The chemicals used in sheep dips have evolved from very basic to more sophisticated products over the past 160 years. There were not many changes from the onset of dipping, when arsenic was the major chemical, until the manufacture of organochlorine pesticides in the mid-1940s (initially DDT, then lindane). The next major change was the appearance of dieldrin and aldrin on the market in 1955. The use of organochlorines only lasted 16 years until they were banned by regulation in 1961. Arsenic remained on the market until 1978, when it was deregistered. Synthetic pyrethroids and organophosphate pesticides continue to be used today.
The risk to human health and the environment from chemicals like carbolic acid, potash and sulphur is much less than the risk from arsenic, organophosphates and organochlorines. The following discussion highlights the history of these latter chemicals, their use in sheep dips in New Zealand and their toxic effects.
Arsenic was one of the earliest chemicals used for lice and blowfly control in sheep. Arsenic is a stomach poison and is only effective for the control of parasites once the target pest ingests it. Derris (see below) was often mixed into the arsenic to try to control keds. Arsenic combined with derris remained one of the standard chemicals used in sheep dips until around 1952, when derris began to be used in combination with other chemicals.
Arsenic had many drawbacks. In the early days, it was already recognised as a hazard to human and animal health, although the full extent of its toxicity was often not realised. Arsenic poisoning in dipped sheep was reasonably common. The sheep could become poisoned in a number of ways, either through direct swallowing and ingestion of the dip contents, absorption through the skin, or aspiration when arsenic was applied in powder form. When sheep were hot from the muster, or even from the sun on a hot day, arsenic could easily scald their skin. As well as adversely affecting livestock, arsenic polluted the soil and water surrounding dip sites, both due to dipping operations and poor disposal methods for the spent dipping fluids and sludges.
Arsenic was withdrawn from the commercial market as a dipping chemical in 1978, with remaining stocks being sold until June 1980. However, it is understood that arsenic sheep dips continued to be used for a number of years as farmers used up supplies of arsenic they had previously bought.
In the 1950s, copper was used for three main reasons in conjunction with pest control of sheep: as a bacteriostat, as a preventive for mycotic dermatitis, and as footrot fluid in a bath. However, copper left high residues in the wool of sheep. These were very hard to remove, and the dyes used to colour the wool would often react with copper residues leaving the wool improperly dyed. Over time, the copper also turned to copper sulphide, which led to discoloration of the wool. When used as a bacteriostat the common name for the copper sulphate used was bluestone.
In small concentrations, copper is an essential element for humans (1−2 mg daily diet intake), but it is toxic to many bacteria and viruses. The free copper (II) ion is potentially very toxic to aquatic life. Copper is normally tightly bound to the soil, greatly diminishing its toxicity, and the likelihood of off-site discharges.
Figure A.1: Advertisement for derris-based dip in New Zealand Journal of Agriculture, 1945
Derris elliptica (poison vine) is native from India to Indonesia and has mainly been cultivated in the tropics for its roots, a source of the insecticide Rotenone (Bailey and Bailey 1976). Derris (Rotenone) was first used in New Zealand in approximately 1911 and was often mixed with arsenic to kill both keds and lice. This made it the most popular and effective dip before World War II.
Because World War II interfered with the supply of the plant, other chemicals came onto the market, including DDT and other synthetic insecticides, as discussed below. By 1961, derris use had declined significantly from its use before the war. More effective chemicals were available, and derris uptake also suffered from not having the necessary lasting effect required to kill larvae hatching from the pupal stage.
The organochlorines group includes DDT and its derivatives, chlorinated cyclodienes (aldrin, dieldrin and heptachlor) and lindane (the commercial name for formulations based on purified γ-hexachlorocyclohexane). All of these chemicals became hugely popular for sheep dipping because they allowed new methods of dipping to be developed due to their ability to dissolve in the wool grease and migrate down towards the skin. This meant that saturation was not necessary to successfully treat sheep, and the traditional dip was often replaced with spray showers.
