M A Gemmell's research while affiliated with University of Cambridge and other places

Publications (86)

Data
Full-text available
A baseline survey of hepatic hydatid disease in sheep in Tunisia was undertaken. Prevalence and intensity (i.e. the number of cysts/liver) were both found to increase with age. The age-speci® c trends in these two parameters indicate that Echinococcus granulosus is in an endemic equilibrium steady-state in Tunisia and therefore amenable to control....
Article
This study attempted control of transmission of Echinococcus granulosus from dogs to sheep in different areas in the Department of Florida, Uruguay, by treating dogs with praziquantel at intervals of 6, 12 and 16 weeks. The 6-week interval was based on the prepatent period of infection with E. granulosus, the 12- and 16-week intervals were based on...
Article
Full-text available
A baseline survey of hepatic hydatid disease in sheep in Tunisia was undertaken. Prevalence and intensity (i.e. the number of cysts/liver) were both found to increase with age. The age-specific trends in these two parameters indicate that Echinococcus granulosus is in an endemic equilibrium steady-state in Tunisia and therefore amenable to control....
Article
Full-text available
A baseline survey of hepatic hydatid disease in sheep in Tunisia was undertaken. Prevalence and intensity (i.e. the number of cysts/liver) were both found to increase with age. The age-specific trends in these two parameters indicate that Echinococcus granulosus is in an endemic equilibrium steady-state in Tunisia and therefore amenable to control....
Article
A control programme directed against Echinococcus granulosus was introduced into the Republic of Cyprus in 1971. In 1974, this was restricted to the areas controlled by the government of the island. The force used in the 'attack' phase included a major dog control programme. This and other measures led to the rapid 'disappearance' of the parasite a...
Article
Full-text available
Sonographic evidence of asymptomatic Echinococcus granulosus lesions in the liver was found in 156 of 9,515 persons in the Department of Florida, Uruguay. The sensitivity of ELISA and latex agglutination serology compared with ultrasound was 47.6% and 28.1%, respectively, and specificity was > 85%. There was a significant positive association betwe...
Article
The theory of the generalized plasticity model is reviewed. A special form for multiaxial behavior, based on the Drucker-Prager flow potential, is proposed. The model is applied to a simplified representation of the behavior of shape-memory alloys, with numerical examples.
Article
A survey was undertaken to determine both the prevalence of, and reinfection rates with Echinococcus granulosus and other cestodes in the Department of Florida, Uruguay. Baseline prevalence was determined in 303 rural dogs which then, in 4 groups, were re-examined 2, 4, 8 or 12 months later. Baseline prevalences for E. granulosus, Taenia hydatigena...
Article
A base-line survey was carried out on the transmission dynamics of Echinococcus granulosus, Taenia hydatigena and Taenia ovis in sheep in the Department of Florida, Uruguay. Mean life expectancy at birth of the sheep population in this Department was estimated at 3.5 and 4.8 years for male and female sheep, respectively. Both intensity and prevalen...
Article
Autopsy of Soay Sheep on St. Kilda revealed the presence of cysticerci of Taenia hydatigena despite the absence of the definitive host on the island. Both the intensity of infection and the prevalence increased with age implying that the sheep did not acquire immunity to reinfection or superinfection. The sheep on average ingested approximately 2.4...
Article
Tablets containing praziquantel, pyrantel embonate, and febantel were tested for efficacy against helminths in dogs. A single treatment with this drug combination gave 100% reductions in Toxocara canis and Taenia hydatigena in experimentally induced infections in dogs. In dogs with naturally acquired infections, treatment gave > 97 to 98% reduction...
Article
A comparison has been made of the interactions between passively transferred and actively acquired immunity in regulating populations of Taenia hydatigena and T. ovis. When ewes were grazed prior to parturition under a high infection pressure, immunity was transferred to their offspring for up to 8 weeks. A qualitative difference between the specie...
Article
For several years after Echinococcus granulosus was introduced into Australasia from Europe, its life cycle was unknown. Before the end of the 19th century, however, Cystic Hydatid Disease (CHD) was recognized in humans as an important cause of death, although its magnitude was not quantified. During the first 50 years of the 20th century, the urba...
