Veterinary Parasitology 117 (2003) 271–283
Acquired resistance of horses to Amblyomma cajennense (Fabricius, 1787) ticks Karina C. Castagnolli b , Luciano B. de Figueiredo a , Danilo A. Santana a , Márcio B. de Castro a , Marco A. Romano a , Matias P.J. Szabó a,b,∗ b
a Universidade de Franca, Franca, SP, Brazil Universidade Estadual Paulista, Jaboticabal, SP, Brazil
Received 12 May 2003; received in revised form 13 September 2003; accepted 14 September 2003
Abstract Acquired immunity of horses to larvae, nymphs and adults of the Amblyomma cajennense tick was evaluated through three consecutive experimental infestations of tick-bite na¨ıve hosts. Data from these infestations were compared to those from field-sensitized horses and donkeys. It was observed that tick-bite na¨ıve horses developed a low level of resistance after two infestations as shown by a significant decrease in larval yield and a tendency for lower engorged weight of nymphs during third infestation. Ticks fed on field-sensitized horses had a similar biological performance to that observed on the third infestation of tick-bite na¨ıve horses but the mean engorged nymph weight was significantly lower than that of the first infestation from tick-bite na¨ıve horses. Donkeys presented the strongest resistance with significantly lower engorged weights of all instars and of the egg mass compared to the first infestation of tick-bite na¨ıve horses. Donkeys also displayed a significantly higher resistance than field-sensitized horses as demonstrated by significantly lower egg mass weights. Overall these results indicate that donkeys but not horses maintain a strong resistance to A. cajennense ticks. The importance of these findings in relation to vectoring of tick-borne diseases is discussed. © 2003 Elsevier B.V. All rights reserved. Keywords: Amblyomma cajennense; Donkey; Horse; Resistance; Ixodidae
∗ Corresponding author. Present address: Rodovia Prof. Paulo Donato Castelane s/n, Depto. de Patologia Veterin´aria, F.C.A.V.J.-UNESP, 14.884-900 Jaboticabal, SP, Brazil. Fax: +55-16-3202 4275. E-mail address:
[email protected] (M.P.J. Szab´o).
0304-4017/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.vetpar.2003.09.004
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1. Introduction Ixodid and argasid ticks are obligate ectoparasites of practically all terrestrial and flying vertebrates (Balashov, 1972). These parasites are cosmopolitan and are of medical and veterinary importance due to both direct damage caused by feeding and transmission of disease causing agents. Horses in Brazil are infested by three tick species: Anocentor nitens, Amblyomma cajennense and less commonly, Boophilus microplus (Labruna et al., 2001). A. cajennense (F), the cayenne tick, is widely distributed throughout the American continent, from the south of the US, through Central America to northern Argentina (Robinson, 1926). This tick species is commonly found on many domestic and wild animals but is also known for being aggressive to human beings (Campos Pereira et al., 2000; Guimarães et al., 2001; Szabó et al., 2001). Although the native capybaras and tapirs are believed to be the main natural hosts for A. cajennense, horses have also been proven to act as primary hosts for all parasitic stages of this tick and are capable of maintaining high tick populations in pastures (Aragão, 1936; Labruna et al., 2001). A. cajennense is the main vector of Rickettsia rickettsii, the causative agent of spotted fever in the Neotropical region (reviewed by Campos Pereira and Labruna, 1998) and a potential vector of the Ricketsia Cowdria ruminatum the pathogen that causes heartwater in Africa (Barre et al., 1987). This tick species might also be involved in the transmission of the unknown causative agent of Lyme-simile disease of humans in Brazil (Guimarães et al., 2001). To date, only a few studies have examined the life cycle, ecology or reproductive behavior of A. cajennense on its primary hosts. At the same time, a few examples indicate that in naturally occurring relationships hosts do not develop resistance to ticks even after repeated feeding (Randolph, 1979; Fielden and Rechav, 1992; Szabó et al., 1995). The knowledge of the immune status of a particular host species to ticks is important for the comprehension of disease vectoring, as resistance to ticks may reduce tick-borne disease transmission (Wikel et al., 1997). Considering the importance of A. cajennense tick in disease vectoring and its maintenance by horses in domestic environment, it would be worthwhile to quantify resistance development of horses to A. cajennense ticks under controlled conditions. Additionally, if alternative control measures such as vaccination against A. cajennense are to be developed, information of naturally occurring resistance might be necessary. Thus, the objective of the present work was to evaluate acquisition of resistance of tick-bite na¨ıve horses to A. cajennense ticks during three consecutive infestations and compare it to resistance prevailing in the field on naturally sensitized horses and donkeys. This last host species was included because empirical field observations indicate it as a tick-resistant host. 2. Materials and methods 2.1. Ticks An A. cajennense tick colony was set up in the laboratory to supply the experiments with unfed ticks. Initially, engorged females were collected from horses of Instituto de Zootecnia de Colina, São Paulo State, Brazil. Once identified, they were kept under constant temperature and relative humidity conditions of approximately 29 ◦ C and 85%, respectively.
