5
Mosquito vectors of Diroflaria immitis in southwestern Ontario J. EKNST AND J. 0. D. SLOC'OMH~ Dc>purtrnctzt oj Ptrtholo~!., Orzttrr-ro Vrter-iritrr~ College, Urzr \*c.r..\it~ of Grrc~ll~h. G'rrc~ll~lz. Orzt., Ccrrztrdcr N 1 G 2 WI Keceived September 2 1 . 1982' ERNST, J., and J. 0. D. SLO(:OMBE. 1984. Mosquito vectors of Dir-ofi'ltrr-itr irnrniti.~ in southwestern Ontario. Can. J. Zool. 62: 212-216. Over 3000 mosquitoes were collected in southwestern Ontario in 1980 and 198 1. using a stable trap housing a dog naturally infected with Dirofilaritr irnrniti,s. The following were collected: 26 Atzo~~lrc~le.~ prrrzc~ti~~t~rrrzi.s ( IOO'X blood fed), 36 Aetlrs trisc~ricrtu.~ (94.4. C/r blood fed), 38 Ardes .stic.tic*zr.s ( I 00% blood fcd ). 44 Aec1t.s tlor.sti1i.s (97.7% blood fed). 73 Anophrlrs bivrlkrri ( 100% blood fed), 16 1 A~YI~.~ .stirn~rltrrz.s -,fi'tthii (97.5% blood fed). 5 10 Actles tri\,ittcrtrrs (XO.XC/cl blood fed), 569 Mc~nsorzicr pc~rturhcrri.~ (93.3% blood fed). 739 Art1c.s \v.vtrrzs (95.4'2 blood fed) and 1 1 14 Cu1c.v 11il1icrz.s-rc~.stutrrz,s (6 1 .6% blood fed). After holding the blood-fed mosquitoes at 26OC for 2 weeks. 1708 were identified and dissected, and the mouthparts were observed for infective third-stage larvae of Dirc?ji'ltrritr imrnitis. Infective larvae were found in 26 Aedc..s .stic.tic.us (96.3% positive), 33 Ardes dor.st~1i.s (89.2% positive). 406 Ac~c1e.s \,evtrrl.s (88.XC;/( positive), 177 Aetlos tri\~itttrtu.s (68.9% positive), 38 Culev pipic.n.s-re.st~rcrrz.s (7.4% positive). and 4 Ardrs .stir~~rrltrrz.s-,fitc.liii (4. 1 ' % positive). Acdes \vc~.vcrrz.s and Ac~t1e.s tri\,ittcrtu,s were considered the major vectors because they were collected in large numbers. fcd readily on dogs. and allowed development of D . irnrnitis to the infective stage. This is the first report of Artlrs rlor.str1i.s allowing developnient of D . in1rniti.s to the infective stage. ERNST, J., et J. 0 . D. SLO(:OMBE. 1984. Mosquito vectors of Dirc?fi'lcrritr irr~rniti.~ in southwestern Ontario. Can. J. Zool. 62: 212-216. Plus de 3000 moustiques ont ete recoltcs en I980 et 1981 dans le Sud-Ouest de I'Ontario, grrlce h I'utilisation d'unc cage en bois ou un chien portant une infection naturelle de Dir~filtrritr irrirniti.~ servait d'apprlt: 26 Arzol~helt~.~ punc~til~c~nrii.~ ( 100% nourris de sang), 36 Artlrs tri.sc~ritrtu.s (94.4% nourris de sang). 38 Aedes .stic,tic.~r.s ( 100'i nourris de sang). 44 Ardc..s dorstr1i.s (97.7% nourris de sang). 73 Anophrlrs \t,trlkeri ( 100% nourris de sang). 161 Aetli~s .~irn~rl(rr~.s-ji'tc+hii (97,S'F nourris de sang), 5 10 Ac~1e.s tri~itttrtu.~ (80.8% nourris de sang). 569 Mtrn.sorzitr prvt~rrhtrn,~ (93.3C/( nourris de sang). 739 Ac)de.s \vr.t-trn.s (95,4% nourris de sang) et 11 14 Cu1r.v pi11irri.s-rc.stutr~z.s (61-6% nourris de sang). Lcs moustiques nourris de sang ont kt@ gardks 5 26°C pour 2 semaines; 1708 d'entre eux ont ete identifies et dissequcs ct leurs pieces buccales ont etC examinees afin d'y chercher des larves de troisieme stade de Dirc?fi'ltrritr irn?riiti.s. Des larves infectieuses ont etd trouvces chez 26 Ardes stic-tic.u.s (96,3% des insectes examines), 33 Aedc.s dorstr1i.s (89.2% des insectes examines). 406 Aetles \lc<vcrrz.s (88.8% des insectes examines). 177 Aedrs tri\,ittertu.s (68,9'k des insectes examinks). 38 C'lrle..