Treatment of Hepatozoon americanum infection: review of the literature and experimental evaluation of efficacy.

There is no labeled treatment for dogs with American canine hepatozoonosis (ACH), but the drug therapies discussed in this article, although not rapidly curative, may be successful in alleviating acute clinical signs, prolonging life, reducing the number of clinical relapses, and enhancing quality of life. This article also describes a pilot trial conducted to assess the efficacy of a novel treatment approach with ponazuril as a stand-alone parasiticide administered for 4 weeks without follow-up decoquinate treatment. Although extended ponazuril treatment in combination with NSAID administration did ameliorate acute clinical signs associated with ACH, the parasite was not completely cleared with this treatment protocol alone. Long-term decoquinate therapy remains a critical component of successful treatment of ACH.

Experimental transmission of Hepatozoon americanum to rodents.

Laboratory-raised cotton rats (Sigmodon hispidus), outbred white mice (Mus musculus), and C57BL/6J-Lystbg-J/J mice (M. musculus) that were administered approximately 50 sporulated oocysts of Hepatozoon americanum (AF176836) by gavage developed inflammatory lesions containing parasitic cystozoites in cardiac and skeletal muscle, kidney, and lung. Sprague-Dawley rats (Rattus norvegicus) similarly exposed showed no evidence of infection. Cystozoites were first detected by histopathologic examination four weeks after exposure to oocysts. Globular, PAS-positive material accumulated around the cystozoites as the duration of infection lengthened. Nested PCR analysis of tissues collected 16 weeks post-exposure was positive for the 18S rRNA Hepatozoon sp. gene and the DNA sequence of the fragment amplified was 99.6% and 99.8% identical to H. americanum sequences previously reported from naturally-infected dogs (AF176836 and AY864676, respectively). Merogonous and gamontogonous stages of the parasite were not detected in any of the cystozoite-infected rodents.

Infectivity of Hepatozoon americanum cystozoites for a dog.

Hepatozoon americanum cystozoites from experimentally infected, laboratory-raised rodents were fed to a Hepatozoon-free dog. Gamonts were detected by examination of blood smear 42 and 56 days post-exposure. PCR analysis of blood was positive for the 18S rRNA Hepatozoon gene on days gamonts were demonstrated. Meronts were detected histologically in a skeletal muscle biopsy 90 days after ingestion of cystozoites. Sequencing confirmed that the parasite in the dog was H. americanum. Xenodiagnosis was conducted by replete feeding of Ambylomma maculatum larvae on the dog; 40 days after detachment, sporulated oocysts were recovered from recently molted nymphs.

Field survey of rodents for Hepatozoon infections in an endemic focus of American canine hepatozoonosis.

Eighteen of 31 (58%) cotton rats (Sigmodon hispidus) and 8 of 24 (33.3%) white-footed mice (Peromyscus leucopus) that were wild-trapped from 4 American canine hepatozoonosis endemic sites in Oklahoma were infected with Hepatozoon species. The predilection organ for merogony of the Hepatozoon species in cotton rats was the liver. Meronts were not detected in any of the white-footed mice. A 488 bp DNA fragment that includes a variable region of the 18S rRNA Hepatozoon gene amplified from blood or tissue of these infected animals. Sequences from eight cotton rats were 100% identical to each other as were sequences from three white-footed mice 100% identical to each other. The cotton rat sequence and the white-footed mouse sequence were 98.8% identical, differing in 6 bp of the 488 bp fragment. The DNA sequence from cotton rats was 97.7% identical to a Hepatozoon sp. described in a large bandicoot rat from Thailand and 97.5% identical to a Hepatozoon sp. in a bank vole from Brazil. The sequence from white-footed mice was 98.6% identical to the bandicoot rat sequence and 98.4% identical to the bank vole sequence. However, the sequences were only 90.6% (cotton rat) and 91.4% (white-footed mouse) identical to H. americanum. These findings suggest that the rodents are obligate intermediate hosts for distinct Hepatozoon spp., but not H. americanum.

Emergence of Veterinary Parasitology in the United States: Maurice C. Hall and the Bureau of Animal Industry.

