Host specificity host ranges and zoonoses

We saw in the chapters on the interactions between predators and their prey that there is often a high degree of specialization of a particular predator species on a particular species of prey (monophagy). The specialization of parasites on one or a restricted range of host species is even more striking. For any species of parasite (be it tapeworm, virus, protozoan or fungus) the potential hosts are a tiny subset of the available flora and fauna. The overwhelming majority of other organisms are quite unable to serve as hosts: often, we do not know why.

There are, though, some patterns to this specificity. It seems, for example, that the more intimate a parasite's association with a particular host individual, the more likely it is to be restricted to a particular species of host. Thus, for example, most species of bird lice, which spend their entire lives on one host, exploit only one host species, whereas louse flies, which move actively from one host individual to another, can use several species of host (Table 12.1).

The delineation of a parasite's host range, however, is not always as straightforward as one might imagine.

host-specific polymorphisms: gentes necroparasites: pioneer saprotrophs natural and accidental hosts

Table 12.1 Specialization in ectoparasites that feed on birds and mammals. (After Price, 1980.)

Percentage of species restricted to:

Scientific name

Common name and lifestyle

Number of species

1 host

2 or 3 hosts

More than 3 hosts

Philopteridae Bird lice (spend whole life on one host)

Streblidae Blood-sucking flies (parasitize bats)

Oestridae Botflies (females fly between hosts)

Hystrichopsyllidae Fleas jump between hosts)

Hippoboscidae Louse flies (are highly mobile)

122 135 53 172 46

87 56 49 37 17

11 35 26 29 24

Species outside the host range are relatively easily characterized: the parasite cannot establish an infection within them. But for those inside the host range, the response may range from a serious pathology and certain death to an infection with no overt symptoms. What is more, it is often the 'natural' host of a parasite, i.e. the one with which it has coevolved, in which infection is asymptomatic. It is often 'accidental' hosts in which infection gives rise to a frequently fatal pathology. ('Accidental' is an appropriate word here, since these are often dead-end hosts, that die too quickly to pass on the infection, within which the pathogen cannot therefore evolve - and to which it cannot therefore be adapted.)

These issues take on not just para-plague: a zoonotic sitological but also medical importance infection with in the case of zoonotic infections: infec-

humans as tions that circulate naturally, and have accidental hosts coevolved, in one or more species of wildlife but also have a pathological effect on humans. A good example is bubonic and pneumonic plague: the human diseases caused by the bacterium Yersinia pestis. Y. pestis circulates naturally within populations of a number of species of wild rodent: for example, in the great gerbil, Rhombomys opimus, in the deserts of Central Asia, and probably in populations of kangaroo rats, Dipodomys spp., in similar habitats in southwestern USA. (Remarkably, little is known about the ecology of Y. pestis in the USA, despite its widespread nature and potential threat (see Biggins & Kosoy, 2001).) In these species, there are few if any symptoms in most cases of infection. There are, however, other species where Y. pestis infection is devastating. Some of these are closely related to the natural hosts. In the USA, populations of prairie dogs, Cynomys spp., also rodents, are regularly annihilated by epidemics of plague, and the disease is an important conservation issue. But there are also other species, only very distantly related to the natural hosts, where untreated plague is usually, and rapidly, fatal. Amongst these are humans. Why such a pattern of differential virulence so often occurs - low virulence in the coevolved host, high virulence in some unrelated hosts, but unable even to cause an infection in others - is an important unanswered question in host-pathogen biology. The issue of host-pathogen coevolution is taken up again in Section 12.8.

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