Host specificy, host change and intermediate hosts in different organisms – with special reference to viruses and Sars-CoV-2

von wirthstef

We recently read a lot about the pandemic consequences of infections with the new corona virus Sars-CoV-2, most are medical issues, hygienic advises and information about political reactions in different countries worldwide. But there is not much known about the biological host reservoir, putative intermediate hosts and how the human infections might be explained. It is a normal lack of information, because the scientific research about topics, being generally new to science, is time costing, especially, when life strategies and the population dynamics of organisms a concerned. Organisms? Viruses are per definitionem not considered organisms, because they lack important aspects, which characterize real life: they cannot reproduce on their own power, they do not have an own metabolism, no ingestion, no excretion. But they are organic and show traces of life by possessing a genome, which might indicate that they evolved from living cells. Viruses represent a diverse group of protein bodies containing nucleic acid, either DNA or RNA.

New corona virus Sars-CoV-2, Wikipedia: CDC/ Alissa Eckert, MS; Dan Higgins, MAM / Public domain

Viruses in general, host specificity, host increase, host change

For reproduction viruses depend on living host cells, which they reprogram by inserting their virus genome into the cell’s genome in order to stimulate the forming of a number of virus copies, all that happening on cost of the host cell’s life. Thus viruses need to be named parasites as they harm their hosts to their own advantage. Different groups of viruses attack different kinds of cells using in detail different methods to enslave their host cells. There are plant viruses, viruses associated with bacteria (named bacteriophages) and animalistic viruses. They all show characters, which are typical for parasite – host – relationships. Parasitic partners of any kind of host – parasite – relationship can be exclusively associated with one host species only (host specificity) or a limited group of systematically closely related hosts, while others can have a wider range of different host species. The latter generally might have evolved out of the former, although also the opposite direction is thinkable. When former host-specific parasites make themselves one or even several further hosts accessible, then this phenomenon is named host-increase (Wirtserweiterung). In case an new host was infested as permanent host, while the former host is given up, then a so called host change (Wirtswechsel) happened. The same term is also used in a different context, namely when a parasite requires in its development a change between different hosts.

Host specificity: A parasite (or an organism with similar life-strategy) is associated with one host only, which requires a specialization and a competition between host evolution and parasite evolution (coevolution). This strategy needs to be separated from generalism, which means that a parasite has a very wide range of not related regular main hosts. Host specificity is more common than generalism. But this also depends on definitions. I herewith define the association with one main host species only as host specificity. But I furthermore consider host specificity also given, when parasite-host relations are specific on a higher taxonomic level, for example, when certain closely related genera of parasites are specialized for certain closely related genera of hosts. This part of my definition has variable borders. In the chapter after next, I describe the parasitic case of the trematode Leucochloridium paradoxum, whose main hosts are represented by different systematically not closer related bird species. A host specificy on the level of birds in general (Aves), then present in only some species with similar food preferences might already need to be named a limited generalism.

Obligatory host change in ticks and lifstyle-change in water mites

Some parasites need several hosts to be enabled to finish their life-cycles. This is another context, in which the German term „Wirtswechsel“ (host change) is used. In that kind of parasite – host – association, the host change is often obligatory, meaning that the parasite cannot survive in the absence of one of the required hosts. The castor bean tick Ixodes ricinus represents a parasite, which needs a host change to successfully go through its full development until adulthood, but there is a wider range of suitable hosts, as intermediate host and as final host. Thus the tick is a generalist with obligatory host change. Water mites (Hydrachnidia) are parasitic as first nymphs (juvenile instar, usually named „larva“) and predators as older nymphs and adults. A host specificity of „larvae“ can appear, but a wider range of host species is common. These mites perform a life style change during their development.

Intermediate host, for example in the parasitic flatworm Leucochloridium paradoxum

An example for a parasite, obligatory requiring a specific intermediate host, is the flatworm Leucochloridium paradoxum („green-banded broodsac“, Trematoda, Platyhelmintes), whose larvae (miracidium) need to infest snails of the genus Succinea. This trematode parasite is host specific for a genus of snails, while there is no specificity for their main hosts. They parasite birds, but infest different bird species, which are not closer related to each other, such as finches, the crow family Corvidae or woodpeckers. Although there is a main host specificity on the very high taxonomic level of Aves, the use of the term (limited) generalism might in this case even be appropriate. Inside the smail’s midgut gland, miracidia (larvae) modify into another larva-form, named cercaria, which invade the liver, where they form so called sporocysts, sac-shaped muscular tubes, which grow through the entire snail host until they reach the snail’s tentacles, which they fill up with their tube-shaped bodies entirely. Lastly the snail is unable to retract her swollen organs. The snail tentacles are now well visible as conspicuous greenish stripes, pulsating permanently. The sporocysts as larval stage of this trematode parasite do even more than only increasing the visibility of the snail for bird predators, which represent the worm’s final host. They additionally manipulate the nervous system of the snail so far that the snail performs an unusual behavior and moves towards very well exposed elevated areas, such as leaves of adjacent plants. Thus the probability to be eaten by birds is remarkably increased.

