Parasitol Res (2009) 104:257–265
DOI 10.1007/s00436-008-1184-0
ORIGINAL PAPER
Setaria tundra microfilariae in reindeer and other cervids
in Finland
S. Laaksonen & M. Solismaa & T. Orro & J. Kuusela &
S. Saari & R. Kortet & S. Nikander & A. Oksanen &
A. Sukura
Received: 10 July 2008 / Accepted: 28 August 2008 / Published online: 23 September 2008
# Springer-Verlag 2008
Abstract Harmful parasites of the wild northern boreal
mammals are still surprisingly poorly studied. In 2003–
2006, a peritonitis outbreak caused by the filarioid
nematode, Setaria tundra, emerged in Finland’s reindeer
population. In order to gain knowledge about the basic
biology, epidemiology, and transmission dynamics of this
parasite, samples for S. tundra were collected from reindeer
and other cervids during the follow-up period 2004–2006.
Using morphology and molecular biology methods, we
describe here S. tundra’s first larval stage, microfilaria
(smf), for the first time scientifically. The prevalence and
densities of smf were higher in reindeer calves than in
adults, overall prevalence being 42%. The overall smf
prevalences for moose, wild forest reindeer and roe deer
were 1.4–1.8%, 23%, and 39%, respectively. The focus of
Electronic supplementary material The online version of this article
(doi:10.1007/s00436-008-1184-0) contains supplementary material,
which is available to authorized users.
S. Laaksonen (*) : M. Solismaa : J. Kuusela : A. Oksanen
Fish and Wildlife Health Research Unit,
Finnish Food Safety Authority Evira (FINPAR),
P.O. Box 517, FI-90101 Oulu, Finland
e-mail: [email protected]
T. Orro
Department of Animal Health and Environment,
Estonian University of Life Sciences,
Kreutzwaldi 62,
51014 Tartu, Estonia
S. Saari : S. Nikander : A. Sukura
Department of Basic Veterinary Sciences,
Faculty of Veterinary Medicine (FINPAR), University of Helsinki,
P.O. Box 66, FI-00014 Helsinki, Finland
R. Kortet
Department of Biology, University of Oulu,
P.O. Box 3000, FI-90014 Oulu, Finland
microfilaremia moved north and settled down in the south
simultaneously with the peritonitis outbreak. The peak
microfilaremia occurred in the first summer after the
infection, and smf disappeared from the blood after 2 years.
Captive reindeer were smf positive over the year. The
prepatent period of S. tundra was estimated to be about
4 months, and the life span at least 14 months. This parasite
likely has an important impact on boreal ecosystems.
Introduction
There is a growing body of literature documenting the
expansion of emerging parasites to subarctic areas including
domesticated and wild ungulates. These studies have
revealed an array of Filarioid nematodes and associated
diseases that appear to be emerging in northern ungulates
(Laaksonen et al. 2007; Nikander et al. 2007; Solismaa et al.
2008).
Therefore, understanding interactions between vectorborne parasites and their intermediate and definite hosts
represent a major challenge for the study of northern boreal
ecosystems. A recent review by Hoberg et al. (2008)
highlighted the impact of pathogenic parasites for keystone
mammalian wildlife species in the Circumpolar North and
emphasized the need to monitor these parasites. Filarioid
nematode parasites are known for their harmful effects on
mammalian hosts, but are still relatively poorly studied in the
boreal northern hemisphere (e.g., Laaksonen et al. 2007).
In 2003, a peritonitis outbreak emerged in the Finnish
semidomesticated reindeer (Rangifer tarandus tarandus)
population. The outbreak, starting from the southern part of
the population, and proceeding north during subsequent
years, was caused by the filarioid nematode, Setaria tundra
(Laaksonen et al. 2007; Nikander et al. 2007). The outbreak
258
resulted in significant economic losses to the reindeer
industry and a negative impact on the welfare of reindeer,
especially reindeer calves. The intensity of S. tundra
infection correlated significantly with the degree of peritonitis and with poor body condition of reindeer calves
(Laaksonen et al. 2007). There is a previous report of a
peritonitis outbreak in moose (European elk, Alces alces) in
Finnish Lapland in 1989 associated with Setaria sp.
nematodes (Nygren 1990). Although this earlier outbreak
took place within the reindeer husbandry area, no reports on
associated increased morbidity in reindeer exist.
Each adult female filarioid worm produces thousands of
larval stages, microfilariae (mf) daily; Setaria labiatopapillosa contains at least 50,000 (Nelson 1966) and S. tundra
over 200,000 (Nikander et al. 2007) mf in its uterus.
