chapter 8. nematode infections - kocwcontents.kocw.net/kocw/document/2014/kangwon/kimsamgyu/... ·...
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Chapter 8. Nematode infections
■ Nematodes, commonly referred to as roundworms, eelworms, or threadworms, are
translucent, non-segmented animals ranging between 0.1mm and several meters in
length
< Diversity of nematode morphology and lifestyle>
■ Many nematode species are associated with insects and the types of relationship
are...
• Facultative parasitism
; some nematodes are able to infect insects as well as having the ability to
complete their life cycle as free-living
• Obligate parasitism (=Pathogens)
; cannot complete life cycle without living insect (usually in the hemocoel)
< Plant parasitic and entomopathogenic nematodes >
http://www.youtube.com/watch?v=D7r1S6-op8E&feature=related
■ Insects serve as vectors or intermediate hosts for a number of nematode
parasites of vertebrates
• mosquitoes transmit Dirofilaria immitis, causal agent of the dog heartworm
< Heartworm infected dog heart >
• black flies transmit Onchocerca volvulus, causal agent of human river blindness
(=onchocerciasis)
< Human onchocerciasis (=river blindness) and causative agent, Onchocerca
volvulus filarial nematode, and black fly vector >
■ Life cycles
• egg
• juvenile (usually have 4 molts J1, J2, J3, J4)
⇨ some insect nematodes have environmentally resistance stage called "dauer
juvenile" (3rd stage juvenile) which ensheathed in the second-stage cuticle
• adult
■ Mode of infection
• insect-parasitic nematodes parasitize their hosts by directly penetrating thru the
cuticle into the hemocoel or by entering thru natural openings (spiracles, mouth,
anus)
• nematodes eggs can be ingested by susceptible hosts and hatched juveniles
penetrate the midgut and enter into the hemocoel
8-1. Mermithid nematodes (Nematoda: Mermithidae)
• infect the early stage of insect hosts (mainly aquatic insects)
• directly penetrate the host's integument or enter via natural openings
(such as spiracle or anus)
• do not reach adulthood in the host's hemocoel
➪ have parasitic (juveniles) and free-living stages (adult)
➪ they emerge from the host as postparasitic juveniles
< Generalized life cycle of mermithid nematodes and emerging postparasitic
juveniles from mosquito larva >
• mermithid nematodes grow until they occupy most of the larval hemocoel,
obtaining all of their nutritional requirements within the host to complete the
parasitic stage of their life cycle
• then the nematode may exit thru the host cuticle, resulting in host death, or
remain in the host thru pupation and into the adult fly
• mermithid infections carrying thru to the adult fly frequently result in structural
and behavioral host feminization
Mermithids emerging
from mosquito larvae
➪ feminization can either be complete or partial (=intersex)
➪ feminized adult black flies exhibit characteristic uninfected gravid female
oviposition behaviors by travelling to upstream oviposition sites and depositing
their mermithids into the water
8-2. Entomopathogenic nematodes (Steinernematidae, Heterorhabditidae)
• obligate pathogens (but recently known that some are facultative scavenger as a
alternative survival strategy)
• recovered from soils throughout the world and their distribution may be primarily
limited by the availability of susceptible hosts
• they can kill the hosts within 1-4 days (usually within 2 days)
• have mutualistic symbiotic bacteria in their guts
- Steinernematid nematodes - Xenorhabdus bacteria
- Heterorhabditid nematodes - Photorhabdus bacteria
• the only stage that survives outside of a host is the non-feeding, non-developing
infective juvenile (IJ) that contain symbiotic bacteria
• in Steinernema species, the symbiotic bacteria are contained inside a
specialized intestinal vesicle (=receptacle), whereas the bacteria of
Heterorhabditis colonize the entire intestine (anterior to mid intestine)
< Location of symbiotic bacteria in intestines of their respective nematode hosts
(A, Steinernema; B, Heterorhabditis) >
• the bacteria usually persist in a quiescent state within the nematode
; some bacteria may grow inside the nematodes
< Xenorhabdus bovienii in Steinernema oregonense >
< Xenorhabdus hominickii in Steinernema monticolum >
1) EPNs and symbiotic bacteria
• nematodes provide symbiotic bacteria with
- shelter
- nutrition
- transportation
• symbiotic bacteria provide nematode with
- toxins to kill the insect host rapidly
➪ some Steinernema spp. can kill the insect host without help of symbiotic
bacteria, but it is less effective
- antimicrobial components (e.g., antibiotics) to protect the cadaver from
contamination by opportunistic organisms including insect scavengers as
well as other common gut inhabiting bacteria including Salmonella,
Pasteurella, and Xanthomonas spp.