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A photoperiod of 12 h:12 h (L:D) for non-parasitic stages was used throughout experiments. Humidity conditions were obtained by placing vials with ticks in a dessicator containing a saturated solution of KCl in the lower compartment. Continuous tick supply as then maintained by feeding each stage on tick-bite na¨ıve New Zealand white rabbits. Unfed ticks used in experiments were from 1 to 3 weeks old in the case of immature stages and at most 4 weeks old in the case of adults. 2.2. Hosts Equines (Equus caballus) and donkeys (Equus asinus) were used as experimental hosts. All animals were generously loaned by neighboring farms or donated by a highway maintenance company (Autovias S/A). Tick-bite na¨ıve horses were obtained by breeding pregnant mares and their offspring in tick-free stalls. For this purpose, pregnant mixed bred mares undergoing final period of pregnancy were taken to Franca University’s Veterinary Hospital and submitted to acaricide bath with cypermethrin (Cypermil® OuroFino) every week for 3 weeks. These animals were also given twice, 1 week apart, an oral moxidectin-based anti-helmintic and acaricide compound (Equestre® Fort Dodge). Mane, ear and tale hair were cut short to permit close inspection for ticks after each treatment and increase acaricide treatment efficiency. Ears, a common site for A. nitens tick infestation, were also treated with a local methilcarbamate-based acaricide (Tanidil® , Bayer). Following all treatments and meticulous inspection for ticks, mares were taken to the new stalls (never used before for animal keeping) of the animal reproduction sector of the veterinary hospital and kept there until birth and weaning of foals which occurred at 3 months of age. To avoid accidental introduction of ticks, mares and colts were fed solely with commercial ration (Equitage Supreme® Guabi) during this period. Colts received anti-helmintic treatment at 3 months of age (Equestre® Fort Dodge). Horses were allowed a daily sunbath in a cemented yard adjacent to the stalls. Stalls and yard were periodically sprinkled with Amitraz-based acaricide (Nokalt® OuroFino) throughout the horse breeding period for the experiments. For the experimental infestations, 5–7-month-old tick-bite na¨ıve horses were taken to the veterinary hospital stalls where they remained till the end of experiments. These stalls but not the animals were acaricide treated after each infestation. Field-sensitized animals, horses and donkeys were taken to Franca University’s veterinary hospital or to Centro de Pesquisa em Sanidade Animal-UNESP (CPPAR) stalls. These animals were given commercial ration and water “ad libitum”. All field-sensitized donkeys and equines were already infested with more than 100 ticks (A. nitens and A. cajennense) and had endoparasites (strongyle fecal eggs, one donkey had also Parascaris equorum fecal eggs). These animals were not treated against ecto or endoparasites in order to represent prevailing field conditions as closely as possible. 2.3. Infestation procedures For each experimental infestation, three feeding chambers consisting of a plastic tube (5 cm in diameter and 3 cm in height) with a rubber and cloth base, were fixed to the shaved back of the hosts with an adhesive glue (Brascoplast® -Brascola Ltd., Brazil) 1 day before infestation. Two chambers were glued on the thoracic region of the animals one on each side
