Analysis of tick hemolymph represents an important source of information on how some pathogens cause disease and how ticks immunologically respond to this infection. The present study demonstrates how to inoculate fungal propagules and collect hemolymph from Rhipicephalus microplus engorged females.
Ticks are obligate hematophagous ectoparasites and Rhipicephalus microplus has great importance in veterinary medicine because it causes anemia, weight loss, depreciation of the animals' leather and also can act as a vector of several pathogens. Due to the exorbitant costs to control these parasites, damage to the environment caused by the inappropriate use of chemical acaricides, and the increased resistance against traditional parasiticides, alternative control of ticks, by the use of entomopathogenic fungi, for example, has been considered an interesting approach. Nevertheless, few studies have demonstrated how the tick's immune system acts to fight these entomopathogens. Therefore, this protocol demonstrates two methods used for entomopathogen inoculation into engorged females and two techniques used for hemolymph collection and hemocytes harvesting. Inoculation of pathogens at the leg insertion in the tick female's body allows evaluation of females biologic parameters unlike the inoculation between the scutum and capitulum, which frequently damages Gené's organ. Dorsal hemolymph collection yielded a higher volume recovery than collection through the legs. Some limitations of tick hemolymph collection and processing include i) high rates of hemocytes' disruption, ii) hemolymph contamination with disrupted midgut, and iii) low hemolymph volume recovery. When hemolymph is collected through the leg cutting, the hemolymph takes time to accumulate at the leg opening, favoring the clotting process. In addition, fewer hemocytes are obtained in the collection through the leg compared to the dorsal collection, even though the first method is considered easier to be performed. Understanding the immune response in ticks mediated by entomopathogenic agents helps to unveil their pathogenesis and develop new targets for tick control. The inoculation processes described here require very low technological resources and can be used not only to expose ticks to pathogenic microorganisms. Similarly, the collection of tick hemolymph may represent the first step for many physiological studies.
The cattle tick, Rhipicephalus microplus, is an hematophagus ectoparasite with an enormous negative impact on livestock in tropical areas. This tick is the vector of pathogenic agents such as Babesia bovis, Babesia bigemina, and Anaplasma marginale that, combined with the direct hemofeeding damage, can reduce milk and meat production, cause anemia and ultimately death. Losses caused by this ectoparasite were estimated in 3.24 billion dollars annually in Brazil1. Sustainable methods are demanded and the use of entomopathogenic agents is considered a promising alternative to reduce the use of chemical acaricides2,3,4.
Entomopathogenic fungi, such as Metarhizium spp., are natural enemies of arthropods including ticks, and some isolates can be used as biocontrollers. These pathogens actively infect the host through the cuticle and colonize their body2,5,6. As the infection develops, cellular and humoral responses are mediated by the tick immune system. Analysis of the tick hemolymph is reported as a useful tool to evaluate the immune responses after the challenging with pathogens7,8.
Arthropods' immune response is divided into humoral and cellular responses. The humoral response involves hemagglutination processes and production of antimicrobial proteins/peptides, whereas the cellular immune response is performed through the hemocytes. These cells are present in the hemolymph from all arthropods and are reported to develop an expressive role in studies involving innate immune response9, as it is directly related to the phagocytosis and encapsulation processes. Accordingly, studies about hemocytes can help to investigate death pathway and understand processes such as autophagy, apoptosis, and necrosis. In some invertebrates as bivalves, the hemocytes' collection faces limitations like cell disruption, low hemolymph volume obtained, and low concentration of recovered cells10. Very frequently, depending on the methodology applied, reduced concentration of cells is obtained, impacting directly on cells quantification and analysis.
The number of hemocytes circulating in the hemolymph is variable among different arthropods and it can change in the same species due to different physiological stages such as sex, age, and the arthropod's developmental stage11. Hemocytes can also be found adhered to some organs and be released into circulation just after the infection process11. Nevertheless, most studies reported use insects, while ticks remain less explored regarding their physiology and pathology. Despite pathogen inoculation and hemolymph collection in ticks are less used techniques, establishing standard methods helps the development of more accurate studies.
The aim of the present study was to compare the most used methods for hemolymph collection and inoculation of pathogens into R. microplus ticks, evaluating the efficacy in the hemolymph acquisition and hemocytes concentration.