By 1883 a Veterinary Division had been established within the United States Department of Agriculture, itself established in 1862. Federal concern about animal health in the U.S.A. emerged as early as 1865 when Congress adopted regulations aimed at controlling importation of livestock. It was not until 1884 that the Bureau of Animal Industry (BAI) was formally created by Act of Congress, and shortly after that the Zoological Laboratory was established and assigned responsibility for study of parasites and the diseases they produce in animals. Classically trained parasitologists working in USDA's BAI soon became internationally recognized for their contributions to basic research and development of programs for prevention and control of parasitic diseases. Leadership by a series of BAI-employed parasitologists led to the emergence of veterinary parasitology as a sub-discipline. Maurice C. Hall who served as president of both the American Society of Parasitologists and the American Veterinary Medical Association was a central figure in development of veterinary parasitology in the U.S.A., which flourished in his country and elsewhere today.

Experimental infection of adult and juvenile coyotes with domestic dog and wild coyote isolates of Hepatozoon americanum (Apicomplexa: Adeleorina).

Each of five adult and four juvenile coyotes (Canis latrans) was exposed to an oral dose of 50 Hepatozoon americanum oocysts recovered from Amblyomma maculatum ticks that previously fed on either naturally infected domestic dogs (Canis familiaris) or naturally infected wild coyotes. All coyotes exposed to H. americanum became infected, regardless of isolate source, and all exhibited mild to moderate clinical disease that simulated American canine hepatozoonosis in naturally infected dogs. At 100 days postexposure, parasitemia was greater in juvenile than adult coyotes (0.9% and 0.3%, respectively); radiographic imaging of femurs revealed moderate exostosis in all juveniles and mild to moderate new bone growth in four of five (80%) adult coyotes. Gross postmortem analysis of bone lesions demonstrated variation between age groups of coyotes but not between isolates of H. americanum. Microscopic evaluation of skeletal muscle revealed that parasite-induced lesions were significantly more numerous (t = 5.0, df = 7, P = 0.001) in juvenile than adult coyotes. Results of this study indicate that juvenile and adult coyotes are equally susceptible to experimental infection with H. americanum isolated from domestic dog and wild coyote sources. The age of coyotes at the time of exposure, and possibly the number of H. americanum oocysts ingested, might influence morbidity and mortality, but it appears that both adult and juvenile coyotes could be reservoirs of H. americanum.

Quantitative factors proposed to influence the prevalence of canine tick-borne disease agents in the United States.

The Companion Animal Parasite Council hosted a meeting to identify quantifiable factors that can influence the prevalence of tick-borne disease agents among dogs in North America. This report summarizes the approach used and the factors identified for further analysis with mathematical models of canine exposure to tick-borne pathogens.

Novel Hepatozoon in vertebrates from the southern United States.

Novel Hepatozoon spp. sequences collected from previously unrecognized vertebrate hosts in North America were compared with documented Hepatozoon 18S rRNA sequences in an effort to examine phylogenetic relationships between the different Hepatozoon organisms found cycling in nature. An approximately 500-base pair fragment of 18S rDNA common to Hepatozoon spp. and some other apicomplexans was amplified and sequenced from the tissues or blood of 16 vertebrate host species from the southern United States, including 1 opossum (Didelphis virginiana), 2 bobcats (Lynx rufus), 1 domestic cat (Felis catus), 3 coyotes (Canis latrans), 1 gray fox (Urocyon cinereoargenteus), 4 raccoons (Procyon lotor), 1 pet boa constrictor (Boa constrictor imperator), 1 swamp rabbit (Sylvilagus aquaticus), 1 cottontail rabbit (Sylvilagus floridanus), 4 woodrats (Neotoma fuscipes and Neotoma micropus), 3 white-footed mice (Peromyscus leucopus), 8 cotton rats (Sigmodon hispidus), 1 cotton mouse (Peromyscus gossypinus), 1 eastern grey squirrel (Sciurus carolinensis), and 1 woodchuck (Marmota monax). Phylogenetic analyses and comparison with sequences in the existing database revealed distinct groups of Hepatozoon spp., with clusters formed by sequences obtained from scavengers and carnivores (opossum, raccoons, canids, and felids) and those obtained from rodents. Surprisingly, Hepatozoon spp. sequences from wild rabbits were most closely related to sequences obtained from carnivores (97.2% identical), and the sequence from the boa constrictor was most closely related to the rodent cluster (97.4% identical). These data are consistent with recent work identifying prey-predator transmission cycles in Hepatozoon spp. and suggest this pattern may be more common than previously recognized.

