PARASITIC PLANTS - Viscum.dk

Taraschewski, H. 2006. Hosts and parasites as aliens. Journal of Helminthology80: 99–128.Torchin, M. E., K. D. Lafferty, A. P. Dobson, V. J. McKenzie, and A. M.Kuris. 2003. Introduced species and their missing parasites. Nature 421:628–630.Torchin, M. E., K. D. Lafferty, and A. M. Kuris. 2002. Parasites andmarine invasions. Parasitology 124: S137–S151.PARASITES, OF PLANTSSEE PARASITIC PLANTS; PATHOGENS, PLANTPARASITIC PLANTSHENNING S. HEIDE-JØRGENSENUniversity of Copenhagen, DenmarkParasitic flowering plants exploit other flowering plants forwater and nutrients with the help of one or more haustoria.Part of the haustorium, the intrusive organ, penetrateshost tissue to establish contact with the conductive tissueof the host. Introduced parasitic plants occur throughoutthe world, and some are considered invasive. They oftencause considerable economic damage when attackingmonocultures in agriculture, orchards, and forestry, andmuch effort is spent to avoid and control invasive, harmfulparasitic plants.HISTORYParasitic flowering plants have been known and describedsince the days of Theophrastus. However, for a long timeeven esteemed botanists were doubtful about the natureof parasitic plants, and the class Sarcophytae was establishedfor monstrous excrescences such as members ofRafflesiaceae and some Balanophoraceae; other plantswere classified as fungi. In 1969 Job Kuijt published hisThe Biology of Parasitic Flowering Plants, for close to40 years the only comprehensive book on parasitic plants.Intensive research on the physiology and control of harmfulparasites began in the 1950s, when a witchweed, Strigaasiatica, was introduced into the United States and threatenedthe cultivation of maize.NUMBER AND TYPES OF PARASITIC PLANTSWith one possible exception among gymnosperms(Parasitaxus usta in New Caledonia), parasitic plants arelimited to eudicotyledons with about 4,500 species inabout 280 genera belonging to 20 families. The majority,about 4,100 species, are hemiparasites (i.e., they aregreen plants meeting most or all of their needs for carbonthrough their own photosynthesis). Hemiparasites maybe attached to roots and called root parasites. In this case,water and other nutrients are achieved partly from thesoil through the roots and partly from the host throughhaustoria. If hemiparasites are attached to stems (andconsequently called stem parasites), then they obtain allwater and inorganic nutrients from the host. A minorityof about 390 species are holoparasites (i.e., they lackchlorophyll and photosynthesis); hence, carbon must beobtained along with water and other nutrients from thehost. Holoparasites may also be either root parasites orstem parasites. A few holoparasitic root parasites developa reduced root system that may contribute to water andnutrient absorption, but this is not well established.Parasitic plants may be either facultative or obligateparasites. The latter cannot survive without a host, whilethe former may survive for a longer period and even producesome seeds, but productivity is better when waterand organic and inorganic nutrients are supplied from oneor more hosts. Only hemiparasitic root parasites can befacultative parasites. However, there are no records fromnature of a parasitic flowering plant that has completed atleast part of its life cycle without haustorial connectionsto host plants. Competition from other species in theplant community will sooner or later eliminate a potentialfacultative parasite. Therefore, the terms facultativeand obligate should be avoided until facultative parasiteshave been demonstrated to occur in nature. They may beused under laboratory conditions where it is possible togrow some hemiparasitic Orobanchaceae throughout thereproductive phase without a host.There are parasite “lookalikes.” These may be greenorchids, bromelias, or ferns sitting on tree branches, butthey neither develop haustoria nor obtain nutrients orwater from the branches supporting them. Such plants arecalled epiphytes. Other “lookalikes” have lost all or nearlyall chlorophyll and therefore look like holoparasites, butthey have a three-part relationship wherein a mycorrhizalfungus interconnects the chlorophyll-free plant with anormal green plant having photosynthesis. Such plantsused to be called saprophytes but are now called mycoheterotrophicplants (mycotrophic) plants. Examples areMonotropa, Sarcodes, some Pyrola, and orchids such asNeottia nidus-avis and Corallorhiza trifida.HAUSTORIAThe development, structure, and function of the haustoriumare essential subjects—as Job Kuijt has put it, “thehaustorium is the defining part, the essence of parasitism.”504 PARASITIC PLANTSFrom Daniel Simberloff and Marcel Rejmánek, editors, Encyclopedia of Biological Invasions,Berkeley and Los Angeles: University of California Press, 2011.

