[show abstract][hide abstract] ABSTRACT: Foot-and-mouth disease (FMD) is highly contagious and one of the most economically devastating diseases of cloven-hoofed animals. Scientific-based preparedness about how to best control the disease in a previously FMD-free country is therefore essential for veterinary services. The present study used a spatial, stochastic epidemic simulation model to compare the effectiveness of emergency vaccination with conventional (non-vaccination) control measures in Switzerland, a low-livestock density country. Model results revealed that emergency vaccination with a radius of 3km or 10km around infected premises (IP) did not significantly reduce either the cumulative herd incidence or epidemic duration if started in a small epidemic situation where the number of IPs is still low. However, in a situation where the epidemic has become extensive, both the cumulative herd incidence and epidemic duration are reduced significantly if vaccination were implemented with a radius of 10km around IPs. The effect of different levels of conventional strategy measures was also explored for the non-vaccination strategy. It was found that a lower compliance level of farmers for movement restrictions and delayed culling of IPs significantly increased both the cumulative IP incidence and epidemic duration. Contingency management should therefore focus mainly on improving conventional strategies, by increasing disease awareness and communication with stakeholders and preparedness of culling teams in countries with a livestock structure similar to Switzerland; however, emergency vaccination should be considered if there are reasons to believe that the epidemic may become extensive, such as when disease detection has been delayed and many IPs are discovered at the beginning of the epidemic.
Preventive Veterinary Medicine 10/2013; · 2.39 Impact Factor
[show abstract][hide abstract] ABSTRACT: Disease transmission between wild ungulates and domestic livestock is an important and challenging animal health issue. The potential for disease transmission between wildlife and livestock is notoriously difficult to estimate. The first step for estimating the potential for between-species disease transmission is to quantify proximity between individuals of different species in space and time. This study estimates second-order statistics of spatio-temporal location data from radio-collared free-ranging deer, elk and cattle in northeast Oregon. Our results indicate, that when observed simultaneously, elk and cattle occur in closer proximity to each other than what would be expected based on general space use of these species. The same is true for deer and elk but not for deer and cattle. Our analysis also demonstrates that average distances between cattle and elk are largely driven by rare events of close co-mingling between the species, which extend over several hours. Behavioral causes for these co-mingling events are currently unknown. Understanding the causes for such events will be important for designing grazing practices that minimize wildlife-livestock contacts.
Preventive Veterinary Medicine 01/2013; · 2.39 Impact Factor
[show abstract][hide abstract] ABSTRACT: Classical swine fever (CSF) outbreaks can cause enormous losses in naïve pig populations. How to best minimize the economic damage and number of culled animals caused by CSF is therefore an important research area. The baseline CSF control strategy in the European Union and Switzerland consists of culling all animals in infected herds, movement restrictions for animals, material and people within a given distance to the infected herd and epidemiological tracing of transmission contacts. Additional disease control measures such as pre-emptive culling or vaccination have been recommended based on the results from several simulation models; however, these models were parameterized for areas with high animal densities. The objective of this study was to explore whether pre-emptive culling and emergency vaccination should also be recommended in low- to moderate-density areas such as Switzerland. Additionally, we studied the influence of initial outbreak conditions on outbreak severity to improve the efficiency of disease prevention and surveillance. A spatial, stochastic, individual-animal-based simulation model using all registered Swiss pig premises in 2009 (n=9770) was implemented to quantify these relationships. The model simulates within-herd and between-herd transmission (direct and indirect contacts and local area spread). By varying the four parameters (a) control measures, (b) index herd type (breeding, fattening, weaning or mixed herd), (c) detection delay for secondary cases during an outbreak and (d) contact tracing probability, 112 distinct scenarios were simulated. To assess the impact of scenarios on outbreak severity, daily transmission rates were compared between scenarios. Compared with the baseline strategy (stamping out and movement restrictions) vaccination and pre-emptive culling neither reduced outbreak size nor duration. Outbreaks starting in a herd with weaning piglets or fattening pigs caused higher losses regarding to the number of culled premises and were longer lasting than those starting in the two other index herd types. Similarly, larger transmission rates were estimated for these index herd type outbreaks. A longer detection delay resulted in more culled premises and longer duration and better transmission tracing increased the number of short outbreaks. Based on the simulation results, baseline control strategies seem sufficient to control CSF in low-medium animal-dense areas. Early detection of outbreaks is crucial and risk-based surveillance should be focused on weaning piglet and fattening pig premises.
