Canine ehrlichiosis in six dogs with persistently increased antibody titers

Molecular characterization of Ehrlichia canis from naturally infected dogs reveals a novel Asiatic-lineage and co-circulation of multiple lineages in India

Canine monocytic ehrlichiosis caused by Ehrlichia canis is an important rickettsial pathogen of dogs transmitted by Rhipicephalus sanguineus sensu lato ticks in India. Globally, molecular characterization of E. canis is done using different E. canis gene targets. This study aimed to characterize genetic diversity of uncultured Ehrlichia species from dogs by 16S rRNA and partial gp200 gene (termed as p43 region) sequences data. Phylogeny based on 16S rRNA gene did not reveal any region-specific lineages. The phylogeny based on 5' region of E. canis gp200 gene (termed as p43 region) revealed four major clusters (A, B, C and D) and the Indian isolates fall under clusters A and B. Cluster A is characterized by an insertion of unique 141 bp tandem repeat sequence. Similar tandem repeat sequence was found in one of the E canis isolates from east-Asia, suggesting a possible divergence within this species. The study shows evidence for divergence of a new lineage within E. canis. The location of this insertion at the 'ankyrin repeat domains' containing region is suggestive of its possible role in modulation of host responses.

Exotic bloodborne parasites and dogs imported from Africa

The number of dogs legally imported to the UK from Africa is on the rise, and so too, therefore, is the risk of co-importing currently exotic parasites. Here, Rebecca Dobinson and Ian Wright report the results of a small study carried out in Sierra Leone, in which visibly healthy dogs were screened for the presence of several bloodborne parasites.

Mapping the distribution and risk factors of Anaplasmataceae in wild and domestic canines in Chile and their association with Rhipicephalus sanguineus species complex lineages

Anaplasma platys and Ehrlichia canis are members of the Anaplasmataceae family that cause disease in dogs and are mainly transmitted by Rhipicephalus sanguineus species group ticks. We performed a cross-sectional study on these pathogens across six bioclimatic regions of Chile, including 719 free-ranging rural dogs, 132 Andean foxes (Lycalopex culpaeus), and 82 South American gray foxes (Lycalopex griseus). Dog and fox blood samples were first screened for DNA of Anaplasmataceae followed by two Ehrlichia-specific protocols. Antibodies against Anaplasma sp. and E. canis were assessed by immunofluorescence in dogs. Ectoparasites were collected and identified, with the determination of the lineages of the Rhipicephalus sanguineus species group by molecular and phylogenetic analyses. Finally, potential risk factors for infection were investigated across the different bioclimatic regions and host species. All DNA amplicons obtained from the screening protocol corresponded to Anaplasma platys. The occurrence of both A. platys DNA and antibodies was confirmed in all six bioclimatic regions, except for regions at high altitude and/or without either R. sanguineus species group lineage present. Dogs infested with R. sanguineus ticks were significantly more prone to be infected and exposed to Anaplasma spp. Prevalence of DNA was significantly higher in juvenile (19%) than in adult dogs (9%), whereas the opposite was found for seroprevalence (19% versus 35%, respectively). Overall prevalence of A. platys DNA was higher in dogs (11%) than in foxes (4%), probably owing to markedly lower tick infestations in the foxes. Ehrlichia canis DNA was not detected in any sample, and antibodies against this pathogen were detected only in four dogs, in areas with both R. sanguineus lineages present. Free-ranging dogs in Chile could be favoring the maintenance of A. platys in all areas suitable for its tick vector. Although apparently infrequent, spillovers from dogs to foxes may be taking place and should be considered in management plans in Chile.