- food (bacterivore)
※ All Xenorhabdus and Photorhabdus spp. are symbionts of EPNs only except
Photorhabdus asymbiotica which is isolated from human wounds in the
absence of nematode vectors
< Skin abscess caused by Photorhabdus asymbiotica >
2) Generalized life cycle of EPNs
• the IJs actively search (cruiser) or sit-and-wait (ambusher) for a suitable insect
host and gain entry into the hemocoel by invading through natural openings
(mouth, spiracles, and anus) or, in Heterorhabditis, enter the hemocoel directly
through the insect’s integument
• upon entry into the host, the IJs resume developmental cycle (=recovery) and
liberate their bacterial symbionts
➪ Xenorhabdus are released from the nematodes (Steinernema spp.) by
defecation whereas Photorhabdus exit through the mouth (regurgitation) of
Heterorhabditis spp.
• the bacterial symbionts multiply in the hemolymph and the guts of the host,
producing toxins and exoenzymes that result in septicemia and bioconversion
(=metabolize) of the insect cadaver suitable for nematode feeding
• near the end of bacterial proliferation, the symbionts produce a variety of
antimicrobial compounds that protect the cadaver from colonization by other
organisms
➪ include antibiotics that are active against other bacteria, fungi, and yeasts
and bacteriocins (e.g., xenorhabdicin and lumicins) that are active against
bacteria closely related to Photorhabdus
• the developing nematodes feed on the bacteria and bioconverted host tissues
and reproduce in the cadaver for 1 ~ 3 generations
• once the food resources in the cadaver are exhausted, the nematodes develop
a new generation of IJs that recruit few bacterial cells and emerge from the
host cadaver in search of a new susceptible host
3) Endotokia matricida
• inside + birth + mother + kill
➪ juveniles hatch inside the uterus and feeding on the mother's tissues
causing her death
< Endotokia matricida of Heterorhabditis bacteriophora >
< Endotokia matricida of Steinernema carpocapsae >
• endotokia matricida usually observed in the starved females with not
enough energy to push their eggs thru the reproductive opening
• its function is to provide optimal conditions for offspring development when
the environmental conditions are becoming detrimental
➪ the intra-uterine juvenile development accompanied by the consumption
of the maternal tissues and body content is a favorable and highly
efficient way to raise well-fed IJ, particularly under conditions of nutritive
depletion
4) Steinernematid nematodes (Rhabditida: Steinernematidae)
• both male and female nematodes must enter a susceptible host for
reproduction (sexual)
➪ exception: Steinernema hermaphroditum - has hermaphrodite (as in
Heterorhabditis) in the first generation
• IJs enter a susceptible host via natural openings (do not directly penetrate the
cuticle)
• each species of Steinernema has only single species of bacterial symbiont
➪ Steinernema carpocapsae - Xenorhabdus nematophila
➪ Steinernema monticolum - Xenorhabdus hominickii
• single Xenorhabdus sp. can colonize multiple nematode species
➪ Xenorhabdus bovienii can colonize 8 Steinernema spp.
• signs
- brown to dark brown
- getting flaccid (for some cases, cadavers getting hardened)
- cadaver do not putrefied (decay, rot) because of antibiotics
- characteristic smells (depend on Xenorhabdus spp.)
< Galleria mellonella larvae infected with Steinernema sp. >
< Flaccid cadaver caused by Steinernema jollietti >
EPN from bait insect cadaver
• Steinernema nematodes can be cultured in an artificial medium
< Lipid agar medium for Steinernema nematode in vitro culture >
⇒ this doesn't mean that they are not obligate parasites
5) Heterorhabditid nematodes (Rhabditida: Heterorhabditidae)
• all infective juveniles become hermaphrodites
➪ only single individual is required to infect a new host for successful
reproduction
• IJs infect a susceptible host by direct penetration of the cuticle
< Typical discoloration and bloating of Heterorhabditis infected larvae >
• signs
- cadaver turn to reddish dark brown
- no typical smells
- cadaver usually bloated
6) EPNs and biological control
• many EPNs have been used as biological control agent for various insect
pests including lepidopteran larvae, mole crickets, and fly larvae
• few EPNs have a broad host range (e.g., Coleoptera, Lepidoptera, Diptera
larvae)
➪ Steinernema carpocapsae, Steinernema riobrave, Heterorhabditis
bacteriophora
• some EPNs have a restricted host range
➪ Steinernema scapterisci is specialized to cricket and mole crickets
➪ Steinernema feltiae is specialized to fly larvae
< Fungus gnat larvae infected by S. feltiae >
➪ Steinernema kushidai and Steinernema scarabaei are restricted to scarab
larvae
; other species such as Steinernema glaseri and Heterorhabditis
bacteriophora also usually infect scarab larvae in natural environment
• Host finding strategies
; EPNs are positively attracted to carbon dioxide
- ambusher(sit and wait)
- cruiser
• Effect of temperature
- 10∼15℃: getting sluggish
- 30∼40℃: inactive
- 20∼30℃: optimum for most species
• Application of EPNs in the field
- when EPNs applied onto the soil surface by using conventional straying
equipment, losses can reach 50% within hours of application due primarily
to UV radiation and desiccation
- generally only 1% of the original inoculum survives after 1∼6 weeks
- it is important that EPN should only be applied when susceptible stages of
the target pest are present