Parasite Leucochloridium paradoxum, sporocysts inside the tentacles of a snail of genus Succinea, Wikipedia: Thomas Hahmann / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Host specificity on humans with side-hosts and coevolution with the ancestor line of Homo sapiens: skin mite Sarcoptes scabiei

An interesting example of a host specificity with numerous side-hosts and even an additional host-increase is the skin parasitic mite Sarcoptes scabiei (also named the „seven-year itch“). It was originally exclusively specific for Homo sapiens and accompanied mankind over its entire evolution (e. g. J. R. H. Andrew’s Acarologia, 1983). Systematical relatives of that mite species can only be found within the Great Apes. Originating from the recent Homo sapiens, S. scabiei conquered the human’s domestic animals, such as dogs or bovine animals within long-term periods, in which humans and their domestic animals had shared the same buildings or even rooms. Domestic animals may transfer the mite-parasite subsequently to wild animals. In case main host (humans) and side hosts (domestic animals, wild animals) can supply everything, which the parasite needs for its development without the necessity to leave its host specimen, one might speak about real hosts. In case side hosts cannot supply the necessary basic equipment, they represent either intermediate hosts or dead-end hosts. It can for example be discussed, whether dogs might in fact be dead-end hosts, as the skin disease can harm them under certain conditions to dead.

Mite Sarcoptes scabiei (Astigmata, Acariformes), Wikipedia: Kalumet / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

Host increase due to the globalisation and human economic interests: example honey bee parasite Varroa destructor (mite)

Another example of a former host specificity on a species‘ level with host increase is the mite Varroa destructor (Parasitiformes, Mesostigmata). It was originally specific for the Eastern honey bee Apis cerana. The mite could only switch over to the Western honey bee Apis mellifera due to a human influence: Men transferred A. mellifera for economic reasons to the natural habitats of A. cerana in Eastern Asia, were it got infected by the mite V. destructor. A subsequent transfer of the Western honeybee back home established the mite parasite in Western countries. As A. mellifera colonies are much more harmed by V. destructor than its original host, our honey bee must be considered as an intermediate case between a new host and a dead-end host. Human international traffic enabled this host-increase primarily, although there are areas between Afghanistan and Iraq, where both bee species coexist due to natural distribution. But there is an almost insurmountable (allopatric) desert border between the population of both species of about 360 to 600 kilometers, although there are evidences for bees rarely surmounting this border. Thus a natural mite transfer between closely related bee species might have happened additionally. Species of animals, plants, fungi or bacteria and even viruses, which successfully established new (additional) living spaces are named neobiota or alien species.

Mite Varroa destructor, Wikipedia: The original uploader was Tullius at German Wikipedia. / Public domain

Can viruses as non-living genome possessing lumps be subject of evolution and complex host – parasite relationships?

Can this high complexity of modes of parasite – host – relationships in living organisms also be found in virus – host – relationships, although viruses do not represent living organisms at all according to biological definitions? The answer is yes, because viruses do not only share a genome with living cells, but based on this genome even are subject to the mechanisms of evolution. And evolution was the most important factor in all the mentioned complex parasite – host – interactions.

Parasitism versus mutualism or harming the host or not harming the host

Two different life-strategies with similar mechanisms as organism – to – organism associations

Are there other organism – to – organism relationships, being subject to a similar complexity than found in parasites with their hosts? Yes, a superordinate term for other close associations between different organism species is mutualism. While parasites need to harm their hosts by using them as final living-sources, mutualists are considered to practice a more neutral host contact, which per theoretic definition means that nobody harms anybody. But the assumption of a neutrality is in fact an artificial construct, as in detail it can come out that some of these organism associations represent unrecognized parasite-relationships, while in other cases a benefit for both partners (symbiosis) or for one partner only might be discovered in future studies. At least so called mutualists share as a feature that harmfulness or benefit are not easily noticeable.

