AS NICK MOONEY states: ‘Most of the research on lethal dose to 50% (LD50) for Australian animals and the potential impacts of 1080 was done on captive animals decades ago by John McIlroy, then at CSIRO, and published in various issues of Australian Wildlife Research. It is doubtful if this work could ever be substantially expanded or repeated because it involves lethal testing.’  (Foxes, quolls, devils and 1080)

With DPIW poised to embark on a decade-long $56 million dollar fox eradication campaign using 1080 meat baits as the principle eradication tool, I believe there are several very good reasons why 1080 testing of non-target Tasmanian species exposed to these baits must now be repeated. For Tasmanian wildlife authorities to rely solely on this unrepeated toxicological data would be reckless. 

John McIlroy commenced his work on the sensitivity of Australian animals to the poison 1080 (Sodium Fluoroacetate) a quarter of a century ago. John was a research scientist working at the CSIRO Division of Wildlife Research at Gunghalin near Canberra. During the period from 1980-86 he conducted a series of dose-response experiments to assess the sensitivity of 1080 on a representative range of Australian animals, covering species in all the main vertebrate taxa. He published 9 scientific papers in this series; 7 as the sole author and 2 in collaboration with others.

In documenting his research findings, John was careful to firstly prepare the theoretical and statistical ground work on which this series of experimentally-based toxicity would be based (McIlroy 1981a).

“In toxicological work the sensitivity of different [species of] animals to a poison is usually expressed as the LD50 or median lethal dose, a statistical estimate of the dose — in milligrams of poison per kilogram body weight, that will kill 50%  of a large population.

The LD50 of a poison and its 95% confidence limits are only an indication of the values that might be expected from repeated trials on the same strain of animals under the same experimental conditions.”

In applying the LD50 values to a test poison, McIlroy states: 

“The necessity for such a standardised procedure has been questioned … [as] statistically significant differences in LD50 values (up to 3.2 fold) within and between laboratories, related to differences in experimental procedure, …  [but] these were not great enough to change the interpretation of the relative hazards of the test chemical involved. However, because I was concerned with a controversial poison [1080] and its toxicity to a variety of wild animals, I felt it was important to assess the effects that differences in experimental procedure might have on LD50 values of 1080 and, if necessary, design a procedure to minimize such sources of variation.” (McIlroy 1981a)   

In his second paper detailing the results of his experimental studies on marsupials and placental mammals, John began on a cautionary note:

“The effect that these [1080] poisoning campaigns are having on non-target or native animal populations is not known, despite occasional reports of individuals of these species being found dead or ‘vanishing’ from areas in which 1080 has been used.” (McIlroy 1981b)

Targeting dingoes

McIlroy was very considered in any reliance of these experimentally derived LD50 values:

“In reality many factors are involved in determining whether an individual or what proportion of a population may be killed by a [1080] poisoning campaign. The preceding theoretical analysis involved mean body weights of only small samples of animals, LD50s obtained under specific experimental conditions, and a particular concentration of 1080 in each bait plus the assumptions about bait intake by free-living species. All are likely to vary in different field situations, altering the risk each individual carnivore faces.”

Based on 1080 baiting campaigns targeting dingoes (& wild dogs), John McIlroy made some thoughtful recommendations when deciding on the most effective bait size and quantity of 1080 per bait for maximal kill of target species and minimal impact to non-target (native) species.

“The data on [1080] sensitivities do provide fundamental information for the planning of dingo-poisoning operations. For example, if the aim is to obtain maximal control with minimum dose it would be best to plan the baiting on the basis of a LD100 based on twice the upper confidence limit of the LD50 and the weight of the heaviest specimen reported. In contrast, to assess the hazard to a non-target species, calculations might be best based on the lower confidence limit of the LD50, or some other lower figure, and either the mean weight or much lower body weights of, for instance, immature animals.”

McIlroy went on to do a theoretical calculation to show this point for dingoes (the target carnivore) and spotted-tail quolls (a non-target carnivore). 