Microfilariae of the subfamily Setariinae are sheathed and
occur in hosts’ blood circulation, where they are available
to arthropod vectors (Anderson 2000) and available for
diagnostics. Occurrence of Setaria spp. mf (smf) have
earlier been reported in reindeer blood in Alaska (Dietrich
and Luick 1971), where reindeer were mf positive (Setaria
yehi) year-round, and in Sweden in reindeer skin samples
(Rehbinder 1990), but there are no reports of smf in other
cervids.
The main aim of this study was to monitor the S. tundra
outbreak dynamics during the parasites’ rapid expansion in
Finnish Lapland and to study basic biology of S. tundra,
including life cycle parameters such as reservoirs, prepatent
period, and life span. Reaching this aim would help us to
understand the impact of this parasite for Northern boreal
ecosystems. One of the aims was also to collect an
extensive data set about the prevalence of S. tundra in
cervids that would work as a reliable basis for future
monitoring. To achieve these goals, we collected spatial and
temporal parasitological samples from captive and freeranging reindeer and other cervids. Since there was no
earlier scientific description of smf, the study included also
morphological and genetic surveys.
Parasitol Res (2009) 104:257–265
20th–April 3rd) and in the winter 2005–2006, 491 animals
(221 adults and 270 calves; Jan 1st–Feb 28th). To compare
the current prevalence of Setaria infection to that of the
previous decade, 251 additional blood samples collected
randomly in 1997 were included in the study. It is worth
noting that no epidemic by this parasite was observed in
1997. These additional samples were stored frozen (−20°C
without anticoagulant).
To estimate the prepatent period of S. tundra infection,
blood samples were collected in Kuusamo reindeer slaughterhouse (Fig. 1) from 119 adults and 205 calves from eight
slaughter batches from October to January 2004–2005. All
batches originated from the same highly S. tundra endemic
pastures. To determine the ability of smf to penetrate the
placenta, blood samples from 90 unborn fetuses (5th–
6th months of gestation) were collected.
All the blood samples taken were collected after
stunning (or shooting other cervids) during exsanguination;
Materials and methods
Blood from semidomesticated reindeer at slaughter
The total summer reindeer population in Finland is about
300,000 individuals of which about 100,000 are slaughtered yearly (Nieminen 2006). To analyze the spatial and
temporal variation in prevalence and density of smf in the
reindeer population during the course of the peritonitis
outbreak, a total of 1,290 reindeer blood samples, selected
randomly from all the regions of the Finnish reindeer
herding area were collected. In the winter 2003–2004, 627
animals were sampled (273 adults and 354 calves; Jan
Fig. 1 The Finnish reindeer husbandry area divided into four areas
for analysis of the S. tundra associated peritonitis outbreak’s spatial
development. Dotted areas mark the northern (a; Kainuu area) and the
southern (b; Suomenselkä area) populations of wild forest reindeer
Parasitol Res (2009) 104:257–265
the blood flow from cut jugular arteries and veins was
directed into opened blood tubes (Vesafe®, EDTA, Terumo,
Belgium).
Blood from other cervids
To study the interactions and possible reservoirs of S.
tundra in nature, 442 blood samples were collected, using
similar methods than above, from wild cervids, moose (n=
324), wild forest reindeer (Rangifer tarandus fennicus n=
92), roe deer (Capreolus capreolus n=17), and white-tailed
deer (Odocoileus virginianus n=9) between Jan 26th 2004
and June 4th 2005. These samples were collected by
hunters and from road-killed animals.
A total of 212 moose blood samples were from the
reindeer herding area (total population about 56,000) and
112 outside the southern border of the reindeer herding area.
The survey included 92 samples from wild forest reindeer,
of which 33 from the northern population (Kainuu area;
~1,000 individuals) and 59 from the southern (Suomenselkä
area) population (~1,000 individuals; Fig. 1). The northern
population is adjacent to the southeastern border of the
reindeer husbandry area, with contacts to semidomesticated
reindeer, whereas the Suomenselkä population resides 300
km further south in the middle of Finland (Fig. 1).
The total population of roe deer is about 18,000 covering
the whole of Finland. For this study, 17 roe deer were
sampled from various locations. White-tailed deer, introduced in 1935 from the USA, with an estimated population
size of about 48,000 animals are found mainly in southwestern Finland, where nine samples were collected for this
study (http://riistaweb.riista.fi/riistatiedot/riistatietohaku.
mhtml).
259
reindeer during rest with minimal disturbance, and mf
values were compared to those detected in samples taken
from the same animals after keeping them in moderate
movement by chasing for 15 min. Tests were made in May
2005 with seven and in July 2006 with six reindeer.
Laboratory analyses
Smf detection and counting
The blood samples (1 ml /animal) were examined for the
presence of smf by the modified Knott’s technique (Georgi
1985), and the smf were counted in temporary wet mounts.
To assess the repeatability of the mf counting method, a test
was made by counting 38 repeats using blood samples of
eight reindeer.