Ehrlichia ewingii infection and exposure rates in dogs from the southcentral United States.

We used PCR and a novel serologic assay to determine infection and exposure rates to Ehrlichia ewingii in dogs from an area of northeast Oklahoma and northwest Arkansas where Amblyomma americanum ticks are abundant. Of 143 dogs assayed, 13 (9.1%) harbored E. ewingii by PCR and 64 (44.8%) had antibodies to E. ewingii detected using a peptide-based microtiter plate ELISA. Dogs were more likely (P=0.001) to be positive by PCR if sampled in August (30.8%) but no association was found between seropositive status and month of collection of sample (P>0.05). Additional testing revealed PCR evidence of Ehrlichia chaffeensis (4/143; 2.8%) and Anaplasma platys (5/143; 3.5%) as well as antibodies reactive to E. chaffeensis (25/143; 17.5%), Ehrlichia canis (2/143; 1.4%), and Anaplasma spp. (8/143; 5.6%). Testing of another 200 dogs from the area revealed additional PCR and/or serologic evidence of E. ewingii, E. canis, E. chaffeensis, and A. platys. None of the 343 dogs evaluated had evidence of Borrelia burgdorferi exposure. These data support the interpretation that E. ewingii may be the primary agent of canine ehrlichiosis in this region, and suggest that diagnostic evaluation of dogs suspected to have a tick-borne disease should include assays targeting this organism.

New developments in canine hepatozoonosis in North America: a review.

Canine hepatozoonosis is caused by Hepatozoon canis and Hepatozoon americanum, apicomplexan parasites transmitted to dogs by ingestion of infectious stages. Although the two agents are phylogenetically related, specific aspects, including characteristics of clinical disease and the natural history of the parasites themselves, differ between the two species. Until recently, H. canis infections had not been clearly documented in North America, and autochthonous infection with H. americanum has yet to be reported outside of the southern United States. However, recent reports demonstrate H. canis is present in areas of North America where its vector tick, Rhipicephalus sanguineus, has long been endemic, and that the range of H. americanum is likely expanding along with that of its vector tick, Amblyomma maculatum; co-infections with the two organisms have also been identified. Significant intraspecific variation has been reported in the 18S rRNA gene sequence of both Hepatozoon spp.-infecting dogs, suggesting that each species may represent a complex of related genogroups rather than well-defined species. Transmission of H. americanum to dogs via ingestion of cystozoites in muscle of infected vertebrates was recently demonstrated, supporting the concept of predation as a means of natural transmission. Although several exciting advances have occurred in recent years, much remains to be learned about patterns of infection and the nature of clinical disease caused by the agents of canine hepatozoonosis in North America.

Experimental transmission of Hepatozoon americanum to New Zealand White rabbits (Oryctolagus cuniculus) and infectivity of cystozoites for a dog.

Inflammatory lesions containing parasitic cystozoites developed in multiple organs and tissues of laboratory-raised Oryctolagus cuniculus that were administered approximately 100 sporulated oocysts of Hepatozoon americanum (Oklahoma isolate, GenBank accession AF176836) orally. The predominantly granulomatous inflammatory lesions were detected histologically 8 weeks after exposure to oocysts. Cystozoites, recognized by cresent-shaped, uninucleated bodies surrounded by an accumulation of globular, PAS-positive polysaccharide material, were evident within macrophages as monozoic and dizoic cysts. Neither meronts nor gamonts were detected in any of the laboratory-raised lagomorphs during the 24-week observation period. Nested PCR assay of rabbit tissues for a 488 bp fragment of the 18S rRNA Hepatozoon spp. gene was positive at 8 and 24 weeks post-exposure. The sequence was 97.1% similar with sequence from the H. americanum carrier used to infect ticks. A Hepatozoon-free dog fed tissues from the 24-week post-exposure rabbit principal developed American canine hepatozoonosis. Gamonts were first detected 5 weeks after the dog ingested the rabbit tissues containing cystozoites. PCR assay of blood from the dog was positive for the Hepatozoon spp. gene fragment. Sequencing confirmed that the parasite in the dog was H. americanum.