African swine fever (ASF) is a very serious contagious disease prevalent in pigs. The clinical symptoms of African swine fever virus (ASFV)-infected pigs include high fever, vomiting, diarrhea, skin hemorrhages, cyanosis, and abortion in pregnant sows, resulting in a mortality rate of up to 100% in acute ASF (1). Subacute and chronic forms emerge as respiratory signs, intermittent fever, chronic skin ulcers, and arthritis, which are caused by lower virulent strains and exhibit lower mortality rates. ASF infection in domestic pigs causes significant direct economic losses and associated indirect economic losses, such as trade restrictions. Since it was tested in Liaoning province in 2018, ASFV has spread to all mainland provinces in China, causing local pork prices to rise (2, 3) and global pork supply to be affected (4).

ASFV, the only known DNA arbovirus, belongs to the family Asfarviridae. ASFV is extremely stable and spreads easily through the infected swine, contamination during the trade of pig products, or blood feeding of infected Ornithodoros vector.

In the first report on ASF, Montgomery et al. speculated that the transmission of ASFV might be related to arthropod vectors, since the virus was not transmitted by contacting between warthogs and domestic pigs. Still, many outbreaks on Kenyan farms could only be attributed to warthogs, as the farms are isolated and no pigs or their products were introduced. However, the biological arthropod vectors of ASFV were not published in Spain until 1962, when Sanchez Botija (1963) confirmed that Ornithodoros erraticus could maintain and transmit ASFV (5). Following an investigation by researchers in East Africa, they demonstrated ASFV infection in Ornithodoros moubata complex in warthog-occupied animal burrows in Tanzania (6). Plowright et al. demonstrated ASFV proliferation, transstadial transmission, and transovarial transmission in the ticks and sexual transmission, showing that ticks could be competent vectors of the ASFV (7–10). Based on the common characteristics of Iridoviridae with ASFV, Plowright et al. suggested that ASFV may have originated from arthropod vectors and evolved in them (6). A recent molecular study supported this opinion by demonstrating ASFV genome-like segments existing in archived specimens of the O. moubata complex. Many studies have shown that ticks were important to the transmission of ASFV, and it might contribute to ASFV evolution (11).

Since Ornithodoros play an essential role in ASFV transmission, an anti-tick vaccine could be an alternative to prevent ASF in the absence of commercially available anti-ASFV vaccines. Therefore, a large number of researchers have examined the immune effects of tick components, starting with mixed proteins such as egg vitellin, salivary gland extract, and midgut protein extract, and later with recombinant single proteins. Although there are many kinds of candidate proteins affecting the life cycle of ticks, the ideal anti-tick vaccine antigen has not been screened. It was not until 3 June 2022 that the launch of ASFV-G-ΔI177L became a boon for the pig industry against ASFV. So far, whether anti-tick vaccines prevent ASF needs to be further investigated.

Here, we will summarize the role of ticks in ASFV transmission to evaluate the necessity of anti-tick vaccine development and further compare its advantages and disadvantages with the anti-ASFV vaccine, providing direction and ideas for the prevention of ASF.

Generally, ASFV transmission occurs through direct contact with infected animals or contaminants or through the bite of a soft tick. Four epidemiological cycles described by Chenais summarize the transmission of ASFV (27). Sylvatic cycle was observed in eastern and southern Africa where the virus was transmitted between warthogs and O. moubata complex (28). Tick–pig cycle mainly occurs in sub-Saharan Africa and Iberian Peninsula (29). Transmission during domestic cycle was only linked with domestic pigs, without the presence of infected free-living pigs or tick vector, such as West Africa and Brazil (30, 31). In wild boar–habitat cycle, ASFV transmission depends on carcasses in the habitat (27). This chapter will detail the role of soft ticks in ASFV epidemiological cycles.

ASFV was classified into 8 serogroups and 24 genotypes based on antibody-mediated hemadsorption inhibition and the B646L gene, respectively (46). Here, we summarized the ASFV strains of experimental transmission by the eight ticks mentioned above.