compression of cells. When the intrusive organ reaches theconductive tissue of the host, a bridge of xylem cells differentiatesand connects host xylem with parental xylemof the parasite. The parasite always maintains a lowerwater potential than the host. The main route for waterand nutrients from host to parasite is through the xylembridge, although the complete interface between the twopartners also plays an important role in nutrient uptake.In the most advanced holoparasites, the intrusive organcomprises all vegetative tissue of the parasite. It splits intocellular strands, which penetrate large parts of the host,although they rarely reach the shoot tips. This internaltissue is called the endophyte, as opposed to the exophytefor external parts such as shoots and flowers.FIGURE 1 Notanthera heterophylla (Loranthaceae), Chile. (A) showsthe adhesive disc of a primary haustorium (center) and several secondaryhaustoria on two cortical roots running parallel with the hostbranch. (B) shows several cortical roots with young leafy shootsemerging above secondary haustoria. (Photographs courtesy of JobKuijt. Reproduced from H. S. Heide-Jørgensen, 2008.)Available space does not allow for many details. From adevelopmental point of view, there are two types of haustoria,primary and secondary. The primary haustorium(Fig. 1) develops directly from the primary root apex; inthe more advanced parasites, it is the only haustorium,and functions throughout the lifetime of the parasite.Evolution of the primary haustorium made holoparasitismpossible, because the generally small-seeded holoparasitesneed water and nutrients from a host immediatelyafter germination. Secondary haustoria (Fig. 1) developon lateral and adventitious roots, and they may be shortlived,sometimes living only a few months. They mayoccur in numbers of up to several hundred per plant.Regarding nutrient absorption, it may be an advantage tohave the secondary haustoria placed on roots from differenthosts, because different hosts absorb various nutrientions in varying amounts.A haustorium may consist of an outer part called aholdfast with an adhesive surface used for preliminaryattachment to the host. Within the holdfast, an intrusiveorgan develops, which penetrates the outer layers ofthe holdfast and then penetrates the host by a combinationof enzymatic dissolution of cell walls and mechanicalSYSTEMATIC AFFINITY OFINVASIVE PARASITESIntroduced invasive parasites are known from 5 or 6of 20 families, including parasites. Most important areOrobanchaceae, a family that now also includes parasitesearlier placed in Scrophulariaceae, and Convolvulaceae(Cuscuta); there are further examples in Loranthaceae,Viscaceae, and Santalaceae. The majority of invasivespecies are hemiparasitic, mostly annual root or stemparasites. Invasive holoparasites are known only fromOrobanchaceae, and they are annual root parasites.HOST RANGEParasitic flowering plants only occur as introduced specieswhen acceptable hosts are available. Parasites with oneor few acceptable hosts have no possibility of becomingintroduced or invasive outside the natural distribution oftheir hosts unless the hosts also become introduced. Topredict the possibility of a species becoming introduced,it is necessary to know the range of its acceptable andpreferred hosts, a factor that is often underestimated. Ifa parasite is not found on a certain species, this speciesmay still be an acceptable host. The reason for the absencemay be ecological, such as the lack of a suitable dispersalagent (e.g., birds), or may have to do with other environmentalconditions (e.g., the light conditions may beinsufficient for the parasite). In bird dispersal, the properbird species must be available. When present, their flyingbehavior is important (e.g., many birds prefer to searchfor food, rest, and seek nesting possibilities in hedges,solitary trees, or wood edges, while the inside of the forestmay be avoided). For root parasites, it may be physicallyimpossible to follow a host root with attached haustoriaback to the mother plant. In herbarium collections, thehost species is rarely identified and noted.PARASITIC PLANTS 505