Phoresy: taking a ride on a taxi-host as example of mutualistic relationships

An example for a more neutral organism, at least not harming association is called phoresy. It is often performed by nematodes and mites. These tiny organisms take a ride on bigger animals in order to become carried from one habitat to another. This „taxi-association“ is considered being of advantage for the phoretic part and harmless for the carrier (in English also often named host). But there are seeming phoretic interactions known, which based on developing technical scientific standards could be identified as unusual cases of parasitism. An example is a phoretic instar of an astigmatid mite (Astigmata, Acariformes), which as all phoretic instars within this big mite clade has no functional mouth, but sucking structures to fix itself to its host. This specific mite species had evolved a mechanism for opening the host cuticle in order to incorporate blood of its host using these sucking organs. This is unlike the common use of homologous suckers in related mite taxa, where they (as far as known so far) only support the adherence.

Another interesting example of a phoretic mite is Histiostoma blomquisti (Histiostomatidae, Astigmata), which is specifically associated with the red imported fire ant (sometimes referred as RIFA) Solenopsis invicta, which worldwide appears as troublesome neozoon, again a result of human global traffic. I am the scientific describer of that mite, and my research about it’s biology and abundance in ant nests refers to populations in Louisiana (USA). An interesting aspect is that the ant is originally native to Southern America. We lack studies, whether the mite appears in the native habitats of the ant also as its specific cohabitant or whether it originally deals with a wider range of phoretic hosts. We do not even know, whether the mite is at all native to the same area, in which S. invicta had its natural distribution. On one hand, we hypothesise that, but there is also a theoretical option that the mite performed a subsequent host change in areas, for example in the Southern USA, where the ant was accidentally established via sandy ballast substrate of ships as neozoon. It is further more not known, whether the mite – ant – relationship is indeed neutral, at least with no noticeable harming features. I discovered (S. Wirth & J. C. Moser, Acarologia 2010) that mite deutonymphs (= phoretic instar) can attach to active nest queens in such extraordinary high numbers (hundreds of mite specimens) that mobility restrictions for the concerned queens were sometimes visible. On the other hand, my video documentations showed that even completely overcrowded queens could still freely move and, much more important: stayed reproductive. The purpose of the mites inside the fire ant nests is unknown. But generally, mites of the Histiostomatidae can appear as beneficial animals in ant nests. At least according to my findings about the mite Histiostoma bakeri, which is a phoretic associate of the leafcutter ant Atta texana in Southern USA. I discovered these mites improving the hygienic conditions inside specific nest chambers (detritus chambers) due to their fungi and bacteria feeding activities (Wirth & Moser, European Association of Acarologists proceedings, 2008).

I will in different chapters of this article repeatedly refer to examples with phoretic mites of the family Histiostomatidae (Astigmata, Acariformes). As mutualism and parasitism follow similar organism-host association patterns, I will in those chapters not each time mention again that examples with these mites do not concern parasitism, but mutualism. It is by the way no accident that both life-strategies share common features, as there are examples known, which indicate that one strategy can evolve out of the other.

Mite Histiostoma blomquisti Wirth & Moser, 2010 (Histiostomatidae, Astigmata, Acariformes) on queens of ant Solenopsis invicta, Pineville/ Louisiana, copyrights Stefan F. Wirth

Mutualism often used as neutral term for organism associations with unknown effect of both partners to each other.

The copepod (Crustacea) Ommatokoita elongata on Greenland and sleeper sharks

So called mutualistic associations can sometimes represent interactions of unknown benefits or damage regarding both of the associated partners. Another interesting example of such an association with a not yet understood status is the copepod Ommatokoita elongata (Crustacea), which was discovered as specific cohabitant on the Greenland shark (Somniosus microcephalus) and the pacific sleeper shark (Somniosus pacificus). Larvae of the crustacean in their copepodit stadium and adult females attach to the ocular globes of the shark, where they can cause visible tissue damages. They are thus considered being parasites, although alternating hypotheses assume a more neutral mutualistic copepod – shark – association, based on the sometimes high abundance of the crustacean on one shark specimen (B. Berland, Nature, 1961). There are even assumptions about a benefit contributed by the copepode to the sharks: reasearchers say that it might improve the shark’s hunting success by attracting suitable prey with bioluminescence signals.