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near the vertebral midline. The third chamber was glued on the midline of the hindquarters. Neck collars were used during every infestation to prevent grooming. For infestations, each thoracic chamber received 100 larvae, 30 nymphs and 6 adult pairs. The third chamber was used as reserve in case the first two were accidentally destroyed. Experimental infestations were the same regardless of host species or immune status. Three sequential infestations of na¨ıve horses occurred at, approximately, 30-day intervals as described elsewhere for other host–tick relationships (Szabó et al., 1995). 2.4. Resistance evaluation Resistance of hosts to ticks was evaluated by analyzing feeding and reproductive data of the parasite. Chambers were inspected daily from the second infestation day on and detached engorged ticks collected and maintained as already described. Detached adult female ticks were placed into individual plastic vials and larvae and nymphs stored separately in daily batches. The following biological parameters, related to tick feeding and reproductive performance were observed during each infestation: tick yield, engorged female (FW), larval and nymphal weights and egg mass weight, feeding and pre-oviposition periods, molting rate and molting period, larval hatchability rates and efficiency rates of female ticks in converting their food reservoir to eggs (ERCE). Each egg mass was weighed 20 days after tick detachment (EW). The engorgement period was defined to be the time that elapsed between the infestation by ticks on the hosts till their detachment, partially or fully engorged; pre-oviposition, the time from detachment until beginning of oviposition; and molting period the time interval between detachment from host and first tick to molt from the daily batch. The larval hatching rate for each egg mass was estimated as the mean value of visual evaluation performed by three persons separately. The efficiency rate of conversion to eggs was calculated as follows: ERCE =
EW × 100 FW
2.5. Experimental groups Six tick-bite na¨ıve horses (three males and three females) were infested three times; seven field-sensitized horses (four males and three females) and seven field-sensitized donkeys (five males and two females) were infested once. Unpredictability of when to animals could be obtained and space constraints for experimental infestations many times impeded simultaneous infestations of animals. But, whenever possible, infestations occurred in groups composed of animals of different experimental status but using the same tick batches. This procedure was intended to minimize tick batch and seasonal effects on results. Table 1 displays experimental groups according to tick batch and time of year. 2.6. Data analysis Each host was infested in two to three feeding chambers to guarantee results from at least one chamber, nevertheless most of the time successful infestations (no perceptible
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Table 1 Experimental groups and infestations according to tick batches used and seasona,b Animal
Infestation 1
Infestation 2
Infestation 3
Field horse
Field donkey
1 2 3 4 5 6 7
A A B C G H –
B B C D H I –
C C D E I J –
A C C C Fc Fc Fc
Ac Cc Ec Ec Ec Ec Ec
a Infestations
designed with the same letter occurred at the same period and same tick batches. autumn 2001; B and C: winter 2001; D–F: spring 2001; G: summer 2001–2002; H–J: autumn 2002. c Infestations that occurred at CPPAR. b A:
loss of ticks due to chamber damage) could be obtained in two chambers. It was thus standardized that for each tick stage (larva, nymph or adult) only data from the chamber with the highest tick yield should be analyzed. Biological parameters for the three groups were submitted to analysis of variance and means compared by Tukey’s test (P < 0.05). Repeated measure tests were used to compare means from the first, second and third infestations of colts. Data from field-sensitized donkeys and equines were compared to first or third infestations of colts separately using non-repeated measure tests.