Host range varies from one acceptable host (e.g., thedwarf mistletoe Arceuthobium minutissimum on Pinus griffithii)to at least 343 different host species for the loranthDendrophthoe falcata. In general, holoparasites have fewerhosts than hemiparasites. To be counted as a host, thespecies must be able to support the parasite throughoutits life cycle. Genetics and biochemical tissue incompatibilitydetermine the maximum number of acceptablehosts, but in practice parasite range is mainly influencedby geographical (host distribution) and ecological (dispersalbiology and environmental factors) relationships.BIOLOGY OF INVASIVE PARASITESGenerally, perennial parasites reduce the vigor of the hostbut do not kill the host, because to do so would destroythe possibility for survival of the parasite. A weakened hostproduces fewer flowers, fruits, and viable seeds and is moresusceptible to fungal diseases and harmful insects. However,annual parasites can allow themselves to kill the host,provided seed set is completed before the host dies. Therefore,some of the most harmful invasive parasites are annuals.As mentioned, dispersal biology and host distributionlimit the possibility of a parasite’s becoming introducedand invasive. When a parasite becomes introduced, thedispersal agent has often been humans, whether deliberately(as for Viscum album, see below) or accidentally.LoranthaceaeThis family contains more than 900 species of hemiparasiticstem parasites and three root parasites, mainly fromtropical and subtropical regions. Flowers are generallyshowy, and birds pollinate the flowers and disperse thefruits. Host range is generally high. Only a few speciesare considered introduced on some tropical islands inSoutheast Asia (e.g., the aforementioned Dendrophthoefalcata). However, several loranths act like invasive speciesin orchards and plantations of monocultures. In India,D. falcata causes enormous damage in plantations of teak(Tectona grandis), and the parasite may lead to death ofentire trees. One reason for the success of D. falcata onteak may be that it is more shade-tolerant than most otherloranths. On average, the parasite receives only 40 percentof the light received by the host, and it will surviveeven when the host leaves block 70 percent of the incidentsolar radiation.In West Africa, some of the larger loranths havebecome real pests. Tapinanthus bangwensis uses a widevariety of hosts but has become invasive in plantationssince cocoa was introduced as a crop in the 1870s. It hasbeen shown that germination of seeds and establishmentof seedlings of this light-dependent parasite are up tothree times more likely in unshaded compared to shadedcocoa trees. Therefore, the problem increased with deforestationand the practice of growing cocoa without shadetrees. Phragmanthera capitata has invaded plantations ofteak and rubber, and the presence of other large species ofTapinanthus, Agelanthus, and Globimetula only worsensthe situation.For many years, the only method to control attacks bymembers of Loranthaceae was cutting down these stemparasites. Some of the host branches must also be cut dueto the spreading endophyte inside the branches and toprevent regeneration from secondary haustoria placed onso-called epicortical roots (Fig. 1). Otherwise, new shootsmay arise from the endophyte or the adhesive disk. Inrecent years, herbicides have been tried, but very few herbicidesare available for a system where both host and pestare dicotyledons. The substance 2,4-D dichlorophenoxyaceticacid has been sprayed onto the leaves of variousmembers of Loranthaceae and Viscaceae or injected intothe trunk of the host, but with inconsistent results. Herbicidesmay be used to control Dendrophthoe falcata onteak if used during the deciduous stage of the host tree.ViscaceaeAll members of Viscaceae are hemiparasitic stem parasites.The distribution is similar to Loranthaceae but with morespecies in the northern temperate zone. Only a primaryhaustorium is present, and the most advanced genera havea widely distributed endophyte. The flowers are smalland inconspicuous, and the fruits are dispersed by birdsexcept in the case of Arceuthobium, which relies on selfdispersalby explosive fruits. Arceuthobium species are themost harmful parasites on conifers in North America, butthe