Modelling the lethargic crab disease

The lethargic crab disease (LCD) is an emergent infirmity that has decimated native populations of the mangrove land crab (Ucides cordatus, Decapoda: Ocypodidae) along the Brazilian coast. Several potential etiological agents have been linked with LCD, but only in 2005 was it proved that it is caused by an ascomycete fungus. This is the first attempt to develop a mathematical model to describe the epidemiological dynamics of LCD. The model presents four possible scenarios, namely, the trivial equilibrium, the disease-free equilibrium, endemic equilibrium, and limit cycles arising from a Hopf bifurcation. The threshold values depend on the basic reproductive number of crabs and fungi, and on the infection rate. These scenarios depend on both the biological assumptions and the temporal evolution of the disease. Numerical simulations corroborate the analytical results and illustrate the different temporal dynamics of the crab and fungus populations.     

A systemic microbial disease in the Dungeness crab,Cancer magister,caused by a Chlamydia-like organism

A Chlamydia-like agent was diagnosed in Dungeness crabs, Cancer magister, collected from Willapa Bay and northern Puget Sound, Washington. Over a 5-year period, infected crabs were found during the months of December through March. Infection by the organism is systemic, resulting in greatly hypertrophied cells with appressed nuclei. Unlike Chlamydia or other Chlamydia-like organisms, the present microbe has a great affinity for connective tissue or connective tissue cells rather than epithelial cells. Transmission electron microscopy revealed the presence of several developmental stages that are similar to chlamydial stages, in addition to what appear to be slightly altered or aberrant stages.     

Isolation and characterization of 11 microsatellite loci in the mangrove crab, Ucides cordatus (Ocypodidae: Ucidinae)

Eleven polymorphic microsatellite loci were described for the mangrove crab, Ucides cordatus, an important fishery resource on the Brazilian coast. The number of alleles observed at each locus varied between eight and 23. Observed and expected mean heterozygosities were 0.791 and 0.893 respectively. Amplification of all loci was highly successful, under the same polymerase chain reaction conditions. With the exception of P2D3, all loci adhered to the assumptions of the Hardy-Weinberg equilibrium and did not present deviations reflecting linkage disequilibrium. Given this, these markers will be extremely useful in future management programmes for U. cordatus stocks.     

Histopathology of the mangrove land crab Ucides cordatus (Ocypodidae) affected by lethargic crab disease

Lethargic crab disease (LCD) has caused extensive epizootic mortality of the mangrove land crab Ucides cordatus (Linnaeus 1763) (Brachyura: Ocypodidae) along the Brazilian coast. Direct culture of tissue samples from sick crabs and subsequent isolation and purification identified the causative agent as an Exophiala species of fungus. The histopathology of crabs with variable signs of LCD indicates that the most affected tissues are the epidermis, connective tissue, heart, hepatopancreas, nervous system, and gills. Gonads, somatic muscles, and digestive system are less affected by the fungus. The observed pathology is compatible with the clinical signs of LCD. Necrosis, tissue degeneration, and congestion of hemal sinuses and vessels are present in heavily infected organs. Nerve fibers may be compressed by accumulations of yeast-like cells. In heavy infections the tissue of gill lamellae is destroyed with subsequent dilation or compression. Cellular immune responses include hemocytic infiltration, agglutination and encapsulation, and phagocytosis. Phagocytosis of yeast-like cells is abundant in the connective tissue associated with the exoskeleton. These results indicate that LCD is the result of a systemic phaeohyphomycosis caused by a species of Exophiala. The present study also suggests that dispersal of the fungus within the crab occurs through the hemal system.     

Bitter crab syndrome in two species of king crabs from the Sea of Okhotsk

The disease caused by parasitic dinoflagellates of the genus Hematodinium was found in the red Paralithodes camtschaticus and blue P. platypus king crabs from the Sea of Okhotsk. The hemolymph of diseased crabs was cream colored, opaque, and dense. Numerous dinokaryotic trophonts and multinucleate plasmodia of the parasite were revealed in the hemolymph and internals of diseased animals. The parasitic infection was recorded in females and juvenile males from August to mid-October.     