Article
Experiments showed that dead blowflies containing eggs of Taenia hydatigena can transmit infection if ingested by lambs during grazing. Rabbits became infected with T. pisiformis after grazing grass exposed to blowflies that had been in contact with dog faeces containing the eggs of this parasite. When captive blowflies were exposed first to dog fa...
Article
Field trials and control programmes demonstrate that ovine echinococcosis (Echinococcus granulosus) and the cysticercoses (Taenia hydatigena and T. ovis) have different stabilities and may not respond in the same way to control. The first-named, which is usually in the endemic state, is readily transformed by a dog-dosing programme to extinction st...
Article
A brief review is given of the transmission dynamics of the ovine cysticercoses Taenia hydatigena and T. ovis. The importance of the basic reproductive rate (Ro) and the parasite, host and socio-ecological factors determining the epidemiological steady states of Taeniidae are described. The transmission dynamics of these ovine cysticercoses have be...
Article
An official control programme against Echinococcus granulosus and Taenia hydatigena has been in operation in New Zealand for more than 28 years and against Taenia ovis for more than 18 years. This unique effort to control three metazoan parasites at the same time has led to a change from endemic to extinction status for E. granulosus but only a cha...
Article
Control programmes against Echinococcus granulosus in its dog-sheep transmission cycle (Fig. 1) have been successful in many parts of the world'. In contrast, the related E. multilocularis presents a much more complex problem for control authorities. Unlike E. granulosus, the life cycle of E. multilocularis predominantly involves sylvatic hosts (e....
Article
An evaluation has been made of the biological and epidemiological parameters that determine the basic reproductive rates of Taenia hydatigena and T. ovis. These host-parasite systems are characterized by (i) no overcrowding in either host; (ii) no parasite-induced mortality of either host; (iii) no density-dependent constraint in the definitive hos...
Article
It is shown that under the conditions that prevailed in New Zealand in the late 1950s, Taenia hydatigena was hyperendemic, the life-cycle being regulated by a density-dependent constraint in the form of acquired immunity, and T. ovis was rare. The control measures that caused Echinococcus granulosus, which was endemic at the time, to decline toward...
Article
An evaluation of surveys, field trials and control programmes suggest that E. granulosus in its steady state dog/sheep life cycle may be endemic and structurally unstable with R0~l due to its low biotic potential. In this state it is amenable to control or even eradication with currently available resources. In contrast, the ovine cysticercoses cau...
Article
A comparative study has been made of the progress in the control of ovine echinococcosis caused by Echinococcus granulosus and the ovine cysticercoses caused by Taenia hydatigena and T. ovis in New Zealand. The methods of control included an educational and a dog-dosing programme. In the Styx field trial, the Otago/Southland Surveillance Programme...
Article
A mathematical model of the life-cycle of Echinococcus granulosus in dogs and sheep in New Zealand is constructed and used to discuss previously published experimental and survey data. The model is then used to describe the dynamics of transmission of the parasite, and the means by which it may be destabilized. It is found that under the conditions...
Article
The numerical distributions of Echinococcus granulosus in an experimental dog population are described. At all dose rates of protoscoleces from 10 to 175000 the distribution of worms was over-dispersed. Host age had no effect. There was a direct proportionality between the infective-stage density and rate of infection, and between the latter and th...
Article
Progress in national and provincial control programmes directed against Echinococcus granulosus is reviewed. The results show that this parasite, because of the low basic reproductive rate in its domestic life-cycle, is unstable and is amenable to control or eradication by currently available methods. The difference between control and eradication...
Article
Taenia hydatigena and Echinococcus granulosus in dogs have been used as a model to evaluate the response of these parasites to increasing doses of mebendazole in different dose forms (powder, micronized powder and tablets of micronized powder). Overall the micronized powder proved to be the most active, the tablets less so and the normal powder lea...
Article
In laboratory studies with 3 species of Calliphora more than half were shown to ingest eggs of Taenia hydatigena from the surface of dog faeces with a maximum of over 5000 eggs in 1 fly. Most eggs were voided within 48 h and between 38 and 48% of these eggs had lost their embryophores. In field studies carried out in the South Island of New Zealand...
Article
Lambs were reared from birth to 3 months of age on pasture contaminated with the eggs of either Taenia hydatigena or Taenia ovis. They were necropsied at 3, 6, 9 or 12 months. Almost all larvae that were viable at 3 months continued to survive throughout the experiment. Larvae of T. hydatigena were infective to dogs at 3 months, whereas those of T....
