Introduction to Nematodes

                              Rev 08/03/2017

Nematodes  are   invertebrate  roundworms   that   inhabit   marine, freshwater, and  terrestrial environments.   They  comprise  the  phylum Nematoda (or  Nemata) which includes parasites of plants and of animals, including humans,  as well  as species  that feed  on  bacteria, fungi, algae,  and on other  nematodes.    Four out of every five multicellular animals on the planet are nematodes (Platt, 1994).  Cobb (1914) calculated that if the nematodes resident in a single acre of soil near San Antonio, Texas, USA, were to proceed in head-to-tail procession to Washington D.C., some 2000 miles away, the first nematode would reach Washington before the rear of the procession left San Antonio!

Background Music:

Ghost Worms in the Sky

Lyrics: Kathy Merrifield

Vocals: Pointless Sisters

The  majority  of  nematodes  are microscopic, averaging less than a millimeter in length, but some of the animal parasites  are quite  large and readily visible to the naked eye. The animal  and plant parasites are of direct importance in agriculture, the environment,  and in  human health;  however, most  nematodes in the environment are not parasites.   Nematodes that feed on other organisms are important  participants in  the cycling of minerals and nutrients in the ecosystem that is fundamental to other biological activity.  Some of these  nematodes  may  have  major  roles  in  decomposition,  including biodegradation of  toxic compounds.   In  fact, the incidence of certain nematode species  is sometimes  used as  an indicator of  environmental quality.   Insect-parasitic nematodes can be of importance in regulating insect populations,  and are  being used  in the  biological control  of insect pests.  

The  developmental  biology  of  one  nematode  species, Caenorhabditis elegans,  is better  characterized than that of any other multicellular organism.   C.  elegans is  studied as  a model  system in molecular and developmental biology, and is providing insights into many other areas of biology and medicine.

Agriculture
   

    The management  of plant-parasitic nematodes has been fundamental to advances in  agricultural production in the United States and worldwide.  However,  the   use  of   certain  pesticides,  such  as  1,2-Dibromo-3-chloropropane (DBCP) to control nematodes, has resulted in contamination of  soil   and  groundwater  in  California.    Current  research  seeks integrated and  sustainable approaches  to the  management  of  nematode
populations through  genetic manipulation  of  host  plants,  design  of cropping systems,  development of  suppressive  chemicals  of  botanical origin, and biological control.
   

    There is  currently a  major effort  to shift  from nematicide-based nematode management  to alternative  methods.  Some 15 million pounds of 1,3-Dichloropropene have  been used  annually on  approximately  200,000 acres in  California.   Alternatives  to  this  approach  are  critical, particularly following  the April  13, 1990 suspension by the California  Department  of   Food  and   Agriculture  (CDFA)  of  the  use  of  1,3-Dichloropropene.   Since alternative approaches are more target-specific and system-specific, priorities will have to be established for research and development  in applied  nematology.    At  the  same  time,  strong linkages to  more basic  areas of  nematode biology must be fostered and maintained to promote the development of new technology.

 

Animals
   

    Of the nineteen Orders in the phylum Nematoda, seven contain nematodes that are  parasites or  associates of  invertebrates,  and  six  include species that are parasites of vertebrate animals.      Nematodes  are   reported  as   parasites  and  associates  of  many invertebrate  animals,   especially  in   the  Annelida,  Mollusca,  and Arthropoda.     In  some   cases,  the  invertebrate  functions  as  the intermediate  host  in  a  life-cycle  that  includes  parasitism  of  a vertebrate.   In other  cases,  the  invertebrate,  usually  an  insect, functions as  a vector  between vertebrate  hosts, or  the  nematode  is passively  transported  by  the  insect.    Several  interesting  plant-
parasitic nematodes fall into this latter group and, significantly, they are closely  related to  nematode species that are parasites of insects. A considerable  research effort  has been  applied toward using nematode parasites of  insects as  biological control agents, e.g., for mosquitos and blackflies (Maggenti, 1981).
   

    Some of  the nematode  associates of  insects are  important because they vector  bacteria that kill the insect.  The nematode invades (or is consumed by)  the insect,  and bacteria  are released  into  the  insect hemolymph.  When the insect is dead or near death, growth and subsequent development of  nematodes  occur  as  they  utilize  essential  steroids supplied by  the insect (Maggenti, 1981).  These nematodes are also used extensively in  the biological  control of  insects and are particularly effective against  those insects  that pass  through at  least one  life
stage in the soil. 
   

