This study presents evidence of ground-foraging songbirds transporting Lyme disease vector ticks to northern latitudes in Canada. We have documented Bbsl in Ixodes
ticks parasitizing migratory songbirds collected from Alberta to Newfoundland and Labrador. Some of these neotropical and southern temperate migrants have wintering ranges in the Caribbean, Central and South America. These long-distance migrants provide insights into how people in northern areas contract Lyme disease. In this study, we detected a Bbsl infection prevalence of 39% in songbird-transported I. scapularis
nymphs which is slightly higher than previous bird-tick-pathogen studies that ranged from 31–35% [5
]. Although Lyme disease vector ticks have been reported as far north as the Yukon [21
], we report the first Bbsl-positive Ixodes
ticks parasitizing migratory songbirds in several northern regions of Canada.
4.2. Obstacles Facing Migratory Songbirds
In this study, we obtained bird-feeding ticks in several different ways, especially during northward spring migration. Some migratory songbirds, such as the Willow Flycatcher, Epidonax traillii
, may fly 8000 km or more between wintering and breeding grounds [29
]. Migration requires high energy reserves, and enhances exposure to predators; thus, it is typically a period of high mortality. Migratory songbirds are susceptible to predatory cats, power transmission lines, vehicles, hunting, agricultural pesticides, houses and tall buildings. Blancher [30
] calculated that outdoor feral and domestic cats kill more than 196 million birds annually in Canada, making them the most significant bird mortality factor. In our study, we collected ticks from 11 different ground-frequenting songbirds either by rescuing birds from domestic cats or birds obtained after collisions with moving vehicles or with tall buildings with reflective glass. When songbirds collide with lighted skyscrapers and glass facades, they normally fall to the ground, and often predator birds, such as gulls, prey on these helpless, stunned birds. In some urban locales, Fatal Light Awareness Program volunteers come to the rescue and rehabilitate birds and, at the same time, collect host-seeking ticks. Specifically, 2 of 3 engorged, I. scapularis
nymphs collected on 2 May 2017 from a Northern House Wren, which collided with a multi-story buildings in downtown Toronto, were spirochetemic for Bbsl. Some researchers suggest that 80% of annual bird mortalities occur during migration [31
4.6. Songbird-Transported Ticks Infected with Lyme Disease Bacterium in Alberta
We provide the first report of Bbsl-positive I. scapularis
ticks parasitizing a bird in Alberta. Previously, tick researchers have documented I. scapularis
immature ticks in this province [4
]. When the amplicons underwent DNA sequencing, they were delineated as Bbss. However, there was considerable heterogeneity between two Bbss strains detected in these two Bbsl-positive nymphs. It is likely that the initial spirochetal infections occurred at two different locations when these ticks were in their larval stage. Alternatively, the host bird may have been harbouring two different Bbss strains, and transmitted them directly to the nymphs during their blood meals. The Bbsl-positive I. scapularis
female has the potential to infect any suitable mammalian host, including humans.
At this northern latitude (56.23° N), the daylength is less than 14 h in late August. Consequently, immature stages of I. scapularis
will not molt to the next life stage in late summer. A photoperiod of >14 h is required for I. scapularis
larvae and nymphs to molt [28
]. Therefore, I. scapularis
cannot become established in this northern location or further north. However, songbird-transported I. scapularis
immatures could molt in early summer after spring migration and, after molting, could bite humans and other suitable hosts.
In retrospect, Bbsl was detected in H. leporispalustris
, collected from a snowshoe hare, Lepus americanus
, at Grande Prairie, Alberta [34
]. Clearly, Bbsl is present in the environment in Alberta, and migratory songbirds facilitate long-distance dispersal of Bbsl-infected ticks, especially during the spring and fall migration. In addition, researchers have reported immature stages of Lyme disease vector ticks (i.e., I. pacificus
, I. scapularis
, I. spinipalpis
) parasitizing songbirds in Alberta [4
], and each of these tick species exhibits vector competency [35
]. These epidemiological findings provide sound evidence of how people can contract Lyme disease and associated tick-borne diseases in this province.