3. Results Results obtained at all infestations and respective statistical analyses are presented in Tables 2–4. Data from egg hatching are not presented and were not analyzed. This parameter was extremely variable throughout the experiments and many of the eggs did not hatch at all, regardless of experimental group. Broadly, tick-bite na¨ıve horses did not acquire an appreciable resistance to A. cajennense ticks from the first infestation to the third one. The only significant change was a decrease of larval yield from first to third infestation (Table 4). Although not statistical significant, other biological parameters displayed a tendency to decrease in tick performance when feeding was on na¨ıve horses that were sensitized by one or two previous infestations. Such a situation was observed in the case of engorged weights of larvae and nymphs and nymphal yield which all decreased slightly during the third infestation (Tables 3 and 4). Most significant differences were observed comparing data from the first infestation of na¨ıve horses to that of field-sensitized donkeys. Mean weights of engorged larvae, nymphs and adults were significantly lower when ticks were fed on donkeys (Fig. 1). Female ticks feeding on this host also produced significantly lighter egg masses and had a decreased efficiency rate of conversion to eggs. Mean larval yield from donkeys was also significantly lower if compared to the larval yield obtained from na¨ıve horses undergoing first infestation (Fig. 1).
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Infestation
Tick yield (%)
Engorged weight (mg)
Egg mass weight (mg)
Engorging period (days)
Pre-oviposition period (days)
Ercec (%)
First Secondd Third Field horses Donkeys
96.7 ± 8.2 Aa (80.0–100.0) 93.3 ± 10.3 A (80.0–100.0) 93.3 ± 10.3 Aa (80.0–100.0) 77.1 ± 33.5 a (20.0–100.0) 62.9 ± 39.0 a (0.0–100.0)
700.1 ± 78.7 Aa (594–837) 702.3 ± 108.0 A (541.0–827.5) 677.3 ± 89.7 Aab (582.2–836.7) 614.9 ± 104.2 ab (491.0–777.4) 539.2 ± 89.2 b (409.5–645.8)
243.1 ± 69.2 Aa (169.0–350.5) 283.5 ± 98.1 A (177.7–430.0) 234.4 ± 73.0 Aa (129.0–313.3) 219.6 ± 52.1 a (170.0–323.2) 71.6 ± 37.9 b (14.1–126.9)
9.3 ± 1.0 Aa (8.2–10.7) 9.4 ± 1.5 A (8.0–11.2) 9.1 ± 1.5 Aa (7.8 + 11.6) 13.8 ± 2.8 b (11.0–17.7) 12.0 ± 3.2 a (9.0–16.2)
7.6 ± 1.1 Aa (6.4–9.2) 5.8 ± 0.9 A (4.7–7.5) 6.6 ± 0.5 Aa (6.2–7.2) 7.1 ± 0.9 a (5.5–8.0) 7.1 ± 1.2 a (5.5–8.5)
34.6 ± 9.2 Aa (28.0–49.9) 39.8 ± 11.0 A (25.40–54.0) 34.3 ± 9.0 Aa (22.2–46.5) 35.8 ± 5.8 a (23.7–41.6) 13.2 ± 7.8 b (3.5–26.8)
Results are expressed as means ± S.D. Range of values is shown in parentheses. Means followed by different letters differed significantly (P < 0.05). Uppercase letters indicate statistical analysis comparing data from three consecutive infestations of colts. Lowercase letters indicate statistical analysis comparing first and third infestations to field-sensitized horse and donkey data. c Efficiency rate of conversion to eggs. d Second infestation of colts was not compared to field-sensitized horse and donkey data. a
b
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Table 2 Biological parameters of adult female A. cajennense ticks that were fed on tick-bite na¨ıve horses during three consecutive infestations or on field-sensitized horses or donkeysa,b
Infestation
Tick yield (%)
Engorged weight (mg)
Engorging period (days)
Molting period (days)
Molting rate (%)
First Secondc Third Field horses Donkeys
88.9 ± 10.5 Aa (76.7–100.0) 72.2 ± 16.9 A (53.3–100.0) 67.2 ± 16.8 Aa (40.0–86.7) 72.4 ± 21.1 a (36.7–93.3) 73.3 ± 22.6 a (43.3–100.0)
