Coendangered hard-ticks: threatened or threatening?

In their review on tick-host specificity from 1982, Hoogstraal & Aeschlimann wrote: "As biomedical researchers, we are charged with the task of improving the quality of human life and welfare precisely, reducing risks of disease, irritation, and debilitation resulting from parasitism by ticks" [1]. From strict medical point of view they might be questionably right. But, ticks, as all parasitic species, are part of global biodiversity which according to current trends should be preserved.

The overwhelming majority of conservation projects in the animal kingdom are focused on vertebrates, despite most of the species on Earth being invertebrates. Estimates state that about half of all named species of invertebrates are parasitic in at least one stage of their development [2]. The dilemma of viewing parasites as biodiversity or pest has been discussed by several authors, regardless if it is about animal parasites [3–8]. General human perception of parasites is usually negative and several dictionaries derogatorily associate this concept with exploitation. Among all parasites, ticks, along with other ectoparasites seem to have one of the most negative reputations [9].

Ticks (suborder Ixodida) are obligate blood-sucking acarines attacking a wide variety of hosts from all tetrapod vertebrate classes (Amphibia, Reptilia, Aves and Mammalia). Three families are currently recognized: Ixodidae (hard ticks), Argasidae (soft ticks) and Nuttalliellidae. The latest taxonomical synopses of the group [10–12] updated by [13] consider valid 700 non-fossil species in Ixodidae (for a review of fossil ticks see [14]). Though, how many of the 'non-fossil' species are still extant is almost impossible to tell, as many of them are known only from type specimens in museums and were never collected since their original description.

Generally, extinction is considered to have four main causes: habitat loss, species invasion, overkill and cascades of extinctions [15]. Cascades of extinctions (or coextinctions) are in most situations cases of habitat loss in species for which the habitat is another species, like the case of mutualists, commensals and parasites. In the case of most symbiotic interactions the extinction of the host could result in the extinction of several associated species [16]. Ticks are no exception.

After the concept of 'coextinction was intuited by Darwin in 1862 and introduced in scientific literature in 1993 [17], the term 'coendangered' arose logically within the next years [18], when estimates stated that 6300 symbiotic species are coendangered with their associated organisms. Nevertheless, the review omitted several groups of parasites like protozoans, cestodes, trematodes, most nematodes, acanthocephalans, fleas, ticks, whale lice etc. Therefore, the number of coendangered parasites could be much higher. For ectoparasites, including ticks, not only the endangered status of the host makes them endangered. As part of conservation efforts of threatened vertebrates, actions often involve artificial breeding, re-introduction or relocations. During these processes, a common practice is the removal of external parasites, with devastating impact on their population [19]. Several cases are documented. One relevant example is of the louse Colpocephalum californici (now extinct) which were intentionally removed from the endangered California condor, Gymnogyps californianus during the captive breeding project at Los Angeles Zoo [20].

In the case of parasites, the coendangered status applies with predilection to species with high host-specificity. Ticks are distributed worldwide from the Arctic to tropical regions. Their geographical distribution is related to the range of their host(s) with the highest diversity in tropical regions. Host specificity in ticks is still a debated issue. In some tick species, the host specificity was evaluated by more or less complex experimental trials, but in the majority of the situations this label comes solely from field reports on tick-host associations. In the first situation, one of the most studied species is the cattle tick, Rhipicephalus (Boophilus) microplus. Several hypotheses were incriminated to explain host specificity in ticks: adaptation by the tick to the particular properties of host's skin, specific sensory stimulus to attachment, specific ability of the tick to evade the host's immune responses or dietary specificity [21–23]. Based on a review of experimental evidence or ecological observations, about 85% of the tick species are considered to have a certain degree of host specificity, especially in their adult stage [1]. However, sometimes ecological specificity (habitat dependence) could explain the apparent specific host association in ticks, reducing the access of certain tick species to a limited number of vertebrate species [24].

The first and single review so far on tick conservation [19] proposed 42 species of Ixodidae as candidates for the endangered status. Following this idea, the echidna tick Bothriocroton oudemansi was listed as coendangered with its host [25]. Similar opinions are available for other groups of parasites. The conservation status of myiasis causing Oestrid flies was discussed recently in detail [6]. In this review, the authors grouped the endangered parasitic flies into three categories, by the cause of possible extinction: treatment-induced, coextinction and neglected, listing a total number of 39 bot-flies. A synoptic review on coextinct lice of birds and mammals is also available [26].