    Some 5,000  species of  nematodes are  estimated to  be parasites of vertebrate animals and humans.  These species are often characterized in a larger  group of  worm parasites  as helminths.  Nematode parasites of domestic vertebrate  animals are  managed  by  strategies  that  include control  of   secondary  hosts  or  vectors  and  the  use  of  chemical anthelminthics.  Helminth infections of wild animals are, of course, not managed,  except  by  attrition  of  infected  individuals.    As  human demography patterns  change in California, and throughout the world, the interface between  the ranges  and habitats of wild and domestic animals change and  overlap.   Consequently, the pattern of exposure of domestic animals to  helminth infections  is also  changing, and new associations continue to  be  reported;  for  example,  the  incidence  of  heartworm (Dirofilaria immitis) infection in dogs is currently increasing in California.
   

    In general,  the nematode parasites of California wildlife have only been studied  descriptively.   There  is  much  interesting  biology  to investigate.  Studies on the diplotriaenid nematode parasites of the air sacs of  California swallows indicate that the birds carry a substantial biomass of  nematode parasites  on their annual migrations.  The studies raise interesting  ecological questions  regarding flight efficiency and energetics, and  also provide models for considering the distribution of parasites.
   

    Both freshwater  and marine fish are subject to nematode infections. The impact  of the  infections on  the health  and longevity  of fish in nature is  generally unknown.  Frequently, nematodes are observed in the tissues of  fish purchased  by consumers.   The  nematodes  are  usually killed during cooking, but certainly the transfer of live fish parasites to humans  can occur  during consumption  of sashimi  and other raw fish products.   Generally, these  nematodes will  not establish  a permanent infection  in  humans,  but  they  may  cause  intestinal  disorders  in
attempting to do so.

 

Humans
   

    There are  other well-known examples of the transfer of nematodes to humans.  In most cases, the incidence of infection is relatively low due to regulatory inspection of food products, public education, and cooking of food.   An  example is trichinosis caused by the nematode Trichinella spiralis.   Humans become  infected by  Trichinella  by  eating  raw  or undercooked pork.
   

    The  nematode  parasites  of  humans  cause  a  variety  of  disease conditions and  symptoms, ranging  from lack  of  energy  and  vigor  to blindness and  malformations.   Pinworms, hookworms,  and roundworms are extremely common  intestinal helminth  infections of  humans; worldwide, roundworms are  probably the  most common,  but in  the  U.S.,  pinworms predominate.   Pinworm transmittal generally occurs through ingestion of
fecal-contaminated material, and infection occurs commonly in children.
   

    Other helminth  infections are vectored as filarial worms by insects such as  mosquitos, or  the filaria  may penetrate  directly through the skin from  water or  soil.   Filarial worms cause such diseases as river blindness (Onchocerca volvulus) and  elephantiasis which  are major  health problems  in  some third-world countries.   In  the United States, most helminth infections of humans  are controlled  by public  health programs, public education,
vector control,  intermediate host  control,  and  anthelminthic  drugs.  However, changing demographic patterns, including the immigration of new California residents  from third-world  countries, has  resulted in  the introduction  of   unfamiliar  helminth   infections  into   the  state. Frequently, the  faculty in  the Departments of Nematology are consulted by public health officials for identification of unfamiliar nematodes.

Forestry
   

    In California,  forestry is  important for  lumber, lumber products, and recreation.   Assessment  of the nematode impact on lumber yield and quality in  forestry, however,  is especially  difficult  since  harvest cycles may  be as  long  as  75  years,  and  land  geography  and  soil composition are  extremely diverse.   Nematode  impact may be via plant-parasitism; fungal  feeding by  nematodes, resulting  in destruction  of mycorrhizae essential  to  forest  growth;  and  above-ground  parasitic nematode  parasites   vectored  by  insects,  e.g.,  pinewood  nematode.
Preliminary investigations  have indicated that there is an entirely new array of nematode pests indigenous to California forests, in addition to known agricultural  pests, which feed on native forestry species.  It is important that  we begin  to accumulate baseline information on nematode pests of  California forests,  particularly since no such programs exist in the western United States.
   