Article
Praziquantel was evaluated for its larvicidal activity against naturally or experimentally induced Taenia hydatigena infections in sheep. The major criterion used for assessing efficacy was an estimate of the dose rate required to free 90 per cent of the sheep from infection (ED90) together with a 95 per cent confidence interval. With both light an...
Article
This chapter describes the environmental factors that modify the survival and infectivity of eggs and the mechanisms and agents that are responsible for their dispersal. The infective pattern of a parasite within a host population is described by both the mean level of infection and its distribution within the population. The factors that influence...
Article
Lambs reared from birth in a 'tapeworm-free' unit were immunised at three months old by a parenteral injection of the embryos of Taenia hydatigena. Subsequently, they were grazed with unimmunised control animals on an egg-free pasture. At one, three, six, nine, 12 or 15 months after immunisation, groups of the treated and control animals were graze...
Article
A review has been made of the advances in knowledge on the treatment of tapeworm infections of man and animals from the time of the introduction to dichlorophen in 1956. This opened up the era of out-patient treatment for human tapeworm infections. The drugs studied have been compared, where possible, on the basis of estimates of the single dose ED...
Article
Dogs infected with Echinococcus granulosus were treated with praziquantel by three routes and at different dosage rates. The results indicated that the dog was of similar efficacy when administered by the oral and intramuscular routes, but less effective by the subcutaneous route.
Article
Tablets of micronised nitroscanate (nominal particle size 2--3 microns) were given to a total of 190 dogs that had been experimentally infected with either Echinococcus granulosus or Taenia hydatigena. The efficiency of the drug in tablet form in freeing dogs from tapeworms, was investigated. The dose rate at which 50 per cent of normally fed dogs...
Article
Trials with oxfendazole carried out on 120 dogs infected with either Echinococcus granulosus or Taenia hydatigena indicated that a single treatment significantly reduced the proportion of dogs infected with tapeworms. Diarrhoea occurred occasionally when the dose rate exceeded 20 mg/kg.
Article
A limited, but controlled trial with mebendazole incorporated in a prepared food and with tablets given with this food showed that with the former there was an overall reduction in the expected dog population harbouring Echinococcus granulosus and Taenia hydatigena. Neither treatment was fully effective.
Article
Diuredosan has been tested against Echinococcus granulosus and Taenia hydatigena infections in dogs. The drug showed significant activity against T hydatigena, but there was no significant dose response curve against E granulosus in this trial. Two treatments at 50 mg/kg or 100 mg/kg, however, reduced the number of dogs infected with hydatid worms....
Article
The effect of an antibiotic of the streptothricin class on Echinococcus granulosus and Taenia hydatigena infections has been investigated in a trial involving 160 dogs. The drug showed significant activity in freeing dogs from Thydatigena, but the evidence for similar activity against E granulosus was less well marked. Vomiting and diarrhoea were c...
Article
Two grazing trials using 'sentinel' lambs were undertaken to measure changes in the availability of eggs within 30 metres of deposition by dogs infected with Taenia hydatigena. In the first trial the experimental pasture was divided into nine equal plots; in the second trial, each of the nine plots was divided into sub-plots. In each trial, infecte...
Article
A 13-year assessment has been made of the effectiveness of a monthly drug treatment programme for the control of tapeworms in dogs in order to prevent hydatidosis (Echinococcus granulosus) and cysticercosis (Taenia hydatigena and T. ovis) in sheep. The age-specific prevalence of T. hydatigena in lambs was used as the principal indicator. The trial...
Article
The effect of praziquantel on Echinococcus granulosus, Taenia hydatigena and T ovis was investigated in a trial involving 360 dogs. The drug was found to be highly toxic to tapeworms. Few dogs remained infected after a single treatment within the range of dose rates tested (0-31 mg/kg to 10-0 mg/kg body weight). No toxicity to dogs was observed.
Article
The lethal effect of some benzimidazoles on Taenia hydatigena infections in dogs have been compared. Fenbendazole, parbendazole and oxibendazole were more effective than cambendazole and thiabendazole, while coarsely ground mebendazole did not differ significantly from any of them.
Article
The effect of niclosamide (particle size 1 micron m) and its piperazine salt (particle size 5-10 micron m) on Echinococcus granulosus and Taenia hydatigena has been investigated. The piperazine salt of niclosamide was also tested for activity against Taenia ovis. These trials involved 326 dogs. Irrespective of particle size, the drug was found to b...