4.9. Songbirds Establish Tick Populations
Tick-laden songbirds have the capacity to initiate new foci of ticks hundreds of kilometres from their original source. Any heavily infested songbird, which is parasitized by Bbsl-infected I. scapularis
, has the makings to establish a new population of I. scapularis
ticks in a suitable habitat [36
]. Oftentimes, migratory songbirds will make stop-overs at Lyme disease endemic areas, and become parasitized by Lyme disease vector ticks. In the present study, a Common Yellowthroat (tick no. 17-5A42), which was parasitized by the 3 Bbsl-positive I. scapularis
nymphs, and has the means to start a Lyme disease endemic area. As well, neotropical songbirds transport ticks from as far south as Brazil, and import them into Canada during northward spring migration [39
]. After these ticks molt, they have the potential to infect suitable hosts in northern latitudes.
Some passerine migrants are parasitized by I. scapularis
immatures that are co-infected with more than 1 tick-borne pathogen. Hersh et al. detected 3 different pathogens (Anaplasma phagocytophilum
, Babesia microti
, Borrelia burgdorferi
) in an I. scapularis
nymph collected from a Veery, Catharus fuscescens
]. This triple co-infection shows that birds have the potential to be parasitized by ticks harbouring a wide range of tick-borne pathogens, such as Babesia duncani
], and subsequently, these tick-associated pathogens can be transmit to other vertebrates. Such bird parasitisms reveal how numerous tick-borne pathogens can be introduced into a breeding colony of I. scapularis
ticks. Similarly, people can be co-infected with multiple zoonotic pathogens during a tick bite.
4.10. Viability of B. burgdorferi Sensu Lato in Bird-Feeding Ticks
The viability of Bbsl in this study has been questioned because ticks were not cultured. Culturing and sub-culturing isolates is a very time-consuming and labour-intensive task, and this procedure is commonly bypassed. Contamination of cultures by unwanted microorganisms often occurs, and is a laboratory bugbear. Regardless of whether culturing is successful or not, nucleic acid testing, namely PCR, is ultimately employed to detect and identify Bbsl.
In this study, many of the fully engorged I. scapularis
ticks completed the nymph-adult molt before they were preserved. During the molt, the midgut of I. scapularis
larva and nymphs remains intact during development of the next life stage; the rest of the tick exoskeleton, including the foregut and hindgut, is shed at the end of the molt [43
]. Lyme disease spirochetes reside in the midgut, and remain viable throughout the molt.
When I. scapularis larvae and nymphs molt, Bbsl remains viable and infective during the typical 5–8 week molt, and the resulting nymphs and females are able to infect subsequent hosts. Any I. scapularis ticks, which tested positive for Bbsl in the present study, would have been harbouring motile spirochetes prior to the 94% ethyl alcohol preservation. Notably, I. scapularis has transstadial transmission of Bbsl and, when this tick species becomes infected, retains Lyme disease spirochetes for the rest of its life.
In previous bird-tick-pathogen studies, Lyme disease spirochetes were isolated from host-seeking ticks collected from songbirds during spring migration [4
]. The only difference between these earlier bird-tick-pathogen studies, and the present study, is the timeframe. Despite our not culturing ticks, 39% of the I. scapularis
nymphs were positive for Bbsl. The bird-feeding ticks in the present study would, no doubt, have been infected with motile Lyme disease spirochetes just before preservation in 94% ethyl alcohol.
In mammalian hosts, non-viable Lyme disease spirochetes are promptly shed by the body. When Straubinger et al. inoculated the skin of beagles with heat-killed Bbsl, the borrelial fragments were no longer present after 3 wk [44
]. Notably, the molt period in I. scapularis
ticks is significantly longer than the time for clearance of Bbsl from mammalian hosts. Blacklegged ticks retain viable Bbsl bacteria in the midgut, whereas mammalian hosts promptly shed them.
Based on culturing of Bbsl from I. scapularis nymphs in previous studies, and the ability of Bbsl to remain in ticks during the current study, we have substantive evidence that Bbsl bacteria in I. scapularis ticks are infective. Even though we preserved ticks for PCR testing, we are assured that if they were kept alive, they would have been able to transmit infective, Lyme disease-causing spirochetes to people.
4.11. Implications of Human Lyme Disease
Because Lyme disease is a zoonosis, it is pathologically important to provide an interconnecting link between vector ticks and humans. These ectoparasites have super-sensitive sensory organs that detect urine, carbon dioxide, and phenols given off by potential hosts [45
]. Ticks can transmit innumerable pathogens to people during engorgement, and these pathogenic microorganisms typically cause multisystem infections, including Lyme disease.