Modelling vertical transmission in vector-borne diseases with applications to Rift Valley fever

We present two ordinary differential equation models for Rift Valley fever (RVF) transmission in cattle and mosquitoes. We extend existing models for vector-borne diseases to include an asymptomatic host class and vertical transmission in vectors. We define the basic reproductive number, ₀, and analyse the existence and stability of equilibrium points. We compute sensitivity indices of ₀ and a reactivity index (that measures epidemicity) to parameters for baseline wet and dry season values. ₀ is most sensitive to the mosquito biting and death rates. The reactivity index is most sensitive to the mosquito biting rate and the infectivity of hosts to vectors. Numerical simulations show that even with low equilibrium prevalence, increases in mosquito densities through higher rainfall, in the presence of vertical transmission, can result in large epidemics. This suggests that vertical transmission is an important factor in the size and persistence of RVF epidemics.     

Global analysis of an SAIS model

This paper is concerned with global analysis of an SAIS epidemiological model in a population of varying size introduced by Busenberg and van den Driessche. In this model the population is divided into three subgroups of susceptible, asymptomatic and infective individuals. It has been shown that this system has no periodic solutions and all its trajectories tend to the equilibria of the system. We use the Poincaré Index theorem to determine the number of the equilibria and their stability properties. We have shown that bistability occurs for suitable values of parameters and found a set of examples of all possible dynamics of the system.     

An exploratory study on grapsid crab zonation in Kenyan mangroves

Despite earlier efforts to understand the role played by grapsid crabs inmangroves, their importance in the structuring and functioning of suchsystems is fully appreciated, particularly with regard to small-scalestudies. The present study provides some new data on the interaction between mangroves and crabs, namely the link between the distribution of particular mangrove tree species and the distribution of certain crab species at the assemblage level. Floristicand faunistic relevés were made in Gazi Bay (2 sites) and Mida Creek(3 sites), 140 km apart on the Kenyan coast, along five transects in aseries of quadrats covering the width of the mangrove belts. Zonation ofboth mangrove vegetation and brachyuran fauna was described and heightabove datum and distance to the mainland (limit of non-flooded area)measured. The relationship between the presence and the absence of crabs andtrees was analysed using detrended correspondence analysis.Summarized, the mangrove tree zonation pattern contains four assemblageswith a particular dominant species: first a landward Avicennia marinazone, followed by a mixed zone with Ceriops tagal, Rhizophoramucronata and Bruguiera gymnorrhiza, and finally a R.mucronata zone and a Sonneratia alba zone, both of which can mixwith seaward A. marina. Ordination results show that the distributionof Neosarmatium meinerti and Sesarma ortmanni is linked to thelandward A. marina zone, that of Neosarmatium smithii, Sesarma guttatum and Sesarma leptosoma corresponds to the R. mucronata zone and that of Metopograpsus thukuhar and Sesarma elongatum to the seaward A. marina and S. albazone. There appears to be one major underlying factor in the zonation of bothcrabs and trees, with most likely a complex multiple causality. In certaincases the association between crabs and trees are causal, whereas in othercases it can be the result of an independent restriction to the same zonesby a common cause.     

Modelling studies to estimate the prevalence of foot-and-mouth disease carriers after reactive vaccination

Foot-and-mouth disease (FMD) is a highly contagious and economically significant viral disease of cloven-hoofed animals. Vaccination can be used to help restrict the spread of the infection, but evidence must be provided to show that the infection has been eradicated in order to regain the FMD-free status. While serological tests have been developed, which can identify animals that have been infected regardless of vaccination status, it is vital to know the probable prevalence of herds with FMD carriers and the within-herd prevalence of those carriers in order to design efficient post-epidemic surveillance strategies that establish freedom from disease. Here, we present the results of a study to model the expected prevalence of carriers after application of emergency vaccination and the impact of this on the sensitivity of test systems for their detection. Results showed that the expected prevalence of carrier-containing herds after reactive vaccination is likely to be very low, approximately 0.2%, and there will only be a small number of carriers, most likely one, in the positive herds. Therefore, sensitivity for carrier detection can be optimized by adopting an individual-based testing regime in which all animals in all vaccinated herds are tested and positive animals rather than herds are culled.     