Preventive Veterinary Medicine 08/2012; · 2.39 Impact Factor
[show abstract][hide abstract] ABSTRACT: When avian influenza viruses (AIVs) are transmitted from their reservoir hosts (wild waterfowl and shorebirds) to domestic bird species, they undergo genetic changes that have been linked to higher virulence and broader host range. Common genetic AIV modifications in viral proteins of poultry isolates are deletions in the stalk region of the neuraminidase (NA) and additions of glycosylation sites on the hemagglutinin (HA). Even though these NA deletion mutations occur in several AIV subtypes, they have not been analyzed comprehensively. In this study, 4,920 NA nucleotide sequences, 5,596 HA nucleotide and 4,702 HA amino acid sequences were analyzed to elucidate the widespread emergence of NA stalk deletions in gallinaceous hosts, the genetic polymorphism of the deletion patterns and association between the stalk deletions in NA and amino acid variants in HA. Forty-seven different NA stalk deletion patterns were identified in six NA subtypes, N1-N3 and N5-N7. An analysis that controlled for phylogenetic dependence due to shared ancestry showed that NA stalk deletions are statistically correlated with gallinaceous hosts and certain amino acid features on the HA protein. Those HA features included five glycosylation sites, one insertion and one deletion. The correlations between NA stalk deletions and HA features are HA-NA-subtype-specific. Our results demonstrate that stalk deletions in the NA proteins of AIV are relatively common. Understanding the NA stalk deletion and related HA features may be important for vaccine and drug development and could be useful in establishing effective early detection and warning systems for the poultry industry.
PLoS ONE 01/2011; 6(2):e14722. · 3.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Populations that are structured into small local patches are a common feature of ecological and epidemiological systems. Models describing this structure are often referred to as metapopulation models in ecology or household models in epidemiology. Small local populations are subject to demographic stochasticity. Theoretical studies of household disease models without resistant stages (SIS models) have shown that local stochasticity can be ignored for between patch disease transmission if the number of connected patches is large. In that case the distribution of the number of infected individuals per household reaches a stationary distribution described by a birth-death process with a constant immigration term. Here we show how this result, in conjunction with the balancing condition for birth-death processes, provides a framework to estimate demographic parameters from a frequency distribution of local population sizes. The parameter estimation framework is applicable to estimate parameters of disease transmission models as well as metapopulation models.
Theoretical Population Biology 09/2010; 78(2):71-6. · 1.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: A duck-origin avian influenza virus (AIV) was used to study viral adaptation and transmission patterns in chickens (Gallus gallus domesticus) and Pekin ducks (Anas platyrhynchos domesticus). Inoculated birds were housed with naïve birds of the same species and all birds were monitored for infection. The inoculating duck virus was transmitted effectively by contact in both species. Viruses recovered from infected birds showed mutations as early as 1 or 3 days after inoculation in chickens and ducks, respectively. Amino acid substitutions in hemagglutinin (HA) or deletions in neuraminidase (NA) stalk regions were identified in chicken isolates, but only substitutions in HA were identified in duck isolates. HA substitution-containing viruses replicated more efficiently than those with NA stalk deletions. NA deletion mutants were not recovered from contact chickens, suggesting inefficient transmission. Amino acid substitutions in HA proteins appeared in pairs in chickens, but were independent in ducks, indicating adaptation in chickens. In addition, our findings showed that a duck-origin virus can rapidly adapt to chickens, suggesting that the emergence of new epidemic AIV can be rapid.
[show abstract][hide abstract] ABSTRACT: Public gene sequence databases have become important research tools to understand viruses and other organisms. Evidence suggests that the identifying information for some of the sequences in these databases might not belong to the sequences they are associated with. We developed two tests to conduct a comprehensive analysis of all published sequences of the hemaglutinin and neuramidase genes of avian influenza viruses (AIVs) to identify sequences that may have been misclassified. One test identified sequence pairs with highly similar nucleotide sequences despite a difference of several years between their sampling dates. Another test, which was applied to samples sequenced and deposited more than once, detected sequences with more nucleotide differences to their own than to their closest relatives. All sequences identified as misclassified were further traced to relevant publications to assess the likelihood of contamination and determine if any conclusions were associated with the use of these sequences. Our results suggested that among 4040 published gene sequences examined, approximately 0.8% might be misclassified and that publications using these sequences may include inaccurate statements. Findings from this report suggest that using laboratory-adapted strains and handling multiple samples simultaneously increases the risk of contamination. The tests reported here may be useful for screening new submissions to public sequence databases.
