HOST SPECIFICITY

Host specificity or niche restriction: the number of different host species that a single species of parasite can infect is limited and the maximum number of different host species recorded for any one species of parasite is probably no more than one hundred. Even when you get a parasite that can develop in a large number of different hosts, the parasite usually develops best in one or two host species and less readily in the rest. However, the vast majority of parasites are restricted to one or a few closely related host species, and this is what is meant by host specificity. Host specificity can of course be different at different stages of the life cycle. The digeneans, for example, are highly specific for their molluscan host, but much less so for their vertebrate host.

Host specificity can be supra-specific, where natural groups of parasites are associated with natural groups of hosts. For example, Gyrodactylid monogeneans are always found on Cypridont fish. Host specificity can also be infra-specific, where a single species of parasite is associated with single species of host.

The Platyhelminthes illustrate the different aspects of host specificity:

Monogeneans, these are parasitic on teleosts, elasmobranchs, amphibians and reptiles, with one record from a mammal (under the eyelid of a hippopotamus). However, the greatest diversity of monogeneans is found on teleosts, and whilst many species are found on elasmobranchs, only a few occur on amphibians and reptiles. Of all the described species of monogeneans 78 % are specific to a single host species. So there is a high degree of infra-specificity as well as a supra-specific relationship with teleosts.

Cestodes occur in all major groups of vertebrates (except possibly crocodiles). The greatest diversity of cestode genera occurs in birds and elasmobranchs. In birds there is a high degree of infra-specificity, with each order of birds and in some cases each sub-order having its own peculiar tapeworm fauna. This association between different groups of tapeworm and different groups of birds is sufficiently good to be used in solving problems in avian taxonomy.

Digeneans are found in all groups of vertebrates with the largest number of genera being found in teleosts. In contrast to monogeneans and cestodes, there is no correlation between taxonomy and the vertebrate host of the digeneans (although the vertebrate hosts are often related ecologically (e.g. Fasciola in browsing animals). The digeneans are, however, highly specific for their molluscan host. In schistosomes, geographical strains of the parasite will only develop satisfactorily in local races of snail. So the restriction extends below the species level. Because of the high specificity of digeneans towards their molluscan host, it is though that digeneans were originally parasites of molluscs and the vertebrate host has been more recently acquired (there are, however, a few species of digenean that have a wide molluscan host range).

Evolution of Host Specificity

Parasites depend, to a large extent, in their evolution on the evolutionary processes of their hosts. So parasites and especially the highly specialised ones will tend to evolve in parallel with their hosts. So when two host species have evolved from a common ancestor, the parasites that were originally present in that common ancestor evolve with the two new hosts. So the modern descendants of the original host ancestor will have parasites descended from those originally present in the ancestral host.

So related hosts tend to be infected with related parasites and the phylogenetic tree of a group of parasites frequently corresponds to the phylogenetic tree of their hosts. In general the evolution of the systematic units amongst the parasites lags behind that of their hosts. So if the host has diverged into two independent species the parasites often retain their status as one species, or differentiate into two sub-species. If two genera are formed from the original host species, the corresponding divergence amongst the parasites reaches the level of two species and so on. This can be illustrated by reference to lice (Anoplura); the degree of parallel evolution between the host and parasite can be seen by comparing the different taxonomic divisions.

Host specificity is not rigid; it is still evolving. In Europe Fasciola hepatica occurs only in the snail Limnea truncatula and in no other species, not even the closely related L. stagnalis. On other continents where F. hepatica has been introduced with domestic livestock it has adjusted to new host snails: L. humilis in North America, L. viator in South America, L. tomentosa in Australia.

How is Host Specificity Maintained?

The life cycle of a parasite can be divided into three phases:

• Localisation of the host
• Establishment on the host
• Growth and reproduction

The mechanisms that contribute to host specificity can operate at all three levels.

Host localisation (the problems of host finding). Parasites are limited to certain ecological or geographical areas and so are limited to the hosts that occur in the same area. Parasites are restricted by the conditions necessary for the development or survival of their free-living or infective stages or of their intermediate hosts or vectors. If a new host species enters the parasites range it changes from being a potential host to becoming an available host. Reindeer, for example, when introduced into Scotland, rapidly acquired gastrointestinal nematodes normally found in Red Deer. Nematodirus battus an important parasite sheep of was unknown in the UK before 1951, was probably introduced in exotic deer.

A parasite's geographical range may encompass a variety of different habitats and the potential host may be restricted to certain locations within the parasite range. So although the host and parasite overlap in their geographical distribution the parasite still has to find the host. Host finding by parasites can be divided into two phases:

• Location of the hosts' area within the parasites range.
• Having located the host area, find the host within the host area.

Location of host area: the infective stages of parasites frequently show behavioural adaptations which help to bring them into the hosts' area and so increase their chances of locating a host. An interesting example is provided by two species of Digenean. Gorgodera vitelliloba and Gorgodera euzeti. They both infect the same species of bivalve intermediate host. However, the cercariae of G. vitelliloba are negatively geotactic and emerge onto the surface of the mud where they are eaten by tadpoles that are the second intermediate host of G. vitelliloba. The cercariae of G. euzeti, in contrast are positively geotactic and burrow into the mud where they are eventually eaten by alder fly larvae (Sialis), which are the second intermediate host for G. euzeti. The different geotactic behaviour of the two species of cercariae brings them into the ecological range of their different second intermediate hosts.