16.4 ± 2.3 Aa (13.0–19.6) 14.8 ± 1.4 A (12.9–16.2) 13.9 ± 1.6 Aab (12.8–17.1) 12.1 ± 2.6 b (9.0–16.1) 11.5 ± 1.0 b (10.3–13.0)
5.7 ± 0.4 Aab (5.2–6.3) 5.0 ± 0.6 A (4.1–6.0) 5.4 ± 1.5 Aab (3.4–7.1) 7.3 ± 2.1 b (5.5–11.5) 5.1 ± 0.6 a (4.5–5.9)
12.2 ± 2.5 Aa (9.3–15.6) 13.3 ± 0.8 A (12.0–14.0) 12.9 ± 1.4 Aa (10.9–13.8) 12.8 ± 0.8 a (11.9–13.8) 13.8 ± 1.3 a (12.7–16.2)
98.9 ± 2.7 Aa (93.5–100.0) 99.1 ± 2.3 A (94.4–100.0) 99.2 ± 2.0 Aa (95.2–100.0) 90.3 ± 10.8 a (68.2–100.0) 95.3 ± 11.5 a (69.2–100.0)
are expressed as means ± S.D. Range of values is shown between parentheses. < 0.05). Uppercase letters indicate statistical analysis comparing data from three consecutive infestations of colts. Lowercase letters indicate statistical analysis comparing first and third infestations to field-sensitized horse and donkey data. c Second infestation of colts was not compared to field-sensitized horse and donkey data. a Results
b Means followed by different letters differed significantly (P
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Table 3 Biological parameters of nymphs of the A. cajennense ticks that were fed on tick-bite na¨ıve horses during three consecutive infestations or on field-sensitized horses or donkeysa,b
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278
Infestation
Tick yield (%)
Engorged weight (mg)
Engorging period (days)
Molting period (days)
Molting rate (%)
First Secondc Third Field horses Donkeys
45.0 ± 14.4 Aa (24.0–68.0) 33.5 ± 27.8 AB (4.0–72.0) 13.2 ± 12.1 Bb (3.0–30.0) 19.9 ± 13.6 b (4.0–42.0) 10.9 ± 17.9 b (0.0–41.5)
0.77 ± 0.09 Aa (0.63–0.89) 0.73 ± 0.12 A (0.51–0.85) 0.68 ± 0.14 Aab (0.45–0.81) 0.69 ± 0.11 ab (0.54–0.85) 0.39 ± 0.37 b (0.05–0.72)
5.4 ± 0.5 Aa (4.9–6.3) 4.5 ± 0.7 A (4.0–5.6) 5.9 ± 1.4 Aa (4.0–7.0) 8.6 ± 2.0 b (6.3–11.7) 7.0 ± 1.2 ab (5.6–8.0)
10.0 ± 1.6 Aa (8.0–12.5) 11.8 ± 3.7 A (6.2–15.0) 10.6 ± 2.2 Aa (8.0–12.0) 11.3 ± 1.1 a (10.0–12.9) 12.4 ± 1.8 a (11.0–14.4)
47.7 ± 29.5 Aa (11.8–93.0) 39.8 ± 43.7 A (0.0–95.2) 43.6 ± 33.5 Aa (0.0–81.8) 57.2 ± 35.9 a (0.0–100.0) 57.7 ± 41.8 a (0.0–100.0)
are expressed as means ± S.D. Range of values is shown between parentheses. < 0.05). Uppercase letters indicate statistical analysis comparing data from three consecutive infestations of colts. Lowercase letters indicate statistical analysis comparing first and third infestations to field-sensitized horse and donkey data. c Second infestation of colts was not compared to field-sensitized horse and donkey data. a Results
b Means followed by different letters differed significantly (P
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Table 4 Biological parameters of larvae of the A. cajennense ticks that were fed on tick-bite na¨ıve horses during three consecutive infestations or on field-sensitized horses or donkeysa,b
K.C. Castagnolli et al. / Veterinary Parasitology 117 (2003) 271–283 engorged female weight
engorged nymph weight 20
weight (mg)
a
ab b
500
weight (mg)
1000
750
279
a b b
10
250
fie
infestation
infestation
larval yield
engorged larva weight 1.00
75
a
ab
0.75
a
b 50
%
weight (mg)
do nk ey s
s ne fie
ld
ld
eq
ui
fir
do nk ey s
s ne eq
ui
fir
st
0 st
0
0.50
b
b
25
0.25
do
nk
ey
s
s ne eq
ui
fir
fie
fie
ld
ui eq ld
do nk ey s
ne
s
st fir
st
0
0.00
infestation
infestation
Fig. 1. Mean weights of engorged females, nymphs and larvae and mean larval yield of A. cajennense ticks fed on either tick-bite na¨ıve horses or field-sensitized horses or donkeys.