The frozen blood samples from the year 1997 were first
thawed at room temperature, and then, five drops of
heparin (0.1 g heparin/1 ml saline) were mixed with 1 ml
of blood. Smf count was conducted 30 min later. The
repeatability of this technique was estimated by counting
the same samples of fresh EDTA blood and blood without
anticoagulant and stored frozen (−20°C) for 21 days from
24 reindeer samples. In calculating the repeatability of smf
counts, the coefficient of variation was 23%. In the test of
smf in blood samples preserved frozen without anticoagulant, the repeatability was 92% to get consistent smf
positive results compared to positive EDTA samples, and
in negative cases, the repeatability was 100%. In smf
counts, the number of smf was on average 50% (range 0–
100%, SD 42%) of the amount of smf detected in EDTA
samples.
Smf morphology
Monitoring of captive reindeer
In March 2004, three male and four female semidomesticated reindeer calves (10 months old) and one 3-year-old female
reindeer were relocated from Kuusamo to the experimental
zoo of the University of Oulu. The zoo is located within the
city of Oulu, in the university campus, outside the reindeer
herding area with no contacts to other reindeer or roe deer
(Fig. 1). All reindeer were naturally infected and had smf in
blood. The density of smf was monitored weekly over
1 year by jugular vein samples collected in evacuated
blood collection tubes (Venoject VP-100SDK, with 18 G
Venoject® needle, Terumo Corporation, Belgium). The
samples were collected at 10:00–11:00 A.M. In September
2004, two of the reindeer (harboring high smf densities)
were killed for parasitological studies.
To estimate the effect of physical activity on microfilarial density in peripheral blood, exercise assessments
were performed. Blood samples were taken from the
To confirm the identity of the parasite in cervid blood
circulation, the mf found in peripheral blood were compared with smf isolated from the uterus of adult female S.
tundra nematodes. The mf (n=30) were measured and
studied under a light microscope after staining with
hematoxylin or May–Grünwald–Giemsa (MGG) methods.
Unstained smf were studied also with the aid of a
microscope equipped with differential interference contrast
(DIC). The structures measured for the smf description
were: length of the larva without sheath, maximum width,
distance of the nerve ring from the anterior end, distance of
the excretory pore from the anterior end, and distance of the
anal opening from the tip of the tail.
PCR
For molecular biological studies, blood from three reindeer
from the Oulu experimental zoo in 2004 and from three
260
reindeer from Kuusamo in 2005, were checked for the
presence of smf by the modified Knott’s technique. Then,
100 μl of blood was diluted to 2 ml sterile distilled water to
lyze erythrocytes and other blood cells and filtered through
Schleicher & Schuell 589/4 medium fast filter papers using
filter holder FP 025/1. The filter papers were placed in a
1.5-ml microfuge tube, 500 μl 5% resuspended Chelex®
100 Resin mixtures (Bio-Rad) were added, and the
mixtures were then vortexed. After that, the samples were
incubated for 20 min at 56°C, vortexed briefly and kept at
99°C for 10 min. Before use in the polymerase chain
reaction (PCR), the Chelex mixture was centrifuged for
2 min, and 3 μl liquid from the uppermost supernatant was
used for each PCR reaction. The presence of smf was
demonstrated by sequencing mitochondrial DNA and
comparing with all the known sequences of S. tundra, as
well as the other Filarioidea species. The specific S. tundra
primers and PCR conditions are described elsewhere
(Laaksonen et al. 2007). Samples from 1997 were not
included in PCR analyses because of the low sample
volume and low smf density.
One adult Setaria sp. specimen from a 1989 parasitized
moose in northern Finland preserved in alcohol and one
adult Setaria sp. specimen from a 2005 parasitized roe deer
in southern Finland were PCR amplified and sequenced
using primers St COI 616L and St COI 1321H which
amplify the 729-bp region of the mtDNA Cox1 gene. The
primers are described in Laaksonen et al. (2007) although
the primer St COI 1321H was erroneously named as St COI
1105L.
Parasitol Res (2009) 104:257–265
Results
Smf
The smf in fresh reindeer blood smears were actively
moving, long, and slender with blunt anterior end and
tapering posterior end (Fig. 2). They had a delicate and
transparent sheath that was otherwise tight-fitting but
projected notably beyond the anterior and posterior end.
The mf occupied on average 89% (SD=3.9%) of the length
of its sheath. The cuticle possessed fine transversal
striations that were visible especially with the aid of a
DIC-equipped microscope and if the mf was bent. Intensively basophilic stained nuclei (in MGG stained specimens) were present up to the tip of the tail. The average
length was 293 µm (range 280–306 µm, SD 6.5 µm),
331 µm with the sheath (range 304–365 µm, SD 17 µm),
and width 6.7 µm (range 5.1–7.7 µm, SD 0.6 µm) with
rounded anterior end and a regularly tapering 35-µm-long
filamentous tail (Fig. 2). The average distances from the
anterior end were 44 µm to nerve ring and 69 µm to
excretory pore.