Article
The hatching characteristics of the eggs of Taenia hydatigena, T. ovis, T. multiceps, and Echinococcus granulosus were examined in vitro. These characteristics varied with the eggs from different segments and different tapeworms, suggesting that the eggs from each segment, although morphologically similar, consisted of populations at different phas...
Article
This chapter discusses the experimental epidemiology of hydatidosis and cysticercosis. Important cestodes responsible for cysticercosis in food animals include Taenia saginata, T. solium, T. hydatigena, and T. ovis. Man is an obligatory definitive host for T. saginata and T. solium as well as an important anomalous intermediate host for T. solium....
Article
A dog with four patent Taenia hydatigena was placed at a fixed site on a pasture for 50 days. The spread of the eggs was determined by grazing lambs for 10 days on various plots, and counting the cysts present at autopsy. Some 'wild' eggs were detected on the pasture before the dog was introduced. Nevertheless, a marked divergence from randomness w...
Article
Some intrinsic factors that may modify the transmission of cysticercosis caused by Taenia hydatigena in lambs have been assessed and the age at which the infective pattern should no longer be modified by the grazing-learning process and by colostrum-derived antibody, estimated. Lambs of 0-1, 2-3, 5-6, 8-9, 11-12 weeks of age, were placed for one we...
Article
The build-up of the eggs of Taenia hydatigena was measured on a pasture following the introduction of infected dogs. Susceptible lambs grazed on the pasture beforehand showed that only small numbers of randomly dispersed infective eggs were present. Two dogs each with four patent T hydatigena were chained to kennels placed in the middle of the past...
Article
A pasture was prepared so that an equilibrium of fresh and ageing eggs of Taenia hydatigena was present. This was done by placing dogs with patent infections on it for six months. Five months after their introduction, groups of lambs were grazed there for two, four, eight, 16 or 32 days. The dogs were then removed. Three and six months later, furth...
Article
The metabolism of unlabelled human monocomponent insulin was studied in a group of six patients being treated with combined estrogen progestogen oral contraceptives (OC) and compared with a group of ten normal subjects. The parameters of insulin metabolism were determined by a priming dose continuous infusion technique which enabled measurements of...
Article
Trials with nitroscanate carried out on 186 dogs infected with either Echinococcus granulosus or Taenia hydatigena indicated that a single treatment at a dose rate of 1000 mg/kg or two treatments at 250 mg/kg eliminated the former, but a single treatment at 64 mg/kg was sufficient to eliminate the latter. At this dosing schedule, vomiting and diarr...
Article
Trials with fospirate carried out with 170 dogs infected with either Echinococcus granulosus or Taenia hydatigena indicated that a dose rate of 40 mg/kg given on three occasions and 10 mg/kg given on one occasion eliminated each species, respectively. Increasing the dose rat substantially above 40 mg/kg did not reduce the number of treatments requi...
Article
ExtractTheoretically, the life-cycle of the sheep metacestodes with their strobilate phase in dogs permits combinations of at least three methods of control. These are the prevention of dogs gaining access to larval cestodes, the chemical elimination of the strobilate and or the larval stage in the appropiate hosts, and the immunization of either t...
Article
Activated oncospheres of T. hydatigena within filtration membrane diffusion chambers implanted intraperitoneally into dogs developed into larvae 3 mm in dia possessing a scolex anlage without hooks. Exogenous antigens released by developing T. hydatigena larvae failed to stimulate any measurable resistance in the dogs to challenge infection with E....
Article
Trials with bithionol sulphoxide involving 130 dogs, infected with either Echinococcus granulosus or Taenia hydatigena, indicated that a single treatment at a dose rate of 400 mg/kg or two treatments at 50 mg/kg eliminated the former, but a single treatment at 50 mg/kg was sufficient to eliminate the latter. Dose rates 200 mg/kg or more were lethal...
Article
Eggs or artificially activated embryos of T. ovis were administered into the rumen or via the jugular vein to nonimmune lambs and to ewes at various times during pregnancy. Autopsy of the nonimmune lambs demonstrated that Cysticercus ovis had been established from organisms administered by either route. In contrast, autopsy of the aged ewes and the...