    Recent plans  for shipment of raw lumber from Siberia for processing in  northern   California  may   assuage   public   concerns   regarding deforestation in  the U.S.;  however, analyses  of  Siberian  logs  have revealed a high potential for introduction of new pests.  Included among the pests  detected in  the  lumber  is  the  nematode,  Bursaphelenchus mucronatus, and  its associated  insect vectors.   B.  mucronatus is not reported from  the U.S. and is a close relative of the pinewood nematode that has  devastated forests  in Japan.   Although the pinewood nematode already occurs  in the  U.S., its  detection in  wood chips  exported to Scandinavia has  resulted in  a ban  on those products.  Certainly there are approaches  to mitigating  the potential  for  introduction  of  new pests, including debarking and fumigation of logs prior to shipment.

 

Rangeland
   

    With respect  to rangeland,  it is necessary to consider a different array of  nematodes than  those which  impact agriculture  and forestry. The nematode  problems  in  range  include  not  only  root  and  foliar parasites, but  also diseases  resulting  from  nematode-microorganismal associations.   The latter  aspect is  perhaps  best  illustrated  by  a nematode-bacterial association  in which  the grass forage becomes toxic to livestock;  this situation  constitutes a  major problem in Australia where large  numbers of  sheep and,  in some  areas,  entire  unattended
flocks have  been killed.   This  nematode (
Anguina  agrostis)  and  the associated  bacterium  (Clavibacter  sp.)  also  exist  in  northern California and  southern  Oregon.    Because  the  livestock  and  dairy industries constitute  a major  component of  agricultural production in California, and  because rangeland  and forests constitute approximately two-thirds  of  California  land  area,  the  significance  of  nematode problems on  rangeland, and  the need  for additional  research in  this area, becomes clear.
   

    Direct  control   techniques  used   for  nematode   management   in traditional  agriculture   are  most   likely  impractical   for   range situations.   The resolution  of range  and forest problems depends upon alternative technologies  which, in  turn, depend  on extensive baseline information    incorporating     nematode     characteristics,     plant characteristics, and environmental and edaphic factors.

 

Aquatic Systems
   

    Nematodes are,  by nature,  aquatic organisms.  It is estimated that about 50% of nematode species inhabit marine environments, although many of these  have yet  to be described and characterized.  The remainder of the species  inhabit soil  and freshwater.   In  the soil, their aquatic requirements are  satisfied by  inhabiting the  water films  around soil particles.   Parasitic nematodes  are biologically active when bathed in moisture films  supplied by  water in  the tissues or body fluids of the host.
   

    Analyses of  nematode communities  in aquatic  environments  reveals that the  incidence and  prevalence of  species in the community reflect the nature  and quality of the environment.  Not surprisingly, the types of species  present (and  the resultant  community structure)  differ in marine, brackish,  and freshwater  environments.    Recent  observations indicate  that   various  nematode   species  respond   differently   to degradation of  environmental quality.   Thus,  the degree and nature of change in  the community  structure  of  aquatic  nematodes  may  be  an excellent indicator of water quality or pollutant levels.
   

    Nematodes in  freshwater aquatic  systems also  serve as  a nutrient source for  invertebrates, small  vertebrates, and fungi.  The source of food for  these nematodes is primarily bacteria, but algae and fungi are also consumed.   A  considerable number  of plant-parasitic nematodes in aquatic systems  are associated  with higher plants, although the impact of their  parasitism on  those plants is generally unknown.  Preliminary research indicates  a potential for management of these nematodes in the biological control  of aquatic  weeds (Gerber  and Smart, 1987).  Genera that parasitize  crop plants  grown in immersed culture (e.g., rice) are well-characterized and are extremely important crop pests worldwide.

 

Marine nematodes

 

The marine environment provides habitat for an enormous diversity of nematodes, from surface, littoral and estuarine zones to the ocean depths.  One interesting group of deep sea nematodes are the Rhaptothyridae, which have no mouth and a very reduced alimentary tract.  The digestive tract is filled with symbiotic chemoaototrophic bacteria . A similar relationship exists in the mouthless genus Astomonema.

 

Urban
   

    We have only begun to understand the nature of the nematode problems and the  biology  of  nematodes  in  the  urban  environment.    As  the demographics, economy,  and  land-use  patterns  in  the  state  change, research in urban nematology will become increasingly important.  Within the scope  of urban nematology, are individual, societal, and commercial components.
   