Ornithodoros marocanus larvae became infected with ASFV, isolated from an infected domestic pig in Portugal in 1986, by feeding on a viremia pig. Although virus titers decreased with tick development, adult ticks were still able to transmit ASFV to susceptible pigs 588 days later and could recover ASFV 655 days after the infective blood meal ( Table 1 ) (19). Experimentally infected O. puertoricensis second-instar nymphs with the Dominican Republic isolate (DR-II) of ASFV could transmit virus 239 days after infection ( Table 1 ) (17). Hess et al. demonstrated that it was able to transmit ASFV transstadially and transovarially ( Table 1 ) (16). Ornithodoros coriaceus can be infected with ASFV strain Tengani, Z1, and DR2 together with different mortalities, and infection with the DR2 persisted for 502 days ( Table 1 ) (16). Ornithodoros coriaceus can pass ASFV transstadially but not transovarially ( Table 1 ) (15, 16). Ornithodoros moubata porcinus-transmitted ASFV strains include Z1, Uganda, Tengani, Chiredzi/83/1 (Ch1), Pretoriuskop/96/4/1 (Pr4), Crocodile/96/1 (Cr1), and Nooitverwacht/96/6 (No6), with at least 70% of ticks multiplying and persisting for 13–15 months over “Uganda” strain virus ( Table 1 ) (8, 16, 47). Sexual transmission, stadial transmission, and ovarial transmission of ASFV were involved in O. moubata porcinus ( Table 1 ) (10, 16). ASFV strains Tomar 87, OUR T88/1, and ASFV/P99 isolate were able to be transmitted by O. erraticus, which harbored ASFV and transmitted it to pigs for at least 588 days after infection ( Table 1 ) (43, 48). ASFV strains CHZ T90/1, SIN T90/1, and VIC T90/1, isolated from O. moubata, together with Georgia 2007/1 and Ukr12/Zapo isolated from domestic pigs, were able to be transmitted by O. moubata ( Table 1 ) (37, 49, 50). Ornithodoros moubata could transmit Georgia2007/1 and Liv13/33 strains to naive pigs 8 months post-infection ( Table 1 ) (50). Only O. moubata has been shown to transovarial transmit ASFV ( Table 1 ) (51). Infected O. turicata transmitted ASFV to susceptible pigs 23 days later ( Table 1 ) (16). Infected O. savignyi can transstadially transmit ASFV and maintain it for 106 days or more ( Table 1 ) (14).

ASFV can continuously infect natural hosts and domestic pigs, which is of great significance in epidemiological studies. As an important transmission medium of ASF, the study of ASFV infection dynamics in ticks has attracted many researchers’ interests. Here, O. porcinus was taken as an example to describe the dynamics after ASFV infection.

ASFV infection happened when O. porcinus porcinus feed on blood of viremic pigs. After infection, the midgut was the first site of ASFV replication as determined by Greig, although there were immunofluorescence and virus titration detection of dissected tissues (52). Although the mechanism has not been reported, it could be speculated that the entry of ASFV into midgut epithelial cells may be related to erythrocyte phagocytosis because intact erythrocytes infected with ASFV were detected in the midgut epithelial cells phagolysosomes. It is important to note that most viremic pigs are related to erythrocytic fractions, and almost all field isolates of ASFV are hemadsorbing (53–57). An entry mechanism unrelated to erythrocyte phagocytosis might exist because non-hemadsorbing ASFV was isolated from O. porcinus porcinus and experimental ASFV infection could also occur in feeding on an artificial membrane with cell-free virus preparations (47, 47). Greig demonstrated that after being ingested by ticks, ASFV rapidly crosses the gut wall and infect the hemocoel, coxal sac, rectal ampulla, and salivary gland, together with ASFV recovered from the hemocoel and other tissues within 24–48 h of infection, suggesting that there was no true gut barrier to ASFV (52). However, Kleiboeker’s study showed a delay of 15–21 days for ticks to reach generalization of ASFV infection, suggesting the presence of midgut barrier (47).

After transmission and infection in multiple different tissues, the ASFV titer remained at 6 log₁₀ HAD₅₀/mg of body weight for 290 days of sampling period, which is closely related to that in naturally infected ticks (34, 40, 47). ASFV titers of the reproductive tissue and salivary glands rose to 5–6 log₁₀ HAD₅₀/mg after 91 dpi, which was the highest of any tissues (47).

Successful ASFV transmission from infected O. porcinus porcinus to pigs was associated with high titers in salivary and coxal glands and excretion virus into secretions (8). Surprisingly, coxal secretions were more ASFV positive and had higher ASFV titer than salivary secretions, showing that they were also an important source of ASFV transmission (47). Regurgitation was unlikely to be an effective transmission mechanism for ASFV because the esophageal terminus of argasid species had a proventricular valve, although the tick midgut had high titers of ASFV (58).

The skin represents the interface where ticks bite and be a site of tick-borne pathogen transmission. Saliva or salivary glands (SG) play a key role in the transmission of most tick-borne pathogens. Tick saliva is a critical biological cocktail that inhibits host defenses and promotes blood flow, containing a large number of molecules involved in cytolytic, anti-coagulant, anti-inflammatory, anti-chemokine, anti-pain, and vasodilating activities (59).

