Aspects of eradicating disease from wildlife vectors

Bovine tuberculosis (Tb) occurs in cattle and in wildlife, with various species infected in a range of countries. The control or eradication of Tb can be empirically based or based on understanding of disease processes such as described in mathematical models. The eradication of Tb from Australia was largely empirically based, with water buffalo (Bubalus bubalis) apparently being the only significant wildlife disease host. Buffalo populations were apparently greatly reduced to achieve Tb eradication. This paper examines the alternative approach of basing control or eradication on mathematical models, and compares some predictions of one-host (wildlife) and two-host (cattle and wildlife) disease models with empirical data on Tb in both cattle and in wildlife. The one-host and two-host models were based on those published and also derived. For example, reported positive regressions between prevalence’s of Tb in cattle and in brushtail possums in New Zealand and also between prevalence’s of Tb in red deer and in brushtail possums in New Zealand, are consistent with two-host disease models assuming frequency dependent transmission. However most models of Tb in wildlife, e.g. the Barlow models, assume density dependent transmission. The implications for disease eradication are that models assuming density dependent transmission have a threshold host (wildlife) density about zero. Hence, if frequency dependent, not density dependent, transmission actually occurs, then host (wildlife) density would have to be reduced much lower, in order for eradication of Tb from cattle.     

Combining surveillance data from, and comparing the relative utility of wildlife sentinels for detecting bovine tuberculosis in New Zealand

Brushtail possums (Trichosurus vulpecula) are the main wildlife host of bovine tuberculosis in New Zealand. The disease cycle can be broken by intensive large-scale reduction of possum populations to low levels. That strategy has been successfully applied over much of New Zealand, to the point that a key question now is whether the disease has indeed been locally eliminated, so that the expensive disease control programme can be stopped. However, it is usually too expensive to survey the few possums left. We have therefore developed a new approach to characterising the Tb detection capabilities of any wild animal species that can host the disease. The approach assigns each individual animal and spatially-explicit “detection kernel” that combines data on average range utilisation with probabilities of becoming infected when Tb is present in possums at the lowest sustainable level. We describe the principles involved, and compare the detection capabilities and cost-effectiveness across the species that are commonly being used for this purpose – wild pigs (Sus scrofa), ferrets (Mustela furo), and deer (Cervus spp.).     

Spatial variation in impacts of brushtail possums on two Loranthaceous mistletoe species

Browsing by introduced brushtail possums is linked to major declines in mistletoe abundance in New Zealand, yet in some areas mistletoes persist, apparently unaffected by the presence of possums. To determine the cause of this spatial variation in impact I investigated the abundance and condition (crown dieback and extent of possum browse) of two mistletoes (Alepis flavida, Peraxilla tetrapetala) and abundance and diet of possums in two mountain beech (Nothofagus solandri var. cliffortioides) forests in the central-eastern South Island of New Zealand. Mistletoe is common and there are long-established uncontrolled possum populations in both forests. Mistletoes were abundant (216?1359 per hectare) and important in possum diet (41?59% of total diet), but possum density was low (c. 2 per hectare) in both areas. Possum impacts were slight with low browse frequencies and intensities over much of the study sites. However, impacts were significantly greater at a forest margin, where possum abundance was highest, and at a high-altitude site where mistletoe density was lowest. Mistletoe crown dieback was inversely proportional to intensity of possum browsing. These results suggest that the persistence of abundant mistletoe populations at these sites is due to mistletoe productivity matching or exceeding consumption by possums in these forests of low possum-carrying capacity, rather than low possum preference for the local mistletoe populations.     

Quantifying Short-Term Foraging Movements in a Marsupial Pest to Improve Targeted Lethal Control and Disease Surveillance