Shark Somniosus pacificus, Wikipedia: National Oceanic and Atmospheric Administration / Public domain

Greenland shark with copepod Ommatokoita elongata, hardly visible, when the shark turns to show his right eye, Youtube: copyrights The Canadian Press, video by Ben Singer, footage Brynn Devine, Marine institute of Memorial University of Newfoundland

Human parasites with mutualistic features: the mites Demodex folliculorum and D. brevis

Can viruses be compared with some mites, nematodes or copepodes by performing mutualistic virus – host – relationships? A priori it must be stated that they are unable for a neutral relationship with another organism, as they need the destruction of living cells for their own persistence. But indeed there are viruses known, causing no known diseases and thus being named passenger viruses. But first, an example of an organismic example of parasitism without harmfulness will be presented: the mites Demodex folliculorum and Demodex brevis (Trombidiformes, Prostigmata), which appear as so named „face mites“ inside hair follicles of humans, preferring eyebrows and eyelashes, but also other hairy body parts. The abundance in humans is high and grows with a human age. According to Schaller, M. (2004), new born children are free of Demodex, while over 70 years old people are at almost 100 percent infested with the mites. The mite in fact is a parasite and feeds on sebum from the sebaceous glands. Incorporating needed human gland secretions must be named parasitism. Nevertheless mites under normal conditions cause no visible damages nor do they seem to harm their host noticeably.

Mite Demodex folliculorum, Wikipedia: Information |Description=Demodex folliculorum |Source|Date=2009-09-08 08:34 (UTC) |Author=: http://www.legart.ru/demodex

So called passenger viruses as mutualists with a more or less neutral affect to their human hosts

Such a parasitic relationship might be comparable with so called passenger viruses, which do not harm noticeably, although they destroy living tissue as all viruses do. They can accompany more harmful viruses and even might harm the pathological success of the diseases, caused by these harmful viruses, and for example might slow the disease’s progression. An example is the GB virus C (GBV-C), which was before known as Hepatitis G virus. The virus is common in humans and shows no pathogenic damaging effect. According to an US-study, about 13 percent of probands, whose blood was examined, possessed antibodies against the virus. GBV-C is considered to slow the effects of an HIV disease by negatively effecting the replication of the HI-virus.

Host increase towards systematically not closer related new hosts

Example for a transfer within related host taxa in mites is the bark-beetle-clade within Histiostomatidae (Astigmata), an example for non related side hosts is the mite Histiostoma maritimum (Histiostomatidae, Astigmata)

Do side-hosts or intermediate hosts as results of host increases commonly need to be systematically close relatives of the main host? The answer is no, although parasites are usually better pre-adapted in infesting a host, which shares a maximum of common characters with the main host. Within the mite family Histiostomatidae, there exists a clade of mites being associated with a clade of beetles. I named it bark beetle-clade (e.g. Wirth, phd thesis, 2004). Mites and bark beetles performed a parallel evolution, which required host increases and host changes towards related hosts and subsequent evolutionary adaptations to harmonize with these new hosts, either to become specific for a new host or to deal with a range of host species.

But the transfer of a parasite to new hosts can also happen towards not closely related host species, representing a scenery being based on a common ecological context between main hosts and side hosts. The phoretic mite Histiostoma maritimum for example is host specific for at least two closely related beetle-species of genus Heterocerus (Heteroceridae). But the mite regularly also appears on predatory beetles of genera Elaphrus and Bembidion (Elaphrus cupreus and Bembidion dentellum, Carabidae) (S. Wirth, phd thesis 2004 and subsequent studies). These beetles partly share the same habitats with Heterocerus: sapropel around ponds, being exposed to sunlight and warmth. In my research about the mite H. maritimum, I hypothesised that the phoretic mite instar might switch over to Elaphrus and Bembidion, for example when these predators feed on adult Heterocerus beetles, larvae or cadavers. Although I could regularly find mites in lower abundances over years on the side hosts (collected in the Heterocerus sampling sites), it is unknown, whether the „switch-over“-scenario was a starting event in an evolutionary past to establish the mite to new additional hosts, where they would today survive more or less independently from the original Heterocerus source, or whether the mites regularly need to switch over in the above mentioned situations, and in consequence side hosts with no Heterocerus-contact would thus lack the mite. A possible support for the latter hypothesis are my laboratory findings about the preferred developmental habitat of the mite, which was cadavers of died Heterocerus beetles. In my experiments the mite remained on its Heterocerus– carrier until this died. Mites subsequently developed on the beetle’s cadavers, feeding there on bacteria and fungi (the phenomenon is named necromeny). Mites under laboratory conditions developed also seemingly successfully on E. cupreus– and B. dentellum-cadavers. But I could so far never continue these studies and don’t know, whether or how well mite colonies with having only cadavers of these two side-hosts available would reproduce compared to mites being reared in Heterocerus settings. In case of a strict substrate specialization for Heterocerus cadavers, the side hosts would be dead-end hosts, and permanent reinfections from the original host source would be required to explain the regular mite abundance in Elaphrus and Bembidion.