“The heaviest individual [dingo] caught in the Eastern Highlands was 25 kg. Thus if the LD100 is assumed to be approximately twice the upper confidence limit of the LD50 (i.e. 0.3mg/kg BW), it would be necessary to get 7.5 mg of 1080 into a dog of this size to kill it. Similar calculations for tiger cats [spotted-tail quolls], using twice the lower confidence limit of the LD50 (i.e. 2.56 mg/kg BW) and taking the mean body weight of 2.8 kg, indicate that 7.17 mg of 1080 is a lethal dose for [this species]. 

Applying McIlroy’s precautionary recommendation to the mean body weight for immature spotted-tail quolls of 1.1 kg, only 2.8 mg of 1080 is a lethal dose.

Obtain a lethal dose

The same theoretical calculation and logic can be applied can be applied to 1080 poisoning campaigns targeting foxes.

For an extreme body weight fox of 6 kg and applying an LD100 that is approximately twice the upper confidence limit of the LD50 (i.e. 0.26mg/kg BW), it would be necessary to get a fox to consume 1.56 mg of 1080 to kill it (not 3 mg of 1080 per bait). If each dried kangaroo meat (DKM) baits contained this amount of 1080, one bait would kill all foxes less than 6 kg. When applying McIlroy’s precautionary calculation to a mean body weight for immature quolls, such animals would need to ingest at least two baits to obtain a lethal dose.

“From the viewpoint of trying to safeguard tiger cats [spotted-tail quolls]; therefore, it is obviously necessary to keep 1080 concentration in baits as low as possible.” (McIlroy 1981b)         
 
One variable that McIlroy particularly commented on was the effect of ambient temperature on the sensitivity of 1080 poison. He was concerned that his experimental trials to set the LD50 for many native marsupials were carried out at about 22°C (in controlled environment rooms). He noted that in relation toxicity studies on the spotted-tail quolls, trials were conducted at 13°C where the LD50 was calculated at 1.85 mg/kg BW.

“… different ambient temperatures cause two to five fold differences in the susceptibility of mice and guinea pigs to 1080. Both species are susceptible at both low and high ambient temperatures than they are at medium temperatures. If similar responses occur amongst other, larger homeotherms, this might explain the relatively low LD50 for the tiger cat [spotted-tail quoll] compared to those for the other native cats [quolls]. The possibility exists, therefore, that if these trials had been carried out at 22°C [instead of 13°C], the LD50 would have been slightly higher than 1.85 mg/kg BW.

Ambient temperatures obviously vary considerably between field poisoning situations, both geographically and diurnally, so a LD50 obtained at 22°C, or a dose that will kill 50% of a population experiencing this ambient temperature, must be regarded as only a general value. Greater population mortality may be expected at much lower or higher environmental temperatures.” (McIlroy 1981b)

In relation to the most susceptible non-target marsupial carnivore, the spotted-tail quoll, 1080 baiting programs targeting foxes and wild dogs are still reliant on McIlroy’s highly qualified toxicology studies and LD50 calculations.

In obtaining his LD50 levels for each species, McIlroy orally dosed between 3 and 5 individuals at dose intervals of 1.26 in 4 distinct dose groupings. For spotted-tailed quoll he used 12 animals. The LD50 was calculated at 1.85 mg/kgm with 95% confidence intervals of 1.28 to 2.68 mg/kgm BW.

Other animals begin to vomit

Clinical observations were made on the experimentally poisoned animals.

“Most commonly, affected animals suddenly became hyper-excited, with rapid breathing, bouts of trembling and sometimes periodic circling within their cages. Again, some animals may then recover while other begin to vomit, convulse, or both. With some animals, particularly the eastern native and tiger cats [quolls] and Tasmanian devils, the first symptom is the sudden onset of vomiting.