Organochlorines accumulate in the body fat of animals and can be very persistent (especially aldrin and dieldrin) because they are not excreted rapidly and remain stored in body tissue. As a result, these chemicals can accumulate in the food chain, allowing higher concentrations to occur higher up the food chain. Organochlorines are also very persistent in the environment, often remaining in soil for years or even decades.
Chlorobenzene derivatives used in sheep dips were DDT and lindane. These chemicals are insoluble in water and so were used in suspension in the dips. As sheep passed through the dip, their fleeces removed chemicals, so the dip became weaker and less effective during the course of the dipping operations. To maintain the dip at the appropriate strength, additional chemicals had to be added.
Although DDT was found to be much better in sheep dips than arsenic and derris, it was not used for very long because lindane was discovered to be much more effective at killing parasites. Lindane was first used in sheep dips around the mid-1940s and rapidly took over from DDT as the newest and most effective dipping chemical.
Aldrin and dieldrin were first used in sheep dips in approximately 1955. These chemicals have the ability to dissolve into the wool grease and then diffuse to the skin, where the parasites are located. Dieldrin was found to be more toxic to invertebrate insects than aldrin, and was therefore used much more extensively as a sheep dip.
Figure A.2: Advertisement for aldrin-based dip in New Zealand Journal of Agriculture , 1956
Aldrin and dieldrin are structurally related. Sunlight and bacteria in the environment convert aldrin to dieldrin reasonably quickly. Freedman (1989) estimated the half-life of aldrin to be 0.3 years, with 95 percent disappearance in three years. For this reason, aldrin is not commonly found in the soil around old contaminated dip sites. Dieldrin attaches to soil and may stay there unchanged for many years. It is not very soluble in water. However, the mobility of aldrin and dieldrin in the soil environment can be enhanced at hazardous waste sites where organic solvents may be present that have the ability to increase their water solubility (Sawhney 1989).
The organochlorines aldrin, dieldrin, DDT and lindane were prohibited as active ingredients in stock treatment under the Stock (Insecticides and Oestrogens) Regulations 1961 due to their persistence in animal fat and concerns about residues which may accumulate in food and impact on the markets. The consequent unavailability of organochlorines in the market led to a rise in the usage of organophosphates (see below), while arsenic and derris dips continued to be used.
Organophosphates such as diazinon or nankor came into use as sheep-dipping chemicals in the 1960s. They became more popular after organochlorines were banned, and in many ways were much better than organochlorines because they were not as persistent: within a relatively short time after dipping the sheep, meat would have no detectable chemical residues. However, the major drawback of organophosphates was their lack of ability to diffuse down the wool, as organochlorines had done, so the treatment methods which had been developed to use with the organochlorines (not including saturation) were almost useless with organophosphates. Some organophosphates, such as diazinon, were still effective in tip-sprayers and dusters.
Organophosphates are still in common use today because they readily break down in the environment in most situations. Complications can occur, however, when organophosphates are used in places where arsenic-based dips have already contaminated the soil. The arsenic residue may remain biotoxic to soil micro-organisms, thereby preventing the breakdown of organophosphates by micro-organisms that normally occurs in the soil.
Synthetic pyrethroids, which were developed after organophosphates in the 1970s, are a synthetic form of naturally occurring pyrethroid chemicals that are found in the flower heads of chrysanthemums. Synthetic pyrethroids present relatively lower risks compared to arsenic or the organochlorines because they have very low dermal toxicity and are used at much lower concentrations. Under normal soil conditions (ie, in the absence of heavy metals such as arsenic), synthetic pyrethroids break down very rapidly. However, they are highly toxic to aquatic species and non-target invertebrates, and care needs to be taken at the time of application and when disposing of used dip wash, especially near waterways.
Insect growth regulators
Insect growth regulators are the most common chemicals used today to treat flies and lice (especially maggots and nymphs). They do not constitute a distinctive chemical class, and are commonly applied by hand-jetting, saturation or spray-on.