Histiostoma maritimum, a adult female with conspicuous copulation opening, b both adult genders in dorsal view, c, d copulation opening in dorsal and sideview, SEM, Berlin 2020/ ca. 2002, copyrights Stefan F. Wirth

Assumed transfer of virus SARS-CoV-2 from bat main hosts via a pangolin as intermediate host towards humans:

There is an ecological context between bats and pangolins

The new corona virus SARS-CoV-2 is assumed to be host specific to a group of animals and from there infesting another animal as intermediatehost, from which presumably humans were opened up as new host source. There are researchers interpreting us humans as an dead-end hosts, as unlike in bats human people can be harmed remarkably with the lung disease COVID-19 (corona virus disease 2019), triggered by SARS-CoV-2. As at least from a general statistical point of view a high majority of infested people shows no or only slight symptoms, thus it can up-to-date not be excluded that Homo sapiens is in order to become a fully potential side host, because all a parasite needs in order to „survive“ before all other requirements is the (statistically) surviving of its host.

There is evidence that bats (Chiroptera) represent the main host, thus representing the „natural virus reservoir“, while pangolins (Pholidota) presumably act as intermediate hosts. This main-host-to-intermediate host context is for example reported as putative scenario by Ye Z.-W et al. (Int Biol Sci, 2020), who stated that based on molecular features the bat Rhinolophus affinis (Rhinolophidae, Chiroptera) is hosting a virus most similar to SARS-CoV-2 differing from all other known corona viruses (Similarity 96.2 %, nucleotide homology). The pangolin species Manis javanica was identified to carry formerly unknown CoV genomes, being according to the same authors with 85-92 % similar to SARS-CoV-2 (nucleotide sequence homology).

Megabat Cynopterus brachyotis as example for a species native to Southeast Asia, Wikipedia: Anton 17 / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Bat Rhinolophus affinis as known reservoir of a virus most similar to Sars-CoV-2. Wiki commons: Naturalis Biodiversity Center

Pangolins and Chiroptera (bats and megabats, this taxon subsequently sometimes refereed as „bats“) are systematically not closer related to each other. Pangolins (Pholidota) are considered to represent the sister taxon of the clade Carnivora. Chiroptera were reconstructed as sister taxon to the clade Euungulata (containing animals such as horses, cattle or whales). But both, Chiroptera and Pholidota, can be connected by an ecological context. Pangolins (Pholidota) are species, which are either adapted to live preferably on the ground, or to spent most of their time on trees. Both types are specialised ant and termite feeders, which use cavities on the ground or inside trees as hideaways. They additionally give birth to their offspring inside these burrows and subsequently use to stay there with their young for a while. Such cavities can accidentally be the same time aggregation and resting places for bats, excluding megabats, which use to rest during daytime on exposed areas on trees. Manis javanica has a semi-arboricol life-style, spending time in trees and on the ground. This pangolin uses different resting cavities, either subterranean burrows or tree cavities.

Chinese pangolin Manis pentadactyla, a ground living species, Wikipedia: nachbarnebenan / Public domain, Zoo Leipzig, Tou Feng

Pangolin Manis javanica as known host of a virus similar to virus SARS -CoV-2. Wikipedia: creative commons Piekfrosch / CC BY-SA

Chiroptera and Pangolins are in South Eastern counties often subject to hunting, as both for example play a role in the traditional Chinese medicine. Thus a virus transfer to humans via main host or via the putative intermediate host is assumed to have happened on animal markets (in the province Wuhan in China).

Which indications point to animal hosts as original source of virus SARS -CoV-2 ?

The scientists Andersen et. al (2020) explain there was no virus-engineering instead of a natural evolution

But which proofs exist that animal hosts sources such as Chiroptera and pangolins are involved in the transfer of the virus SARS -CoV-2 to humans? The lack of general knowledge is still fundament for conspiracy theories, such as an artificial creation of the new corona virus in laboratories with biological warfare purposes.