The third infestation of na¨ıve horses also produced ticks with better biological performance in comparison to ticks fed on donkeys but differences were less extreme. In those comparisons, female ticks fed on donkeys produced significantly lower mean egg mass weight and had also a significantly lower efficiency rate of conversion to eggs (Fig. 2). Ticks from field-sensitized horses had an intermediate feeding and reproductive performance when compared to other two experimental groups. Engorged nymphal weight and larval yield of ticks fed on field-sensitized horses were significantly lower when compared to first but not the third infestation of na¨ıve horses. On the other hand, field-sensitized horses produced, in relation to ticks fed on donkeys, significantly heavier egg mass weights and also displayed ticks with a significantly superior efficiency rate of conversion to eggs. It was curious to observe, however, that ticks fed on field-sensitized horses had a prolonged feeding period of larvae, nymphs and adults when compared to donkeys and colts during first and third infestation. These extended feeding periods were often statistically significant (Tables 2–4).
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400
Erce
50
a
a
40
a %
200
b 100
30
b 20 10
fie
s
fie
ld
ld
do
nk
ey
s ne ui eq
s ey nk
ui eq
do
ne
s
ird th
ird
0
0
th
weight (mg)
a 300
infestation
infestation
Fig. 2. Mean egg mass weight and efficiency rate of conversion to eggs of engorged A. cajennense females fed on either the third infestation of tick-bite na¨ıve horses or field-sensitized horses or donkeys.
4. Discussion To the best of our knowledge, this work is the first to determine A. cajennense biological parameters using tick-bite na¨ıve horses as hosts, to evaluate horse resistance acquisition to A. cajennense ticks through repeated experimental infestations, and to determine the feeding data of larvae and nymphs on horses and donkeys. There are a few published reports on feeding and reproductive data of A. cajennense ticks but they cannot be directly compared to our observations as other hosts such as bovines (Serra-Freire and Furlong, 1993) and rabbits (Daemon and Ishizuka, 1995; Prata et al., 1997; Sanavria and Prata, 1996) were used. In addition, in the few works that used horses as hosts for adult ticks, non-parasitic phases were maintained at lower temperatures (Prata and Daemon, 1997; Sanavria et al., 1996). But, even considering such differing conditions, many results from these other reports are roughly similar to ours and indicate the existence of an A. cajennense species-associated biological pattern in Brazil. Among the biological parameters observed in the present work egg hatching and larval molting data were lower than the values observed previously. For example, Prata et al. (1997) collected engorged females from horses, fed their larvae on rabbits and obtained approximately 95% egg hatching and a mean molting rate of engorged larvae of 95%. Our egg hatching was extremely variable throughout the experiment and many of the eggs did not hatch at all regardless of experimental group and molting rate was around 50% in all groups. We do not know the reasons for these lower values and our experimental conditions, such as higher incubation temperature (29 ◦ C), might be blamed for them. In truth, A. cajennense ticks seem to be very susceptible to some environmental conditions. Labruna et al. (2001) observed a requirement for mixed overgrowth pastures with several species of bushes and shrubs interposed with grasses for A. cajennense, since mowing all pastures once a year prevented high infestation levels on farms. On the other hand, other biological parameters from our experiments had values equal to or above those observed elsewhere. Sanavria et al. (1996) infested the neck of equines with adult A. cajennense ticks under controlled