Article
Single large doses of the eggs of Taenia hydatigena and T. ovis were fed to lambs either simultaneously or separated by an interval of 3 months. The proportion of the cysticerci of T. hydatigena which failed to survive was greater than expected when the eggs of T. ovis were fed prior to or simultaneously with those of T. hydatigena. In contrast, th...
Article
The distribution of 12,310 cysts of Taenia ovis in 212 experimentally infected lambs and of 46 cysts in 24 naturally infected 5-year-old sheep, has been analysed. It was determined that irrespective of the number of cysticerci in an infected group of animals, about two-thirds were present in the carcase meat, and the remainder were distributed betw...
Article
Rabbits immunized with 6 or 12 injections of the embryos of Taenia hydatigena or T. ovis were subsequently challenged with the eggs of T. pisiformis. A significant reduction in the proportion of cysts of T. pisiformis surviving was detectable only following 12 injections of either sheep metacestode.Lambs immunized with 6 or 12 injections of T. pisi...
Article
Lambs born of dams naturally immunized against at least one species of larval cestode and of similar naturally immunized dams, which had been artificially hypersensitized by the injection of activated embryos of T. hydatigena before parturition, were allowed to suckle for 3 days. At specific intervals after birth, the lambs were fed per os with egg...
Article
Following activation, the embryos of Taenia hydatigena and T. ovis were formalized or frozen before being injected parenterally into sheep. These treatments were performed in an attempt to prevent embryos developing into cysts at the site of injection as occurs when normally activated embryos are injected. The protection induced by these treated em...
Article
Resistance to Taenia hydatigena can be artificially induced by an injection of the homologous activated embryos, and also, but to a lesser degree, by those of T. ovis. Two experiments were undertaken to determine whether the immunity so induced to the establishment of T. hydatigena could be due to a passively transferable cellular immunity.Cells fr...
Article
Immunization of sheep by the intramuscular injection of artificially activated embryos of Taenia hydatigena or of T. ovis stimulated the production of passively transferable humoral factors which provided some protection against a challenge infection with the eggs of the homologous and heterologous species.
Article
A study, involving 52 lambs, has been made to define the efficiency of immunization with the activated embryos in protecting sheep against T. hydatigena when the challenge infection was administered before, simultaneously with, or after the immunization procedure. Absolute resistance developed only in those animals in which the immunization procedu...
Article
From time to time, reports of anthelmintic trials are published indicating that a compound is, or is not, effective against Echinococcus granulosus in dogs. In some of these reports, an attempt is made to assess the efficacy of the compound from inadequate data. This paper reviews the problems associated with selecting, screening and defining the t...
Article
An assessment is made of the present state of knowledge on acquired immune responses developed by the intermediate and definitive hosts to tapeworm infections. From this evaluation, some gaps in knowledge and some of the problems associated with the development of practical immunization techniques are described. The principal conclusion reached is...
Article
An assessment was made of the effectiveness of the generally accepted methods recommended for controlling hydatid disease during the course of a field-trial, initiated in 1943 in an isolated region of New Zealand. The results obtained during the first 21 years are described.Basically, the trial was an attempt to compare the effectiveness of a gener...
Article
Rabbits vaccinated by intramuscular injection with the viable eggs or the activated embryos of Taenia pisiformis, T. ovis or T. hydatigena, were subse-quently challenged per os with eggs of T. pisiformis. A single intramuscular injection of viable eggs or activated embryos of T. pisiformis induced a relatively solid immune response to the establish...
Article
Sixty lambs were divided into three groups each comprising twenty animals. Half the lambs in each group were vaccinated with the viable eggs and half with the activated embryos of Taenia ovis, T. hydatigena or T. pisiformis. These lambs together with ten control animals were subsequently challenged with eggs of T. ovis.Only one of the lambs in the...
Article
Ninety sheep, comprising three groups each of thirty animals, were vaccinated intramuscularly with the eggs or activated embryos of Taenia hydatigena, T. ovis or T. pisiformis. These, together with ten controls, were subsequently challenged with eggs of T. hydatigena. Each of these three groups of thirty animals were subdivided into three sub-group...
Article
EVIDENCE that immunity to tæniid infestations in the intermediate host may be encountered first when embryos attempt to penetrate the small intestinal wall soon after hatching was recently reviewed by Froyd and Round1. They were able by oral dosing with eggs of Taenia saginata to infect young calves, but not older animals, when the majority of the...