Ticks used in the present study were obtained from an artificial colony, mantained at Federal Rural University of Rio de Janeiro, which methods have been approved by the Committee on Ethics for the Use of Vertebrate Animals (CEUA-IV/UFRRJ #037/2014).
1. Tick engorged females
2. Pathogens inoculation between the scutum and capitulum
NOTE: In the present study, entomopathogenic fungal suspensions were used as an example.
3. Inoculation of entomopathogenic fungi between the leg thigh and the tick female's body
4. Dorsal hemolymph collection
5. Hemolymph collection from the tick leg
6. Hemolymph processing
7. Hemocyte sampling slide preparation
This article approaches inoculation and hemolymph collection methods applied to ticks. After the inoculation between the leg thigh and the tick female's body, some fluid (hemolymph) can be secreted during the process; however, it is important to note that when the inoculation is finished, no liquid or tissues were present in the needle tip or at the inoculation site, ensuring that the fungal suspension was completely inoculated. When the inoculation process was correctly performed, the needle insertion did not cause tick females' death. On the other hand, ticks died approximately 48 h after inoculation of the entomopathogenic fungus. Inoculation between the leg thigh and tick female's body can damage tick intern organs such as midgut and the Malpighian tubules, while damage to Gene's organ can occur during inoculation between the scutum and capitulum.
When the tick midgut is disrupted by the needle during the dorsal hemolymph collection, the fluid obtained is red, not colorless. This indicates an incorrect hemolymph collection. In these cases, hemolymph shall be discarded since it is contaminated with intestinal content.
The correct hemolymph collection is crucial to properly conduct the experiments and obtain reliable results. When hemolymph was collected from cut tick legs (n = 20 ticks homogenously weighed), the volume obtained was lower than the total hemolymph obtained from the dorsal collection (n = 20 ticks homogenously weighed) (Figure 4). It is suggested that, due to the low volume of each drop that pulls out of the cut leg, hemolymph clotting can be frequently present during this process of hemolymph collection. This may negatively interfere in the hemocytes acquisition and classification (Figure 5 and Figure 6). Despite the higher volume achievement of hemolymph, the dorsal collection is considered more difficult to be performed.
Figure 1: Ticks'cuticle hygienization. Rhipicephalus microplus fully engorged females were washed using tap water and immersed in 0.5% sodium hypochlorite solution (v/v) for 3 min in a 500 mL glass beaker recipient. Please click here to view a larger version of this figure.
Figure 2: Ticks'cuticle drying. Rhipicephalus microplus fully engorged females were dried using sterile paper towels. Please click here to view a larger version of this figure.
Figure 3: Metarhizium blastospores. Fungal propaluges used in tick inoculation. Scale bar = 50 µm. Please click here to view a larger version of this figure.
Figure 4: Representative graph demonstrating hemolymph volume obtained with each collecton method. Rhipicephalus microplus hemolymph volume obtained after dorsal or leg collection. A pool of 20 homogenously weighed ticks were used for each method. These ticks were not previously inoculated. Mean values ± standard deviation followed for the same letter do not differ statistically after analysis of variance (ANOVA) test (P ≥ 0.05). Please click here to view a larger version of this figure.
Figure 5: Hemocytes concentration obtained with each hemolymph collection method. Rhipicephalus microplus hemocytes concentration obtained after resuspension in Leibovitz's L-15 culture medium after dorsal or leg collection. Mean values ± standard deviation followed for the same letter do not differ statistically after Kruskal-Wallis test (P ≥ 0.05). Please click here to view a larger version of this figure.
Figure 6: Hemocytes found in Rhipicephalus microplus hemolymph smear. R. microplus hemocytes were stained by Giemsa. Black arrows indicate the different cells in the tick hemolymph. Scale bar = 100 µm. Please click here to view a larger version of this figure.
Inoculation of pathogens is useful when the study aims to investigate the in vivo action of microorganisms in experimental arthropod models because it assures that the pathogen is inside the host. The technique can also be applied to inoculate molecules such as RNA interference (RNAi). Inoculation between the scutum and capitulum is considered easier to perform but frequently damages Gené's organ, impairing the eggs viability12,13. Gené's organ is anatomically located close to the anterior part of capitum, and it is an important organ for the tick oviposition14. Accordingly, inoculation between the leg tigh and the female's body is more appropriate if the study requires the observation of females' biological parameters since the Gené's organ will not be injured. Despite the inoculation method between the leg thigh and the tick's body is considered more difficult and easily damage or expose the tick internal organs, such as the midgut and the Malpighian tubules, when it is well performed, it will not disrupt these organs.