maximum dispersal distance is 20 m from the motherplant, and long-distance dispersal rarely occurs. Althoughpresent in Washington State and British Columbia, noArceuthobium has spread to any of the minor west coastislands. A population on Mt. Constitution, Orcas Island,is interpreted as an Ice Age relict. There are seven genera,but only Viscum album occurs as introduced.Around the year 1900, the European Viscum album ssp.album (Fig. 2) appeared in Sonoma County north of SanFrancisco, California, not spread by birds but introducedby the highly respected plant breeder Luther Burbank. By1984, the parasite had spread by birds to about 114 km 2 .The average distance of spread from the point of introductionwas 5.8 km. In 1991 the corresponding figures were184 km 2 and 8 km. Viscum album ssp. album occurs onmore than a hundred different hosts of broad-leaved trees,506 PARASITIC PLANTS

FIGURE 2 Viscum album ssp. album on apple tree, March. The Europeanmistletoe is introduced and invasive in California. Female plant with ripefruits, seven years after sowing. (Photograph courtesy of the author.Reproduced from www.viscum.dk/abstracts/text/snylteplanter.pdf.)and in California it has at least 22 hosts. Many of thoseare introduced species from Europe, but native NorthAmerican species are also attacked, such as Acer saccharinum,Robinia pseudoacacia, Alnus rubra, Populus fremontii,and Salix lasiandra. Because mainly ornamental trees inurban areas are attacked, damage is considered moderate.Further spread is expected to be limited due to thepresence of few acceptable hosts in the surrounding area.However, if spread by humans to gardens at long distancesfrom Sonoma County, the parasite could be a realpest. Viscum album was recently also reported in Victoria,Canada. As for loranths, the control method is cutting offhost branches. The cut must be at least 30 cm below thehaustorium to ensure removal of all endophyte tissue.are at least 150 species (and possibly more), but there aremany unsolved taxonomic problems. All species are herbaceous,winding, stem parasites with only secondary haustoria.Host range is high for most species but often difficultto determine, because Cuscuta haustoria attach to any subjectwithin reach. However, many haustoria develop onlya holdfast and no intrusive organ or endophyte. In suchcases, the supportive species is not counted as a host.Cuscuta species are fast growing (Fig. 3). This may inpart be explained by faster nutrient translocation becausethe xylem bridge is accompanied by phloem. The presenceof both xylem and phloem continuity is a uniquefeature in Cuscuta, and only one species of Orobanche isreported to have a similar advanced haustorium. Cuscutaspecies are annuals, and this life form, along with the fastgrowth, makes several species serious invasive weeds inagriculture, where crops such as tomato, potato, carrot,sugar beet, alfalfa, clover, avocado, coffee, and citrus speciesare attacked. The seeds are less than a millimeter insize. Very little is known about seed dispersal, but bothbirds and wind may be dispersal vectors. It is known thatseeds survive the passage of the digestive canal of sheep.However, introduced invasive Cuscuta species probablyalways originate from contaminated seeds of crop plants.The invasive species causing most problems in manycountries is the North American C. campestris (Fig. 3). InAsian countries, yield loss in sugar beet crops has been onthe order of 3,500 to 4,000 kg/ha. In addition, Cuscutamay also be toxic to some domestic animals. No fullyeffective control method seems available. Mechanicalmethods such as flaming, harrowing, and hand-pullingSantalaceaeThe sandalwood family, with 35 genera, consists of bothwoody and herbaceous species, which, with a few exceptions,are hemiparasitic root parasites. Here, too, the fruitsare dispersed by birds. The family is represented in all climaticzones except the arctic zone. The mainly AfricanThesium is by far the largest genus, with about 250 species.The small, white-flowered, Eurasian T. arvense is reportednear Calgary in Canada and in Montana and North Dakotain the United States. It most likely arrived with seeds ofagricultural plants. The root parasite is mainly a grasslandspecies