Citations

... The present study noted that adult goats were significantly more infected than young, which is in agreement with the previous studies, and they found that the infection is strongly influenced by the age of the host (Bayu et al., 2013;Lahmar et al., 1999). Older animals are more likely to acquire infection due to their prolonged period of exposures during feeding, which is influenced by the reduction of immunity to battle infection (Shuvo et al., 2021). ...
... Using the latter technique, the prevalence for taeniids was almost three times higher. The use of a highly sensitive method is important especially for Echinococcus species which tend to produce lower egg loads given their lower biotic potential (Gemmell et al., 2001). Moreover, identification of Taenia species and Echinococcus strains from the isolated eggs allows to trace the sources of infection as well as to clarify the life cycle of the parasites, as it was done in this study. ...
... In the past decades, numerous studies have been devoted to effectively modeling the pseudo-elastic effects of shape memory alloys (SMAs) and significant results have been obtained based on either a micromechanismsbased or a macroscopic standpoint. Early results may be found in, for example, [3][4][5][6], and subsequent results in, for example, [7][8][9][10][11][12][13][14][15][16][17], and many others. For recent results, reference may be made to [18][19][20][21], and others. ...
... For example, the mean intensity of infection in the present studyÐ 2534 E. granulosus/infected dog (Table 1)Ð was more than 12-fold higher than the 202 worms/infected host reported by Gemmell (1990) in Australasia. This may re¯ ect differences in the mean age of sheep in local ¯ ocks, Tunisian sheep being relatively old and therefore more likely to have large cysts (and therefore large numbers of protoscolices ) than their Australasian counterparts (Lahmar et al., 1999). Although this may cause a relatively high infection pressure on dogs, a particularly large infection of protoscolices may be more likely to stimulate the immune response in a dog, preventing reinfection . ...
... The initial immune response occurs within the intestine upon ingestion of cestode eggs and involves both innate and acquired defences. A second distinct antibody-mediated immune response is induced by the larva once it reaches the predilection site (Gemmell 1962; Blundell et al. 1968). Several studies have also examined transfer of immunity from ewes to their offspring via colostrum. ...
... Each terminal proglottid of mature T. multiceps can contain more than 30,000 eggs [3][4][5]. Proglottids containing eggs detach from the end of the worm and pass out in the feces onto pasture where they can be ingested by goats and sheep [9][10][11]. ...
... Disturbance of this stability by reduction in the prevalence of infected definitive hosts could reduce the incidence if infection in pigs, leaving many highly susceptible to any future exposure to the parasite. This situation is believed to have led to the occurrence of " cysticercosis storms " in New Zealand where disturbance of endemic stability of T. ovis in sheep led to massive infections in flocks that were exposed to T. ovis eggs (Gemmell, 1978 ) and is a well understood phenomenon in relation to other infectious diseases (Coleman et al., 2001). Most of the interventions that have been tested for T. solium to date have relied on the treatment of the definitive hosts (humans) to remove tapeworm infections (Lightowlers, 2010) and this may have increased the overall susceptibility of the pig population to cysticercosis. ...
... Oxfendazole is a benzimidazole drug which has been used in veterinary medicine to control nematode infections and was found to be effective against the intestinal stage of E. granulosus, as well as other cestodes in the gastrointestinal tract, thus could be used to treat infections in dogs, the principal reservoir for human infection (Gemmell et al., 1979). Gonzales et al. (1996) examined the effect of the drug on the tissue stage of tapeworm infections and found that a single dose of 30 mg/kg of body weight of oxfendazole in pigs completely eliminated all tissue cysts of Taenia solium, a medically important human tapeworm. ...
... Lawson and Gemmell [19,20,[54][55][56] performed several experiments to determine the possible infection route via invertebrates and dispersal in the field. Lambs that were allowed to graze downwind of dog kennels or in close proximity to a plot where infected dogs had been previously kept contained a much higher level of cysticerci, detected during autopsy, than those grazing elsewhere. ...
... G Model VETPAR-7715; No. of Pages 17 purgation and coproantigen/coproPCR surveys). The use of sentinel animals for estimation of levels of pasture contamination (Gemmell and Johnstone, 1977; Eckert et al., 1982; Lloyd et al., 1991 ) is generally considered a form of active surveillance. ...