Pathogen–host–environment interplay and disease emergence

Engering, Anneke; Hogerwerf, Lenny; Slingenbergh, Jan

In some cases, pathogen adaptations in the novel host are not required for successful emergence in a novel host. [31] [32] [33] [34] Certain pathogens can shift from one host to the next via ecological fitting, using traits already present, 31 or by pre-adaption in the reservoir host. 34 One example is the 2009 pandemic H1N1 influenza A virus that showed evidence of pre-adaption to humans during its circulation in swine. 35 However, adaptation in the novel host is often required for successful infection and subsequent sustained transmission between new hosts. The probability of adaptation success is influenced by the number of primary infections, the initial R 0 , the number of genetic changes required, the pathogen evolvability and other factors. 28 Occasionally, several rounds of stuttering chains of transmission in a novel host population are required for successful emergence. Even if the initial establishment of a pathogen fails, the novel host population can become partially immune and in this way primed for more prolonged epidemics upon reintroduction of the pathogen. 36 For example, a recent study on the Nipah virus dynamics in Malaysia suggested that repeated introduction from bats into the same pig farm had created a conducive pig herd immunity status supporting on-farm virus circulation and spread to other pig farms. 37 Pathogens prone to emerge in a novel host A species jump requires utmost pathogen evolvability, and appears facilitated by a broad host range, frequent re-assortment or recombination events, a segmented genome structure, genetically conserved receptors, replication without nuclear entry and quasispecies formation. 8, 24, 28, 32, 33, 38 These features are especially common in singlestranded RNA viruses, dominant among species jumpers. These traits alone do not guarantee success in species cross-over. 32, 33 The collective pathogen trait profile and the ensuing transmission ecology specifics will have to be supportive. For example, pathogens with good environmental survival or transmitted by a range of arthropod vectors may stand a higher chance of infecting new host types than sexually transmitted pathogens. 39 Certain pathogens are more prone to periodic species jumps and may again do so in the future. Influenza A viruses underwent multiple species jumps in recent history, including from avian host sources to horses, humans and pigs, from horses to dogs, from pigs to human, and from dogs to cats. [40] [41] [42] [43] Although the virus causing H5N1 highly pathogenic avian influenza (HPAI) has so far only shown rare stuttering transmission in humans, a mere five mutations would render this virus air-borne transmissible. 44 For comprehensive inventories of pathogens in the different stages of emergence in a novel host, see elsewhere. 8, 9, 45 Drivers of emergence in a novel host Disease emergence in a novel host species depends on the contact rate between the reservoir host and the novel host, as well as on the suitability of the novel host for the concerned pathogens. 9 In addition, major flare-up of disease in a reservoir host can increase spill over to other host types. 7, 9 In most cases, the main driver behind the emergence of a pathogen in a new host is increased contact between different host types. A set of global factors changes ecological landscapes worldwide and brings animals and humans in closer contact. Microbial reservoirs with potentially hazardous disease agents circulate in non-human primates, wi