CHRONIC WASTING DISEASE – NORTH AMERICA (06): TICKS AS POTENTIAL
MECHANICAL VECTORS
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Date: Wed 17 May 2023
Source: News Medical [edited]
https://www.news-medical.net/news/20230517/Ticks-found-to-harbor-and-release-prions-of-chronic-wasting-disease.aspx
In a recent study published in the Scientific Reports journal,
researchers combined experimental tick feeding with tick evaluations
from wild white-tailed deer (Odocoileus virginianus) to determine
whether ticks may contain transmission-relevant amounts of chronic
wasting disease (CWD)-associated prions (PrPCWD).
Natural means of the spread of CWD among free-roaming cervids have not
been extensively investigated, and they may prolong the endemic
increase and the disease’s vast geographic distribution.
Because cervids can carry significant tick infestations and exhibit
allogrooming, a common tick defense technique between conspecifics,
the presence of PrPCWD in the blood may constitute a danger for
indirect transmission via hematophagous ectoparasites functioning as
mechanical vectors. The contribution of ticks as mechanical carriers
of CWD is unknown.
The present study investigated whether ticks harbor and excrete CWD
prions and contribute to their indirect transmission. Pathogen-free
Ixodes scapularis (black-legged tick) female and male adults were
housed and fed a blood meal inoculated with a 10exp-3 dilution of
CWD-positive brain (10exp6 ng). They were exposed to CWD through
membrane feeding.
Subsequently, ear tissue and ticks attached to hunter-harvested, wild
white-tailed deer were obtained. Tick, blood, frass, and ear tissue
homogenates were prepared for real-time quaking-induced conversion
assays (RT-QuIC). CWD status was determined using enzyme-linked
immunosorbent assays (ELISA).
Protein misfolding cyclic amplification (PMCA) assays were performed
to cross-analyze the pooled engorged tick samples tested by RT-QuIC.
Genomic deoxyribonucleic acid (DNA) was extracted from the ear
tissues, and the prion protein (PRNP) gene sequence was amplified
using polymerase chain reaction (PCR).
The researchers evaluated whether they could retrieve and identify
CWD-associated prions from the spiked blood and tick homogenates as
opposed to the CWD-infected tissue or the brain, i.e., the source
tissue and whether the rates of recovery varied depending on the
sample type (CWD-infected brain, prion-spiked homogenates from ticks,
or spiked blood).
Relationships between the type of sample and dilution were included to
investigate whether detection or recovery was responsive to sample
quantity throughout the series of 10-fold dilutions.
Subsequently, the researchers investigated whether ticks fed blood
implanted with CWD-infected brain homogenate could absorb and release
prions.
Amyloid formation rate (AFR) results were compared according to the
type of sample (CWD-infected tissue of the brain, frass obtained from
experimentally fed tick species, and ticks feeding on prion-infected
blood) and the relationships between the type and dilution of the
samples.
Further, the team investigated whether the status of CWD could be
determined using simpler to obtain tissues (e.g., ectoparasites or ear
tissues), which might offer support for less intrusive antemortem CWD
testing.
RT-QuIC mean AFRs were obtained from the deer for the 3 types of
sample types (pooled tick specimens, ear tissues, and lymph nodes) to
determine correlations in the AFRs (i.e., whether deer who had
elevated AFRs in the lymph nodes had greater AFRs in tick or ear
samples in comparison with deer who had reduced AFRs). Linear
regression modeling was performed for the analysis.
Furthermore, using the estimated quantity of PrPCWD seeding substance
in the samples concerning the experimentally established per-tick
infectious dose (ID50) for a comparable quantity of material within
the brain, the team calculated whether prion levels found among ticks
from freely roaming deer could cause infections.
The artificial membrane feeding assays showed that I. scapularis
could ingest and excrete PrPCWD. The specificity and sensitivity of
PrPCWD-exposed ticks with RT-QuIC could validate the absence or
presence of PrPCWD among wild-fed ticks.
The RT-QuIC and PCMA assay results showed seeding activity in 40% (6
of 15) of pooled tick samples, which were obtained from Odocoileus
virginianus infected with CWD. The findings indicated a 7.0% to 40%
CWD prevalence in Ixodes scapularis that had fed on CWD-infected
white-tailed deer.
The findings indicated that a single black-legged tick that fed on a
CWD-infected white-tailed deer could contain 0.30 to 42 ID50,
indicating that tick consumption by deer during bouts of allogrooming
might facilitate oral exposure of PrPCWD from ticks consuming blood
meals from CWD-infected deer.
RT-QuIC seeding activities among wild-fed ticks were analogous to 10.0
to 1000.0 ng of CWD-infected retropharyngeal lymph node (RPLN) from
every animal, and the peripheral samples were less sensitive to RPLN
using RT-QuIC. The quantity of PrPCWD present in tick samples was near
the detection threshold for each method.
The optimization methods for whole blood and blood-engorged ticks
allowed for sensitive and specific detection of PrPCWD from the sample
types by RT-QuIC for the spiking experiments. However, differences in
sensitivity were obtained for the 2 assays, which detected only one
sample in common.
The PRNP genotype may have influenced the naturally occurring PrPCWD
loads from ticks and ear tissue obtained from the 15 CWD-positive
white-tailed deer.
The 4 positive PCMA results for the same pooled tick samples showed
the detection of PrPCWD in one CWD-positive 96G/96S and 3 CWD-positive
96G/96G. Prion loads were different in each animal based on the RPLN
dilution series.
Overall, the study findings showed that infection-relevant loads of
seeding substances are present in individual ticks. The findings could
inform CWD research and adaptive management efforts and deepen our
understanding of ecologically critical drivers of CWD dynamics.
Future studies could evaluate the rate, preferences, and frequency of
cervid allogrooming across different cervid species to improve our
understanding of host behavior, the status of the disease, and
conspecific exposure risk.