[show abstract][hide abstract] ABSTRACT: The spread of highly pathogenic avian influenza virus (AIV) (H5N1) underlines the potential for global AIV movement through birds. The phylogenies of AIV genes from avian hosts usually separate into Eurasian and North American clades, reflecting limited bird migration between the hemispheres. However, mounting evidence that some H6 sequences from North America cluster with Eurasian subtype H6 sequences calls the strict hemispheric divide into question. We conducted a comprehensive phylogenetic analysis of the extent and timing of cross-hemisphere movements by the H6 gene. Results suggested that Eurasian H6 subtype has invaded North America several times, with the first invasions occurring 10 years before the first detection of invading isolates. The members of the North American clade decreased from 100% in the 1980s to 20% in the 2000s among H6 isolates from North America. Unraveling the reasons for this large-scale gene movement between hemispheres might identify drivers of global AIV circulation.
[show abstract][hide abstract] ABSTRACT: Prevention of Chagas disease depends mainly on control of the insect vectors that transmit infection. Unfortunately, the vectors have been resurgent in some areas. It is important to understand the dynamics of reinfestation where it occurs. Here we show how continuous- and discrete-time models fitted to patch-level infestation states can elucidate different aspects of re-establishment. Triatoma infestans, the main vector of Chagas disease, reinfested sites in three villages in northwest Argentina after community-wide insecticide spraying in October 1992.
Different methods of estimating the probabilities of bug establishment on each site were compared. The results confirmed previous results showing a 6-month time lag between detection of a new infestation and dispersal events. The analysis showed that more new bug populations become established from May to November than from November to May. This seasonal increase in bug establishment coincides with a seasonal increase in dispersal distance. In the fitted models, the probability of new bug establishment increases with increasing time since last detected infestation.
These effects of season and previous infestation on bug establishment challenge our current understanding of T. infestans ecology and highlight important gaps in knowledge. Experiments necessary to close these gaps are discussed.
[show abstract][hide abstract] ABSTRACT: 1. Prevention of Chagas disease is mainly dependent on control of the insect vectors that transmit infection. Unfortunately, this control is not wholly successful and the vectors have been resurgent in some areas. Where re-infestation has occurred, it is important to understand the dynamics of the process. We investigated how a metapopulation framework can elucidate key aspects of re-infestation and thereby contribute to more efficient disease control.2.Triatoma infestans, the main vector of Chagas disease, re-infested sites in three villages in north-west Argentina after community-wide insecticide spraying in October 1992. Ten surveys were carried out at 6-monthly intervals from November 1994 to May 1999.3. Comparisons were made of different methods of estimating the sources of dispersal and the number of sites in which bug infestations became established.4. The results indicated that (i) the number of dispersing Triatoma infestans from a given site was proportional to the number of bugs found at the site; (ii) there was a 6-month time lag between detection of a new infestation and dispersal events; (iii) the relationship between infestations and new establishments varied by season.5. Three of 156 sites at which bugs were found were estimated to be the source of more than 50% of establishment events. These three sites were the only ones with large, persistent bug populations.6.Synthesis and applications. To reduce the risk of human Chagas disease, identifying those few sites infested with large, persistent bug populations and targeting control measures at those sites should greatly improve the efficiency of vector control. The appropriate seasonal timing of vector control could also greatly increase its efficiency. Specific recommendations for the timing of insecticide spraying require further research to establish how the observed temporal pattern of bug establishment is associated with the seasonality of bug dispersal.