~ ~~il~ic)rz.s - r-c~.st~rtr~z.s (7-4% des insectes examines) et 4 Aee1c.s .stimulun.s-ji'tc.hii (4.1 C/r des insectes examines). Aetles \'c<vtrrz.s et Aetles tri\~itttrtu.s sont considcres comme les principaux vecteurs parce qu'ils ont ete rccoltcs en grands nonibres, qu'ils se nourrissent volontiers de sang de chien et que D . immitis y atteint le stade infectieux. Le developpenient jusqu'au stade infectieux de I). irnnzitis chez A . dorsrr1i.s est signale ici pour la premiere fois. [Traduit par le journal 1 Introduction Dirofilariu immitis Leidy, prevalent mainly in dogs, is a mosquito-borne filarial nematode which causes canine heart- worm disease. This disease has been reported with increasing frequency in dogs which have not previously left Canada (Slocombe 1978; Slocombe and McMillan 1979, 1980, 198 1 ). In Canada two foci of infection exist, one in the Winnipeg. Manitoba, area and the other in southwestern Ontario. The greatest spread of infection has occurred in southwestern Ontario. The vectors of heartworm in Canada are unknown. How- ever, of species of mosquitoes reported to be potential vectors (Ludlam et ul. 1970; Crans and Feldlaufer 1974; Christensen 1977: Todaro et ul. 1977: Rogers and Newson 1 979), 24 occur in southwestern Ontario (Wood et cd. 1979). The objective of this study was to identify species of mosquitoes in southwestern Ontario which may be vectors of D. immitis. Materials and methods Mosquitoes were collected in the Windsor, Ontario. area in 1980 and 198 1 using a portable stable trap as described by Magoon ( 1953) and modified to accommodate a dog (Ernst 1982). The trap was made 'Revised manuscript received September 26, 1983. from plywood with most of the upper portion of the sides of the trap consisting of mosquito screening. Entrance baftles, made to trap en- tering mosquitoes, were small. V-shaped, longitudinal openings ta- pering from outside inwards. The baftles were placed horizontally on three sides of the trap directly beneath the screening. Three dogs naturally infected with D . immitis were used as bait. Two dogs were used in 1980 and only the third was used in 198 1. The number of microfilariae per millilitre of peripheral blood of the dogs was estimated prior to each collection as described by Church et al. ( 1976). For each collection, the dog was restrained in a wire cage which was placed within the trap from early evening to the following midmorning. Trapped niosquitoes were aspirated by a sucking tube and transferred to Plexiglas cages (30.5 cm x 30.5 cm x 30.5 cm). Mosquitoes which had not fed on the heartworm-infected dogs were identified to species using the keys of Wood et trl. (1979) and dis- carded. Blood-fed mosquitoes in the Plexiglas cages were supplied with cotton batting saturated with 10% sucrose and maintained at 26 ? 1°C and 80 ? 5% relative humidity with a 16-h photoperiod. The 10% sucrose was replaced every 2nd day and the humidity wicks were replaced once a week. After 4 to 9 days of incubation, some mosquitoes were anaes- thetized by placing them at 4OC for 30 min and identified. and the Malpighian tubules were dissected in physiological saline on micro- scope slides to detect first- ( L I ) and second-stage larvae (L?) of D . irnrnitis. From 2 to 4 weeks after incubation, the remainder of mosquitoes were dissected and the mouthparts, head, thorax, and tubules were examined for third-stage larvae (L3).When large num- Can. J. Zool. Downloaded from www.nrcresearchpress.com by SAVANNAHRIVNATLABBF on 11/14/14 For personal use only.