The partial 18S ribosomal RNA gene sequences of S.
tundra microfilaria in all six samples were similar to the
sequence of S. tundra parasitizing reindeer in northern
Finland (Laaksonen et al. 2007) and are deposited in the
GenBank under accession number EF081341.
Statistical analyses
Statistical analyses were performed with Stata 9 (StataCorp
LP, USA) software. Overall effect of the age group and year
on the prevalence and density of smf in blood was analyzed
using logistic regression and Poisson model, respectively.
The reindeer husbandry area was divided into four subareas
(Fig. 1) for analysis of the spatial development of the
outbreak. Subarea was used in models as a hierarchical
dummy variable, and as a result, differences between
adjacent subareas were evaluated. S. tundra mf counts in
blood were divided into five groups for Poisson models
(0, 1–10, 11–100, 101–300, and >300 mf/ml). The
prevalence analyses among wild cervids were conducted
using Pearson’s chi square test and density analyzes in
wild forest reindeer by the Poisson model. In Poisson
model, S. tundra mf counts in blood of wild forest reindeer
were divided into five groups as in reindeer. As only five
moose were positive for smf and only 17 roe deer blood
samples were studied, statistical analysis for moose and roe
deer data was not performed. The level of significance was
set at 5% ( p<0.05).
Fig. 2 a S. tundra microfilaria based on May–Grünwald–Giemsa
stained specimen. s Sheath, n nerve ring, ep excretory pore, G1–4 germ
cells, ap anal pore. Bar=50 µm. b S. tundra microfilaria as seen under
light microscope equipped with differential interference contrast (DIC)
Parasitol Res (2009) 104:257–265
261
The partial mtDNA Cox1 sequence of the one adult
Setaria sp. from moose in 1989 (GenBank EF661848) and
the one adult Setaria sp. from roe deer in southern Finland
in 2005 (GenBank EF661849) were identical along the
sequenced 680 bp. The sequences confirmed that they were
S. tundra, but another haplotype separated by eight
nucleotide differences (1.18%) from the reindeer S. tundra
causing the present outbreak.
In addition to smf, mf of other species were discovered
from reindeer and other cervid species (unpublished data).
Semidomesticated reindeer at slaughter
The mean prevalences of smf in the whole sample material
from all the reindeer herding areas in 2004 were 56% in
calves and 35% in adults. Corresponding numbers in 2006
were 54% and 28%. The spatial prevalence and density in
calves and adult reindeer during the years 2004 and 2006
are presented in Table 1. In all areas, smf infection was
more prevalent ( p<0.001) and intense ( p<0.001) in calves,
and there was no overall significant difference between the
years 2004 and 2006. In 2004, smf prevalences were higher
in more southern areas ( p<0.05 and p<0.001; area 1 vs.
area 2 and area 2 vs. area 3, respectively). In 2006, smf
appeared in reindeer in area 4; the prevalence was lower
than in area 3 ( p<0.001). Statistical differences in smf
intensity between areas in different years were the same as
in smf prevalence. In 1997, smf were present in very low
density (mean 9.3, range 2–24) in 4% of samples (n=54)
from reindeer in area 2 and were not found in other areas.
No smf were detected in reindeer calves in the beginning of
the slaughter season in late October, whereas 4.8% of adult
Table 1 The spatial smf prevalence and density (smf/ml) in reindeer
in 2004 and 2006
Area
Year
Group
Number
(N)
Prevalence
(%)
1
2004
2006
2004
2006
2004
2006
2004
2006
2004
2006
2004
2006
2004
2006
2004
2006
Calf
Calf
Adult
Adult
Calf
Calf
Adult
Adult
Calf
Calf
Adult
Adult
Calf
Calf
Adult
Adult
116
114
133
87
150
56
82
37
31
25
17
25
57
75
41
72
87
75
39
30
61
71
40
17
7
17
18
60
0
4
0
7
2
3
4
Mean
(range)
71 (1–500)
66 (1–700)
38 (1–269)
29 (4–84)
24 (1–233)
63 (1–950)
15 (1–89)
8 (1–27)
53 (2–138)
33 (1–200)
12 (1–27)
15 (1–55)
0
1
0
66 (7–196)
reindeer were already positive. First microfilaremic calves were
observed in the beginning of November, and the prevalence
increased to its peak, 80% until the end of slaughter season in
the beginning of January. In adult reindeer, the prevalence also
increased, but only to 20%. No smf were found in blood
samples taken from reindeer fetuses in slaughterhouse.
Wild cervids
The prevalences of smf in moose within and outside the
reindeer herding area were similar (1.4% and 1.8%), the
densities being very low (1–3 smf/ml blood). There was
significant difference between smf prevalence in the two
wild forest reindeer populations (36% of 33 in Kainuu vs.
