Table 1

Examples of SPEs. Care was taken to represent all major groups of horizontally transmitted parasites. Priority was given to studies with more replicates and to studies with controls. Studies with small transfer numbers were omitted (see text). Fitness refers to competitive ability relative to the ancestral strain. Growth refers to within-host growth. Attenuation indicates reduced survival, reproduction, growth, or virulence in the former host. Dashes indicate no information available; yes/no indicates that some replicates or combinations within a study showed attenuation, whereas others did not.

Parasites/hostsIncrease in passaged hostAttenuation (former host)Reference
FitnessVirulenceGrowth
RNA viruses
Poliomyelitis virus/cell cultureyesyes (monkeys)(33)
Influenza A seal virus/chicken eggsyesyes(61)
Neurovirulence influenza virus/cell culturesyesyesyes/no (mouse)(62)
Sindbis virus/cell cultureyes (mouse)(27)
Venezuelan equine encephalitis virus/cell culturesyes (mouse)(12)
Infectious bursal disease virus/chickenyes(18)
Chicken influenza virus/chickenyes(5)
Yellow fever virus/rhesus, mouseyesyesyes (man)(63)
Borna disease virus/mouse, ratyesyesyes/no (rat, mouse)(64)
Vesicular stomatitis virus/cell culturesyesyesyesyes (cell culture)(21)
Foot-and-mouth disease virus/cell cultureyesyes (mice, cattle)(20)
Tobacco mosaic virus/legumesyesyesyes (other legumes)(14)
Avian reovirus/cell cultureyesyes(25)
Hepatitis A/cell cultureyesyesyesyes (primates)(65)
Dengue-4 virus/cell cultureyes (monkey)(17)
Japanese encephalitis virus/cell cultureyes (mouse)(66)
Equine infectious anemia virus/cell cultureyesyes (ponies)(7)
DNA viruses
Nucleopolyhedrosis virus/Pseudoplusia includens yesyes (other moths)(15)
Duck hepatitis virus/ducklingsyes(4)
Alcelaphine gammaherpes virus/cell cultureyesyes (rabbit)(67)
Influenza B virus/chicken eggsyes (man)(68)
Bacteria
Salmonella typhimurium/miceyesyes(48)
Rhodococcus equi/axenic mediumyesyesyes (mouse)(31)
Borrelia anserina/axenic mediumyes (chicken)(69)
Bacillus anthracis/in vitroyes (livestock, human)(70)
Legionella pneumophila/in vitroyes (guinea pig)(71)
Fungi
Septoria nodorum/wheat, barleyyesyesyes (wheat, barley)(9)
Phytophthora infestans/potatoyesyesyesyes (other potatoes)(6)
Paracoccidiodes brasiliensis/in vitroyes (mouse)(72)
Coccidioides immitis/in vitroyesyes (mouse)(73)
Protozoa
Eimeria tenella and E. necatrix/turkeynoyes(74)
Trypanosoma brucei/mouseyesyes(39)
T. cruzi/axenic cultureyesyesyes (mouse)(45)
Plasmodium knowlesi/manyes(75)
P. berghei/mouseyes(43)
Naegleria fowleri/cell cultureyes (mouse)(11)
Theileria annulata/cell cultureyes (cattle)(10)
Toxoplasma gondii/ratyes (mouse)(76)
Babesia bovis/splenectomized cattleyes (cattle)(13)
Helminth
Nematospiroides dubius/Quackenbush miceyes (CH3 mice)(8)
Arthropoda
Tetranychus urticae(mite)/beanyesyesyes (tomato, cucumber)(46)
Ostrinia nubilalis(moth)/cornyes (corn)(77)
Nilaparvata lugens(planthopper)/riceyesyes(78)