.WHO/VBC/80.785
VBC/BCDS/80.11
WORLD HEALTH ORGANIZATION
ORGANISATION MONDIALE DE LA SANTE
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ENGLISH ONLY
DATA SHEET ON THE BIOLOGICAL CONTROL AGENT(l)
Isomermis lairdi Mondet, Poinar and Bernadou 1977
Isomermis lairdi is a nematode parasite of larval, pupal and adult stages of blackflies,
especially Simulium damnosum s.l. (Theobald 1903), a species complex whose blood-sucking
females are responsible for the transmission of human onchocerciasis ("river blindness")
West Africa.
~n
I. lairdi bionomics are very well suited to ensure the long term persistence of this
mermithid parasite in temporary rivers constituting the breeding places of certain important
vectors of onchocerciasis. They would not facilitate the operational use of this parasite as
a biological agent for the control of these vectors.
l.
Identification and synonymy
Mermithidae
Nematoda:
Isomermis lairdi (Mondet, Poinar and Bernadou 1977) (1977-4).
2.
Origin (1977-4, 1979-2, 1979-4, 1980-l)
Jl. lairdi has been found parasitizing larval stages of Simulium adersi, S. alcocki, S.
cervicornutum, S. unicornutum and S. vorax in temporary rivers in the savanna-of West Africa.
Larvae and adults of S. sirbanum a~d S. damnosum s.s. (savanna species belonging to the
~· damnosum complex) ~re also parasitized. The p~e;ence of I. lairdi in other species of this
complex remains to be confirmed.
3.
Natural geographical distribution
Small to large temporary streams in Guinean and Sudanese savanna with a flow rate that may
3
exceed 1000 m /sec.
All known foci of paras~t~sm in the Ivory Coast and Mali (West Africa) are now under
insecticide treatments by the Onchocerciasis Control Programme in the Volta River Basin Area.
I. lairdi is the most common mermithid parasite of S. damnosum s.l. in the Ivory Coast. The
;pecies is probably present in other West African c~untries (19l9~4).
4.
Biological characteristics
4.1 ·Description (1977-4, 1979-4)
I. lairdi is a nematode endoparasite of blackflies. The free-living stages are aquatic.
The mouth is terminal. Adult females range in length from 8.7 to 18.6 mm (average 14.5 mm),
with a strongly S-shaped vagina and no vulval lips. Males range in length from 5.2 to
14.4 mm (average 9.9 mm) and have two spicules of medium length (1.7-2.8 times diameter 'of the
body at the genital opening). The amphids are circular and medium-sized, with round amphidial
openings.
(l)
.
Informat~on
document produced by t h e WHO
. . .
D~v~s~on
o f v ector
.
B~o 1 ogy
an d contro 1
July 1980
I
l
The issue of this document does not constitute
Ce document ne constitue pas une publiCation.
formal publication.
It should not be reviewed.
11 ne doit faire l'objet d'aucun compte rendu ou
abstracted or quoted without the agreement of
resume ni d'aucune citation sans J'autorisation de
the World Health Organization.
Authors alone
!'Organisation Mondiale de Ia Sante. Les opinions
are responsible for v1ews expressed in signed
exprimees dans les articles signes n'engagent
que leurs auteurs.
articles.
WHO/VBC/80.785
VBC/BCDS/80.11
page 2
Parasitic and post-paras1t1c stages are distinguished by a short pointed tail-appendage.
Eggs are almost spherical (diameter of 60 x 65,~m). Preparasites measure between 250 and
300~;
their tail appendage is curved.
4.2
Life cycle
After fertilization, free-living females lay their eggs in the sand of the river bottom.
The eggs are covered with a slightly adhesive substance which attaches them to the substrate.
In the laboratory, eggs hatch between 5 and 8 days after oviposition. The first moult takes
place inside the egg. Preparasites are mobile and live between two and three days. Preparasites caught by host mouth fans penetrate the simuliid larvae in the cephalic region. The
parasitic cycle is completed entirely in the larval host if the preparasite penetrates an early
instar, or in the larval, pupal and adult stages if the penetration takes place in a late stage
blackfly larva. In either case, the parasitic cyle lasts between 10 and 16 days. Adults mate
after the two last simultaneous moults, 3-4 days after emerging from the host. Oviposition
takes place 2 to 5 days after mating. The life cycle is completed in the laboratory in about
one month. Females can lay between 2000 and 3000 eggs with almost 100% being viable (1977-3,
1979-4).
In nature, the life-cycle can be much longer. After the rivers stop flowing, females
apparently die soon after oviposition but males may survive for months. Eggs await eclosion
until the rivers begin flowing again (5 to 8 months). The chemical composition of the water
stagnating in the river bed may be the reason for this quiescence. Hatching of both blackfly
and nematode eggs commences when the river resumes flowing (1979-1).