K.G. Andersen et al. („The proximal origin of SARS-CoV-2“, Nature Medicine, 2020) concluded based on their molecular research that the genetic template for specific spike proteins forming structures, which the virus body possesses on its outside for holding on and penetrating into the host cells, showed evidence for a natural evolution and not for an engineering. They argue with the strong efficiency of the spikes at binding human cells, which makes an engineering implausible and evolution based on natural selection highly probable. The authors additionally examined the overall molecular structure of the backbone of SARS-CoV-2. Backbone can be explained as the „skeleton spine“ of a macromolecule as a continuous row of covalent bond atoms. This overall backbone structure of the new corona virus is according to the authors similar to viruses, which were isolated from Chiroptera and pangolins and dissimilar to other corona viruses, which are already known to science.

Spikes (here in red) in Sars-CoV-2 hold on and penetrate into host cells, Wikipedia: CDC/ Alissa Eckert, MS; Dan Higgins, MAM / Public domain

Can a host increase happen more or less spontaneously with a subsequent enormous success (as for example in virus SARS-CoV-2)?

And: Can the complexity of adaptations to a main host decide for the option of a host increase?

An example for a tendency to spontaneous temporary host changes is mite Histiostoma piceae (Histiostomatidae, Astigmata)

Is it imaginable that a host change or a host increase happens spontaneously and subsequently having such a remarkable impact to the new host, as it is recently ongoing with the SARS-CoV-2 pandemic? Host specificity, host changes and parasitism or mutualism in general are result of evolution. The most common case of evolutionary changes in organisms or viruses is a slow process of stepwise modifications being based on mutations and natural selection.

But it needs also to be stated that as more complex the pattern of characters is (genome, morphology, behavior, function-morphology, reproduction biology etc.), which binds a parasite or mutualist to a specific host, as more evolutionary steps are necessary to perform a host change and as longer an exposure to mutation and selection would need to take place. However it is alternatively possible that a host specificity is only based on a few, but important features. Slighter ecological pressures focusing towards these features might then theoretically allow rather fast host changes.

As an example with a putatively reduced complexity of host adaptations I herewith introduce the phoretic mite Histiostoms piceae (Astigmata, Histiostomatidae), which I repeatedly studied and reared under laboratory conditions. The scientific describer of this species (Scheucher, 1957) discovered a strict host specificity to the bark beetle Ips typographus. According to my and her research, the mite has along the geographic distribution of that bark beetle a high abundance, beetles without the mite are rare. In 2016 I discovered H. piceae being additionally associated with Ips cembrae as a second regular host. I cembrae represents the sibling species of I. typographus (Wirth, Weis, Pernek, Sumarski List, 2016). Exceptions are smaller bark beetle species, which regularly burrow their galleries into those of I. typographus or I. cembrae. It is unknown, whether these small bark beetles as cohabitants of I. typographus carry the mite temporarily or regularly. But the former might be confirmed by the following interesting phenomenon in the mite H. piceae:

In case of very high numbers of mites inside bark beetle galleries and a relatively low numbers of corresponding Ips species, the phoretic instar of the mite attaches under natural field conditions all available arthropods inside or adjacent to the galleries of the main hosts, including bigger mite species, different beetle species or – as already mentioned – smaller bark beetle species (for example my studies in the area of the city Tyumen, Siberia, Russia, 2015-2016). This indiscriminateness for specific hosts under certain conditions might indicate that the substrate specificity of the mite H. piceae is more developed than the phoretic specificity for the host insect itself as a carrier . In such a case, I would generally expect that a host change or a host increase might faster happen in future evolutionary steps than in mite species, which are strictly choosy for their specific host carrier. In H. piceae the tolerance for a variety of carriers (unlike the specificity for substrate conditions) might in a future evolution even succeed as pre-adaptation, which under suitable circumstances might spontaneously allow a regular transfer to new hosts. A second important step towards a real host increase would require that the mite becomes able to stay permanently on its new host. In the H. piceae context the evolution of a tolerance for different substrate conditions might once become an important selective factor in may be opening up new permanent host-associations.

Temporary side hosts, as described in the above explained observations, would represent nothing then dead-end hosts, as they are unable to carry the phoretic mite to suitable habitats for its development. But under favorable circumstances, a former dead-end host might even become a new permanent host.