In New Zealand, the introduced marsupial brushtail possum (Trichosurus vulpecula) is a pest species subject to control measures, primarily to limit its ability to transmit bovine tuberculosis (TB) to livestock and for conservation protection. To better define parameters for targeted possum control and TB surveillance, we here applied a novel approach to analyzing GPS data obtained from 44 possums fitted with radio-tracking collars, producing estimates of the animals’ short-term nocturnal foraging patterns based on 1-, 3- or 5-nights’ contiguous data. Studies were conducted within two semi-arid montane regions of New Zealand’s South Island High Country: these regions support low-density possum populations (<2 possums/ha) in which the animals’ home ranges are on average larger than in high-density populations in forested habitat. Possum foraging range width (FRW) estimates increased with increasing monitoring periods, from 150-200m based on a single night’s movement data to 300-400m based on 5 nights’ data. The largest average FRW estimates were recorded in winter and spring, and the smallest in summer. The results suggest that traps or poison-bait stations (for lethal control) or monitoring devices (for TB surveillance), set for > 3 consecutive nights at 150m interval spacings, would likely place >95% of the possums in this type of habitat at risk of encountering these devices, year-round. Modelling control efficacy against operational expenditure, based on these estimations, identified the relative cost-effectiveness of various strategies that could be applied to a typical aerial poisoning operation, to reduce the ongoing TB vectorial risk that possums pose in the High Country regions. These habitat-specific findings are likely to be more relevant than the conventional pest control and monitoring methodologies developed for possums in their more typical forested habitat.     

Predicting Summer Site Occupancy for an Invasive Species,the Common Brushtail Possum (Trichosurus vulpecula), in an Urban Environment

Invasive species are often favoured in fragmented, highly-modified, human-dominated landscapes such as urban areas. Because successful invasive urban adapters can occupy habitat that is quite different from that in their original range, effective management programmes for invasive species in urban areas require an understanding of distribution, habitat and resource requirements at a local scale that is tailored to the fine-scale heterogeneity typical of urban landscapes. The common brushtail possum (Trichosurus vulpecula) is one of New Zealand’s most destructive invasive pest species. As brushtail possums traditionally occupy forest habitat, control in New Zealand has focussed on rural and forest habitats, and forest fragments in cities. However, as successful urban adapters, possums may be occupying a wider range of habitats. Here we use site occupancy methods to determine the distribution of brushtail possums across five distinguishable urban habitat types during summer, which is when possums have the greatest impacts on breeding birds. We collected data on possum presence/absence and habitat characteristics, including possible sources of supplementary food (fruit trees, vegetable gardens, compost heaps), and the availability of forest fragments from 150 survey locations. Predictive distribution models constructed using the programme PRESENCE revealed that while occupancy rates were highest in forest fragments, possums were still present across a large proportion of residential habitat with occupancy decreasing as housing density increased and green cover decreased. The presence of supplementary food sources was important in predicting possum occupancy, which may reflect the high nutritional value of these food types. Additionally, occupancy decreased as the proportion of forest fragment decreased, indicating the importance of forest fragments in determining possum distribution. Control operations to protect native birds from possum predation in cities should include well-vegetated residential areas; these modified habitats not only support possums but provide a source for reinvasion of fragments.     

Combining livestock and wildlife surveillance data to assess the likelihood of freedom from bovine tuberculosis in New Zealand

Bovine tuberculosis (Tb) has historically occurred in livestock throughout most of New Zealand, and also in wildlife over about 40% of the country. Using a test-and-cull strategy to eliminate Tb from livestock, intensive control of the major wildlife host (brushtail possums), and surveillance of key spillover hosts (wild ferrets, pigs and deer), the Animal Health Board has reduced the number of infected cattle herds by over 90% since 1994. Many formerly infected areas appear to now be free of the disease, so a key question is whether the disease control programme can be stopped in those places. We have developed a Bayesian framework for combining spatially explicit data derived from livestock testing and surveys of Tb prevalence in key wildlife hosts, and using those data to updating or refine an estimate of the probability of Tb freedom from wildlife provided by a priori prediction derived from a model of how Tb levels in possum are likely to have been affected by the duration and intensity of control. This paper outlines the conceptual framework, and some preliminary “case-study” steps toward implementation.     

Assessing Movements of Brushtail Possums (Trichosurus vulpecula) in Relation to Depopulated Buffer Zones for the Management of Wildlife Tuberculosis in New Zealand

In New Zealand, managing the threat of bovine tuberculosis (TB) to livestock includes population reduction of potentially infectious wildlife, primarily the brushtail possum (Trichosurus vulpecula). Population control is often targeted on forested buffer zones adjacent to farmland, in order to limit movements of possums across the buffer and reduce the risk of disease transmission to livestock. To assess the effectiveness of buffers in protecting livestock we analysed GPS telemetry data from possums located in untreated forest adjacent to buffers, and used these data to characterise patterns of movement that could lead to possums reaching farmland during the season when most dispersal occurs. Analyses of movement data showed that the direction of dispersal by sub-adult and adult possums and the extent of long exploratory movements were not biased toward forest buffers, even though these provided vacant habitat as suitable for possums as untreated forest. Instead, dispersal and exploratory movements were uncommon even for sub-adult possums and such events typically lasted <10 days. Dispersing possums settled predominantly in river valleys. A simulation model was developed for the 3-6-month dispersal season; it demonstrated a probability of <0.001 that an infected possum, originating from a low-density population with low disease prevalence in untreated forest, would move across 3 km of recently controlled forest buffer to reach farmland. Our results indicate short-term reduction in the risk of TB transmission from possums to livestock in New Zealand by the use of depopulated buffer zones, while acknowledging that the threat of disease spread from untreated forest is likely to increase over time as possum population density and, potentially, TB prevalence among those possums, increase in the buffer zone.     

