Supplementary Text 1. Well-Studied Bird Pathogens

1Electronic Supplementary Material

2Supplementary Text 1. Well-Studied Bird Pathogens.

3We selected these diseases because they have been the subject of extensive studies and affect

4humans and livestock as well as migratory birds. In particular, we chose avian influenza and

5avian malaria because in order to predict the effects of climate change on a parasite, we need to

6know the present-day geographic distribution of the parasite and its host. Avian influenza and

7malaria are two of only a few parasites for which we have this detailed knowledge at present . In

8the interest of comprehensiveness, we also wanted to include a bacterial example. We selected

9Salmonella because a great deal is known from human strains . Furthermore, there is a literature

10on seasonal variation in Salmonella prevalence in wild birds and Salmonella genotypes in avian

11migrants .

13Avian malaria

14Plasmodium and related haemosporidians (Haemoproteus and Leucocytozoon) are among the

15best-studied bird pathogens. Today, information for ~1,000 parasite mtDNA lineages and their

16distribution in 600 species of birds are available . Birds wintering in the tropics annually bring

17hundreds of parasite species to temperate breeding areas. Most tropical lineages are not

18transmitted within migrant populations or to local resident species in breeding locations, even

19though gametocytes circulate in the blood of the birds . Nevertheless, resident northern birds may

20become infected with tropical parasites and there is evidence that vectors in tropical areas are

21expanding their ranges due to climate change . If such parasites can establish transmission cycles

22at northern latitudes, previously unexposed species might be severely affected, as famously seen

23in the decline of endemic birds of Hawaii upon introduction of Plasmodium in the 20th century .

25Salmonella

26Salmonella is highly clonal with close to 2,500 described serological variants (serovars). Host

27range varies from narrow to broad, and considerable variation also exists between lineages

28within serovars . Symptoms vary from asymptomatic to death depending on both host and

29parasite, as well as environmental stressors. Salmonellosis is frequently recorded in seed-eating

30birds at feeder tables where fecal contamination may spark an outbreak . Public health concerns

31have initiated studies on fecal contamination of recreational water or pastures by geese and

32gulls . Gulls frequently pick up Salmonella from waste; the most frequent serovars are also

33common in human or food animal sources . Spread of anthropogenic Salmonella could induce

34epizootics in susceptible species , or increase in frequency if concomitant factors reduce the

35health status of individuals. Salmonella has been isolated from gulls and passerines during the

36migratory period , but the extent to which infected birds can transport the bacteria over long

37distances requires further study.

39Influenza A virus

40Influenza A viruses are common in aquatic birds, especially dabbling ducks . The segmented

41RNA genome and high mutation rate result in considerable genetic variation, particularly in the

42surface proteins that interact with host immune systems. Host shifts occur frequently: to

43gallinaceous poultry where it may cause AIV, and to humans and other mammals where it may

44cause flu. In dabbling ducks, prevalence follows seasonal patterns and is higher in juveniles .

45Exposure to low-pathogenic subtypes may infer transient partial immunity to other subtypes,

46including highly-pathogenic variants . Infections in dabbling ducks have been associated with

4 47lower condition and ecological costs . Non-reservoir bird species have other infection patterns

48and seem sometimes more strongly affected by infection . Highly pathogenic H5N1 has caused

49considerable mortality in wild populations, including range restricted species in Asia . The

50mechanism by which highly pathogenic H5N1 spreads to new areas remains controversial. For

51example, the international poultry trade and long-distance movements by migratory birds have

52both been hypothesized to explain the introduction of H5N1 to Europe from Asia in 2006.

6 53Supplementary Text 2. Pathogen Ecology.

54Generalists vs. specialists.

55Many pathogens are host specific, being restricted to one or few host species to which they are

56well adapted. Intracellular parasites like viruses, rickettsia, and protozoa ultimately depend on

57the survival of their specific host and have evolved strategies to balance their own reproductive

58success against the impairment to the host . This includes highly specialized mechanisms during

59the viral replication cycle starting with the entry of the host cell. The host specificity of Avian

60influenza viruses is mediated through the hemagglutinin glycoprotein, which binds to sialic acids

61on the host cell membrane, while the Circumsporozoite protein of Plasmodium falciparum

62provides the specific binding to host liver cells . Pastoret et al. provide an overview of the avian

63immune system.

65 Generalist pathogens are less selective in the choice of their hosts; for example, many

66bacteria species have a broad host range and are not necessarily dependent on a specific host. As

67extracellular parasites, they are able to survive and replicate even outside their host and have

68evolved strategies for long term survival in the environment. The most impressive examples are

69sporulating bacteria such as Bacillus anthracis, which can survive for decades in the

70environment . Pasteurella multocida, the causative agent for avian cholera is another example of

71a generalist pathogen. It is distributed worldwide and is part of the natural oral flora of

72carnivores, but causes peracute septic infections in bird species .

74 Often adaption to a major host can be observed, while the replication of the pathogen in a

75minor host is less effective and can even be interrupted in a dead end host, which either does

8 76not replicate the virus or dies of severe symptoms of disease. For the example, wild waterfowl

77appear to be a major host of avian influenza but geese, while are susceptible to the virus, are less

78important for the perpetuation of the infection and can be considered a minor host .

80 Wild bird populations are the natural reservoir for several zoonotic pathogens and spill-over

81infections from wild birds to livestock or humans are of special concern for public health and

82safety. Spill-over infections have been reported for several avian pathogens such as for instance

83AIV or duck plague virus .

84For the maintenance of an infection within the population, many factors are relevant. Most

85important is the success of a pathogen within the individual host (see Supplementary Fig. 1),

86which is mainly influenced by the pathogenic properties of the pathogen and the immune system

87of the host. Like avian reoviruses, which are resistant to interferon , many pathogens have

88evolved strategies to evade the host’s immune system.

90 Another important aspect of pathogen ecology is the transmission of the pathogen from one

91host to another. Directly-transmitted pathogens infect the host via contact with another

92infected host. Vector-borne pathogens, defined as pathogens transmitted to the host via an

93arthropod or fomite that does not cause the disease itself , can be highly specialized or can

94depend on multiple host species. In the case of avian malaria, arthropod hosts are essential for

95the replication of the parasite and can transmit the parasite between host populations .

10 96Supplementary Figure

97Supplementary Figure 1. From infection to disease. Successful infection of a host can lead to

98different outcomes ranging from asymptomatic infection to symptomatic disease. A highly

99specialized and adapted parasite causes a chronic infection with little impairment of the host.

100Depending on the host’s immune response, an asymptomatic infection can later become

101symptomatic.

12 102 Supplementary Figure 1.

14 103

16 104

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