Tick saliva has the ability to suppress innate immune response, complement system, and adaptive immunity of host. In terms of innate immunity, tick saliva strongly suppresses the recruitment of blood-borne innate immune cells. Evasin-1, evasin-2, and evasin-3 are chemokine-binding proteins from Rhipicephalus sanguineus SG, which present selectively for different chemokines (60, 61). Ir-LBP, a lipocalin from I. ricinus, interfered with neutrophil chemotaxi and activation (62). The saliva of several tick species also contains macrophage migration inhibitory factor (MIF), which inhibit migration of macrophages (63, 64). Tick saliva also inhibits inflammation by reducing or enhancing pro- or anti-inflammatory cytokines secretion, respectively. Hyalomin-A and hyalomin-B exerted significant anti-inflammatory functions by inhibiting the secretion of C–C motif chemokine ligand 2 (CCL2), tumor necrosis factor alpha (TNF-α), and interferon gamma (IFN-γ) and increasing the secretion of interleukin (IL)-10 (65). PGE2 and purine nucleoside adenosine (Ado) also impaired the production of IL-12p40 and TNF-α and increased IL-10 level by murine DC (66).

The complement system links host innate and adaptive immune responses. IRAC I and II from I. ricinus inhibited C3 convertase formation of the alternative pathway through blocking complement factor B binding to complement C3b (67, 68). Ornithodoros moubata OmCI, TSGP2, and TSGP3 specifically target C5 activation (69, 70). Salp14 and Salp9Pac from I. scapularis inhibited mannan-binding lectin binding to the polysaccharide mannan, preventing complement pathway activation (71).

Tick saliva also suppressed the adaptive immunity, including cellular immune response and humoral immune response (72). Iripin-3 from I. ricinus reduced IL-6 production by MPs and reduced T-helper type 1 immune response (73). Sialostatin L (SialoL) from I. scapularis reduced IFN-γ and IL-17 production and impaired specific T-cell proliferation (74). Salp14 also inhibits pro-inflammatory cytokine production and impaired T-cell proliferation (75). Tick salivary components inhibited humoral immunity by interfering with B-cell-derived immune responses; for example, several ticks could secrete a series of IgG-binding proteins (IGBPs) to suppress specific antibodies (76).

In addition to suppressing immune responses, tick saliva has the ability to block hemostasis and mitigate itching and pain.

ASF causes major economic losses with mortality rates approaching 100% and limiting pork production. In addition to anti-viral vaccines, anti-tick vaccine is theoretically an alternative to prevent ASF. In the last decade of the twentieth century, two commercial anti-tick vaccines (TickGARD^© and GAVAC^©) against Rhipicephalus microplus were developed based on the glycoprotein antigen Bm86. Regarding soft ticks, antigens that can be used as candidate vaccines are being developed to prevent tick-borne diseases (77).

To date, vaccine development for soft ticks has been significantly less studied than that for hard ticks, and only a few Ornithodoros and Argas genera species have been involved. Here, the protective effects of candidate antigen of eight soft ticks associated with ASFV transmission are summarized, which include salivary and concealed proteins.

Current strategies to control ASF depend on rapid testing for the virus and the policies of quarantine and slaughter, which result in large numbers of animals being culled and are ineffective. Therefore, prevention is the best way to protect pigs against ASF. Strengthening feeding management, avoiding contact with ASFV-infected pigs, and strictly controlling the quality of pig feed are effective ways to control ASF (95).

In areas where ticks are involved, either in a domestic or sylvatic cycle, tick prevention is also an important part of ASF prevention. Using chemical acaricides is the most basic method of tick control, which has serious shortcomings, including pollution of the environment, contamination of animal products, and the emergence of drug-resistant tick strains (96–98). In addition, there is no guarantee that acaricides will completely kill hidden ticks because they have a nidicolous/endophilic lifestyle, making the use of this strategy inefficient (89). The high cost of developing and commercializing acaricides has prompted the search for alternative methods to control tick.