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Page 1: Mosquito vectors of               Dirofilaria immitis               in southwestern Ontario

Mosquito vectors of Diroflaria immitis in southwestern Ontario

J . EKNST AND J . 0 . D. SLOC'OMH~ Dc>purtrnctzt o j Ptrtholo~!., Orzttrr-ro Vrter-iritrr~ College, Urzr \*c.r..\it~ of Grrc~ll~h. G'rrc~ll~lz. Orzt., Ccrrztrdcr N 1 G 2 WI

Keceived September 2 1 . 1982'

ERNST, J . , and J . 0. D. SLO(:OMBE. 1984. Mosquito vectors of Dir-ofi'ltrr-itr irnrniti.~ in southwestern Ontario. Can. J . Zool. 62: 212-216.

Over 3000 mosquitoes were collected in southwestern Ontario in 1980 and 198 1 . using a stable trap housing a dog naturally infected with Dirofilaritr irnrniti,s. The following were collected: 26 Atzo~~lrc~le.~ prrrzc~ti~~t~rrrzi.s ( IOO'X blood fed), 36 Aetlrs trisc~ricrtu.~ (94.4. C/r blood fed), 38 Ardes .stic.tic*zr.s ( I 00% blood fcd ). 44 Aec1t.s tlor.sti1i.s (97.7% blood fed). 73 Anophrlrs bivrlkrri ( 100% blood fed), 16 1 A ~ Y I ~ . ~ .stirn~rltrrz.s -,fi'tthii (97.5% blood fed). 5 10 Actles tri\,ittcrtrrs (XO.XC/cl blood fed), 569 Mc~nsorzicr pc~rturhcrri.~ (93.3% blood fed). 739 Art1c.s \v.vtrrzs (95.4'2 blood fed) and 1 1 14 Cu1c.v 11il1icrz.s-rc~.stutrrz,s (6 1 .6% blood fed). After holding the blood-fed mosquitoes at 26OC for 2 weeks. 1708 were identified and dissected, and the mouthparts were observed for infective third-stage larvae of Dirc?ji'ltrritr imrnitis. Infective larvae were found in 26 Aedc..s .stic.tic.us (96.3% positive), 33 Ardes dor.st~1i.s (89.2% positive). 406 Ac~c1e.s \,evtrrl.s (88.XC;/( positive), 177 Aetlos tri\~itttrtu.s (68.9% positive), 38 Culev pipic.n.s-re.st~rcrrz.s (7.4% positive). and 4 Ardrs .stir~~rrltrrz.s-,fitc.liii (4 . 1 '% positive). Acdes \vc~.vcrrz.s and Ac~t1e.s tri\,ittcrtu,s were considered the major vectors because they were collected in large numbers. fcd readily on dogs. and allowed development of D . irnrnitis to the infective stage. This is the first report of Artlrs rlor.str1i.s allowing developnient of D . in1rniti.s to the infective stage.

ERNST, J . , et J . 0 . D. SLO(:OMBE. 1984. Mosquito vectors of Dirc?fi'lcrritr irr~rniti.~ in southwestern Ontario. Can. J . Zool. 62: 212-216.

Plus de 3000 moustiques ont ete recoltcs en I980 et 1981 dans le Sud-Ouest de I'Ontario, grrlce h I'utilisation d'unc cage en bois ou un chien portant une infection naturelle de Dir~filtrritr irrirniti.~ servait d'apprlt: 26 Arzol~helt~.~ punc~til~c~nrii.~ ( 100% nourris de sang), 36 Artlrs tri.sc~ritrtu.s (94.4% nourris de sang). 38 Aedes .stic,tic.~r.s ( 100'i nourris de sang). 44 Ardc..s dorstr1i.s (97.7% nourris de sang). 73 Anophrlrs \t,trlkeri ( 100% nourris de sang). 161 Aetli~s .~irn~rl(rr~.s- j i ' tc+hii (97,S'F nourris de sang), 5 10 Ac~1e.s tri~itttrtu.~ (80.8% nourris de sang). 569 Mtrn.sorzitr prvt~rrhtrn,~ (93.3C/( nourris de sang). 739 Ac)de.s \vr.t-trn.s (95,4% nourris de sang) et 1 1 14 Cu1r.v pi11irri.s-rc.stutr~z.s (61 -6% nourris de sang). Lcs moustiques nourris de sang ont kt@ gardks 5 26°C pour 2 semaines; 1708 d'entre eux ont ete identifies et dissequcs ct leurs pieces buccales ont etC examinees afin d'y chercher des larves de troisieme stade de Dirc?fi'ltrritr irn?riiti.s. Des larves infectieuses ont etd trouvces chez 26 Ardes stic-tic.u.s (96,3% des insectes examines), 33 Aedc.s dorstr1i.s (89.2% des insectes examines). 406 Aetles \lc<vcrrz.s (88.8% des insectes examines). 177 Aedrs tri\,ittertu.s (68,9'k des insectes examinks). 38 C'lrle..~ ~~il~ic)rz.s - r-c~.st~rtr~z.s (7 -4% des insectes examines) et 4 Aee1c.s .stimulun.s-ji'tc.hii (4.1 C/r des insectes examines). Aetles \'c<vtrrz.s et Aetles tri\~itttrtu.s sont considcres comme les principaux vecteurs parce qu'ils ont ete rccoltcs en grands nonibres, qu'ils se nourrissent volontiers de sang de chien et que D . immitis y atteint le stade infectieux. Le developpenient jusqu'au stade infectieux de I ) . irnnzitis chez A. dorsrr1i.s est signale ici pour la premiere fois.