4.3
Host-parasite relationship
Pupation is inhibited in blackfly larvae infected at an early instar because the host
cannot develop histoblasts and the fat body is completely depleted. Larvae infected as late
instars pupate normally. Adult longevity in parasitized S. damnosum females is considerably
reduced; however they still can take at least one blood ;eal, which may be necessary for the
mermithids to mature inside the adult flies (1976-1, 1977-2, 1979-4).
I. lairdi-infected blackflies cannot transreit onchocerciasis because the development time
for the Onchocerca volvulus microfilariae is longer than that for the mermithid. Moreover,
98% of the parasitized females have atrophied ovaries incapable of egg maturation (1976-1,
1976-2, 1977-1). Effects of parasitism on male blackflies are unknown but in all cases, the
host (larvae, males or females) die when the mermithid escapes. The sex of the mermithid is
a function of the number of parasites per host and of the sex of the host. Males of S.
damnosum produce essentially male parasites. However in females of ~· damnosum parasite sexratios range from 6.6 to 100% males when the number of parasites per host ranges from 1 to 5
respectively.
Female blackflies with parasite(s) ready to emerge return to the breeding sites where
uninfected females lay their eggs. The behaviour of these infected females allows
naintenance and sometimes even dissemination of parasite foci (1979-2, 1980-1).
4.4
Susceptibility to pesticides
Regular insecticide treatment of the rivers drastically reduces the populations of
I. lairdi. This is due to the absence of the host Simulium larvae necessary for the development
of the nematode (1979-1, 1979-3).
Some observations (B. Mondet, personal communication, 1980) indicated a high tolerance of
I. lairdi postparasites to temephos (Abate) in concentrations lethal to S. damnosum larvae.
No tests were made with preparasites.
WHO/VBC/80. 785
VBC/BCDS/80.11
page 3
5.
Effectiveness against target organ1sms
5.1
Under natural conditions (1976-1, 1978-l, 1979-l)
Parasitism occurs in temporary rivers during the rainy season. Thus blackflies are only
infected when these rivers are flowing (from about May to December). During this period, the
first generations of S. damnosum females are parasitized at a high rate due to the hatching of
the mermithid eggs which had spent the dry season in quiescence. The next blackfly generation
is similarly infected. There are three cycles of !· lairdi during the rainy season (B. Mondet,
personal communication, 1980) during which 10-12 generations of blackflies take place.
Parasitism can affect 80% of the nulliparous females and 30% of the total blackfly population
(i.e. parous and nulliparous females).
Levels of parasitism in females and larvae are not
necessarily the same because of the ability of females to disperse from their native breeding
sites; also access to rivers is often difficult during the wet season making sampling of
larval populations difficult.
In some cases the levels of parasitism in larvae were estimated
at 30 to 40/[,. These percentages included only visible infected larvae which died before
pupation and did not include infected pupae.
5.2
Under laboratory conditions (1977-3)
In the laboratory, 30% of S. damnosum larvae were infected at a dosage rate of 12 preparasites per host.
But these ~esults under the conditions imposed (tanks with magnetic
stirrers) cannot predict what might happen in nature.
Infections in still water are very low
and do not simulate natural life conditions.
6.
Effectiveness against non-target organisms
There have been no trials to infect non-target organisms with I. lairdi, but, in nature,
mermithids found in these insects (particularly Chironomids and Eph~meroptera) belong to other
genera (Spiculimermis, Capitomermis, Hydromermis, etc.) (B. Mondet, personal communication,
1980).
7.
Production and Stability (1978-1, 1980-2)
No efforts were made to cultivate I. lairdi in vitro.
In v1vo production is not yet
possible because a suitable host rearing system has yet to be developed.
It is feasible to
rear S. damnosum from eggs to adults but the remaining part of the cycle (mating, blood-sucking,
ovipo~ition) is currently impossible under laboratory conditions.
Possibly I. lairdi could be reared in an alternate blackfly host which 1s more easily
colonized than-S. damnosum, but none are yet available.
At present collection, maintenance and storage of the free-living and egg stages of the
nematode is the only method by which the nematodes can be accrued for use in field trials.
Free-living stages of !· lairdi are easy to maintain (at least in small quantitites and
living material might be possible in two ways:
(l) adults are cold resistant (one
month at 4°C with no development and no mortality);
their development is very slow.if
sexually segregated; if mating takes place after a one month's isolation females lay viable
eggs after two days and (2) eggs seem also to be cold-resistant (1977-3).