that can attack vegetables, but due to its sporadicoccurrence, it is not a threatening invasive species.Cuscuta (Convolvulaceae)Cuscuta (dodder) is the only parasitic genus in Convolvulaceae.It has a worldwide distribution and is absent only inthe most northern parts of the northern hemisphere. ThereFIGURE 3 Cuscuta veatchii on Bursera sp., Baja California. The speciesis native, but its habit looks like the American C. campestris, which isinvasive in many countries. The fast development of Cuscuta is illustratedby the fact that germination of the pictured species took placeless than three weeks before this photograph was taken. (Reproducedfrom H. S. Heide-Jørgensen, 2008.)PARASITIC PLANTS 507

have been used, and selective herbicides are also availablebut do not give full seasonal control. In 2004 the AsianC. japonica was discovered in California, and by 2007 ithad appeared in 14 counties, indicating very fast dispersal.Furthermore, the growth rate is about 15 cm/day, and thehost range is very wide. This indicates that C. japonicamay soon be a troublesome invasive species.OrobanchaceaeThe broomrape family now includes witchweeds andother parasitic figworts. Of the about 90 genera, 75, representing1,700 species, are hemiparasitic root parasitestransferred from Scrophulariaceae. Furthermore, the familyincludes 17 genera of holoparasitic root parasites. Thefamily is represented in all climatic zones and on all continentsexcept Antarctica. Orobanchaceae contains themost troublesome introduced invasive parasites.Parentucellia viscosa (Fig. 4) and P. latifolia from theMediterranean region are annual root parasites in moistpastures and on heath land. Like other hemiparasitic rootparasites, they have a wide host range, which includesnative species in countries where they are introduced.They spread by tiny seeds carried by wind and water. P.viscosa occurs as introduced around the world in placessuch as Hawaii, the west coast of North America, Texas,Kansas, Denmark (where it is not a problem species),Japan, and Western Australia, and it is spreading furtherinto Australia. Both species have recently been observedFIGURE 4 Parentucellia viscosa introduced to Hawaii, the mainlandUnited States, and many other countries. (Photograph courtesy of Forestand Kim Starr. Reproduced from H. S. Heide-Jørgensen, 2008.)FIGURE 5 Striga asiatica on partly wilted sorghum. This is an invasivespecies in the United States and Australia causing serious lossesin crops from the grass family. (Photograph courtesy of Arne Larsen.Reproduced from H. S. Heide-Jørgensen, 2008.)in the South Gippslands east of Melbourne. They can befairly invasive and can degrade pastures if left unattended,but they may be controlled by use of selective herbicides.Striga is another annual root parasitic genus. Sevenof the 40 species are considered to be among the mostdamaging weeds within their mainly tropical African–Asian distribution. Striga is most common in semidryvegetation, where most species use grasses or sedges ashosts. In crop plants, Striga and Orobanche have foundwell-nourished, abundant hosts, allowing the parasites todevelop extremely well and set lots of seeds. Therefore,these parasites become real pests, whether occurring asnatural or introduced species. Two harmful Striga species,S. asiatica and S. gesnerioides, are known to be invasive inseveral countries. Long-distance dispersal is by wind or byinsufficiently rinsed seed corn. Short-distance dispersalalso occurs through water and by seeds sticking to claws,hoofs, footwear, wheels, and machinery.S. asiatica (Fig. 5) was introduced into North andSouth Carolina, where it appeared in the 1950s, and intoSoutheast Australia. It is a serious threat in fields of maize,sorghum, and sugar cane. S. gesneroides was introducedinto Florida. It mostly uses dicotyledons as its host—inparticular, legumes. The seed set of Striga is on the orderof 100,000 per plant, and the primary haustorium is soeffective that by the time the parasite is visible aboveground, it is too late to save the crop. In the most severeattacks, the yield loss may be up to 100 percent.The large number of tiny seeds and a viability of morethan 20 years are major problems for effective control ofStriga. The most effective control is the development ofresistant crop strains, and some success has been achieved in508 PARASITIC PLANTS