Detection of specific IgM and IgG antibodies in acute canine monocytic ehrlichiosis that recognize recombinant gp36 antigens

The efficacy of antibody detection tools for all stages of Ehrlichia canis infections and for various genotypes remains unclear. We produced recombinant gp36 (rgp36) antigens from different isolates of Thai E. canis to confirm the immunoreactivities to these recombinant proteins from naturally infected dogs. Sera and blood samples were taken from 21 dogs naturally infected with E. canis and in the clinical stages of acute phase ehrlichiosis. The expression vectors and competent E. coli produced two isolates of rgp36. These two major rgp36s were recognized by the dogs' sera in Western blotting, with both anti-dog IgM and IgG used as secondary antibodies. The two different genotypes of these local recombinant immunoreactive proteins were gp36 subgroup A (isolate 1055) and subgroup B (isolate 533). The Western blot analyses successfully identified both specific IgM and IgG from the dogs' sera. Of all 21 cases, five dogs presented specific IgM, twenty dogs presented specific IgG, and the commercial test used found fifteen seropositive dogs. There were four dogs that presented both specific IgM and IgG. Only one dog presented specific IgM only. This report is the first identification of a specific IgM in dogs in response to acute infections with E. canis. The recombinant gp36 isolates may be useful as potential antigenic material for subsequent serological tests that have a high possibility for differentiating between acute, chronic, primary, and nonprimary infections with E. canis.

Ehrlichia canis in dogs experimentally infected, treated, and then immune suppressed during the acute or subclinical phases

Background

Concerns for recrudescence of Ehrlichia canis infection arise when immunosuppressive drugs are used to treat immune‐mediated diseases in dogs previously infected with E. canis.

Objectives

Determine whether administration of prednisolone and cyclosporine would reactivate E. canis infection in dogs previously treated with doxycycline during the acute or subclinical phases.

Animals

Seven beagles previously experimentally infected with E. canis and administered doxycycline for 4 weeks were included. Three of the 7 dogs were incidentally concurrently infected with Anaplasma platys and Babesia vogeli and were administered 2 doses of imidocarb 2 weeks apart before enrollment in the current study.

Methods

Experimental study. Each dog was administered prednisolone and cyclosporine for 6 weeks. Clinical signs, complete blood cell count (CBC), polymerase chain reaction (PCR) assays for E. canis, A. platys, and B. vogeli DNA in blood, E. canis indirect fluorescent antibodies (IFA) titers, and flow cytometry for antiplatelet antibodies were monitored.

Results

All dogs completed the immunosuppressive protocol. No evidence for recrudescence of E. canis, A. platys, or B. vogeli were detected based on clinical signs or results of CBC, PCR, IFA, and flow cytometry for antiplatelet antibodies. E. canis IFA titers were negative in 5/7 dogs at the end of immunosuppressive protocol and were negative 6 months after the protocol in 5/5 dogs available for testing.

Conclusions and Clinical Importance

Dogs administered with a 4‐week course of doxycycline with or without imidocarb failed to show evidence of activation of E. canis infection after administration of a commonly used immune suppressive protocol.

An update on the treatment of canine monocytic ehrlichiosis (Ehrlichia canis)

Canine monocytic ehrlichiosis (CME), caused by Ehrlichia canis, a gram-negative, obligate intracellular bacterium, is a tick-borne disease of worldwide distribution. Experimentally, the course of E. canis infection can be sequentially divided into acute, subclinical and chronic phases, although distinction of these phases is challenging in the clinical setting. Spontaneous clinical recovery of acutely infected dogs is common; however, dogs at this stage require medical treatment in order to hasten their clinical recovery, and to prevent clinical exacerbation or death. An unpredictable proportion of subclinically infected dogs will eventually develop the chronic, severe form of ehrlichiosis, characterized by aplastic pancytopenia and high mortality. The aims of antimicrobial treatment in CME include the achievement of clinical remission, resolution of the clinicopathologic abnormalities, and eradication of the infection, although the latter is not always feasible or diagnostically confirmable. Treatment of dogs with aplastic pancytopenia should be undertaken with the clear understanding that medical management will require long-term care, will be expensive, and may eventually prove ineffective. This manuscript reviews the current state of knowledge regarding treatment of ehrlichiosis, caused by E. canis infection in dogs, provides expert opinion guidelines for the management of the CME-associated aplastic pancytopenia, and outlines methods for evaluation of treatment outcomes.