When Bbsl-infected I. scapularis
bite, they commonly transmit Lyme disease spirochetes in 24–48 h [46
]; however, Cook [47
] reported transmission in less than 16 h, particularly if the tick salivary glands are infected. Whenever I. scapularis
ticks harbour other tick-borne pathogens, such as Anaplasma phagocytophilum
(the causative agent of human anaplasmosis), they can often transmit these pathogens in less than 24 h [48
]. Additionally, Powassan virus can be transmitted in less than 15 min [49
]; thus, there is no grace period between tick attachment and transmission. Because Babesia
sporozoites reside in tick salivary glands, they can be transmitted immediately when the tick starts to take a blood meal [50
After transmission, spirochetes disseminate throughout the body, and lodge in tissues and organs. Lyme disease patients may have an erythema migrans rash (i.e., bull’s-eye, homogenous, erythema multiforme, atypical); however, 40% or less have erythematous rashes [51
]. Only 14% of Lyme disease patients recall a tick bite [55
]. As spirochetes advance in the body, patients typically experience a wide array of symptoms, including fatigue, flu-like symptoms, muscle aches and pain, radicular pain, arthritis, peripheral neuropathy, cognitive impairment, increased impulsivity, sensory hypersensitivity (to sound, touch, smell, taste and/or light), and intense emotional lability [56
]. Spirochetes evade and slip by host defenses, lodge intracellularly, and form more resilient forms, such as biofilms [57
]. These stealth pathogens also attach to, invade and kill B and T lymphocytes [59
]. When Bbsl bacteria are killed off, the byproducts (biotoxins) induce inflammatory cytokines (i.e., interleukin 1, interleukin 6, TNF-alpha) [60
]. These biotoxins typically cause fever, muscle ache and pain, headaches, cognitive impairment, and sometimes, skin discoloration [61
]. For instance, acrodermatitis chronica atrophicans shows up as skin discoloration on body extremities [62
]. At an elevated level, biotoxins can cause tick paralysis [63
]. In addition, borrelial biotoxins will induce mitochondrial dysfunction, oxidative stress, hormonal abnormalities, depressive tendencies, and neuropsychiatric manifestations [61
]. Patients will often experience a Jarisch-Heixheimer reaction when treatment is initiated [66
If left untreated or inadequately treated, Bbsl will sequester and persist in deep-seated tissues including brain [69
], collagenous tissue (i.e., ligament, tendon) [72
], bone [74
], eye [75
], muscle [76
], glial and neuronal cells [77
], synovium [79
], and fibroblasts/scar tissue [80
]. In addition, live Bbsl spirochetes have been cultured from human blood after the patient was bitten by a blacklegged tick [81
]. Since Bbsl is pleomorphic, these diverse forms (i.e., spirochete, blebs, granules, spherocytes) facilitate intracellular Borrelia
sequestration in these tissues [83
]. Collectively, these aggregates combine to form slime-coated, polysaccharide matrices, called biofilms, and exacerbate persistence of infection [58
]. Bbsl has persister cells and sleeper cells, which are known to survive antimicrobials, and subsequently, recrudescence of infection may occur [68
]. Since Lyme disease spirochetes are in human testicles, seminal and vaginal secretions, this spirochetosis has the potential to be sexually transmitted [87
Early treatment is paramount; delayed treatment may be arduous and challenging [89
], and result in fatal outcomes [69
]. Lyme disease can have traumatic social and psychological effects on patients, partners, and family that include extreme temper tantrums, increased irritability, oppositional behaviors, aggressiveness, violence, suicide, homicidality, and homicide [92
In conclusion, we report the northernmost locations in North America, where bird-feeding, Bbsl-positive ticks have been collected. Passerine migrants widely disperse Bbsl-infected I. scapularis from Alberta to Newfoundland and Labrador, and people do not have to visit an endemic area to contract Lyme disease and associated tick-borne diseases. Our flagship findings reveal that 39% of the songbird-transported I. scapularis nymphs are positive for Bbsl, and these spirochetes belong primarily to B. burgdorferi sensu stricto, which is pathogenic to humans. Based on our data, we provide substantive evidence to show how people in northern latitudes of Canada may contract Lyme disease via the bite of a Bbsl-infected, songbird-transported ticks. The medical profession should be cognizant that Ixodes ticks infected with Lyme disease spirochetes are a major public health risk across Canada.