[Traduit par le journal 1

Introduction Dirofilariu immitis Leidy, prevalent mainly in dogs, is a

mosquito-borne filarial nematode which causes canine heart- worm disease. This disease has been reported with increasing frequency in dogs which have not previously left Canada (Slocombe 1978; Slocombe and McMillan 1979, 1980, 198 1 ). In Canada two foci of infection exist, one in the Winnipeg. Manitoba, area and the other in southwestern Ontario. The greatest spread of infection has occurred in southwestern Ontario.

The vectors of heartworm in Canada are unknown. How- ever, of species of mosquitoes reported to be potential vectors (Ludlam et ul. 1970; Crans and Feldlaufer 1974; Christensen 1977: Todaro et ul. 1977: Rogers and Newson 1 979), 24 occur in southwestern Ontario (Wood et cd. 1979). The objective of this study was to identify species of mosquitoes in southwestern Ontario which may be vectors of D . immitis.

Materials and methods Mosquitoes were collected in the Windsor, Ontario. area in 1980

and 198 1 using a portable stable trap as described by Magoon ( 1953) and modified to accommodate a dog (Ernst 1982). The trap was made

'Revised manuscript received September 26, 1983.

from plywood with most of the upper portion of the sides of the trap consisting of mosquito screening. Entrance baftles, made to trap en- tering mosquitoes, were small. V-shaped, longitudinal openings ta- pering from outside inwards. The baftles were placed horizontally on three sides of the trap directly beneath the screening.

Three dogs naturally infected with D . immitis were used as bait. Two dogs were used in 1980 and only the third was used in 198 1 . The number of microfilariae per millilitre of peripheral blood of the dogs was estimated prior to each collection as described by Church et a l . ( 1976). For each collection, the dog was restrained in a wire cage which was placed within the trap from early evening to the following midmorning. Trapped niosquitoes were aspirated by a sucking tube and transferred to Plexiglas cages (30.5 cm x 30.5 cm x 30.5 cm). Mosquitoes which had not fed on the heartworm-infected dogs were identified to species using the keys of Wood et trl. (1979) and dis- carded. Blood-fed mosquitoes in the Plexiglas cages were supplied with cotton batting saturated with 10% sucrose and maintained at 26 ? 1°C and 80 ? 5% relative humidity with a 16-h photoperiod. The 10% sucrose was replaced every 2nd day and the humidity wicks were replaced once a week.

After 4 to 9 days of incubation, some mosquitoes were anaes- thetized by placing them at 4OC for 30 min and identified. and the Malpighian tubules were dissected in physiological saline on micro- scope slides to detect first- ( L I ) and second-stage larvae (L?) of D . irnrnitis. From 2 to 4 weeks after incubation, the remainder of mosquitoes were dissected and the mouthparts, head, thorax, and tubules were examined for third-stage larvae (L3). When large num-

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Page 2: Mosquito vectors of               Dirofilaria immitis               in southwestern Ontario