Ehrlichioses: An Important One Health Opportunity

Ehrlichioses are caused by obligately intracellular bacteria that are maintained subclinically in a persistently infected vertebrate host and a tick vector. The most severe life-threatening illnesses, such as human monocytotropic ehrlichiosis and heartwater, occur in incidental hosts. Ehrlichia have a developmental cycle involving an infectious, nonreplicating, dense core cell and a noninfectious, replicating reticulate cell. Ehrlichiae secrete proteins that bind to host cytoplasmic proteins and nuclear chromatin, manipulating the host cell environment to their advantage. Severe disease in immunocompetent hosts is mediated in large part by immunologic and inflammatory mechanisms, including overproduction of tumor necrosis factor α (TNF-α), which is produced by CD8 T lymphocytes, and interleukin-10 (IL-10). Immune components that contribute to control of ehrlichial infection include CD4 and CD8 T cells, natural killer (NK) cells, interferon-γ (IFN-γ), IL-12, and antibodies. Some immune components, such as TNF-α, perforin, and CD8 T cells, play both pathogenic and protective roles. In contrast with the immunocompetent host, which may die with few detectable organisms owing to the overly strong immune response, immunodeficient hosts die with overwhelming infection and large quantities of organisms in the tissues. Vaccine development is challenging because of antigenic diversity of E. ruminantium, the necessity of avoiding an immunopathologic response, and incomplete knowledge of the protective antigens.

Guideline for veterinary practitioners on canine ehrlichiosis and anaplasmosis in Europe

Canine ehrlichiosis and anaplasmosis are important tick-borne diseases with a worldwide distribution. Information has been continuously collected on these infections in Europe, and publications have increased in recent years. Prevalence rates are high for Ehrlichia and Anaplasma spp. infections in dogs from different European countries. The goal of this article was to provide a practical guideline for veterinary practitioners on the diagnosis, treatment, and prevention of ehrlichiosis and anaplasmosis in dogs from Europe. This guideline is intended to answer the most common questions on these diseases from a practical point of view.

Co-infection with Anaplasma platys, Bartonella henselae, Bartonella koehlerae and ‘Candidatus Mycoplasma haemominutum’ in a cat diagnosed with splenic plasmacytosis and multiple myeloma

Anaplasma platys ( Apl), ‘ Candidatus Mycoplasma haemominutum’ ( CMh), Bartonella henselae ( Bh) and Bartonella koehlerae ( Bk) were confirmed by polymerase chain reaction (PCR) amplification and DNA sequencing in a cat diagnosed with multiple myeloma. Other inconsistently documented hematologic abnormalities included anemia, thrombocytopenia, eosinophilia and hypoglycemia. Persistent Apl infection was confirmed for the first time in a North American cat by sequencing three bacterial genes ( 16S rRNA, p44 and GroEL) in peripheral blood samples collected 100 days apart. Following doxycycline treatment for Apl, multiple myeloma was diagnosed based upon a monoclonal gammopathy and splenic plasmacytosis, and the cat was treated with melphalan, chlorambucil and prednisolone. Apl DNA was not amplified from post-treatment blood samples and the hyperglobulinemia resolved temporarily following chemotherapy. Retrospective PCR analysis of stored DNA extracts identified CMh, Bk and Bh infections. Retrospective PCR for antigen receptor rearrangements (PARR) of splenic aspirates did not confirm B- or T-cell clonality. Co-infection with multiple vector-borne pathogens should be a diagnostic consideration in cats with chronic hypergammaglobulinemia, monoclonal gammopathy and splenic plasmacytosis.