Histiostoma piceae, a adult female in side view, b in dorsal view, c mouthparts and digitis fixus, d adult male in dorsal view, e in ventral view, Berlin 2020/ ca. 2002, copyrights Stefan F. Wirth

Phoretic instar of Histiostoma piceae, ventral view, lightmicroscope with dig contrast, Tyumen (Siberia, Russia), 2016, copyrights Stefan F. Wirth

Two possible ways of virus transfer from bats to humans according to Andersen et al. (Nature Medicine, 2020)

Did the virus evolution towards the recent state happened prior to a first human infection, namely inside animal main host populations, or did it happen afterwards inside human populations?

As there is not yet much known about the presumed host specificity of the virus SARS-CoV-2, Andersen et al. (Nature Medicine, 2020) reconstructed based on their up-to-date knowledge two possible ways of a virus transfer from bats to humans and finally to the recent pandemic situation in the world:

The virus might have evolved its recent human-pathogenic features within the main host populations of bats. Natural selection must have been the corresponding major driving force. The relevant adaptations are represented by the above mentioned two molecular characters of the spike proteins in SARS-CoV-2 (receptor-binding domain for host cell binding and cleavage sites for an opening up of the virus). Under such circumstances the authors expect that the infection of humans could have happened with an immediate effect, leading at once into the pandemic situation of today. An intermediate host would in this option be not obligatory. A direct transfer from bats to humans might be imaginable.

The second option is based on findings that corona viruses in pangolins possess similar receptor-binding domains (RBD) as in the human SARS-CoV-2 version. Thus the authors reconstruct a version according to which a non or less pathogenic form of the new corona virus was via pangolins transferred to humans and circulated there for an unknown period of time. Even further possible intermediate hosts, such as ferrets or civets, are considered to have been involved in that scenario. During its time inside human populations the virus would have developed its recent features due to evolution and finally was able to be spread explosively between human populations on a pandemic level.

A higher probability for one of the two scenarios can according to the up-to-date knowledge not be assumed

I am not sure, whether the authors take under consideration with their second option that pangolins might even represent a main host and whether bats would not necessarily be involved in the animal-human transfer of the virus. But according to Ye Z.-W. et al. (Int Biol Sci, 2020) the context between bats, pangolins and humans was stated: „We cannot exclude the possibility that pangolin is one of the intermediate animal hosts of SARS-CoV-2“. But whether the pangolin is intermediate host or main host would at this point not effect the general conclusion of each of the two scenarios. The virus was either pre-adapted regarding efficient spike protein characters and then infested human populations rapidly or was transferred to humans via an animal host and subsequently evolved its key-features for a pandemic „success“ within human populations. Although the authors have up-to-date no indications allowing a preference for one of the scenarios, they point out that the potential of new SARS-CoV-2 outbreaks after the extinction of the recent human pandemic would be much higher in case of the scenario one, as the pathogenic virus would under these conditions survive in the animal main host populations.

I would as addition to scenario two suggest to test a modified hypothetic scenario, in which the non pathogenic ancestral version of the virus did not only circulate between human populations until it reached its pandemic key-features, but even circulated between humans and animal hosts forth and back for a longer time. This would according to my understanding of evolution improve the probability of a stepwise evolution of important key-features.

Special and unusual features of main hosts can improve the diversity within their parasites, important conditions for subsequent host changes: a very efficient immune system in bats pushes the evolution of their viruses

Chiroptera (bats and megabats) are not only known as putative main hosts for SARS-CoV-2, but also for Mers, Sars, Marburg and ebola viruses. Scientists did a research about the question, whether there are specific features existing, which explain, why Chiroptera are favorable hosts for viruses with a seemingly potential for epidemic and pandemic effects in human populations.

C. E. Brook et al. (eLife, 2020) discovered an unusual efficient immune system in Chiroptera, which they think protects these hosts from harmful diseases by their virus parasites. This bat immune system is considered being the evolutionary driving force for the variety of viruses and their relatively fast modifications, as they would need to compete with immune system responses by regularly evolving new adaptive features.

The authors discovered that the antiviral messenger substance interferone-alpha is released in most mammals as a response to the detection of viral genetic material inside body cells. Whereas they found Chiroptera releasing this messenger substance permanently. This would according to the scientists enhance the virus defense in bats and might explain that the above named viruses do not trigger noticeable diseases in their main host recervoir.

I would resume that such conditions might support the scenario one of Andersen et al. (Nature Medicine, 2020), according to which viral key features to infest humans had evolved prior inside the animal host populations. Regular new virus modifications as result of the competition between these viruses and their bat-host immune responses might support the randomness of the development of features, which as pre-adaptations could support a relatively fast host change. Even when I generally prefer scenarios of stepwise adaptations of organisms to new conditions, a higher probability of the availability of suitable pre-adaptations might at least accelerate evolutionary proceedings.