Convulsions were triggered by disturbance, such as the opening of a door, sudden movement by an observer, or convulsion by a neighbouring animal. In rough order, these symptoms include: restlessness; increased hyperexcitability or response to stimuli; bouts of trembling; rapid, shallow breathing; incontinence[involuntary passing of urine and/or faeces] or diarrhoea; excessive salivation; twitching of the facial muscles; nystagmus (involuntary eyeball movement exposing the whites of the eyes)or bulging eyes with large (dilated) pupils and rapid blinking plus, in domestic cats, discharge of mucus from the eyes); slight lack of coordination or balance; abrupt bouts of vocalisation; and finally, sudden burst of violent activity such as racing around the cage, or biting the cage mesh or other objects. All affected animals then fall to the ground in a tetanic seizure, with hind limbs or all four limbs and sometimes the tail extended rigidly from their arched bodies. At other times the front feet are clasped together, clenched or used to scratch frantically at the cage walls. This tonic phase is then followed by a clonic phase in which the animals lie and kick and ‘paddle’ with the front legs and sometimes squeal, crawl around or bite at objects. During this phase the tongue and penis may be extruded, the eyes rolled back so that only the whites show and the teeth are ground together. Breathing is rapid but laboured, with some animals partly choking on their saliva. Finally such animals begin to relax, breathing more slowly and shallowly and lying quietly with the hind legs still extended but apparently semiparalysed (paresis).

From then on individual animals either: (1) gradually recover; (2) die shortly afterwards; (3) after a short or long delay (e.g. 5 min or 3-4 h) experience another one or two series of convulsions and then die shortly afterwards or eventually recover; (4) remain lying quietly, scarcely breathing or moving, until death up to 6 days later.

It is noteworthy that in McIlroy’s observations on carnivorous marsupials exposed to sub-lethal doses of 1080, he noted that animals that did not die but ‘remained weak for 2 or more days’. From this we can infer that the sub-lethal consequences of 1080 poisoning may therefore affect an animal’s ability to evade predation by other animals and affect their ability to find safe refuge.

McIlroy also makes the following observations:

“The pouch young of tammar wallabies are significantly more susceptible to 1080 than adults (P>0.01. The pouch young of brush-tailed possums and northern native cats, Dasyurus hallicatus, similarly appear to be more sensitive than adults. More pouch young pouch young possums than adults died at each dose level, although only their mothers were dosed with 1080; presumably the young ingested lethal amounts of 1080 in the milk. The eight pouch young of one northern native cat also died within 24 h after their mother received a non-lethal dose (84% of a LD50 )but the five pouch young of a tiger cat, Dasyurus maculatus, survived in similar circumstances (74% of a LD50 ). [There are] similar reports of young rats killed by milk from their poisoned mothers.” (McIlroy 1981)

Fox entry into Tasmania

Fox entry into Tasmania has ALWAYS been a biosecurity/biodiversity risk for Tasmania, yet it is remains unclear whether foxes have established breeding populations in Tasmania.

Despite the unsubstantiated stories of intentional introductions of foxes the most likely source of single-fox introductions into Tasmania has been slack and inadequate quarantine measures. In the decades of inadequate quarantine measures at our ports, any foxes that have arrived and escaped into Tasmania, the questions remains which locations have the highest frequency of receiving fox-risk materials?  Might these be the places where foxes might just get lucky and breed?

Over fifty years of 1080 use in Tasmania to control native herbivores like Bennett’s wallaby, Tasmanian pademelon and brush-tail possum coupled with the high sensitivity of red foxes to secondary 1080 poisoning (i.e. through eating a poisoned carcass) is rarely acknowledged.

Where will they ‘get lucky’ in the landscape? Closest to the farms & feedlots that have historically received container-loads of stock feed grain; agri-businesses that transport or deal with used farm equipment; freight forwarding depots. The highly reliable sighting reports of foxes in remote areas (where 1080 poisons have not been used) like the western Central Plateau or our National Parks must be the basis for intensive investigation. Maybe the remote camera used by the DFT team can be now deployed for fox studies.

It ALWAYS comes down to validating the risk assessment.

References:
McIlroy, JC (1981) The Sensitivity of Australian Animals to 1080 Poison I. Intraspecific variation and Factors affecting Acute Toxicity. Australian Wildlife Research 8, 369-383.
McIlroy, JC (1981) The Sensitivity of Australian Animals to 1080 Poison II. Marsupial and Eutherian Carnivores. Australian Wildlife Research 8, 385-399.

Comment on this article, and subject, HERE 

David Obendorf

With DPIW poised to embark on a decade-long $56 million dollar fox eradication campaign using 1080 meat baits as the principle eradication tool, I believe there are several very good reasons why 1080 testing of non-target Tasmanian species exposed to these baits must now be repeated. For Tasmanian wildlife authorities to rely solely on this unrepeated toxicological data would be reckless. 

Comment on this article, and subject, HERE