Specific primers for the detection of the black-yeast fungus associated with lethargic crab disease (LCD)

Lethargic crab disease (LCD) is an emerging infirmity that has been causing extensive mortalities in populations of the mangrove land crab Ucides cordatus (Ocypodidae) along the Atlantic coast of Brazil. Previous studies have indicated that LCD is associated with a dematiaceous fungus, Exophiala cancerae de Hoog et al. In the present study, we sequenced the internal transcribed spacer (ITS) of the rDNA region of this black yeast species and developed species-specific PCR primers. Sensitivity tests indicated that the developed protocol is capable of detecting very small amounts of target DNA. Also, the application of the protocol to a variety of other dematiaceous fungi did not generate any false positives. The specific primers provided in the present study represent an important tool for rapidly surveying a large number of crab individuals, as well as environmental samples. Such knowledge will be instrumental in understanding the epidemiological dynamics of LCD.     

Modelling seasonal variations in the age and incidence of Kawasaki disease to explore possible infectious aetiologies

The average age of infection is expected to vary during seasonal epidemics in a way that is predictable from the epidemiological features, such as the duration of infectiousness and the nature of population mixing. However, it is not known whether such changes can be detected and verified using routinely collected data. We examined the correlation between the weekly number and average age of cases using data on pre-vaccination measles and rotavirus. We show that age-incidence patterns can be observed and predicted for these childhood infections. Incorporating additional information about important features of the transmission dynamics improves the correspondence between model predictions and empirical data. We then explored whether knowledge of the age-incidence pattern can shed light on the epidemiological features of diseases of unknown aetiology, such as Kawasaki disease (KD). Our results indicate KD is unlikely to be triggered by a single acute immunizing infection, but is consistent with an infection of longer duration, a non-immunizing infection or co-infection with an acute agent and one with longer duration. Age-incidence patterns can lend insight into important epidemiological features of infections, providing information on transmission-relevant population mixing for known infections and clues about the aetiology of complex paediatric diseases.     

A disease transmission model in a nonconstant population

A general SIRS disease transmission model is formulated under assumptions that the size of the population varies, the incidence rate is nonlinear, and the recovered (removed) class may also be directly reinfected. For a class of incidence functions it is shown that the model has no periodic solutions. By contrast, for a particular incidence function, a combination of analytical and numerical techniques are used to show that (for some parameters) periodic solutions can arise through homoclinic loops or saddle connections and disappear through Hopf bifurcations.Supported in part by NSERC grant A-8965, the University of Victoria Committee on Faculty Research & Travel, and the Institute for Mathematics and its Applications, Minneapolis, MN, with funds provided by NSF     

一类具有暂时免疫传染病模型的Hopf分支

本文应用Hassard的“规范形”方法,讨论了一类具有暂时免疫传染病模型的动力形态．给出了该系统发生Hopf分支的参数曲线,进一步计算出了决定分支方向及稳定性的参数条件,并给出了生态解释．     

Pest control through viral disease: mathematical modeling and analysis

This paper deals with the mathematical modeling of pest management under viral infection (i.e. using viral pesticide) and analysis of its essential mathematical features. As the viral infection induces host lysis which releases more virus into the environment, on the average 'kappa' viruses per host, kappain(1,infinity), the 'virus replication parameter' is chosen as the main parameter on which the dynamics of the infection depends. We prove that there exists a threshold value kappa(0) beyond which the endemic equilibrium bifurcates from the free disease one. Still for increasing kappa values, the endemic equilibrium bifurcates towards a periodic solution. We further analyse the orbital stability of the periodic orbits arising from bifurcation by applying Poor's condition. A concluding discussion with numerical simulation of the model is then presented.     

Observations on the histopathology and probable progression of the disease caused by Trichomaris invadens,an invasive ascomycete,in the Tanner crab,Chionoecetes bairdi

Fifty-three Tanner crabs with varying degrees of surface encrustations and internal invasion by Trichomaris invadens were necropsied, processed, and examined to determine the extent of internal tissue invasion and histopathological effects of the infection. Sufficient data were accumulated to provide some insight on the progression, but not the rate, of tissue invasion. The epidermis and subepidermal layers are invaded and virtually replaced by proliferating hyphae. The invasion then proceeds internally, predominantly via the connective tissue. Major muscles and blood vessels are invaded, then invasion of deeper organs, including the hemopoietic tissue, gastrointestinal tract wall, and wall of the heart follows. The eyestalk is infected, usually early in the disease, and the invasion follows a similar pattern to that in the cephalothorax. Massive invasion and destruction of the retina is the major pathological effect. The gill may also be involved, with destruction of the lamellar tissues and invasion and destruction of podocytes in the stem of the gill.     