    Individuals create  a personal  environment involving trees, shrubs, other annual  and perennial  ornamentals, ground  covers, lawns,  indoor plants, and  vegetable gardens.    Frequently,  the  plants  are  placed directly into  soil already  containing plant-parasitic nematodes from a previous planting;  nematodes may  also be  introduced with  the current planting in  associated soil and roots.  Obviously, the layout and plant species   involved   in   urban   gardening   are   not   selected   for incompatibilities of  their  associated  nematode  species.    Nematodes
introduced or  supported by  one plant  species can  be very damaging to neighboring or  companion plants.  Such individual plants have aesthetic value or  generate emotional  attachments and,  therefore, have  a  high individual value.   Nematode  damage  to  lawns  and  groundcover  often results in  poor growth,  bare spots, and chlorotic appearance which the individual attempts  to remedy  by increasing  the application  of  both fertilizer and water.
   

    Societal components  of urban nematology include nematode effects in parks, recreation  areas, landscape  plantings, highways,  schools, etc. Nematode problems of turf constitute an area of concern for recreational and landscape  industries.   Again, all  these plantings have tremendous value in  terms of  management input  and public perception.  Damage and lack of  plant vigor  due to  nematode parasitism  result  in  unsightly
areas, increased  weed competition,  and increased  water and fertilizer usage.   Commercial components  of urban nematology include the costs of nematode management-or  of lack  of nematode  management-by producers of plants in  nurseries, turf  farms, and  greenhouses.   The economics  of urban nematology also include the activities of agrichemical industries, wholesalers, retailers, advisors and consultants.
   

    As with  other areas  of nematology, we should consider the positive and negative  aspects of  nematodes in the urban environment.  Nematodes can act  primarily as  pests or  pathogens, or  can also  participate in decomposer food  chains by using dead organisms as sources of carbon and energy.   Often, the  most productive  soils contain high populations of non-parasitic nematodes  participating in  the cycling  of minerals  and nutrients.   In fact, high population levels of such nematodes may be an indicator of productive soils with a healthy biological status.
   

    As knowledge  of nematode problems in urban situations increases, so will the  potential to design spatial patterns and sequences of backyard gardens  to   provide  the  ultimate  in  nematode  management  options.  Planting sequences and companion plantings can be selected with nematode susceptibilities, or  even allelopathic effects, in mind.  Enhancing the biological activity  of soil to improve nutrient and moisture status, as
well as the biological antagonism of nematodes, can be accomplished with minimal cost  through the  intensive efforts of avid gardeners in small-scale areas.   The  backyard garden offers the possibility of creating a microcosmic "sustainable agriculture" situation.  Here is an environment that the homeowner can control and manipulate experimentally without the overriding  concerns   of  marketability   or  profitability  of  crops.
Experimentation with soil manipulation, incorporation of organic matter, companion plants,  sequences, and  spatial patterns  of plants  may be a rich source  of innovations  and provide  important biological insights. Biological antagonists  of  nematodes  can  be  introduced  and  tested; beneficial nematodes  for the  biocontrol of  insect pests  can also  be introduced  and   conserved  in  the  system.    The  opportunities  for observation, formulation,  and testing  of hypotheses  are more  readily provided by  the backyard  garden than  by commercial  agriculture.   To capitalize on  those opportunities,  many  research  questions  must  be addressed; these include plant species compatibilities (in terms of host status or  effects on  various nematode  species), and  knowledge of the nematodes present  at a  given site  and their  virulence on  ornamental plants.     Conceivably,  increased   regulation  may  be  required  for industries that  impact urban  nematology in  order to minimize nematode
introductions and spread in the urban environment.

 

References:

Cobb, N.A. 1914. North American free-living fresh-water nematodes. Reprinted from Trans. Am. Micr. Soc. 33 in Cobb, N.A. Contributions to a Science of Nematology.

Giere, O. 1995. The bacterial endosymbiosis of the gutless nematode, Astomonema southwardorum - structural aspects.  J of Mar. Biol Assoc of the UK 75:153-164.

Platt, H. M. 1994, Foreword. In The Phylogenetic Systematics of Free-living Nematodes, S. Lorenzen, ed., pp.i-ii, The Ray Society, London. 383p.

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