Ticks are vectors of many zoonotic pathogens, including viruses, parasites, and bacteria, making them a major threat to human and animal health. Alarmingly, more tick-borne diseases are being discovered, such as Alongshan virus and Songling virus (99, 100). Therefore, immunization control with anti-tick vaccines is a promising strategy to prevent tick-borne diseases. Bm86 antigen was identified in Boophilus microplus and used as a commercial vaccine against the same tick species. Studies showed that BM86 can effectively inhibit the weight and egg-laying capacity of female adult ticks and total weight of nymphs. Calves immunized with the BM86 vaccine have an ability to resist B. microplus, Boophilus decoloratus, and Hyalomma dromedarii ( 101). In addition to damaging the life cycle of ticks, it needs to be evaluated whether anti-tick vaccines block pathogens transmission. Malan showed that Rhipicephalus appendiculatus cement protein (64TRP) protected mice against the lethal challenge by infected ticks, which had a protective effect comparable to commercial TBEV vaccine and a better effect in transmission blocking (102). Andaleeb and his colleagues demonstrated that I. scapularis salivate gland protein (19ISP) mRNA vaccine inhibited the blood feeding of I. scapularis and impeded Borrelia burgdorferi transmission from I. scapularis to immunized guinea pigs (103). These two cases fully demonstrate the advantages of anti-tick vaccines. The hope for a vaccine against ticks is that it will both reduce tick bites, affect the life stages of ticks after they feed on blood, and reduce pathogens transmission.

The current challenge of an anti-tick vaccine against ASFV transmission is that no candidate anti-tick vaccine has been reported to inhibit transmission of ASFV from ticks to pigs. All the successful anti-tick vaccines described above are used for hard ticks, which differ greatly from soft ticks in their blood feeding habits. Hard ticks attach to hosts for blood meals and last up to 10 days for adulthood (24). Unlike hard ticks, soft ticks feed for a short time; for example, O. porcinus porcinus ticks feed in 1 h or less (47). Therefore, it is uncertain whether anti-tick vaccines can prevent ASFV transmission from ticks to pigs. It takes the nymphs 36 h to transmit B. burgdorferi to the naive host after feeding on the host, whereas guinea pigs immunized with the 19ISP mRNA vaccine reported by Andaleeb elicited an immune response as early as 18 h after tick challenge, thus blocking B. burgdorferi transmission (103, 104). The transmission of the virus is usually within a few hours, so it is necessary to confirm whether the immunized animal can induce an effective immune response to prevent the blood feeding of soft ticks. The effectiveness of the candidate vaccine reported by Ricardo et al. in inhibiting tick blood feeding is limited, so the ability of the vaccine against ASFV transmission is also questionable, which is a problem with all anti-soft tick candidate vaccines reported so far. On the other hand, vaccines against soft ticks have been reported to be effective in the reduction in survival and female fecundity, which is beneficial in reducing tick populations and the risk of ASF and other tick-borne diseases. Interestingly, Jennifer et al. demonstrated that pigs co-inoculated with ASFV and SGE of O. porcinus presented increased fever, and SGE had corresponding regulatory effects on skin tissue lesions, Langerhans cells disappearance in epidermis, macrophages recruitment in dermis, and virus dissemination in ASFV infection, suggesting the important role of SGE in the transmission of ASFV (105). Therefore, we are very optimistic about the screening of soft tick antigen and the development of anti-tick vaccine.

Anti-ASFV vaccine is also one of the effective means of prevention and control of ASF. ASFV-G-ΔI177L, marked at 3 June 2022, has become the first commercially available anti-ASFV vaccine due to its protective effect and good safety against Georgia strain. Manuel et al. discovered that ASFV-G-ΔI177L was clinically asymptomatic during the 28-day observational period and exhibited an efficacious protection against the epidemiologically relevant ASFV Georgia isolate (106). Interestingly, Oronasal administration of ASFV-G-ΔI177L provides a protective effect similar to intramuscular administration through mediating ASFV-specific antibody response, such as IgG1, IgG2, and IgM (107). Surprisingly, ASFV-G-ΔI177L also protected pigs from the isolated virulent ASFV circulated and produced in Vietnam (108). Fortunately, virulence regression studies of domestic pigs and large-scale tests of virus shedding and transmission confirmed that ASFV-G-ΔI177L is a safe vaccine (109).

As for the anti-ASFV vaccine, it is undoubtedly the best way to prevent ASF. However, marketed vaccines must be safe without the risk of regaining virulence, which is unpredictable with attenuated vaccines. Since ASFV-G-ΔI177L is a proven safe and effective vaccine, its marketing is good news for pig producers around the world. As there are many serotypes of ASF, the broad spectrum of ASFV-G-ΔI177L needs to be verified. In addition, the duration after vaccination also needs to be verified. Thus, although there is one vaccine available, the development of vaccines for different serotypes in different regions needs to be carried out.

In conclusion, given the critical role of soft ticks in ASFV transmission, the development of both anti-tick and anti-ASFV vaccines is an important strategy for preventing ASF.

YC, WZ and TL conceived and designed the study. TL, XX, NS, SZ, YD, KL, LD, XC and SJ wrote the manuscript. All authors contributed to the article and approved the submitted version.