Cross-species transmission

Cross-species transmission, (CST) or spillover, is the ability for a foreign virus, once introduced into an individual of a new host species, to infect that individual and spread throughout a new host population. Steps involved in the transfer of viruses to new hosts include contact between the virus and the host, infection of an initial individual leading to amplification and an outbreak, and the generation within the original or new host of viral variants that have the ability to spread efficiently between individuals in populations of the new host Often seen in emerging viruses where one species transfers to another, which in turn transfers to humans. Examples include HIV-AIDS, SARS, ebola, swine flu, rabies, and avian influenza. Bacterial pathogens can also be associated with CST. Cross-species transmission, (CST) or spillover, is the ability for a foreign virus, once introduced into an individual of a new host species, to infect that individual and spread throughout a new host population. Steps involved in the transfer of viruses to new hosts include contact between the virus and the host, infection of an initial individual leading to amplification and an outbreak, and the generation within the original or new host of viral variants that have the ability to spread efficiently between individuals in populations of the new host Often seen in emerging viruses where one species transfers to another, which in turn transfers to humans. Examples include HIV-AIDS, SARS, ebola, swine flu, rabies, and avian influenza. Bacterial pathogens can also be associated with CST. The exact mechanism that facilitates transfer is unknown, however, it is believed that viruses with a rapid mutation rate are able to overcome host-specific immunological defenses. This can occur between species that have high contact rates. It can also occur between species with low contact rates but usually through an intermediary species. Bats, for example, are mammals and can directly transfer rabies to humans through bite and also through aerosolization of bat saliva and urine which are then absorbed by human mucous membranes in the nose, mouth and eyes. A host shifting event is defined as a strain that was previously zoonotic and now circulates exclusively among humans. Similarity between species, for example, transfer between mammals, is believed to be facilitated by similar immunological defenses. Other factors include geographic area, intraspecies behaviours, and phylogenetic relatedness. Virus emergence relies on two factors: initial infection and sustained transmission. Cross-species transmission is the most significant cause of disease emergence in humans and other species. Wildlife zoonotic diseases of microbial origin are also the most common group of human emerging diseases, and CST between wildlife and livestock has appreciable economic impacts in agriculture by reducing livestock productivity and imposing export restrictions. This makes CST of major concern for public health, agriculture, and wildlife management. A large proportion of viral pathogens that have emerged recently in humans are considered to have originated from various animal species. This is shown by several recent epidemics such as, avian flu, Ebola, monkey pox, and Hanta viruses. There is evidence to suggest that some diseases can potentially be re-introduced to human populations through animal hosts after they have been eradicated in humans. There is a risk of this phenomenon occurring with morbilliviruses as they can readily cross species barriers. CST can also have a significant effect on produce industries. Genotype VI-Avian paramyxovirus serotype 1 (GVI-PMV1) is a virus that arose through cross-species transmission events from Galliformes (i.e. chicken) to Columbiformes, and has become prevalent in the poultry industry. CST of rabies virus variants between many different species populations is a major wildlife management concern. Introduction of these variants into non-reservoir animals increases the risk of human exposures and threatens current advances toward rabies control. Many pathogens are thought to have host specialization, which explains the maintenance of distinct strains in host species. Pathogens would have to overcome their host specificity to cross to a new host species. Some studies have argued that host specializations may be exaggerated, and pathogens are more likely to exhibit CST than previously thought. Original hosts usually have low death rates when infected with a pathogen, with fatality rates tending to be much higher in new hosts Due to the close relation of humans and nonhuman primates (NHP), disease transmission between NHP and humans is relatively common and can become a major public health concern. Diseases, such as HIV and human adenoviruses have been associated with NHP interactions. In places where contact between humans and NHPs is frequent, precautions are often taken to prevent disease transmission. Simian foamy viruses (SFV) is an enzootic retrovirus that has high rates of cross-species transmission and has been known to affect humans bitten by infected NHPs. It has caused health concerns in places like Indonesia where visitors at monkey temples can contract SFV from temple macaques (Macaca fascicularis). TMAdV (titi monkey adenovirus) is a highly divergent, sharing <57% pairwise nucleotide identity with other adenoviruses, NHP virus that had a high fatality rate (83%) in monkeys and is capable of spreading through human hosts. Prediction and monitoring are important for the study of CSTs and their effects. However, factors that determine the origin and fate of cross-species transmission events remain unclear for the majority of human pathogens. This has resulted in the use of different statistical models for the analyzation of CST. Some of these include risk-analysis models, single rate dated tip (SRDT) models, and phylogenetic diffusion models. The study of the genomes of pathogens involved in CST events is very useful in determining their origin and fate. This is because a pathogens genetic diversity and mutation rate are key factors in determining if it is able to transmit across multiple hosts. This makes it important for the genomes of transmission species to be partially or completely sequenced. A change in genomic structure could cause a pathogen that has narrow host range to become capable of exploiting a wider host range. Genetic distance between different species, geographical range, and other interaction barriers will also influence cross-species transmission. One approach to risk assessment analysis of CST is to develop risk-analysis models that break the ‘‘process’’ of disease transmission into component parts. Processes and interactions that could lead to cross-species disease transmission are explicitly described as a hypothetical infection chain. Data from laboratory and field experiments is used to estimate the probability of each component, expected natural variation, and margins of error.