Journal of Applied Ecology 03/2007; 44(1):220-227. · 4.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: Classical theory in community ecology assumes that smaller-scale details such as individual traits can be abstracted safely and that community dynamics can be simply characterized in terms of net changes in population densities. Here we use a mechanistic simulation model of a three-level food web to explore the effect of initial body size variation among herbivores on final plant abundance resulting from a non-linear relationship between population demography and body size. We show that initial herbivore body size variation has a negative effect on their survival and consequently a positive effect on the final plant biomass. We then use trait distribution, in combination with body size-survival and body size-fitness curves estimated through simulations, to generate predictions for comparison with observed food web effects. We show that, owing to frequency-dependence, our ability to predict herbivore population dynamics is limited. However, at the community-level, this frequency-dependence, as well as changes in herbivore population size, can be abstracted safely and the strength of plant–herbivore interactions can be simply predicted from initial body size distribution in combination with the survival curve. Our findings suggest a need to revisit classical theory in community ecology. Doing so will require the mechanistic study of population demography and experimental testing of the effect of trait variation on community dynamics.
[show abstract][hide abstract] ABSTRACT: 1. The dynamics of parasitic organisms depend critically upon the frequency distribution of parasite individuals per host. However, the processes giving rise to this frequency distribution have rarely been modelled and tested for organisms with complex host selection behaviour. 2. In this study Microrhopala vittata, a chrysomelid beetle, was used to investigate how oviposition behaviour, movement and density of host plants interact in shaping the frequency distribution of egg clusters per host in the field. 3. Enclosures were stocked with two different host species and different beetle densities and various stochastic process models were fitted to egg cluster count data obtained from these enclosures. The different models were derived considering different scenarios, in particular whether or not plant density limits oviposition rate, whether or not ovipositing females actively seek out the most attractive plant within their perception radius and whether a female's oviposition rate is determined by plant intrinsic factors, the plant's egg cluster load or the surrounding beetle density. 4. The model parameters fitted to cage data were used to describe the frequency distribution of egg cluster counts obtained in a release experiment in the field. A total of 220 beetle pairs were released at five locations in a field where this beetle was not observed previously. Each release point was at a border between the two host species. 5. One model predicted for the preferred host species the egg cluster count frequencies in the field from parameters estimated in the cages. This model assumed that egg clusters present on a plant increased subsequent oviposition on this plant. All other models could not describe the distribution of egg cluster counts for either of the two host species. 6. The results suggest that females seek out attractive hosts actively and the attractiveness of a plant increases with its egg cluster load. This behaviour creates a frequency distribution of egg clusters per host that depends only on beetle density but not on plant density. This conclusion has important implications for modelling insect-plant interactions.
Journal of Animal Ecology 04/2006; 75(2):387-98. · 4.84 Impact Factor
[show abstract][hide abstract] ABSTRACT: We observed patch-use behavior by two gerbil species in a field setting and investigated how aggression and intrinsic decision-making interact to influence patch residence times. Results were interpreted by using a competing risk analysis model, which uniquely enabled us to estimate the intrinsic patch-leaving decisions independently of external interruptions of foraging bouts by aggression. The experiment was conducted in two 1-ha field enclosures completely surrounded by rodent-proof fences and included allopatric (only Gerbillus andersoni allenbyi) and sympatric (G. a. allenbyi and G. pyramidum) treatments. We predicted that increased food patch quality (i.e., habitat quality) should decrease intrinsic patch-leaving rates and increase rates of aggressive interactions involving the forager feeding in the patch (i.e., the occupant individual). We also anticipated that increasing population density should result in an increase in the rate of aggressive interactions involving the occupant individual. Our results supported the first two predictions, indicating a trade-off between foraging and aggression. However, the third prediction was realized only for G. a. allenbyi in allopatry. Furthermore, in allopatry, occupant G. a. allenbyi individuals with high competitive ranks were involved in aggressive interactions at lower rates than those with low competitive ranks. However, in sympatry, patch-use behavior of occupant G. a. allenbyi individuals was mainly influenced by aggressive behavior of G. pyramidum, which did not respond to their competitive rank. Thus, it should pay less for G. a. allenbyi to be aggressive in sympatric populations. The observed reduction in intraspecific aggression among individual G. a. allenbyi in the presence of G. pyramidum supports this assertion. We suggest that this reduction likely weakens the negative effect of intra- and interspecific density on the per capita growth rate of G. a. allenbyi. Because this would change the slope of the isocline of G. a. allenbyi, it could be an important mechanism promoting coexistence when habitat selection is constrained. Copyright 2003.