Insect growth regulators have very low toxicity for mammals, making them extremely safe to use for both the farmer and the sheep. However, they are toxic to aquatic invertebrates and take a long time to break down in the environment, so special precautions must be taken when disposing of used dip wash.
Toxicity of chemicals
The human health and ecological effects of sheep-dip chemicals of concern − such as arsenic, DDT and its metabolites DDD, DDE (∑DDT), dieldrin and lindane − are summarised in Table A.2. For more detail, refer to the sources shown in the table.
Table A.2: Summary of toxicological effects of chemicals of primary concern
Arsenic can cause cancer and non-cancer effects. Skin cancer is a well-documented effect, and more recently chronic ingestion of inorganic arsenic has been linked with bladder and lung cancer. Non-cancer effects include dermal lesions, pigmentation, keratoses and peripheral vascular disease.
Arsenic is classified as a known human carcinogen by the IARC (Class 1; IARC 1987) and the US EPA (Class A, US EPA 1993).
NRC 1999/2001; WHO 2001; Baars et al 2001
DDT acts on the central nervous system, and has been shown to cause developmental, reproductive and liver toxicity, primarily lesions or tumours. The limited data on DDD and DDE indicate a similar pattern of toxicity at exposure levels to DDT.
DDT is classified as a possible human carcinogen (class 2B) by IARC (1987), and a probable human carcinogen (Class B2) by the US EPA (1988).
Baars et al 2001; ATSDR 2002a
The primary site of action of dieldrin is considered to be the central nervous system, although chronic exposure to low concentrations can result in liver damage. Dieldrin is a potent inducer of liver enzymes, and can cause suppression of the immune system.
Dieldrin was unable to be classified (Class 3) by the IARC (1987), while the US EPA (1993) classified dieldrin as a probable human carcinogen (Class B2).
ATSDR 2002b; Baars et al 2001; IARC 1987; US EPA 1993
The chronic effects of lindane are primarily liver and kidney damage, although neurotoxic (eg, tremors) and immunotoxic effects may also be observed.
Three isomers of hexachlorocyclohexanes (α, β, γ) were classified as possibly carcinogenic to humans (Class 2B) by the IARC. The US EPA did not classify γ−HCH, while α-HCH was classified probably carcinogenic to humans (Class B2) and β-HCH was classified as possibly carcinogenic to humans (Class C).
Baars et al 2001; ATSDR 2005
Arsenic compounds can cause acute and chronic effects in animal and plant individuals, populations and communities, including death; inhibition of growth, photosynthesis and reproduction; and behavioural effects. The toxicity of arsenic is largely dependent on the form (eg, inorganic or organic) and the oxidation state of the arsenic compound. In general, inorganic arsenicals are more toxic than organoarsenicals, and arsenite is more toxic than arsenate. The primary mechanism of arsenite toxicity is considered to result from its binding to protein sulfhydryl groups. Arsenate is known to affect oxidative phosphorylation by competition with phosphate, because they are structurally similar. In environments containing high phosphate levels, arsenate toxicity to biota is generally reduced. In plants, phosphate can decrease arsenate uptake due to competitive uptake. Toxicity to plants typically occurs at lower concentrations than toxic effects on soil organisms.
US EPA 2005b; CCME 1999
∑DDT Aldrin/dieldrin Lindane
Current environmental concerns regarding organochlorine insecticide residues primarily arise from the accumulation of residues through the food chain and sub-lethal effects of exposure. Numerous sub-lethal effects on animals have been observed, including growth impairment or deformities, tumour growth, impairment of immune systems, and impairment of reproductive systems, including eggshell thinning. Other sub-lethal effects include suppression of the immune response, which can lower resistance to disease and infection; or induction of the immune response, which can cause hypersensitivity. Dieldrin has been found to depress the immune system. Eggshell thinning in birds has been the most widely documented form of reproductive impairment, primarily as a result of exposure to DDE. Toxicity to soil organisms and terrestrial vertebrates occurs at much lower concentrations than toxicity to plants.
Carey et al 1998; de Bruijn et al 1999