaphos emits very toxic fumes of sulfur oxides, phosphorous oxides, and chlorides (15). These gases can pollute (14). Efforts should be made to keep containers of this insecticide cool, as they may explode in the heat of fire (8). The runoff from fire control water may give off poisonous gases or cause pollution (14).

Depending on formulation and use rate, coumaphos should not be used to treat lactating dairy cattle, or any cattle within three weeks of calving. It should also not be used for treatment of any animals or poultry within two weeks of shipping, vaccinations or other stressful situations (11).

Persons who work with organo-phosphate materials for long periods of time should have frequent blood tests of their cholinesterase levels. If the cholinesterase level falls below a critical point, no further exposure should be allowed until it returns to normal (22).

Protective clothing must be worn when handling coumaphos. Before removing gloves, wash them with soap and water. Always wash hands, face and arms with soap and water before smoking, eating or drinking.

After work, remove all work clothes and shoes. Shower with soap and water. Wear only clean clothes when leaving the job. Wash contaminated clothing and equipment with soap and water after each use. Keep contaminated work clothes separate from regular laundry.

Exposure Guidelines:

ADI: 0.0005 mg/kg (11)

No occupational exposure limits have been established for coumaphos by OSHA, NIOSH or ACGIH (19).

Physical Properties:

CAS #: 56-72-4
Specific gravity: 1.474 (19); 30.06 lb/cu ft (21)
H2O solubility: insoluble in water (5); 1.5 mg/liter at 20 degrees C (6)
Solubility in other solvents: slightly soluble in acetone, chloroform, corn oil; soluble in organic solvents; slightly soluble in ethanol (5)
Melting Point: 90-92 degrees C (1)
Boiling point: 20 degrees C at 10 to the minus 7 mm Hg (20)
pH: 7.23 at 1 g/100 ml (20)
Vapor pressure: 1 x 10 to the minus 7 mm Hg at 20 degrees C (5)
Partitioning model: PCMC1 (mole fraction) - 7.45 x 10 to the minus 8; PMCM2 (mg/l): 1.5; PCMC3 (mcm/l): 4 (16)
Chemical Class/Use: Organophosphate insecticide

Anthelmintic for animal use (a remedy for the destruction or elimination of intestinal worms)


Miles, Inc.
Agriculture Division
P.O. Box 390
Shawnee Mission, KS 66201-0390
Telephone: 913-631-4800

Review by Basic Manufacturer

Comments solicited: November, 1992
Comments received: January, 1994


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