15% of 59 in Suomenselkä) (p<0.05). The density of smf
was also higher (p<0.01) in North (mean in infected animals
144 smf/ml blood, range 1–830). Seven (41%) of the 17 roe
deer examined had S. tundra infection (mean 187 smf/ml
blood, range 10–650), and the animals originating from
North Finland (n=9) and the southern coastal part of Finland
(n=8) were infected alike, 33% and 50%, respectively.
No smf were detected in the white-tailed deer samples.
Monitoring of captive reindeer
In the experimental reindeer group, the peak period of high
microfilaremia was from the beginning of June to midSeptember 2004, with the mean number of 950 smf/ml
blood (range 62–4,000; Fig. 3). In autumn, the amount of
smf began slowly to decrease. In January, three experimental reindeer were free of smf, whereas another three reindeer
maintained low microfilaremia (Fig. 3) in the beginning of
next summer. Also, these three reindeer were clean of smf
in autumn 2005. In the same summer, three of the animals
in the experimental group gave birth in the zoo, and all the
calves got S. tundra infection during the summer.
In exercise assessments, moderate movement of reindeer increased the density of smf on average 130%
(range 10–447%).
In autopsy, one 16-month-old yearling reindeer (mean of
1,015 smf/ml during follow-up) had seven adult fertile female
S. tundra nematodes in the abdominal cavity (mean length
75 mm), while the other slaughtered (mean 244 smf/ml) was
parasitized by only one adult fertile female (64 mm long).
No dead or male nematodes were found.
Discussion
Ecology of S. tundra in reindeer hosts
S. tundra microfilariae were notably prevalent in Finnish
cervids, especially in reindeer, during the peritonitis
262
Parasitol Res (2009) 104:257–265
1600
Standard Error
1400
Mean
smf/ml blood
1200
1000
800
600
400
200
.4
26
26
.3
.2
00
4
26 .20
.5 04
.2
26 00
.6 4
.
26 200
.7 4
.2
26 00
.8 4
.2
26 00
.9 4
26 .20
.1 04
0
26 .20
.1 04
1.
26 20
.1 04
2.
2
26 00
.1 4
.2
26 00
.2 5
.
26 200
.3 5
26 .20
.4 05
.2
00
5
0
Fig. 3 S. tundra microfilaria periodical densities in the Oulu zoo
group of eight naturally infected reindeer
outbreak. Smf were more prevalent and dense in calves
than in adult reindeer. These results are concomitant
with the reindeer meat inspection findings about the S.
tundra infection during the same period; severe peritonitis
was more common in calves (Laaksonen et al. 2007). The
same pattern had been noticed earlier in the prevalence of
Setaria sp. infection in black-tailed deer (Odocoileus
hemionus columbianus) and in white-tailed deer: fawns
and yearlings had higher prevalence than older deer
(Weinmann 1973; Prestwood and Pursglove 1977). Similarly, in moose, the prevalence of S. cervi was highest
among the young (<2 years old) animals (Nygren 1990;
Laaksonen et al. 2007). Lower smf prevalence and density
in adults, and the fading of the outbreak in the south while
moving north, may indicate a developing immunity against
S. tundra in reindeer populations. This conclusion is
supported by the findings obtained when the smf values
were monitored in reindeer at the experimental zoo in Oulu:
all naturally infected calves cleared the infection within
2 years.
According to the reports of the Chiefs of District of all
56 Reindeer Herding Cooperatives in Finland, 73% of adult
reindeer got annual ivermectin injection in autumn or early
winter in the years 2003–2005. This can partly explain,
together with the growing immunity, the lower prevalence
and density of smf in adult reindeer. The variable efficiency
of avermectins against Setaria spp. infection have been
demonstrated mostly in domestic animals (Klei et al. 1980;
Shirasaka et al. 1994; Sharma and Siddiqui 1996), but the
efficacy of ivermectin against S. tundra in reindeer is not
properly known. Moreover, it is evident that in the present
situation, the high medication rate of reindeer with
ivermectin would not prevent the parasites’ rapid invasion
to the north.
Obviously S. tundra has existed in low numbers in the
Finnish reindeer population at least since the previous large
outbreak in 1973 (Laaksonen et al. 2007). To support this,
smf prevalences and densities in the blood samples from the
year 1997 were low, and S. tundra-associated pathological
changes were rarely reported in reindeer meat inspection
records before the peritonitis outbreak in 2003 (Laaksonen
et al. 2007). However, according to our results, it is evident
that even a low S. tundra prevalence and density can
maintain the infection in reindeer population, since three
hinds in the follow-up zoo animals with low smf value
managed to transfer the infection to their offspring. Low
infection prevalence can probably exist for years and cause
an outbreak in calves, when other conditions of transmission are favorable. Another possibility for the emergence of
the present outbreak is the introduction of a new S. tundra
strain that could have been more pathogenic for reindeer
(Nikander et al. 2007).