Extinction of single host species could result in the immediate extinction of several associated species (parasites, commensals, mutualists) [16]. In the case of Ixodidae, there are certain threatened vertebrates which host more than one tick species. For instance, the extinction of the sambar deer (Rusa unicolor) could lead to the coextinction of four specifically associated ticks. Moreover, ticks harbor themselves internal symbiotic microorganisms, most of them not studied. Thus, the resulted chain of extinctions is much more complex and difficult to estimate.

Most species included in our review (n = 31) belong to genus Amblyomma Koch, 1844. Their host specificity is high, especially in their adult stage [1] which makes them candidates for extinction if their hosts become extinct. Within the genus Ixodes, we propose 16 species, parasitic on tropical birds or mammals, as coendangered. All coendangered species (n = 9) from the genus Haemaphysalis Koch, 1844 are restricted to Asian threatened mammals. Only three species of the genus Dermacentor Koch, 1844 are included in our synopsis. The genus Hyalomma Koch, 1844, parasitic on mammals and tortoises includes two coendangered species. The genus Bothriocroton Keirans, King and Sharrad, 1994, recently erected to genus level, was initially described as a subgenus of the former genus Aponomma (now synonym of Amblyomma) [12]. Seven species are currently included here, all with Australian distribution, with a single species coendangered (B. oedemansi). The monospecific genus Cosmiomma Schulze, 1919 is found on large threatened mammals from Africa, hence its single species, Cosmiomma hippopotamensis is considered coendangered.

Ticks described from fossil deposits are omitted. We consider extinct one species, namely Ixodes nitens, described from two female ticks collected on Rattus macleari on Christmas Island. The last report of the host species was in 1903 [28]. As the other endemic rat species Rattus nativitatis (sympatric with R. macleari), did not harbor I. nitens, we can assume this tick was specifically associated with its type host. Thus, we exclude the possibility that I. nitens might have re-adapted as a parasite of the introduced black rats, Rattus rattus[29]. However, no direct evidence is available.

Probably the most important issue regarding tick-host associations is vectorial transmission of microbial pathogens. Ticks are able to transmit viruses, bacteria and protozoans to a variety of hosts. One of the most pathogenic tick-borne microbes are piroplasms (genera Babesia and Theileria). The potential impact of babesiosis on conservation actions was discussed mainly as a consequence of stress-mediated relapse of chronic infections during translocation [30]. Otherwise, tick-borne diseases of threatened vertebrates are rarely fatal to their hosts. The following accounts consider only reports from hosts of coendangered ticks. Mortality associated with pathogens (Babesia bicornis and Theileria bicornis) acquired from ticks has been documented in black rhinoceros in Tanzania and South Africa [31–34]. The specific vector for these two haemoprotozoans is not known, but Dermacentor rhinocerinus and Amblyomma rhinocerotis were suggested [34]. Hence, extinction of these ticks is expected to result in the eradication of disease caused by B. bicornis and T. bicornis. A recent study from South Africa showed that 36.41% white rhinoceros were infected with Theileria bicornis and 9.23% with Theileria equi[35]. However, no pathology associated with the infection was recorded in white rhinoceros. Babesia loxodontis was described from asymptomatic African elephants, Loxodonta africana[36]; babesiosis in Asian elephants can be associated with weakness, fever, jaundice, constipation and haemoglobinuria [37]. Babesia pattoni was reported in Rusa unicolor but no associated pathology was described [38].

So far, no tick-borne diseases with impact on the health of threatened birds or reptiles have been described. Asymptomatic infections with Hemolivia mauritanica have been reported in Testudo graeca over its distribution range [39]. The zoonotic bacterial pathogen Anaplasma phagocytophilum have been isolated in ticks Amblyomma flavomaculatum collected on monitor lizards (Varanus exanthematicus) [40]. Although neither the host nor the ticks are endangered, there is high probability that other Amblyomma species could transmit Anaplasma to lizards of genus Varanus.

Expectedly or not, we came back to the question of Hoogstraal and Aeschlimann from the beginning of this paper. Should we decide on conservation of rare ticks? Or are they a real threat to their coendangered host and should be eliminated? Ticks as such are not dangerous. Disease, if present, is in most of the situations caused by vectored microbes. Moreover, pathology induced by tick-borne diseases in wild animals is seldom dangerous and is usually related to supplemental stressing factors (i.e. translocation). Last but not least, there is no proof to date that ticks listed by us as coendangered are competent vectors for pathogens of endangered animals.

Nevertheless, IUCN should reconsider the criteria of indexing species in its database as threatened. All symbiotic species (mutuals, commensals, parasites) specifically associated with their host should be listed as co-endangered. As previously suggested [3], some host-specific parasites are more endangered than their host. Moreover, parasites have their own evolutionary importance, and as suggested even in the early 1990's, parasites should have equal rights with their host [3, 5].