ERNST AND SLOCOMBE

TABLE 1. Species and numbers of mosquitoes collected in 1980 and 1981 in Windsor. Ontario. in a stable trap housing a dog naturally infected with Dirofiltiriti irnmitis and the percent blood fed on the dog, the percent mortality of those blood fed. the number of surviving, blood-fed mosquitoes dissected. and the percent found with infective third-stage

larvae (positive] after 2 weeks at 26OC

C/c No. No. 7%

Species collected Blood fed Mortality dissected positive

Aedes dorsalis" Aedes sticticus Aedes stimulans-fitchii Aedes triseriatus Aedes trivittatus Aedes vexans Anopheles punctipennis Anopheles walkeri Culex pipiens - restuans Mansoni perturbans

Totals

3 7 27 98

2 257 45 7

ND' N D'

51 1 45 3

1.842

"Collected in 1981 only. "Collected in 1980 only. ' N D , No dissections were possible because mosquitoes did not survive the ?-week incubation period.

-t Ae&s vexans

+ Aedes stimulans-fitchii

-t Aedes trivittatus

Culex pipiens-restuans

MAY 23 JUNE6 JUNE 2 0 JULY4 JULY 18 AUG. 1 AUG.15 AUG.

FIG. 1. Species and numbers of mosquitoes collected in Windsor. Ontario, froni May 22 to August 27. 1981, in a stable trap housing a dog naturally infected with Diroji'l~iria irnmitis.

bers of mosquitoes were collected, only the mouthparts were exam- ined. Only L3 found in the mouthparts were considered infective. Larvae were identified to larval stage as described,by Fortin (1979). Infective L3 of D. irnrnitis were identified as described by lyengar ( 1957) and Nelson ( 1959). Larvae were characterized as dead if they were granular, immobile, and with the internal organs ill defined.

Results The mean number of microfilariae per millilitre for the two

dogs used in 1980 was 26 820 and 5 230 and that for the one used in 198 1 was 1 500.

In 1980 and 198 1 a total of 33 10 mosquitoes were recovered in 20 collections (Table 1 ) . The species and numbers of mos-

quitoes collected, percent blood fed on the infected dog, per- cent mortality of those blood fed, and the result following dissection of mosquitoes surviving the 2-week incubation period are shown in Table 1. The numbers of the seven most abundant mosquitoes collected at various times in 1981 are presented in Fig. 1.

In 1980, only 134 blood-fed mosquitoes collected survived the 2-week incubation period. Infective L, were not recovered from the mouthparts of these mosquitoes. However, pre- infective stage larvae were found in two Aedes triseriatus (Say) ( 100% positive), 5 1 Mansonia perturbans (Walker) (5.9% pos- itive), and 8 1 of the Culexpipiens Linnaeus and Culex restuans Theobald group ( C. pipiens-restuans) (3.7% positive). In the

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Page 3: Mosquito vectors of               Dirofilaria immitis               in southwestern Ontario

214 C A N . J ZOOL VOL. 62. 1983

latter two species the majority of larvae were dead. In 198 1 , after 4 to 9 days of incubation, 13 of 24 C. pipiens-

restuans, 18 of 23 M. perturbclrzs, all of seven Ae. \Ie.rarz.s (Meigen), and five of six Ae. tri~littatlrs (Coquillet) contained L, in the tubules. Five of 13 L, in Ae. tri\ittcltu.s, 3 1 of 44 in M. perturbarzs, and 2 1 of 42 in C. pipierzs-restuclrls were found dead. In five of six Ae. tri~iittatus, 2 1 L2 were found in the tubules. In 2 of 24 C. pipiens-restuans, three L7 were found, of which one was dead.

In 1981, after 2 weeks of incubation, the mean number of infective L3 found was 4.4 in Ae. vexarzs, 3.6 in C . pipiens- restuans, 3.4 in Ae. dorsalis (Meigen), 3.2 in Ae. stic-ticus (Meigen), 3.0 in Ae. trivittatus, and 1.5 in the Ae. stimuluns (Walker) and Ae. fitc'hii (Feld and Young) group ( A e . stimulans-fitc'hii). The maximum number of L, observed in any one mosquito was I I and this was found in Ae. \!e.rclrzs. Preinfective L3 were found also in the head, thorax, and tu- bules. In 37 Ae. dorsalis 70 of 183 L, were preinfective, as were 33 of 123 L3 in 27 Ae. stic'tic-us, 116 of 408 L3 in 134 Ae. trivittatus, and 4 18 of 12 16 L3 in 237 Ae. \!e.rans. In one Ae. vexans collected in July, there were two encapsulated L, in the tubules. One L, was completely encapsulated and the other partially and at the anterior and posterior ends. Two infective L, were present in the mouthparts of the same n~osquito.