Persistence of Tb in possums: Insights from a spatially explicit, stochastic model

The majority of models for predicting the dynamics of bovine Tb in brushtail possums in New Zealand owe much to the pioneering work of the late Nigel Barlow. These non-spatial, deterministic models subsumed local disease dynamics by using a heterogeneous mixing term that assumed Tb was confined to a fraction of the population (in patches). However, the underlying mechanism(s) that could result in this heterogeneity of infection risk were obscure. We present a new individual-based, spatial, stochastic simulation model of Tb in possums that provides an explicit mechanism for simulating heterogeneous risk of infection based on a model of individual home range utilisation and disease susceptibility. The manipulation of parameters governing individual utilisation of space also means that processes such as non-linear contact structure can be handled naturally. We use the model to predict the persistence of Tb in possums under scenarios currently implemented for the management of bovine Tb in wildlife and determine conditions under which Tb might be predicted to persist despite control efforts.     

First direct estimate of the detection probability of bovine tuberculosis in possums by possum transmission

We conducted a field trial to determine whether percutaneous challenge of wild brushtail possums (Trichosurus vulpecula) with virulent Mycobacterium bovis, the causative agent of bovine tuberculosis (TB), resulted in subsequent transmission of M. bovis to other possums. We then used our results to make a preliminary first direct estimate of the ‘TB detection probability’ or transmission rate in wild possums. Seven free-living adult possums in the Rimutaka Forest Park, New Zealand, were captured, GPS-collared, experimentally challenged and then released. Possum populations surrounding the challenged individuals were examined for secondary cases 4–8 months post-challenge. Of 37 possums caught within the home ranges of the challenged individuals, one had confirmed TB. As no TB was detected in 355 other possums examined in this study, or 659 possums in a concurrent study in the immediate vicinity, we conclude that this represents the first documented case of onward transmission of M. bovis from an experimentally challenged possum in the wild. This pilot trial therefore gives some confidence that the new percutaneous challenge model for TB in possums can be used to obtain empirical estimates of TB detection probabilities. The calculated probability of onward transmission (‘detection’) here was just 0.027; however, with the possibility that further transmission events would have occurred from the one challenged possum removed from the trial before it had died, or if our trapping missed secondary cases, this value could be an underestimate. The implications of this low preliminary estimate are discussed.     

Native plant/herbivore interactions as determinants of the ecological and evolutionary effects of invasive mammalian herbivores: the case of the common brushtail possum

In their native range, mammalian herbivores exist in a suite of direct and indirect ecological and evolutionary relationships with plant populations and communities. Outside their native range these herbivores become embedded in a multitude of new ecological and evolutionary interactions with native plant species in the new range. Sound knowledge of the plant/herbivore interactions in the herbivores’ native range provides an ideal framework to better understand their effects in the introduced range. The example of the common brushtail possum (Trichosurus vulpecula) and its introduction to New Zealand from Australia provides an excellent case study. In Australia, the common brushtail possum is a widespread generalist herbivore and it is thought that this generalist lifestyle has equipped the species well for successful colonisation of New Zealand. In Australia the brushtail possum has co-existed with highly chemically defended foliage since the Oligocene and recent papers have supported the role of possums as agents of selection on eucalypt defences. While the chemical profile of New Zealand foliage is comparatively poorly understood, possums do show clear selectivity between and within populations and some of these interactions may be mediated by the animals ability to ‘cope’ with PSMs, coupled with maintaining its generalist diet. While possums have had less time to effect evolutionary change in New Zealand species, their impacts on plant fitness have been well documented. However, further knowledge on variation and heritability of foliage traits driving possum preferences is needed to elucidate the ecological and evolutionary plant/possum interactions in the invasive range.     