Ehrlichiosis, babesiosis, anaplasmosis and hepatozoonosis in dogs from St. Kitts, West Indies

Background

Although tick-borne diseases are important causes of morbidity and mortality in dogs in tropical areas, there is little information on the agents causing these infections in the Caribbean.

Methodology

We used PCRs to test blood from a cross-section of dogs on St Kitts for Ehrlichia (E.) canis, Babesia (B.) spp., Anaplasma (A.) spp. and Hepatozoon (H.) spp. Antibodies against E. canis and A. phagocytophilum/platys were detected using commercial immunochromatography tests. Records of the dogs were examined retrospectively to obtain clinical and laboratory data.

Principal findings

There was serological and/or PCR evidence of infections of dogs with E. canis (27%; 46/170), Babesia spp. (24%; 90/372) including B. canis vogeli (12%; 43/372) and B. gibsoni (10%; 36/372), A. platys (11%; 17/157) and H. canis (6%; 15/266). We could not identify the Babesia sp. detected in nine dogs. There was evidence of multiple infections with dual infections with E. canis and B. canis vogeli (8%; 14/179) or B. gibsoni (7%; 11/170) being the most common. There was agreement between immunochromatography and PCR test results for E. canis for 87% of dogs. Only 13% of exposed dogs had signs of a tick-borne disease and 38% had laboratory abnormalities. All 10 dogs presenting for a recheck after treatment of E. canis with doxycycline were apparently healthy although all remained seropositive and six still had laboratory abnormalities despite an average of two treatments with the most recent being around 12 months previously. Infections with Babesia spp. were also mainly subclinical with only 6% (4/67) showing clinical signs and 13% (9/67) having laboratory abnormalities. Similarly, animals with evidence of infections with A. platys and H. canis were largely apparently healthy with only occasional laboratory abnormalities.

Conclusions

Dogs are commonly infected with tick-borne pathogens in the Caribbean with most having no clinical signs or laboratory abnormalities.

Ehrlichiosis and anaplasmosis in dogs and cats

In the time since canine ehrlichiosis due to Ehrlichia canis was first described in 1935 and first recognized in the United States in 1962, many key advances have been made in our understanding of the diversity of the rickettsial organisms responsible for ehrlichiosis and anaplasmosis in dogs and, occasionally, cats, the vectors capable of transmitting these agents, and the role these organisms play as both important veterinary pathogens and zoonotic disease agents. Despite considerable progress in the field, much remains to be learned regarding mechanisms contributing to pathogenesis, effective treatment modalities, and prevention strategies that best protect pet health. This article highlights current understanding of the transmission, diagnosis, and management of ehrlichiosis and anaplasmosis in dogs and cats.

Magnetic Resonance Imaging Lesions in the Central Nervous System of a Dog with Canine Monocytic Ehrlichiosis

A ten-year-old neutered male dog was examined for tetraparesis, vestibular dysfunction, ataxia, and vertebral column hyperesthesia of 10 days duration. On magnetic resonance imaging (MRI), there were multifocal, punctate, T2-hyperintense lesions in the brain and cervical spinal cord and intracranial leptomeningeal contrast enhancement. Cerebrospinal fluid (CSF) analysis revealed a predominantly mononuclear pleocytosis and mildly elevated protein.Ehrlichia caniswas diagnosed by serum immunofluorescent antibody (IFA) testing. The dog improved with administration of doxycycline. This report describes MRI central nervous system findings in a dog with ehrlichiosis.

Multiple myeloma in 16 cats: a retrospective study

BACKGROUND

There is limited published information regarding feline multiple myeloma. Diagnostic criteria are derived from canine studies and to our knowledge, have not been critically reviewed for cats.

OBJECTIVE

To evaluate the clinical and laboratory findings in cats with multiple myeloma and appraise diagnostic criteria.