Longtime parasite – host – relationships, a dead-end for the parasite?

Are relationships between organisms over longer time periods of advantage or disadvantage for parasitic or mutualistic passengers? A longtime host specificity of a parasite (or mutualist) requires a strict specialisation, which means complex morphological, ecological and behavioral adaptations.

According to the acarologists P. B. Klimov & B. Oconnor (Systematic Biology, 2013) long-term specialisations could impede the flexibility of such organisms to react to environmental changes via evolutionary adjustments. Thus parasites with long-term relationships to the same hosts might be endangered to reach a dead-end. They would die out. A possible way out from such a disastrous end can be a re-evolution of the parasite back to its ancestral free living conditions, a situation prior to the evolution of its parasitic host specificity. But Dollo’s law states that a complex trait cannot re-evolve again. Thus long-term parasitism could according to the law not other than leading into a dead end. Nevertheless the authors could present an impressive example as proof to the contrary: based on their complex research about house dust mites, the acarologists reconstructed that these mites were originally parasites of warm blooded animals and subsequently evolved into free living associates of mammals, as which they are of medical relevance due to the remarkable allergic reactions in humans.

I think that the access of this paper does contain enough general biological aspects to ask, whether the dead-end scenario of long-term parasite relationships might also concern viruses, which don’t have an option for a free living existence, as they don’t live at all and are unable to perform independent strategies. At least might this long-term scenario support the findings of C. E. Brook et al. (eLife, 2020) that only unusual and regularly changing features of a long-term host might trigger regular corresponding responses by the parasite, another option to prevent a parasite from a dead-end due to a long-term host relation. This might explain, why certain viruses often parasite bats and successfully persist there, while other suitable hosts lack the very efficient immune system of bats and thus cannot host a specialized virus permanently. Regarding SARS-CoV-2 such theories might indicate that the virus would finally move towards dead-ends in humans and other host species, but might permanently survive in chiropterans. It’s a statement only being worth of consideration, in case of scenario one of Andersen et al. (Nature Medicine, 2020). And only in case, it would come out that the virus adapts well to humans, which would require a much reduced harmfulness, as parasites cannot survive by killing their hosts. In case of a dead-end host due to high mortality rates instead of a normal host increase, aspects of a long-term relationship with such a host don’t need to be discussed, as a shorter temporary outbreak and no beginning of a long-term relationship at all would result out of it. One needs additionally to consider that viruses as non living organic bodies with genome and with an unusual ability for fast modifications might often not fit into biological models based on living organisms.

House dust mite Dermatophagoides pteronyssinus. Wikipedia creative commons: Gilles San Martin from Namur, Belgium / CC BY-SA

Summary

Host specificity must be differed from generalism. Known host-parasite specializations include a complexity of strategies. And even different kinds of hosts must be named, such as main host, side-host, intermediate host or dead-end host. Evolutionary steps such as host increase, host change or temporary hosts can appear. Parasitism and mutualism differ from each other as life-strategies, but share common features as association between different organisms: host specificity follows similar rules, an indication that both life-modes can evolve out of each other. The human globalization sometimes supports the spreading of parasites or their hosts over the world, host changes or host increases can thus be performed including organisms, which would under normal conditions get no contact to each other.

Viruses do not represent living organisms, but protein lumps with a genome and depend on living host cells for their reproduction and „survival“. like in living organisms, also viruses underlay the mechanisms of natural selection and evolution. Viral parasite – host – relationships show general similarities with features in living organisms, including options for a host change or host increase, the use of intermediate hosts or a kind of mutualism (passenger viruses).
There is evidence that the main host reservoir of SARS-CoV-2 are Chiroptera, while pangolins (and other mammals) might represent intermediate hosts. Humans are either dead-end hosts (preferred by most authors) or result of a successful host increase. Researchers could not yet decide, whether features to infest humans in a pandemic context evolved prior to the transfer to humans inside animal main host populations or whether a harmless version changed to humans and in their populations evolved its pandemic potential. A major drive motor for a long-term successful relationship with bats is the unusual immune system in chiropterans.

Copyrights Dr. Stefan F. Wirth (phd), all rights reserved, excluding photos labeled as creative common content from Wikipedia sources. Berlin, 2 April 2020

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https://www.sciencedaily.com/releases/2020/03/200317175442.htm