Host switch
In parasitology and epidemiology, a host switch is an evolutionary change of the host specificity of a parasite or pathogen. For example, the human immunodeficiency virus used to infect and circulate in non-human primates in West-central Africa, but switched to humans in the early 20th century.
All symbiotic species, such as parasites, pathogens and mutualists, exhibit a certain degree of host specificity. This means that pathogens are highly adapted to infect a specific host—in terms of but not limited to receptor binding, countermeasures for host restriction factors, and transmission methods. They occur in the body of a single host species or—more often—on a limited set of host species. In the latter case, the suitable host species tend to be taxonomically related, sharing similar morphology and physiology.
Speciation is the creation of a new and distinct species through evolution and so unique differences exist between all life on earth. As dogs and birds are very different classes of animals—for one, dogs have fur coats and birds have feathers and wings—we therefore know that their fundamental biological makeup is as different as their physical appearance, ranging from their internal cellular mechanisms to their response to infection, and so species-specific pathogens must overcome multiple host range barriers in order for their new host to support their infection.
File:Hominine lice.png|thumb|A likely scenario of cospeciation between Hominines and their lice. The phylogeny of Hominine species in grey and that of their blood-sucking lice in red. Co-speciation resulted in similar topologies except for two evolutionary events. First, gorilla lice host-switched to humans to found the species Crab louse. Second, the human louse duplicated into two forms, the Head louse and Body louse.
Types of host switching
Recent studies have proposed to discriminate between two different types of evolutionary change in host specificity.According to this view, host switch can be a sudden and accidental colonization of a new host species by a few parasite individuals capable of establishing a new and viable population there. After a switch of this type, the new population is more-or-less isolated from the population on the donor host species. The new population does not affect the further fate of the conspecific parasites on the donor host, and may finally lead to parasite speciation. This type of switch is more likely to target an increasing host population that harbours a relatively poor parasite/pathogen fauna, such as the pioneer populations of invasive species. The switch of HIV to the human host is of this type.
Alternatively, in the case of a multi-host parasite host-shift may occur as a gradual change of the relative role of one host species, which becomes primary rather than secondary host. The former primary host slowly becomes a secondary host, or may even, eventually, be totally abandoned. This process is slower and more predictable, and does not increase parasite diversity. It will typically occur in a shrinking host population harbouring a parasite/pathogen fauna which is relatively rich for the host population size.
Host switching features
Reason for host switch events
All diseases have an origin. Some disease circulate in human populations and are already known to epidemiologists, but evolution of the disease can result in a new strain of this disease emerging that makes it stronger for example, multi-drug resistant tuberculosis. In other cases, diseases can be discovered that have not previously been observed or studied. These can emerge due to host switch events allowing the pathogen to evolve to become human-adapted and are only discovered due to an infection outbreak.A pathogen that switches host emerges as a new form of the virus capable of circulating within a new population. Diseases that emerge in this sense can occur more often through human over exposure to the wildlife. This can be as a result of urbanisation, deforestation, destruction of wildlife habitats and changes to agricultural practise. The more exposure humans have to the wild, the more spillover infections occur and pathogens are exposed to human-specific selection pressures. The pathogen is therefore driven towards specific-specific adaptation and is more likely to gain the necessary mutations to jump the species barrier and become human-infective.
Host switch and pathogenicity
The problem with diseases emerging in new species is that the host population will be immunologically naïve. This means that the host has never been previously exposed to the pathogen and has no pre-existing antibodies or protection from the infection. This make host switching dangerous and can result in more pathogenic infections. The pathogen is not adapted to surviving in this new host and this imbalance of coevolutionary history may result in aggressive infections. However, this balance must be brought under control for the pathogen to maintain its infection in the new host and not burn through the population.Stages of host switch
A pathogen undergoing a host switch is driven by selection pressures to acquire the necessary changes allowing for survival and transmission in the new host species. According to a 2008 Microbiology and Molecular Biology Review, this process of host switching can be defined by three stages:- Isolated infection
- Local spillovers
- Epidemic
- Pandemic
Zoonosis and spillover
A zoonosis is a specific kind of cross-species infection in which diseases are transmitted from vertebrate animals to humans. An important feature of a zoonotic disease is they originate from animal reservoirs which are essential to the survival of zoonotic pathogens. They naturally exist in animal populations asymptomatically - or causing mild disease - making it challenging to find the natural host and impossible to eradicate as the virus will always continue to live in wild animal species.Those zoonotic pathogens that permanently make the jump from vertebrate animals to human populations have performed a host switch and thus can continue to survive as they are adapted to transmission in human populations. However, not all zoonotic infections complete the host switch and only exist as smaller isolated events. These are known as spillovers. This means that humans can become infected from an animal pathogen, but it does not necessarily take hold and become a human transmitted disease that circulates in human populations. This is because the host switch adaptations required to make the pathogen sustainable and transmissible in the new host does not occur.
Some cross-species transmission events are important as they can show that a pathogen is getting closer to epidemic/pandemic potential. Small epidemics show that the pathogen is getting more adapted to human transmission and gaining stability to exist in the human population. However, there are some pathogens that do not possess this ability to spread between humans. This is the case for spillover events such as rabies. Humans infected from the bite of rabid animals do not tend to pass on the disease and so are classed as dead-end hosts.
An extensive list of zoonotic infections can be found at Zoonosis.
Case studies
The following pathogens are examples of diseases that have crossed the species barrier into the human population and highlight the complexity of the switch.Influenza
—also known as the flu—is one of the most well-known viruses that continues to pose a huge burden on today's health care systems and is the most common cause of human respiratory infections. Influenza is an example of how a virus can continuously jump the species barrier in multiple isolated instances over time creating different human infecting strains circulating our populations for example, H1N1, H5N1 and H7N9. These host switch events create pandemic strains that eventually transition into seasonal flu that annually circulates in the human population in colder months.Influenza A viruses are classified by two defining proteins. These proteins are present in all influenza viral strains but small differences allow for differentiation of new strains. These identifiers are:
IAVs naturally exist in wild birds without causing disease or symptoms. These birds, especially waterfowl and shore birds, are the reservoir host of the majority of IAVs with these HA and NA protein antigens. From these animals, the virus spills into other species creating smaller scale infections until the virus has acquired significant mutations to spread and maintain itself in another species. The RNA polymerase enzyme of influenza has low level accuracy due to a lack of a proofreading mechanism and therefore has a high error rate in terms of genetic replication. Because of this, influenza has the capacity to mutate frequently dependent of the current selection pressures and has the capability to adapt to surviving in different host species.