Vertebrate Pesticide Toxicology Manual (Poisons)

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3.2 Arsenic

Chemical Name: Arsenic trioxide (As2 O3).

Synonyms: White arsenic, arsenous oxide.

3.2.1 Physical and chemical properties

Arsenic is a naturally occurring toxin found in combination with other metals, particularly iron as arsenic pyrite (FeAs), and iron-arsenic sulphide. Two other forms of arsenic sulphide, orpiment and regular white arsenic (arsenic trioxide), are less common.

3.2.2 Historical development and use

The first recorded use of arsenic in New Zealand and Australia was in the 1880s where it was used in a variety of baits including oats, wheat, root crops, apples, and pollard pellets.

In the early 1970s arsenic was discontinued from use in rabbit control due to its inability to decompose. In addition to this, a number of people were reported to have been affected by arsenic’s cumulative properties.

When still in use in the 1970s, arsenic was available to the public. Under the Pesticides (Vertebrate Pest Control) Regulations 1977, the operator had to either hold a licensed operator’s certificate or be working under the supervision of a certificate holder. All of these certificates were withdrawn by the Pesticides Board on the advice of the then Agricultural Pest Destruction Council when arsenic was withdrawn from use in New Zealand.

3.2.3 Fate in the environment

Arsenic from baits is converted into various inorganic and organic arsenic compounds, most or all of which will be toxic to a varying extent. Arsenic baits were not considered safe to livestock until they had decomposed or had been completely disintegrated by rain.


3.2.4 Toxicology and pathology

Onset of signs

Death from a single dose appears to be a painful process occurring over several hours or days.

Mode of action

Arsenic causes severe gastroenteritis, vomiting, copious watery or bloody diarrhoea, with convulsions and coma preceding death. The corrosion of the gastrointesinal tract is thought to lead to shock as well as haemorrhage.

Fate in animals

Substantial elimination of a sub-lethal dose will occur in 1–6 weeks in most animals. Arsenic is well distributed throughout all tissues and remains for long periods in bone, skin, and hair.

Species variation in response to arsenic:

One of the main disadvantages of arsenic, in addition to its extremely inhumane mode of action, is its low toxicity to rats and possums as compared to its toxicity to humans (Table 24).

Table 24. Acute oral toxicity (LD50mg/kg) of arsenic (Hone & Mulligan 1982)

Species LD50mg/kg

Human 1.43

Possum 8.22

Mouse 45.00

Rat 138.00

3.2.5 Current use

Arsenic is no longer available for use in New Zealand.



3.3 Strychnine


Chemical Name: Strychnine alkaloid.

Synonyms: Strychnine.

3.3.1 Physical and chemical properties

The empirical formula for strychnine is C21H22N2O2 and its molecular weight is 334.4. Strychnine is an odourless bitter white powder. Its melting point is 270º–290ºC. Strychnine has a solubility in water at room temperature of 143 mg/L. Its salts are more soluble in water; for example, the sulphate is soluble in water at 30 g/L at 15ºC. Strychnine is soluble in chloroform, slightly soluble in benzene, and less soluble in diethyl ether and petroleum ether.

3.3.2 Historical development and use

Strychnine is found in the seeds of the Indian tree Strychnos nux-vomica where it is one of a number of different alkaloids. It has a long history as a rodenticide (Schwartze 1922), being used first in Germany in the 16th Century. It affects the central nervous system, leading to paralysis a few minutes after intake and to death within half an hour in rodents (Prakash 1988).

The first recorded use of strychnine in New Zealand and Australia was in the 1880s where it was used in conjunction with a variety of baits including oats, wheat, root crops, thistle roots, apples, and pollard pellets. It was also used in grain to control problem birds.

In the early 1980s strychnine was withdrawn from use in New Zealand on the grounds that the type of death that it caused was inhumane.


When strychnine was used in the early 1970s, an operator was required, under the Pesticides (Vertebrate Pest Control) Regulations 1977, to hold a licensed operators certificate or to be working under the supervision of a certificate holder. All licences were then cancelled by the Pesticides Board on the advice of the then Agricultural Pest Destruction Council on the grounds that the toxin was inhumane and dangerous to staff.

3.3.3 Fate in the environment

Strychnine is a stable alkaloid that retains its toxicity indefinitely in the bait and in the carcass of the poisoned animals. Strychnine bait must be completely decomposed or washed out before a poisoned area could be determined as toxin-free.

3.3.4 Toxicology and pathology

Onset of signs

Poisoned animals often die in less than an hour as a result of respiratory failure (asphyxia), but death may take 24 hours or longer if the dose is low. The typical signs of strychnine poisoning are restlessness and muscular twitching that progresses to convulsive seizures continuing for 45 minutes or more before death. Violent muscular spasms extend the limbs and curve the neck upwards and backwards; the jaws fix and the eyes protrude (Osweiler et al. 1985).

Poisoned animals are generally found close to the bait because of the poison’s rapid action. Strychnine and its salts (especially strychnine sulphate) are highly toxic to all mammals, less so to birds. The LD50 to the Norway rat is 5–6 mg/kg (Prakash 1988). The oral LD90 for strychnine in mice is approximately 5 mg/kg (Mutze 1989). Strychnine induces poison shyness in rats and similar shyness is thought to occur in other vertebrate pests (Prakash 1988). The bitter taste is usually masked by a sweetening agent (icing sugar) in baits.


Mode of action

Strychnine is a fast-acting poison that is readily absorbed into the circulatory system from the intestinal tract. Highest concentrations of strychnine are found in blood, liver, and kidney. Even though it is a neurotoxin, strychnine does not appear to concentrate preferentially in nervous tissues (Hayes 1994).

Fate in animals

Strychnine is highly persistent in baits and poisoned carcasses.
Species variation in response to strychnine:

Strychnine is highly toxic to most domestic animals and wildlife (Table 25). Studies in the USA have shown a 50% reduction in horned lark populations after using strychnine (Apa et al. 1991). Some individual non-target bird species have been reported killed in mouse control operations in Australia (Anthony et al. 1984; Mutze 1989).

Table 25. Acute oral toxicity (LD50mg/kg) of strychnine (Hone & Mulligan 1982; Osweiler et al. 1985)

Species LD50mg/kg

Cow 0.5

Horse 0.5

Cat 0.75

Norway rat 6.8

Duck 2.9

Chicken 5.0

Pigeon 2.1

House sparrow 4.0

3.3.5 Current use

This toxin had been in use since the late 1800s for pest control but has now been banned from use in New Zealand.

It is still used in Australia for mouse plagues and in the USA to control several pests, including skunks, targeted for control in rabies-infected areas, using strychnine-injected eggs. It is also used in Fiji and other islands in the Pacific.

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