several crop plants. However, there are a number of methods,both mechanical and chemical, to avoid seed set, seeddispersal, and germination. These includes deep plowing tobury parasite seeds, hand-pulling, burning, cleaning toolsand shoes, covering the soil with polyethylene to increasetemperature, fallowing, fertilizing the soil, crop rotation,intercropping with catch crops, sowing early ripening strainslate, practicing biological control using fungi and herbivorousinsects, using chemical germination stimulants beforesowing, fumigating soil with methyl bromide (for example),and using herbicides. None of these methods are effectiveor practical when used alone; it is necessary to use several ofthe methods simultaneously or successively as an integratedcontrol system. It may also be noted that the biochemicaland biological control methods are so expensive that theyare not feasible in developing countries.Orobanche is the largest genus in the family, with about150 species (including Phelipanche). These are holoparasiticroot parasites and mostly annuals. The root systemis highly reduced, and several species have only a primaryhaustorium (Fig. 6). Seed production is enormous (up to350,000 per plant). Dispersal biology is similar to that ofStriga. At least six species are as problematic in agricultureas the harmful Striga species. They attack only dicotyledonouscrops: mainly legumes (Fabaceae), but also otherssuch as tomato, carrot, tobacco, hemp, and sunflower.The Mediterranean O. minor and Phelipanche ramosa(O. ramosa) have been introduced into several countries—O.minor into the United States, Chile, southernAfrica, Australia, and New Zealand, and P. ramosa intoMexico, Cuba, Australia, New Zealand, and several U.S.FIGURE 6 Orobanche hederae on roots (“white”) of ivy, Hedera helix.From the primary haustorium (center), a tubercle develops. Threeinflorescences with chlorophyll-free, scaly leaves rise from inside thetubercle, along with a number of very short adventitious roots. Notethat the host root has wilted beyond the primary haustorium, indicatingvery effective water and nutrient absorption by the parasite.(Reproduced from H. S. Heide-Jørgensen, 2008.)FIGURE 7 Orobanche flava on Petasites hybridus (large leaves). Bothspecies are introduced to Denmark. (Photograph courtesy of the author.Reproduced from www.viscum.dk/abstracts/text/snylteplanter.pdf.)states (it arrived in Texas as recently as 2000). The controlmethods and problems are the same as mentioned forStriga. In addition, soil application of an inhibitor of gibberellinsynthesis prevents seed germination. Orobancheamethystea has recently been introduced to Israel, whereit is invasive in vetch fields. Interestingly, introducedOrobanche species may be used to control other introducedspecies. Orobanche fl ava (Fig. 7) is introduced intoDenmark, where it locally takes a heavy toll on the introducedand invasive Petasites hybridus.OTHER TAXAMore introduced parasitic species than mentioned aboveare known, mainly from Australia and New Zealand.Most of the species are annual hemiparasitic root parasitesthat are not yet considered invasive. Cassytha filiformis(Lauraceae) is a still-spreading stem parasite with a similarpotential to be invasive as Cuscuta. As a curiosity, it maybe mentioned that Euphrasia frigida (Orobanchaceae)arrived in 2000 as the first parasitic plant on the volcanicisland Surtsey, formed 33 km south of Iceland after aneruption in 1963.SEE ALSO THE FOLLOWING ARTICLESDispersal Ability, Plant / Forestry and Agroforestry / Horticulture /Invasion Economics / Seed Ecology / WeedsPARASITIC PLANTS 509