Having got into the general host area, the parasite then has to find its host. The infective stages of many parasites are attracted to their hosts by chemical cues. Miracidia and cercariae often show positive chemotaxis to skin lipids. Parasites of birds and mammals are often attracted to warmth. Schistosome cercariae are stimulated by shadows. Bitting Diptera are attracted to herds of ungulates by the increased CO₂ and smell of propionic acid.

An alternative strategy is to attract the host to you, brightly coloured sporocysts may attract birds as may the brightly coloured cystacanths of some Acanthocephala. Tapeworm eggs may produce some form of chemical attractant for beetles to eat them.

Another approach to locating a host is to use an intermediate or secondary host that forms part of the food chain of the definitive host. A problem here is that predators only eat a proportion of their prey (10 %). However, a number of intermediate hosts, when they are infected with the intermediate stages of parasites show behavioural changes which may increase the chances of the infected intermediate host being eaten by the final host. These behavioural changes include:

• Loss of vigour in infected intermediate hosts, so the final host more readily catches them. For example, sticklebacks infected with Schistocephalus no longer shoal.
• Disorientation, minnows infected with the metacercariae of Diplostomum swim in circles near the surface of the water (the parasite invades the inner ear). Beetles infected with cysticercoids of the tapeworm Hymenolepis diminuta no longer respond to aggregation pheromone.
• Change of habitat, clams infected with the metacercariae of Gymnophallidae no longer burrow into the mud, but roam on the surface.

Establishment (host selection). Many skin penetrating larvae are limited by their ability to penetrate the tegument of potential hosts, they may be thwarted by thick mucous layers or by the presence of fur or feathers. The phenomenon of age resistance, where older animals are more resistant to skin penetrating larvae than younger ones, may be due to thickening of the epidermis. Many parasites infect their hosts passively via the alimentary canal and the hatching or exsheathing of these parasites often requires complex stimuli, depending on pCO₂, pO₂, redox potential, pH and temperature and in some cases digestion of external layers by host enzymes and the presence of specific bile salts are also required. The kinds of stimuli needed and the order in which they are required may help to differentiate between the guts of different hosts and so limit host range. For example, nematode eggs that hatch in the rumen require high CO₂ levels and a high redox potential, conditions that are only found in the rumen.

The metacercariae of many digeneans, cestode cysticerci, acanthocephalan cystacanths and coccidial oocysts all require specific bile salts for activation. The variation in bile salt composition in different vertebrates may again restrict the parasite's host range.

There can also be morphological and physical features of the alimentary canal in animals that can prevent a parasite from becoming established, the gut may be too short, the rate of gut emptying may be too fast, or the morphology of the villi or crypts may be unsuitable.

In order to explain the high degree of host specificity found in some parasites it has often been suggested that the host must provide some highly specific or unique chemical stimulus. However, such highly specific or unique compounds have not been found to be involved in animal parasites, it is more the overall pattern of stimuli that is important. Egg hatching in the plant parasitic nematode Globodera rostochiensis (potato cyst nematode) is stimulated by a hatching factor is released by the roots of host plants. The factor appears to be a complex mixture of different compounds.

Growth and Reproduction. Once a parasite has established itself there are a whole series of factors that can influence growth and reproduction and thus restrict the range of host species further:

• Lack of host stimuli. The infective larvae of many parasitic nematodes and some digeneans undergo extensive tissue migrations in their final hosts. In the 'wrong host' the larvae often undergo aberrant migrations and finish up in unusual parts of the body. Presumably non-susceptible hosts do not supply the right stimuli. Other parasites (e.g. Polystoma may require specific stimuli for sexual development).
  
  
• Host immune response. Parasites have developed a number of methods of avoiding their hosts' immune response. Some such as schistosomes incorporate host antigen into their teguments and so are not recognised by their hosts as foreign. Others like the African Trypanosomes keep changing their antigenic surface and so stay one jump ahead of the hosts' immune response.
  
  
• Innate resistance. Many potential hosts show innate immunity to infection. Innate immunity is an active response on the part of the host, but unlike acquired immunity it is not dependent on previous infection. The actual cellular and humoral mechanisms involved in innate resistance are not well understood, but include serum factors such as high density lipoprotein and complement as well as natural killer cells and platelets.
  
  
• Essential nutrients. It seems likely that parasites are restricted more by their overall nutritional requirements than by the need for obscure or specific metabolites. Some parasites, however, do have highly specific requirements, the tapeworm Diphyllobothrium accumulates vitamin B₁₂ (sufficient to cause pernicious anaemia in the host) and the malaria parasite requires an exogenous supply of ATP. The molar ratios of amino acids in the vertebrate intestine are remarkably constant and this is related to the amino acid uptake mechanisms in the mucosa. Parasites such as tapeworms or acanthocephalans that rely on their hosts digestive processes to provide them with low molecular weight nutrients have to compete with the uptake mechanisms of their host's intestine. So the amino acid transport systems of cestodes and acanthocephalans, like those of the mucosa, have become adapted to function with particular amino acid ratios. Since the amino acid ratios may differ in different hosts, this could further restrict the parasites' range. The free-fatty acid composition of the intestine is also relatively constant and could be similarly important.

Conclusions

The mechanisms by which a parasite is restricted to one or a few host species involves first an ecological restriction in that parasites are restricted to those hosts that occur in the same ecological area (often referrred to as an ecological filter). The parasite must locate its host and having located it, the parasite has to be able to establish itself, grow and reproduce. The mechanisms of host specificity can operate at all four levels.