Oral vaccination reduces the incidence of tuberculosis in free-living brushtail possums

Bovine tuberculosis (Tb) caused by Mycobacterium bovis has proved refractory to eradication from domestic livestock in countries with wildlife disease reservoirs. Vaccination of wild hosts offers a way of controlling Tb in livestock without wildlife culling. This study was conducted in a Tb-endemic region of New Zealand, where the introduced Australian brushtail possum (Trichosurus vulpecula) is the main wildlife reservoir of Tb. Possums were trapped and vaccinated using a prototype oral-delivery system to deliver the Tb vaccine bacille Calmette–Guerin. Vaccinated and control possums were matched according to age, sex and location, re-trapped bimonthly and assessed for Tb status by palpation and lesion aspiration; the site was depopulated after 2 years and post-mortem examinations were conducted to further identify clinical Tb cases and subclinical infection. Significantly fewer culture-confirmed Tb cases were recorded in vaccinated possums (1/51) compared with control animals (12/71); the transition probability from susceptible to infected was significantly reduced in both males and females by vaccination. Vaccine efficacy was estimated at 95 per cent (87–100%) for females and 96 per cent (82–99%) for males. Hence, this trial demonstrates that orally delivered live bacterial vaccines can significantly protect wildlife against natural disease exposure, indicating that wildlife vaccination, along with existing control methods, could be used to eradicate Tb from domestic animals.     

Differential susceptibility to Eucalyptus secondary compounds explains feeding by the common ringtail (Pseudocheirus peregrinus) and common brushtail possum (Trichosurus vulpecula)

The effect of two plant secondary metabolites, tannins and formylated phloroglucinol compounds (FPCs), on the intake of Eucalyptus foliage by common ringtail (Pseudocheirus peregrinus) and common brushtail possums (Trichosurus vulpecula) was studied. We manipulated the amount of tannin that was free to bind with protein by coating foliage with polyethylene glycol 4000 (PEG) and relied on natural intraspecific variation in FPC concentrations. In contrast to ringtail possums, brushtail possums showed a greater tolerance to FPCs and ate more foliage when it was coated with PEG, suggesting that tannins limited their food intake. Brushtails detected the effects of tannins through immediate oral sensations rather than through systemic effects. Ringtail possums appeared highly tolerant of foliar tannins yet susceptible to low concentrations of FPCs. We could not detect any interaction between tannins and FPCs that affected the intake of Eucalyptus foliage by either species of possum. Although ringtail and brushtail possums are widely regarded as specialist and generalist folivores, respectively, their differential susceptibility to co-occurring secondary metabolites suggests greater complexity. Each possum species appears to be a specialist in its own right, which leads to a partitioning of available foliage. Brushtails avoid tannins and ringtails avoid FPCs.     

Clinical,Microbiological and Pathological Findings of Mycobacterium ulcerans Infection in Three Australian Possum Species

Background

Buruli ulcer (BU) is a skin disease caused by Mycobacterium ulcerans, with endemicity predominantly in sub-Saharan Africa and south-eastern Australia. The mode of transmission and the environmental reservoir(s) of the bacterium and remain elusive. Real-time PCR investigations have detected M. ulcerans DNA in a variety of Australian environmental samples, including the faeces of native possums with and without clinical evidence of infection. This report seeks to expand on previously published findings by the authors'' investigative group with regards to clinical and subclinical disease in selected wild possum species in BU-endemic areas of Victoria, Australia.

Methodology/Principal Findings

Twenty-seven clinical cases of M. ulcerans infection in free-ranging possums from southeastern Australia were identified retrospectively and prospectively between 1998–2011. Common ringtail possums (Pseudocheirus peregrinus), a common brushtail possum (Trichosurus vulpecula) and a mountain brushtail possum (Trichosurus cunninghami) were included in the clinically affected cohort. Most clinically apparent cases were adults with solitary or multiple ulcerative cutaneous lesions, generally confined to the face, limbs and/or tail. The disease was minor and self-limiting in the case of both Trichosurus spp. possums. In contrast, many of the common ringtail possums had cutaneous disease involving disparate anatomical sites, and in four cases there was evidence of systemic disease at post mortem examination. Where tested using real-time PCR targeted at IS2404, animals typically had significant levels of M. ulcerans DNA throughout the gut and/or faeces. A further 12 possums without cutaneous lesions were found to have PCR-positive gut contents and/or faeces (subclinical cases), and in one of these the organism was cultured from liver tissue. Comparisons were made between clinically and subclinically affected possums, and 61 PCR-negative, non-affected individuals, with regards to disease category and the categorical variables of species (common ringtail possums v others) and sex. Animals with clinical lesions were significantly more likely to be male common ringtail possums.

