Ehrlichia phagocytophila (Genus Ehrlichia, Order Rickettsiales) is the
pathogen responsible for Tick-borne fever, a disease of high morbidity in susceptible
ruminants. These bacteria appear to be almost identical at serological and molecular
level to granulocytic Ehrlichia species recently diagnosed in humans, dogs and
horses of Europe and the United States.
A molecular description of different isolates of the pathogen is given.
Samples were derived from wild and domestic vertebrate hosts from the UK, where
Tick-borne fever is endemic. Molecular characterisation of a fragment from the groE
operon gene showed higher nucleotide variation than at 16S rDNA level. Human and
equine isolates from Europe differed from North American samples, which at 16S
appeared to be identical. Further differences were also found between ruminant and
non-ruminant granulocytic samples from Europe. Genomic analysis of less conserved
genes appears necessary to provide more useful phylogenetic information that will
help to clarify the relationship between closely related bacterial species.
Populations of the vector tick, Ixodes ricinus, were sampled and analysed to
determine the prevalence of infection and clarify their role in the epidemiology of the
disease. The studies indicated a low infection prevalence that seems, however,
enough to maintain the pathogen in nature. The prevalence varied according to
widespread sites across Britain but it was always lower than expected from
information in the literature. Attempts to determine the efficiency of latent infection
in sheep to transmit Ehrlichia to ticks were unsuccessful.
A seroepidemiological survey was undertaken using IFAT and involving
samples from suspected vertebrate reservoirs of infection such as dogs, cats, horses,
and deer in order to determine if those species were exposed to the pathogen and the
range of hosts for the bacteria in widespread sites across Britain. The results
suggested high rates of exposure in dogs from rural areas and wild roe deer. Cats
showed also a high seroprevalence indicating the three vertebrate hosts were exposed
to E. phagocytophila and mounted an immune response towards the pathogen. It
remains to be elucidated if dogs, cats and horses are accidental or competent
reservoirs of infection. The presence of E. phagocytophila in roe deer blood and
spleen samples was confirmed by PCR. Tick counts from deer legs ratified that all
three stages of tick (larvae, nymphs and adults) were able to feed simultaneously
upon roe deer thus supporting their role as competent reservoirs for both ticks and E.
phagocytophila together with the serological and molecular evidence.
Cytoecetes ondiri, an African relative of Ehrlichia phagocytophila, was
shown to cross-react in immunoblots with E. equi and in IFAT with E.
phagocytophila antigens thus confirming a close antigenic relationship.
ELISA were developed using crude E. equi and E. phagocytophila as antigens
and samples from several vertebrate species. The assays were validated with previous
results obtained by IFAT. Data suggested that E. equi is a useful surrogate antigen for
serologic studies until E. phagocytophila is routinely grown in culture. The antigenic
structure of Ehrlichia was further characterised using mitochondria as surrogate
antigens under the evidence of the phylogenetic relationship between the organelles
and the bacteria. Ehrlichia are classified in the a-subgroup of Proteobacteria, which
are believed to be the closest relatives to mitochondria. Sera from experimentally
inoculated animals recognised mitochondrial antigens prior and after exposure but
the responses were significantly higher after infection and challenge. Further work
should be directed towards the successful cultivation of the pathogen as for HGE and
E. equi in order to develop more reliable serological tests for E. phagocytophila for
future epidemiological surveys. Identification of the major antigenic components of
the organism will also help towards vaccine development.