Sylvatic hosts and reservoirs
Moose, the most abundant wild cervid in reindeer herding
area, was evidently not a suitable host and reservoir for the
present S. tundra haplotype. The moose population in
northern Finland peaked in the years 2004 and 2005, but
only very low prevalence and density of smf were found in
moose both in and outside the reindeer herding area. The
present molecular biological studies revealed that the
peritonitis outbreak in moose in 1989 was caused by a
genetically different S. tundra haplotype than the later
epidemic. Although the analysis of the S. tundra strain
associated with peritonitis outbreak in moose was based
only on one retained specimen, the results are congruent
with those presented by Laaksonen et al. (2007), who
reported only a few preadult S. tundra nematodes encapsulated on the surface of the liver in samples taken from
moose from high-prevalence S. tundra area during the
reindeer peritonitis outbreak in 2005.
The role of wild forest reindeer as a reservoir of S.
tundra for reindeer may be significant, but it can also work
the other way around. Microfilarial densities and prevalence
were similar between these two subspecies. The S. tundra
outbreak started from the south, and at the same time, there
was a crash in the wild forest reindeer population in Kainuu
adjacent to a reindeer herding area (from 1,700 to 1,000
individuals), with a high percentage of peritonitis. This was
not observed in the southern population (Suomenselkä)
where the smf density was also significantly lower.
The roe deer appears to be another important natural host
and may have a role as a long-distance vector of different
haplotypes of S. tundra in Finland with relatively high
prevalences and densities of smf. Moreover, S. tundra
Parasitol Res (2009) 104:257–265
found in roe deer in northern Finland and in Italy
(Laaksonen et al. 2007) were genetically similar to S.
tundra in the current outbreak in reindeer, and the
haplotype found in roe deer in southern Finland was similar
to the causative agent of peritonitis in moose in northern
Finland in 1989. S. tundra seems also to be a well-adapted
universal parasite of the roe deer, as there were no gross
pathological changes associated with infection (Laaksonen
et al. 2007). The roe deer population has emerged recently
in northern Finland and is now present in the whole
reindeer herding area. It is worth noting that roe deer,
especially young males, can migrate many hundreds of
kilometers from their birthplace (Cederlund and Liber
1995).
Microfilaremia
Although smf are present in reindeer blood round the year,
microfilarial production and circulation tend to be most
intense in midsummer, a few weeks after the calving
season, when calves’ passive immunity is lowest (Orro et
al. 2006). The peak microfilaremia period coincides with
the adult animals’ almost total hair loss due to haircoat shift
and mass appearance of several blood-sucking insects
(= “räkkä”-time), all features favoring transmission.
Räkkä-time is a stressful period for reindeer, since they
are in constant movement round the clock trying to get rid
of mosquitoes, black flies, horse flies, stable flies, horn
flies, warble flies, throat bot flies, biting midges, and other
harassing insects. Some mf repeatedly migrate to peripheral
blood from the small vessels of the lungs, where they stay
during the periods of low microfilaremia (Hawking 1973;
Bogitsh et al. 2005). Host exercise increases the microfilarial density in the peripheral blood where they are
possibly taken up by the arthropod vector with its blood
meal and consequently promote transmission. Thus, the
behavior and niches of smf have not only been selected
under the pressure of vectors’ feeding and activity habits
(Bain and Babayan 2003) but also likely under the pressure
of annual changes in the host’s physiological stage. The
absence of smf in blood samples taken from reindeer
fetuses indicates that smf are probably not capable of
transplacental infection in reindeer.
The impact of microfilaremia on cervid health is
unknown and difficult to separate from the impact of adult
worms. The reindeer in the experimental zoo appeared to
suffer, and they had inexplicable symptoms resembling
those described in chronic Setaria spp. microfilariosis in
buffaloes (Sharma et al. 1981; Kumar et al. 1984; Kumar
and Sharma 1994; Venu 2000) such as anorexia, purulent
discharge from eyes, pale mucous membranes, rough and
dry skin coat, and stiff gait. Because these reindeer also had
263
adult S. tundra and other parasites, it is impossible to
evaluate the role of smf in their unwell-being. Moreover,
round-the-year corralled reindeer are usually more or less
hypoconditioned compared to free ranging reindeer, maybe
because they are not able to display normal patterns of
migrative and selective feeding behavior.
Information about S. tundra life cycle
The life cycle of S. tundra in reindeer is not well-known,
but as the blood sucking insects play a major role, the
infection has to take place during summer months. The
prepatent period of S. tundra, according to the temporal
monitoring of reindeer slaughter batches, is about 4 months.
This is less than the reported prepatent period (224 days) of
Setaria cervi larvae in East European red deer (Cervus
elaphus maral; Shol and Drobischenko 1973).