~torage of
Life cycle discrepancies observed between I. lairdi and S. damnosum s.l. would not
facilitate the use of this parasite as a biological agent for the control-of blackfly larvae 1n
West Africa.
8.
Formulation and specifications
None.
WHO/VBC/80.785
VBC/BCDS/80.11
page 4
9.
References
1976
1976-1
Mondet, B., Pendriez, B. and Bernadou, J. (1976). Etude du paras1t1sm des Simulies
(Diptera: Simuliidae) par des Mermithidae (Nematoda) en Afrique de l'Ouest.
1. Observations preliminaires sur un cours d'eau de savane. Cah. ORSTOM, ser. Ent.
med. Parasit., 14: 141-149.
1976-2
Mondet, B. and Poinar, Jr., G. 0. (1976). Recent developments on Mermithid paras1t1sm
of Simulium damnosum in West Africa. Proc. Int. Colloq. Invert. Path., Kingston,
Canad·a: pp. 232-235.
1977
1977-1
Baccam, D. (1977). Biologie et ecologie de Simulium damnosum Theobald, 1903 (Diptera:
Simuliidae). Recherches sur le fonctionnement ovarien et sur !'influence des
Mermithidae parasites (Nematoda). These 3eme cycle, No. 1994. Univ. Toulouse, France.
99 pages.
1977-2
Berl, D., Bernadou, J. and Vidal, G. (1977). Etude de la survie en laboratoire d'une
population de femelles gorgees de Simulium damnosum s.l. Theobald, 1903, parasitees
par Isomermis sp. Coman, 1953 et Onchocerca volvulus-L;uckart, 1893. OCCGE/IRO
mimeographed d~ument No. 33/0ncho/Rap/77, 8 pages.
1977-3
Mondet, B., Berl, D. and Bernadou, J. (1977). Etude du parasitisme des Simulies
(Diptera: Simuliidae) par des Mermithidae (Nematoda) en Afrique de l'Ouest. III.
Elevage de Isomermis sp. et infestation en laboratoire de Simulium damnosum s.l ..
Cah. ORSTOM, ser. Ent. med. Parasitol., 15: 265-269.
1977-4
Mondet, B., Poinar, Jr., G. 0. and Bernadou, J. (1977). Etude du paras1t1sme des
Simulies (Diptera: Simuliidae) par des Mermithidae (Nematoda) en Afrique de l'Ouest.
IV. Description de Isomermis lairdi ~· sp., parasite de Simulium damnosum. Can. J.
~·· 55:
2011-2017.
1978
1978-1
Mondet, B. (1978). Les Mermithidae parasites de Simulies comme agents de lutte
biologique en Afrique de l'Ouest. Int. Gong. Parasitol., ICOPA IV., Varsovie, Poland,
19-26 August 1978.
1979
1979-1
Mondet, B., Elouard, J. M. and Bernadou, J. (1979). Mermithidae parasites de
Simulies. Evolution du parasitisme chez les femelles de Simulium damnosum s.l. et
influence de traitements insecticides. OCCGE/IRO mimeographed document No.-6/0ncho/
Rap/79, 19 pages.
1979-2
Mondet, B., Bellec, C., Hebrard, G. and Prud'hom, J. M. (1979). Relations hateparasite chez les adultes de Simulium damnosum ~-·• determinisme du sexe des
parasites et comportement de vol des adultes parasites. OCCGE/IRO mimeographed
document No. 7/0ncho/Rap/79, 26 pages.
1979-3
Mondet, B. (1979). Etude des possibilites d'utilisation des nematodes mermithidae
pour une lutte biologique contre les simulies vectrices de l'onchocercose en Afrique
de l'Ouest. OCCGE/IRO mimeographed document No. 8/0ncho/Rap/79, 38 pages.
1979-4
Poinar, Jr., G. 0. (1979). "Nematodes for Biological Control of Insects".
Boca Raton, Florida, USA, 277 pages.
CRC Press,
WHO/VBC/80/785
VBC/BCDS/80.11
page 5
1980
1980-1
Mondet, B., Prud'hom, J. M., Bellec, C. and Hebrard, G. (1980). Etude du parasitisme
des Simulies (Diptera: Simuliidae) par des Mermithidae (Nematoda) en Afrique de
l'Ouest. V. Croissance et sex-ratio de deux especes parasites d'adultes de
Simulium damnosum ~·!·• Cah. ORSTOM, ser. Ent. med. Parasitol, 18: 49-57.
1980-2
Poinar, Jr., G. 0. (1980). Mermithid nematodes of Blackflies. In: "Blackflies. The
future for biological methods in integrated control". Public CRDI (Laird, M., ed.)
Academic Press, London, New York (in press).
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