METHODS

Retrospective evaluation of medical records was performed. Inclusion required an antemortem diagnosis of multiple myeloma using 2 of 4 criteria: 1) >or=20% plasma cells in the bone marrow, or >or=10% if atypical plasma cells; 2) paraproteinemia; 3) radiographically-evident osteolysis; 4) light chain proteinuria. Alternatively, a postmortem diagnosis was based on the findings of multiple plasma cell neoplasms, with marrow involvement.

RESULTS

Sixteen cats were diagnosed with multiple myeloma between 1996 and 2004, with a median age of 14.0 years; 9 of 16 (56%) were castrated males, and 7 of 16 (44%) were spayed females. Laboratory abnormalities included hyperglobulinemia (14/16, 87.5%), with 11/14 (78.5%) monoclonal and 3/14 (21.4%) biclonal gammopathies; hypoalbuminemia (4/16, 25%); light chain proteinuria, (4/9, 44.4%); hypocholesterolemia (11/16, 68.7%); hypercalcemia, (3/15, 20%); nonregenerative anemia, (11/16, 68.7%); regenerative anemia, (1/16, 6.2%); neutropenia (5/15, 33.3%); thrombocytopenia (8/16, 50%); and marrow plasmacytosis (14/15, 93.3%). Plasma cells were markedly immature, atypical, or both in 10 of 12 (83.3%) cats. Focal or multifocal osteolysis was noted in 6 of 12 (50%) cats for which radiographs were available for review; generalized osteopenia was found in 1 (8.3%) cat. Noncutaneous, extramedullary tumors were found in all cats assessed, 7/7 (100%), including spleen (6), liver (3), and lymph nodes (4). The disease in 1 of 2 cats with cutaneous tumors progressed to plasmacytic leukemia.

CONCLUSIONS

Common findings in feline multiple myeloma include atypical plasma cell morphology, hypocholesterolemia, anemia, bone lesions, and multi-organ involvement. Based on the results of this study, we advocate modifying diagnostic criteria in cats to include consideration of plasma cell morphology and visceral organ infiltration.

Ehrlichiosis and related infections

Ehrlichiosis is a term that has been used to describe infection with any of a number of related intracellular, vector-borne pathogens. A recent reclassification has resulted in the transfer of several species previously known as Ehrlichia to the genus Anaplasma or Neorickettsia. Ehrlichia and Anaplasma are transmitted largely through the bite of infected ticks, while vectors for Neorickettsia include trematodes and the intermediate hosts (i.e., fish, snails, and insects) involved in the trematode life cycle. Dogs (and cats) are susceptible to infection with several of these pathogens, and veterinarians should be aware of the similarities and differences between E canis and related infections. Pets with suggestive clinical signs and laboratory abnormalities may be started on doxycycline pending specific diagnostic testing. The veterinarian practicing in endemic areas must understand the implications and limitations of serologic and molecular testing to confirm a diagnosis. For animals in endemic areas, prevention of exposure to vectors can lessen the risk of disease for pets and might lessen the potential for animals to become carriers of disease for their human companions.

Large granular lymphocytosis, lymphocyte subset inversion, thrombocytopenia, dysproteinemia, and positive Ehrlichia serology in a dog

A 7-year-old, mixed-breed dog was presented for evaluation of a possible lymphocytic leukemia. Results of laboratory testing included thrombocytopenia, large granular lymphocytosis, inverted CD4:CD8 ratio, hyperglobulinemia, and hypoalbuminemia. Results of a tick-borne disease panel indicated a positive immunoglobulin G serum titer (1:2,048) to Ehrlichia canis, supporting exposure to this organism. The dog responded to a combination treatment of doxycycline and prednisone. A review of the literature and novel diagnostic methods that aided in the diagnosis of this case are discussed.