The life span of female S. tundra parasites in the
definitive host is at least 14 months, probably much longer.
Two slaughtered experimental reindeer in this study had
adult fertile S. tundra nematodes in September 2005, and
three had mild but steady microfilaremia through the winter
and spring 2005. This is congruent with the life span of
Setaria marshalli, which is 1 year after infection (Fujii et al.
1995) and of S. labiatopapillosa, about 16 months (Osipov
1972). The absence of male parasites in the two reindeer
autopsied probably indicates that they are more short-lived
and redundant after fertilization. Microfilariae can also be
long-lived and perhaps survive for several years (Nelson
1966), but our data indicates that the vast majority of smf
live only for months, at most.
Tools for survey
Blood samples proved practical in monitoring S. tundra
infection in semidomesticated and wild cervids. During the
S. tundra outbreak’s rapid expansion, the fast and extensive
sampling in geographically wide-ranging wilderness was
possible only with the help of reindeer slaughterhouses and
active hunters. Microfilaria counting and identification by
morphological feature analyses at family level were
possible using modified Knott’s method and combined
with PCR methods at species level. It was also possible to
distinguish and identify mf from blood samples preserved
frozen for years without anticoagulant. In the freezing
experiment, the observed smf density decreased, probably
due to getting caught by the blood cell clots; however, the
smf were well preserved, in the samples. According to our
knowledge, there are no earlier morphological descriptions
of S. tundra mf, and the descriptions of other Setaria spp.
mf in literature are few, and reported measurements are
divergent, e.g., the lengths of S. cervi are 271 μm (Ansari
264
1963), 205–231 μm (Anderson 2000), 280 μm (Wajihullah
2001), so taxonomic research is still needed.
This study revealed the absence of basic data concerning
filarioid nematodes in arctic and subarctic wildlife. Heavy
S. tundra infection is likely detrimental at least to
semidomesticated reindeer (Laaksonen et al. 2007). The
rapid emergence of this vector-borne parasite among
reindeer population brought out a number of challenges,
the greatest of which are the understanding of the factors
promoting the parasite’s expansion and its impact on
reindeer/caribou populations in the future. Investigating
the pathogenicity, the development of immunity associated
with S. tundra, possible means of prevention, and the
vectors and vector biology, in the arctic and subarctic
nature are worth future research efforts. This topic is also
highly timely, since the predicted change in temperatures
indicates a particularly strong warming trend at the high
latitudes in the northern hemisphere. This may increase the
incidence of climate-sensitive mosquito-borne diseases, like
setariosis (see Patz et al. 1996; Anderson 2000; Kutz et al.
2004).
Acknowledgements All the animal handling procedures for this work
were accepted by the Experimental Animal Committee, the University of
Oulu (license no. 030/04). The authors thank the Kallioluoma Reindeer
Herding Cooperative for assistance and patience, Orion Pharma Oy for
part-funding this study and the Ministry of Agriculture and Forestry
(MAKERA) for funding the Reindeer Health Care Program, which
constituted the basis of this study. We also thank all the Finnish reindeer
meat inspecting veterinarians and hunter organizations and especially
Kauko Kilpeläinen for their kind collaboration and Tuire Nygren for
providing us the historical Setaria sp. sample from moose, the personnel
of the Oulu University Experimental Zoo, and the laboratory personnel
of Evira in Oulu, for assistance. We thank also Arlo Pelegrin who gave
us valuable comments on the manuscript.
References
Anderson RC (2000) The superfamily filarioidea in nematode parasites of vertebrates; their development and transmission, 2nd edn.
CABI Publishing, New York
Ansari JA (1963) Studies on Setaria cervi (Nematoda: Filarioidea). Part
I. Morphology of the microfilaria. Zeitschrift für Parasitenkunde
4:245–248
Bain O, Babayan S (2003) Behaviour of filariae: morphological and
anatomical signatures of their life style within the arthropod and
vertebrate hosts. Filaria J 2:16
Bogitsh BJ, Carter CE, Oeltman TN (2005) Blood and tissue
nematodes in Human parasitology, 3rd edn. Elsevier Academic,
Burlington, MA, USA
Cederlund G, Liber O (1995) Rådjuret, viltet, ekologin och jakten.