Biological features of a puzzling symbiotic association between the hermit crab Dardanus insignis and the porcellanid crab Porcellana Sayana (Crustacea)

The symbiotic lifestyle is widespread among porcellanid crabs, which maintain ecological and co-evolutionary associations with annelid polychaetes, poriferans, cnidarians, echinoderms, gastropod mollusks, and other crustaceans such as shrimps and hermit crabs, among others. We investigated the ecological association between the hermit crab Dardanus insignis and the porcellanid Porcellana sayana, in southeastern Brazil. Porcellanid crabs, hermit crabs, and available shells were collected monthly from July 2001 to June 2003, with a shrimp boat equipped with two double-rig trawl nets. The majority of P. sayana specimens were collected in shells occupied by D. insignis (96.6%); a few were found in empty shells (3.4%). The catch of both symbionts and hosts increased with increasing depth, with the highest occurrence at 35 m. The P. sayana crabs of various sizes could be found solitary or forming aggregations of up to 14 individuals per host, showing no sex or size segregation. In spite of the high diversity of shell species occupied by the hermit crabs and also available in the field, only a few of them were also utilized by P. sayana. The majority (93%) of shells utilized by P. sayana also hosted other symbiont species, constituting the basis of extensive symbiotic complexes. Thus, the ecological relationship between D. insignis and P. sayana may be classified as a non-obligate and non-specific symbiosis that may also involve other facultative organisms such as sea anemones.     

A mathematical consideration for the optimal shell change of hermit crab

Shell of the adult hermit crab has some important roles for its fitness. In the same time, the shell size often limits the body growth of its owner. To grow the body size larger, the individual must change the shell to another larger shell. If the individual cannot get another larger one, the individual has to suppress the body size growth as the occupied shell size allows. Growth suppression would result in the lower fitness. With a simple mathematical model, we consider the criterion about whether the individual should try to change the shell or not in order to get the higher fitness. We show that the optimality of a shell change behavior has a relation with the body size and the season length for the shell change. They also affect the optimal timing for the shell change. It is implied that the probability of the success in a shell change and the cost for the shell change behavior do not affect the optimal timing for the shell change at all but significantly do the optimality of the behavioral choice.     

Modelling the effect of urbanization on the transmission of an infectious disease

This paper models the impact of urbanization on infectious disease transmission by integrating a CA land use development model, population projection matrix model and CA epidemic model in S-Plus. The innovative feature of this model lies in both its explicit treatment of spatial land use development, demographic changes, infectious disease transmission and their combination in a dynamic, stochastic model. Heuristically-defined transition rules in cellular automata (CA) were used to capture the processes of both land use development with urban sprawl and infectious disease transmission. A population surface model and dwelling distribution surface were used to bridge the gap between urbanization and infectious disease transmission. A case study is presented involving modelling influenza transmission in Southampton, a dynamically evolving city in the UK. The simulation results for Southampton over a 30-year period show that the pattern of the average number of infection cases per day can depend on land use and demographic changes. The modelling framework presents a useful tool that may be of use in planning applications.     

Black mat syndrome,an invasive mycotic disease of the tanner crab, Chionoecetes bairdi

Black mat syndrome, caused by an encrusting fungus on the exterior of the carapace of tanner crabs, has been known for many years. Although it is a nuisance when processing crabs for the commercial market, it has been thought to be restricted to the external surfaces of the crab and, therefore, nonpathogenic. In the present study, 11 tanner crabs from the Kodiak area of Alaska with and 9 without grossly recognizable masses of the fungus on the carapace were necropsied and examined histologically utilizing special stains selective for fungi. In all individuals with the syndrome, hyphae of the fungus, previously identified as Phoma fimeti, penetrated the carapace and virtually replaced the underlying epidermis. In the more advanced cases, the eyestalk was invaded and the epidermis destroyed, and hyphae extended into the eyestalk musculature and nervous tissue. To date, infections of the connective tissue sheaths surrounding the esophagus, stomach, heart, hemopoetic tissue, thoracic ganglion, antennal gland, and ovary have also been observed. None of the crabs without the syndrome contained internal hyphae. Although data on the lethality of the disease are not yet available, the ease with which the hyphae penetrate the chitinous exoskeleton, their extensive proliferation in the epidermis, and their ability to invade deep tissues causing obvious pathological effects, are highly suggestive that it is a virulent, probably fatal, disease that may have a significant impact on tanner crab population dynamics.