Conclusions/Significance

There is significant disease burden in common ringtail possums (especially males) in some areas of Victoria endemic for M. ulcerans disease. The natural history of the disease generally remains unknown, however it appears that some mildly affected common brushtail and mountain brushtail possums can spontaneously overcome the infection, whereas some severely affected animals, especially common ringtail possums, may become systemically, and potentially fatally affected. Subclinical gut carriage of M. ulcerans DNA in possums is quite common and in some common brushtail and mountain brushtail possums this is transient. Further work is required to determine whether M. ulcerans infection poses a potential threat to possum populations, and whether these animals are acting as environmental reservoirs in certain geographical areas.     

Absence of Ross River virus amongst Common brushtail possums (<Emphasis Type="Italic">Trichosurus vulpecula</Emphasis>) from metropolitan Sydney,Australia

Ross River virus (RRV) is a mosquito-transmitted Alphavirus emerging in urban centres throughout Australia. The Common brushtail possum (Trichosurus vulpecula), a native marsupial that has successfully adapted to human settlement, has been implicated as a maintenance reservoir for RRV. In the present study, RRV exposure was assessed amongst 72 urban-adapted possums from Northern Sydney and ten possums from a woodland area, remote from urbanisation. Serological screening was performed using an enzyme-linked immunosorbent assay to detect RRV antibodies in possum sera. Findings indicated that both possum populations from urban and woodland habitats were negative for the presence of RRV antibodies. Lack of exposure to RRV highlights that the host status of possums is contingent upon factors other than their abundance and proximity to human settlement. In view of the potential for climate change to favour transmission of mosquito-borne disease in Australia, identification of wildlife populations entirely absent of RRV may prove useful for monitoring the predicted spread of the virus.     

Leptospirosis serology in the common brushtail possum (Trichosurus vulpecula) from urban Sydney, Australia

The common brushtail possum (Trichosurus vulpecula) is indeed a common marsupial in major cities of Australia. This species is known to be susceptible to leptospirosis and often lives in close contact with humans, raising concerns about the potential for transmission of this disease in urban areas. A total of 192 brushtail possum blood samples were collected from 136 individuals in suburban areas of metropolitan Sydney from November 2002 to November 2004. Sera were screened against a reference panel of 21 Leptospira spp. using the microscopic agglutination test. Leptospiral antibodies were detected in 9.6% (13/136) of tested brushtail possums and represented two serovars; antibodies to Leptospira interrogans serovar Hardjo were most frequently identified (11/136). A representative of the exotic sero-group Ballum, most likely serovar Arborea, was found in two of 136 brushtail possums. Exposure to leptospirosis seemed to be associated with age, as older animals had a higher incidence, but there was no distinction in relation to gender. Antibody prevalence varied between the different sampling sites and seropositive animals were clustered and restricted to a few sites. These data support the possible role of brushtail possums as a maintenance host for Leptospira spp. in urban environments and also identified them as a previously unknown and potential source of serovar Arborea.     

The formulation of a national strategy for biological control of possums and bovine Tb

Abstract

The Government has designated the control of possums and bovine Tb a National Science Strategy (NSS) research area. An NSS Committee has been appointed by the Ministry of Research Science and Technology, commissioned to “identify, coordinate and promote national priorities for possum and tuberculosis (Mycobacterium bovis) related research in order that threats both to New Zealand's export markets and to conservation values can be eliminated”.     