Hemolymph analysis is essential to understand the arthropod immune system as well as to understand pathogenesis15,16. Accordingly tick hemolymph can be used to unveil tick physiology, assure tick infectivity, understand tick-pathogen interactions, and the cellular and humoral immune responses9,17,18.
Tick hemolymph collection through the leg cutting is reported in several studies19,20,21. Despite this method being simple with no or very low hemolymph contamination, it is considered counter productive for studies that require high concentrations of hemocytes or large volumes of hemolymph. On the other hand, the correct execution of hemolymph collection through the dorsal region of engorged females is not easy to achieve since the gut occupies almost the entire tick body and rupturing it with the needle causes hemolymph contamination. Contamination due to gut disruption can also be observed when the tick is naturally infected with high loads of hemoparasites (viz., Babesia spp.), or in the final steps of the death process caused by entomopathogens8. In these cases, hemolymph shall be discarded since hemocytes and plasma analyzes can be affected.
The centrifugation speed of hemolymph samples for hemocytes harvesting is also important and directly influence the hemocytes concentration, since high relative centrifugal field (RCF) speeds contribute to: i) cell pellet difficult to resuspend, ii) cells disruption, and iii) hemocytes degranulation. The ideal cell pellet is a soft pellet easy to resuspend. For this reason, the centrifugation at 500 x g for three minutes at 4 °C8 was used. In the present study, hemocytes were resuspended in a culture medium and not in phosphate-buffered saline (PBS) because the culture medium supports better cell viability.
The methods presented here may face limitations when applied to immatures stages (larva and nymph) of ticks or unfed adults. The inoculation method, for example, is only applied for adult engorged females because the needle size will damage immature stages and possibly unfed adult ticks. To inoculate these stages, a microinjector shall be used. Similarly, hemolymph collection through the dorsal region is more effective when applied to engorged ticks, while hemolymph collection by cutting the legs can be used in studies with unfed adult ticks or immature stages. Despite this, our goal here was to show methods that require very low technological resources. In addition, centrifugation technique has to be performed at low centrifugation force because high force or extended spin time can damage cells.
The methods disclosed here can be used as guidelines for studies involving inoculation of entomopathogens into ticks and hemolymph/hemocytes harvesting. The techniques presented here require very low technological resources and are classical steps for studies about tick physiology, pathology, and tick's immune response.
The authors have nothing to disclose.
This study was financed in part by the Coordenacão de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) from Brazil, finance code 001. CAPES provided Ph.D. scholarship for A.F. Marciano. We thank National Council for Scientific and Technological Development (CNPq) of Brazil for providing Ph.D. scholarship for J. Fiorotti. This research was also supported by grants of Carlos Chagas Filho Foundation for Research of the State of Rio de Janeiro (FAPERJ) and CNPq. V.R.E.P. Bittencourt is a CNPq researcher.
Alkaline Hypochlorite solution | Sigma-Aldrich | A1727 | |
D-(+)-Glucose | Sigma-Aldrich | G8270-1KG | |
EDTA | Synth | 2706 | |
Fetal Bovine Serum | Gibco | 16000036 | |
Flexible rubber | BD | ||
Giemsa stain | Sigma-Aldrich | 48900-500ML-F | |
Glass capillary | CTechGlass | CT95-02 | |
Insulin syringe (needle) | BD | SKU: 324910 | |
KH2PO4 | Vetec | 60REAVET014512 | |
Leibovitz's L-15 culture medium | Gibco | 11415-064 | |
Methanol | Sigma-Aldrich | 34860-1L-R | |
Microscope slides | Kasvi | K5-7105 | |
Microtubes | BRAND | Z336769-1PAK | |
Na2HPO4 | Vetec | 60REAVET014593 | |
NaCl | Sigma-Aldrich | S7653-1KG | |
Neubauer chamber | Kasvi | K5-0111 | |
Penicillin | Gibco | 15140163 | |
Protease inhibitor cocktail | Sigma-Aldrich | P2714 | |
Tween 80 | Vetec | 60REAVET003662 |