FURTHER READINGHeide-Jørgensen, H. S. 2008. Parasitic Flowering Plants. Leiden: Brill.Joel, D. M., J. Hershenhorn, H. Eizenberg, R. Aly, G. Ejeta, P. Rich, J.Ransom, J. Sauerborn, and D. Rubiales. 2007. Biology and managementof weedy root parasites. Horticultural Reviews 33: 207–349.Kuijt, J. 1969. The Biology of Parasitic Flowering Plants. Berkeley: Universityof California Press.Mathiasen, R. L., D. L. Nickrent, D. C. Shaw, and D. M. Watson. 2008.Mistletoes: Pathology, systematics, ecology, and management. PlantDisease 92: 988–1006.Press, M. C., and J. D. Graves. 1995. Parasitic Plants. London: Chapmanand Hall.Press, M. C., and G. K. Phoenix. 2005. Impacts of parasitic plants onnatural communities. New Phytologist 166: 737–751.Sand, P. F., R. E. Eplee, and R. G. Westbrooks, eds. 1990. Witchweedresearch and control in the United States. Weed Science Society ofAmerica Monograph 5: 1–154.PATHOGENS, ANIMALGRAHAM J. HICKLINGUniversity of Tennessee, KnoxvilleAnimal pathogens are disease-causing agents of wildand domestic animal species, at times includinghumans. In the context of invasion biology, the termusually refers to infectious microorganisms such as bacteriaand viruses and excludes nonliving agents such astoxins and toxicants. These infectious organisms aresometimes termed microparasites to distinguish themfrom macroparasites. Introduction of new pathogensinto areas occupied by susceptible animal host speciesthreatens native wildlife, disrupts animal-based foodproduction systems, and puts human and companionanimal health at risk.INVASIVE ANIMAL PATHOGENS ANDEMERGING INFECTIOUS DISEASESPathogens are a natural component of all ecosystems.Long-term association with their vertebrate hosts resultsin coevolutionary responses that reduce the virulence ofthe pathogens or boost the ability of the host to resistor recover from infection. Human-induced changes tothe environment disrupt these natural pathogen–hostrelationships, often with adverse consequences. A diseaseagent transported to a new area may trigger outbreaksof disease among hosts previously naive to that pathogen—theintroduction of rinderpest virus to Africa in the1880s provides a grim example. Changes in transmissionpathways of endemic pathogens can trigger unanticipatedepizootics, such as when the 1827 introduction of Culexquinquefaesciatus mosquitoes triggered outbreaks of avianpox among birds on the Hawaiian Islands; the pox viruswas already present on the islands but had not previouslybeen causing significant disease. Change in habitat orclimate can alter the biogeographic distribution of thevectors and hosts of animal pathogens, leading to diseaseinvasion (or reinvasion of areas previous cleared of thatdisease). Collectively, these kinds of outbreak are termedemerging infectious diseases (EIDs), defined as infectionsthat have newly appeared in a population or thatare rapidly increasing in incidence or geographic range(see Table 1 for examples of EIDs affecting animals). Themajority of EIDs affecting humans originate from pathogensoriginally carried by other animal species; these diseasesare termed zoonoses. Examples of zoonoses includesome strains of avian influenza, and flaviviruses such asWest Nile virus.FACTORS RESPONSIBLE FOR ANIMALPATHOGEN INTRODUCTION AND INVASIONIncreased frequency and speed of local and internationaltravel, increased human-assisted movement of animalsand animal products, and changing agricultural practiceshave all favored the introduction of animal pathogens tonew areas. Genetic and environmental changes also facilitateanimal pathogen invasion.Changes in the Genetic Make-up of PathogensAnimal pathogens sometimes become invasive as a consequenceof natural changes in their genetic make-up,producing new strains with increased transmission ratesor pathogenicity. For example, a new calicivirus closelyrelated to the virus responsible for European brown haresyndrome emerged in rabbits in China in 1984 and spreadto other countries via trade in farmed rabbits. The resultingoutbreaks of rabbit hemorrhagic disease were highlylethal to unvaccinated European rabbits.Humans, animals, and environmental sites are allreservoirs of bacterial communities that include somebacteria resistant to common antimicrobial agents. Ouragricultural practices are increasingly providing environmentsin which these resistant bacteria can amplify andspread, so there is growing concern that enhanced microbialresistance will lead to future pathogen outbreaks.Nevertheless, most animal pathogen introductions aretriggered by the movement of humans and other animals,or are a consequence of human-induced environmentalchange.510 PATHOGENS, ANIMAL

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