Discussion The transmission of D. immitis is dependent on mosquitoes,

the only known vectors, and the results of the present study showed that Ae. vexans should be considered one of the major vectors in southwestern Ontario. Ae. \le.rans was one of the most abundant species collected in Windsor, Ontario. Large numbers were found in all collections from late May to late August 198 1 , with the greatest numbers occurring in July and August. More than 9570 of those collected had taken a blood meal from the infected dog, and in almost 90% of those dis- sected there was development of D. immitis to the infective stage.

Aedes vexans is widespread and occurs in large numbers throughout southwestern Ontario (Wood et al. 1979; Helson et al. 1980). It is the most abundant Aedes sp. in the Windsor area and is generally present from the end of May to early September with greatest abundance in July and August (Helson et al. 1980). Some other factors make Ae. vexans a suitable vector. The adults are found in a variety of environments around houses in towns as well as in suburban and rural areas (Barr 1958; Horsfall et al. 1973). They are fierce biters and in the present studies they fed readily on dogs. Fortin (1979) reported the same eagerness to feed on dogs in the laboratory. Aedes vexans are rapid breeders and may have several gener- ations a year (Horsfall et al. 1973; Helson et al. 1980), es- pecially in areas such as Windsor where heavy and frequent rainfall (Environment Canada data for 1976- 198 1 (Anony- mous 1976- 1981)) results in frequent flooding of breeding sites for subsequent hatching of eggs laid either in the same or the previous year (James and Harwood 1969). The life span of Ae. vexans ranges from 3 to 6 weeks (Horsfall et al. 1973), and in the present studies most infected Ae. vexans survived the 2-week incubation period. It seems likely, therefore, that Ae. vexans will survive long enough in nature to allow complete development and transmission of D. immitis. The flight range of Ae. vexans has been reported to be up to 15 mi (24 km) (Sjogren 1976; Brust 1980), thereby allowing transmission to occur a considerably distance from the source of infection.

Aedes vesans is Canada's worst mosquito pest (Wood et al. 1979) and may be an important vector of D. imrnitis across Canada.

In most Ae. \'e.rcln.s dissected in the present studies, L, and L, were found in the Malpighian tubules concurrent with infec- tive L3 in the proboscis. A similar asynchrony of development has been observed in Ac.. triseriatus (Fortin and Slocombe 198 1 ; Ernst 1982). This asynchronous development may be important. A mosquito may infect a dog with its infective L, while acquiring its third blood meal, and the same mosquito may transmit D. imnzitis a second time if the L, and L, in the tubules develop to infective L3 before acquisition of the fourth bloodmeal.

There appears to be a marked difference in susceptibility of Ae. vexarzs to D . immitis in different parts of North America. In the northern United States, complete development to the infective L3 occurred in Ae. vexarzs (Yen 1938; Bemrick and Sandholm 1966; Jankowski and Bickley 1976; Todaro et al. 1977; Arnott and Edman 1978; Fortin 1979; Hendrix et al. 1980, 1981; Lewandowski et al. 1980; Otto and Jachowski 1981). In more southern states, there was incomplete devel- opment of D. immitis in Ae. \lcxans (Arnott and Edman 1978; Magnarelli 1978; Buxton and Mullen 1980). Factors influ- encing infection of niosquitoes with D. immitis have been de- scribed (Kartman 1953; Otto and Jackowski 198 I) , and two explanations may be advanced for the difference in the vectors potentials of Ae. vexans in the northern and southern areas of North America. First, D. immitis in the Great Lakes and sur- rounding area may be a different strain from that in the southern states of the United States and can complete development in Ae. vexails in the north but not in the south. Otto (1975) suggested that a genetic difference between D. immitis in the north and south may be the reason for the northward spread of heartworm into the temperate regions of North America. Secondly, there may be several strains of Ae. vexans with only those in the north allowing complete development of D. irnmitis.