Almqvist and Wiksell Tryckeri, Uppsala, pp 113–117
Dietrich RA, Luick JR (1971) The Occurence of Setaria in reindeer. J
Wildl Dis 7:242–245
Parasitol Res (2009) 104:257–265
Fujii T, Hayashi T, Ishimoto A, Takahashi S, Asano H, Kato T (1995)
Prenatal infection with Setaria marshalli (Boulenger 1921) in
cattle. Vet Par 56:303–309
Georgi JR (1985) Diagnostic parasitology. In: Georgi JR, Georgi ME
(eds) Parasitology for Veterinarians. 4th edn. W.B. Saunders,
Philadelphia, pp 261–262
Hawking F (1973) The responses to various stimuli of microfilariae of
Dirofilaria corynodes, of Dipetalonema marmosetae and of
unidentified species of Filaria in Saimiri saimiri and Cacajaco
monkeys. Int J Par 3:433–439
Hoberg EP, Polley L, Jenkins EJ, Kutz SJ, Veitch AM, Elkin BT
(2008) Integrated approaches and empirical models for investigation of parasitic diseases in northern wildlife. Emerg Inf Dis
14:10–17
Klei TR, Torbert BJ, Ochoa R (1980) Efficiacy of ivermectin (22,23
dihydroavermectin B1) Against Adult Setaria equine and Microfilarie of Onchocerca cervicalis in Pones. J Parasitol 66:859–851
Kumar M, Sharma SP (1994) Haematological and biochemical studies
in setariasis in buffaloes. J Appl An Res 5:41–46
Kumar B, Joschi HC, Kumar M (1984) Clinico-haematological
changes in microfilarie affected buffaloes (Bubalus bubalis).
Ind J Vet Med 4:45–47
Kutz SJ, Hoberg EP, Nagy J, Polley L, Elkin B (2004) “Emerging”
parasitic infections in arctic ungulates. Integr Comp Biol 44:109–118
Laaksonen S, Kuusela J, Nikander S, Nylund M, Oksanen A (2007)
Parasitic peritonitis outbreak in reindeer (Rangifer tarandus
tarandus) in Finland. Vet Rec 160:835–841
Nelson GS (1966) The pathology of filarial infections. Helminthol
Abstr 35:311–336
Nieminen M (2006) Suomen poronhoidon historia ja kehitys. (The
history and development of Finnish reindeer management).
Poromies 6:26–29
Nikander S, Laaksonen S, Saari S, Oksanen A (2007) The
morphology of the filaroid nematode Setaria tundra, the cause
of peritonitis in reindeer Rangifer tarandus. J Helminth 81:49–55
Nygren T (1990) Riistantutkimusosaston tiedote. [Bulletin of Finnish
Game and Fisheries Institute] 104
Orro T, Nieminen M, Tamminen T, Sukura A, Sankari S, Soveri T
(2006) Temporal changes in concentrations of serum amyloid-A
and haptoglobin and their associations with weight gain in
neonatal reindeer calves. Comp Imm Microb Inf Dis 29:79–88
Osipov AN (1972) Observations on microfilarial numbers and on the life
span of Setaria labiatopapillosa in cattle. Byulleten`Vsesoyuznogo
Instituta Gel`mintologii im. K. I. Skryabina 52–54
Patz JA, Epstein PR, Burke TA, Balbus JM (1996) Global climate
change and emerging infectious diseases. JAMA 275:217–223
Prestwood AK, Pursglove SR (1977) Prevalence and distribution of
Setaria yehi in southeastern white-tailed deer. J Am Vet Med
Assoc 171:933–935
Rehbinder C (1990) Some vector borne parasites in Swedish reindeer
(Rangifer tarandus tarandus). Rangifer 10:67–73
Sharma SP, Siddiqui AA (1996) Efficacy of three antihelmintics in
experimentally induced setariosis in lambs. J Vet Parasitol
10:159–163
Sharma V, Narayan R, Choudhuri PC (1981) Biochemical changes in
the blood of buffaloes suffering from microfilariasis. Ind J Vet
Med 1:58–65
Shirasaka S, Suzuki M, Endou G, Adachi Y, Taira N (1994) Efficacy
of ivermectin against Setaria microfilariae in calves and
cerebrospinal setariosis in sheep and goats. J Vet Med Sci
56:1213–1214
Shol` VA, Drobischenko NI (1973) Development of Setaria cervi
(Rudolphi, 1819) in Cervus elaphus maral. Helminthologia
Bratislava 14:214–246
Parasitol Res (2009) 104:257–265
Solismaa M, Laaksonen S, Nylund M, Pitkänen E, Airakorpi R,
Oksanen A (2008) Filarioid nematodes in cattle, sheep and horses
in Finland. Acta Vet Scand 50:20
Venu R, Radhakrisna Murthy P, Sreedeyi C (2000) Prevalence of
microfilariasis in graded murrah buffaloes in west godavari
district of Andra Pradesh. Ind Vet J 77:272–273
265
Wajihullah BA (2001) Comparative account of developing larvae
of Setaria cervi and Diplotriaena tricuspis. J Vet Par 15:117–
120
Weinmann CJ, Anderson JR, Longhurst WM, Connolly G (1973)
Filarial worms of Columbian black tailed deer in California. 1.
Observations in the vertebrate host. J Wildl Dis 9:213–220