Significance of serological testing for ehrlichial diseases in dogs with special emphasis on the diagnosis of canine monocytic ehrlichiosis caused by Ehrlichia canis

Dogs are susceptible to a number of ehrlichial diseases. Among them, canine monocytic ehrlichiosis is an important and potentially fatal disease of dogs caused by the rickettsia Ehrlichia canis. Diagnosis of the disease relies heavily on the detection of antibodies and is usually carried out using the indirect immunofluoresence antibody (IFA) test. The IFA test may be confounded by cross-reactivities between a number of the canine ehrlichial pathogens. This article presents a review of the ehrlichial diseases affecting dogs with reference to their immune responses, host specificities, cross-reactivites and diagnosis. Diagnostic means such as Western immunblot, dot-blot and PCR are discussed. The use of the IFA test as a diagnostic means for E. canis is presented along with its potential pitfalls. The review emphasizes that the disease process, cross-reactivites with other ehrlichial species, multiple tick-borne infections and persistent IFA antibody titers post-treatment, should all be considered when interpreting E. canis serological results.

Ancillary diagnostic techniques for the lame patient

The cause of lameness is usually identified through evaluation of signalment, history, and physical examination. Radiographic evaluation provides a definitive diagnosis in many cases; however, laboratory evaluation may prove useful in the characterization of certain conditions or in the prognostication of the condition. In some instances where the definitive diagnosis remains elusive, arthroscopic examination is a minimally invasive diagnostic tool that may be more affordable and available than computed tomography. This article reviews laboratory evaluation of the lame patient and arthroscopic evaluation of joint abnormalities.

Acute blindness associated with monoclonal gammopathy induced by Ehrlichia canis infection

Ehrlichia canis infection was diagnosed in a Labrador retriever presented with a primary complaint of acute blindness. Ocular signs on admission included bilateral hyphema, retinal haemorrhage and retinal detachment. Serum protein electrophoresis results revealed monoclonal gammopathy. This report discusses and suggests the pathogenesis of ocular bleeding in canine monocytic ehrlichiosis. Blood hyperviscosity, elevation in oncotic pressure, vasculitis, thrombocytopenia and platelet dysfunction are all proposed to be important factors in the pathogenesis of acute blindness in canine monocytic ehrlichiosis.

Recurrent deep pyoderma in German Shepherd dogs with underlying ehrlichioses and hypergammaglobulinaemia

Four German Shepherd dogs with recurrent deep pyoderma and associated ehrlichioses are described. All the dogs had already received appropriate long-term antibacterial therapy but the clinical signs had consistently returned within 3-4 weeks of cessation of the treatment. Haematological and serum biochemical findings, and indirect fluorescent antibody (IFA) tests were indicative of Ehrlichia spp. infection. Treatment with doxycycline monohydrate was associated with the return of blood values to within normal ranges and diminution of the IFA titres. No further episodes of pyoderma were observed during a 1-2-year follow-up period.

Serum protein alterations in canine ehrlichiosis

Serum protein electrophoresis was performed in 42 dogs with naturally occurring Ehrlichia canis infection and in 15 clinically healthy dogs (control dogs). The infected dogs were found to have a significant hypoalbuminaemia, hyperglobulinaemia and hypergammaglobulinaemia compared to the control dogs (P < 0.001). A polyclonal gammopathy was found in all but one of the infected dogs which presented a monoclonal gammopathy. alpha-1 globulin was lower while alpha-2 and beta-2 globulin concentrations were significantly higher in the infected dogs (P < 0.0001, P < 0.05 and P < 0.005, respectively). The infected dogs were divided into two subgroups according to haematological parameters, defined as pancytopenic (n = 13) and non-pancytopenic (n = 29). When compared, the pancytopenic group revealed significantly lower concentrations of total protein, total globulin and gammaglobulin (P < 0.01, P < 0.05 and P < 0.005 respectively). The lower concentrations of the gammaglobulins coupled with the pancytopenia suggest that the immune state of the pancytopenic E. canis infected dogs is more compromised, and therefore secondary infections should be expected more frequently in these dogs.