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conditions and obtained very similar biological data to those of field-sensitized horses from the present work. These authors obtained female ticks with a mean engorged weight of 570.38 and 236.40 mg of mean egg mass weight. At the same time, their adult tick yield was slightly less (43.57%) as was the nymphal yield of Prata et al. (1996) with a recovery of 53.44% of the released ticks. Overall these observations indicate that our experimental conditions permitted the proper development of nymphs and adults although not so for larval molting and egg hatching. Comparing data of the biological parameters from ticks infesting na¨ıve horses three times or field-sensitized horses and donkeys from this work, it was observed that equines developed a modest degree of resistance to A. cajennense ticks following successive infestations when compared to other host–tick relationships such as guinea pig—R. sanguineus (Szabó et al., 1995). In the present report, there was a tendency of decreased biological performance of ticks to the third infestation. Furthermore, biological parameter values of ticks from the third infestation of na¨ıve horses were close to those seen on field-sensitized horses. These observations suggest that, although variation might exist over time, the resistance expression of horses in the field is close to that observed following three infestations and is only marginal. That observation explains the susceptibility of horses to A. cajennense ticks and the association between horses and this tick species in pastures. Donkeys seem to maintain a stronger resistance that interferes with the feeding and reproductive performance of the tick on this host species. The difference between the expression of resistance between horses and donkeys may have an important role in infectious disease vectoring. As already mentioned, resistance to ticks may lessen tick-borne disease transmission (Wikel et al., 1997). It is conceivable that A. cajennense will have a lower vectoring capacity to donkeys. Such a speculation seems to be supported by the recent observation of negative serum titers to the spotted fever agent, R. rickettsii, of donkeys from Pedreira, São Paulo, whereas 77% of horses from the same county displayed positive titers (Horta, 2002). Positive titers of horses could be explained by an inefficient immunity against ticks at feeding sites, which is also inefficient against intracellular parasites. Immune response inadequate to ticks, which is also inefficient against intracellular parasite, was demonstrated in a mice model by Ferreira and Silva (1999). Such inadequate immune reaction thus allows for tick feeding as well as pathogen diffusion at tick feeding sites and bactaeremia. The influence of different breeds on the infestation level by B. microplus ticks is well known (de Castro and Newson, 1993). In our work, mixed breed horses were used and the effect of different breeds on acquired resistance to A. cajennense ticks could not be determined. In this regard, Labruna et al. (2001) evaluated infestation levels of naturally infested horses in São Paulo State and could not demonstrate any significant difference among the nine evaluated breeds. On a broad context it can be assumed that outcome of tick–host relationships is dependent on the resistance expressed by the host. Hosts developing strong resistance to a tick species will not maintain a parasite population in the environment and will be parasitized only if another host susceptible to the tick is also present. In the case of A. cajennense ticks, a Neotropical species (Camicas et al., 1998), the recently introduced horses are a physiologically acceptable host and maintain this tick in pastures throughout Brazil. The lack of a strong resistance in this host is also permissive for disease vectoring for horses. At the same time, A. cajennense is a very aggressive tick species attaching to various hosts
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including humans (Guimarães et al., 2001) and thus horse–A. cajennense relationship is very important not only for horses but also for other animal species and humans.
Acknowledgements The authors would like to acknowledge FAPESP and FUNADESP for financial support.
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