Mortality of North Island tomtits (

Aerial poisoning operations with carrot or cereal baits are used to control brushtail possum (Trichosurus vulpecula) populations in New Zealand forests for ecosystem conservation and to stop the spread of bovine tuberculosis to cattle and deer herds on adjacent farmland. Although various measures have been implemented to reduce the incidence of bird kills, dead birds continue to be found after poison operations. Colour- banded North Island tomtits (Petroica macrocephala toitoi) were monitored in treatment and non-treatment areas in Pureora Forest Park to determine the costs and benefits of aerial 1080 possum poisoning operations to tomtit populations. The August 1997 operation (carrot baits with very little chaff, 0.08 % w/w 1080, 10 kg ha(-1)), resulted in 11 (79%) of 14 tomtits disappearing, but none of nine from the non-treatment area. Whether the birds died of primary or secondary poisoning is unknown. No tomtits in either treatment or non-treatment areas disappeared following the August 1998 operation (cereal baits, 0.08% w/w 1080, 5 kg ha(-1)). The carrot bait operation resulted in almost all possums and rodents being killed, but a few possums and rodents survived the cereal bait operation, apparently because of a gap in bait distribution. During the 1997/98 nesting season, tomtit pairs in the 1997 treatment area had high nesting success (80% of nests fledged chicks, mean of four fledglings per nest). Even so, by the following spring it seemed that the population had not recovered to its pre-poison level. Further research on this topic is warranted, the priority being to monitor tomtit mortality during more aerial 1080-cereal bait operations in order to assess the likely risks of using those baits.     

Natural immigration of brushtail possums,Trichosurus vulpecula,onto Aroha Island,Kerikeri Inlet,Bay of Islands,New Zealand

Abstract

Aroha is a 5 ha island in Kerikeri Inlet, Bay of Islands, which is joined to the North Island mainland by a 200 m long causeway. Introduced Australian brushtail possums (Trichosurus vulpecula) spread into the general area only in the mid 1970s, and were first trapped on Aroha Island in June 1979. Since then possums have regularly crossed the causeway, and have been systematically trapped to keep the island free of them. Two hundred and sixty two possums have been trapped on Aroha Island to December 1990, and the annual catch has increased steadily since 1981. About equal numbers of males and females have been trapped. Most possums were caught in March-April and fewest in July-August. Captures on the island were clustered, particularly during the breeding seasons. Average body weight was 2.2 kg; 97% were greys and 3% blacks; the young of females trapped on the island had been born in all months except December to February. In colour, body weight, and breeding pattern the possums trapped on Aroha Island were similar to other possum populations from the northern North Island. Most of the possums invading Aroha Island were 1-2 years old: in contrast to findings from previous studies of possum dispersal, Arona Island was invaded year-round by young male and female possums equally. Some older animals may also have been attracted to the island by fruits available in an orchard there. Maintenance of the possum-free state of Aroha Island will require continuous management.     

Diet and impacts of brushtail possum populations across an invasion front in South Westland, New Zealand

Impact of the irruptive fluctuation in abundance of brushtail possum populations since their initial colonisation was investigated in the forests of South Westland, New Zealand. Possum abundance, fecundity, and diet, the condition of common possum-palatable tree species, and the abundance of common forest birds were measured at three sites occupied by possums for c. 10, 20, and 30 years. Possum densities were highest at the site where possums had been present for c. 20 years. Possum fecundity was higher at the site they had recently colonised than at the two sites where they had been present for c. 20–30 years. Diet of possums where they had recently colonised was dominated by highly preferred foods (72% of total diet), whereas these same foods contributed just 36% of total diet at the site where possums had been present for more than two decades. Canopy condition of common possum-preferred trees was scored progressively lower in areas with increasing length of possum occupation, especially at the site where the possum population had apparently declined from its maximum density. Native forest bird abundance also declined with increasing length of possum occupation. The implications of these results for management are discussed.     

Evaluation of non-toxic bait interference for indexing brushtail possum density

Percentage non-toxic bait interference is currently used by local authorities to monitor brushtail possum control operations but the validity of the method has not been established. Two models have been proposed to estimate an index of possum density (possums per bait) from a log-transformation of percentage bait interference. In two trials, percentage bait interference and the density index derived from percentage bait interference using the Bamford (1970) model usually increased from night to night. Variation in the number of possums per bait was reduced by dividing the Bamford index by the number of nights baits were put out, as proposed by Jane (1981). The spatial distribution of bait interference along lines of bait stations was random for the first night and usually for at least 5 nights. Rhodomine dye and cyanide poison, alternated at bait stations after at least 5 nights non-toxic baiting, showed that about 83% of possums in one trial and 29% of possums in the other trial interfered with more than one bait on the same night. Increasing the spacing between bait stations reduced the proportion of possums interfering with more than one bait. The number of possums per line killed by cyanide baits after at least 5 nights of non-toxic baiting was strongly correlated with percentage bait interference, with and without logarithmic transformation, on the first night. The possums' apparent ability to remember the location of bait stations from one night to the next means that, in the absence of better advice, the original Bamford (1970) model should be modified as proposed by Jane (1981) to estimate possum density and change in possum density (e.g., from pre- to post-poison).