Post-therapy antibody titers in dogs with ehrlichiosis: follow-up study on 68 patients treated primarily with tetracycline and/or doxycycline

The clinical and serological responses to therapy were evaluated for at least 1 year in 68 dogs with antibody titers positive for Ehrlichia canis. Treatments were of variable periods with primarily tetracycline hydorchloride and/or doxycycline. Sixteen dogs had initial titers of 1:20 and, at the end of the year, were asymptomatic, no longer receiving medication, and had negative serology. The average length of treatment with tetracycline HCl and/or doxycycline was 85 days (range, 14 to 360 days). Of 39 dogs with initial titers of 1:2,560 or greater, 1 died, 25 were asymptomatic, and 13 were lost to follow-up at the end of the study. The average length of treatment was 210 days (range, 21 to 630 days). Twenty-seven dogs were seropositive at > or = 1:2,560 when the sera was last tested. Thirteen dogs had initial titers of 1:80 to 1:1,280. Of these 13 dogs, 2 died, 2 were lost to follow-up, and 9 were asymptomatic and had titers ranging from negative to > or = 1:2,560 at the end of the study. The persistence of antibodies, prolonged subclinical phase, and delayed relapses despite long-term medication, suggest inadequate chemotherapeutic agents or may be natural features of latency of ehrlichiosis in dogs.

Survey of Ehrlichia canis antibodies among dogs in Israel

A seroepidemiological survey was made of the prevalence of antibodies reactive with Ehrlichia canis in stray and pet dogs from five geographical regions of Israel. Serum samples from 410 dogs were assayed by an indirect fluorescent antibody test and serum reactive at a 1:20 dilution or greater was considered positive. The overall seroprevalence was 30 per cent with 23.9 per cent in pet dogs and 37.5 per cent in stray dogs. The seroprevalence in apparently healthy pets (17.6 per cent) was lower but did not differ significantly from the seroprevalence in sick animals (26.6 per cent). The seroprevalence was highest in the north of Israel and lowest in the central hilly area, and followed the same trend in both the populations sampled. Young dogs (under one year old) had a lower seroprevalence (14.9 per cent) than adult (one to eight years) (36.4 per cent) or older dogs (over eight years) (34.2 per cent). The results demonstrate that dogs in Israel are extensively exposed to E canis and indicate that ehrlichiosis is endemic in this area of the Middle East.

Immunosuppression in sheep experimentally infected with Ehrlichia phagocytophila

The effect of tick-borne fever (TBF) on antibody formation and lymphocyte proliferation in sheep was studied following experimental infection with Ehrlichia phagocytophila. All infected sheep developed fever within three to four days. The sheep recovered clinically within eight days. Both infected and non-infected control sheep were immunised twice with different antigens, that is, on days 9 and 35 following the experimental infection. The levels of antibodies produced against tetanus toxoid and influenza virus in the infected sheep were significantly lower than in the control animals. The findings indicated that a TBF-infection may impair both primary and secondary antibody responses for up to six weeks. Immunisation with Actinomyces pyogenes resulted in significantly higher antibody titres in the TBF-infected group than in the control group, as measured by an enzyme-linked immunosorbent assay (ELISA). It is believed that TBF-induced neutropenia may lead to increased exposure to A pyogenes-antigens and thereby enhance antibody production. Antibodies to E phagocytophila were measured by the indirect fluorescent antibody test and by an ELISA. The inoculated sheep responded with the formation of antibodies to E phagocytophila at one week (P < 0.025), and showed a peak response at four weeks (P < 0.0005) after inoculation. The antibody titre decreased between four and six weeks, but was still high at six weeks (P < 0.0005). The lymphocyte responses to phytohaemagglutinin (PHA) concanavalin A (Con A) and pokeweed mitogen (PWM) were lower than in the control group and this difference was significant at most time points after infection.(ABSTRACT TRUNCATED AT 250 WORDS)