There are 67 other species of mosquitoes which allow devel- opment of D. immitis to the infective stage (Ludlam rt al. 1970; Crans and Feldlaufer 1974; Knight and Stone 1977; Seeley and Bickley 1974; Christensen 1977; Todaro et al. 1977; Rogers and Newson 1979). In the present studies, several of these species were collected and many allowed complete devel- opment of D. immitis.

Aedus tri\~ittatus is another likely vector in southwestern Ontario. In the present studies, it was unusual in its abundance throughout the summer of 198 1 . Normally, this species is col- lected in small numbers in southern Ontario (Helson et al. 1980). Females can survive for 5 to 6 weeks (Barr 1958), will take more than one blood meal (Christensen 1977, 198 1 ), and can tolerate large numbers of developing larvae (Christensen and Andrews 1976; Christensen 1981). Although the flight range ofAe. trivittatus is limited (Ban 1958), it may play a role in transmission when conditions are such that large populations occur.

In the present studies, although a large percentage of Ae. dor.sa1i.s and Ae. sticticus allowed development to the infective L,, they must be considered poor vectors because of the small numbers of mosquitoes collected. Small numbers of these spe- cies are normal for southwestern Ontario (Helson et al. 1980). This is the first report of Ae. dorsalis allowing complete devel- opment of D. immitis, and it increases the number of potential vectors, worldwide, to 69 species.

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Page 4: Mosquito vectors of               Dirofilaria immitis               in southwestern Ontario

ERNST A N D SLOCOhlBE 215

The most abundant species collected in these studies was C. pipiens-restuans and more than 50% of the mosquitoes took a blood meal. However, less than 5O/c of the mosquitoes dissected contained infective L?. L, and L1 were found in the tubules, but many were dead. Other studies have found C. pipiens-re.stuun.s and C. r~stuatl.s to be a poor vector (Kartman 1954; Bemrick and Sandholm 1966; Todaro ct ell. 1977; Buxton and Mullen 1980; Lewandowski et ell. 1980). Although C. pipiCrz.s-r~.stuci~~s is a poor vector, it fed readily on dogs in the present studies. This activity was unusual be- cause this species feeds predominantly on birds and prefers not to feed on mammals (Hayes 196 1 : Wood c-'t a / . 1979).

Aedes stimulans-,fi'tchii were collected throughout most of the summer in 1981, but of those dissected, less than 10% contained infective larvae and, therefore, was not considered a good vector.

Infective L3 were not found in 453 blood-fed M. perturbclns and this mosquito is unlikely to be a good vector in south- western Ontario. In this species, L , and L1 were found, but many of these were dead. Other studies have found M. per- turbans to be a poor vector (Ben~rick and Sandholrn 1966; Christensen and Andrews 1976; Todaro e-'r a / . 1977; Buxton and Mullen 1980; Lewandowski et ell. 1980). However, Yen ( 1938) found complete developn~ent in M. prrturbans.

There was high mortality early in the incubation period of Anopheles pun(-tipennis (Say), An. wcilkeri (Theobald), and Ae. triseriatids and this prevented the possible development of D. irnmitis to the infective stage. The mortality of the Anopheles may have been due either to the developing D. immitis or the inability of the mosquitoes to survive under laboratory conditions. When Ae. trisericttu.~ were collected, they were baited with the dog that had high microfilaremia, and it is believed that the large burden of parasites was the cause of death.

The present natural vector studies and other field and labo- ratory studies show how varied a vector's suitability may be from one area to another and emphasize the importance of determining the local vectors in all areas where heartworm is found. Further work is necessary to clarify the vectors in Canada and consideration should be given to two species which were not collected in the present study. These were Ac. curza- densis (Theobald) and Ae. excruc-iarzs (Walker) which occur in Canada (Wood et a / . 1979) and are considered the most likely vectors in western Massachusetts (Arnott and Edman 1978). Aedes c*anadensis has been frequently collected in southern Ontario (Helson et a / . 1980).

Acknowledgements This study was made possible by grants from the Canadian

Veterinarv Research Trust Fund and several kennel clubs , across Canada and their support is gratefully acknowledged.

Appreciation is expressed to Dr. B. Ramsey and other staff at St. Clair College for allowing use of their facilities in Windsor, Ontario. We wish to thank the Cassidy and Depuis families for their permission to carry out the research on their

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