An Overview of American Canine Hepatozoonosis

Deborah S. Ludlow, DVM; Susan Little, DVM, PhD; Kenneth S. Latimer, DVM, PhD; Bruce E. LeRoy, DVM, PhD; Heather L. Tarpley, DVM

Class of 2006 (Ludlow), and Department of Pathology (Latimer, LeRoy, Tarpley), College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388, and Department of Parasitology (Little), Oklahoma State University, Stillwater, OK

Introduction

Hepatozoonosis is a relatively new disease in North America. It was first identified and described in a dog from Texas in 1978. ¹ Since that time, the disease has been reported from other Southeastern states. ² This report reviews the etiology, life cycle, pathologic findings, diagnosis, and treatment of American canine hepatozoonosis (ACH). Also included is new information regarding the identity and life cycle of this parasite which practicing veterinarians, especially in the Southeastern United States, should be aware of.

Etiology

When hepatozoonosis was first identified in North America in 1978, the etiologic agent was presumed to be Hepatozoon canis. ¹ However, the observed disease was more severe than had been previously reported for canine hepatozoonosis and was often fatal. In the late 1990s, researchers determined that the American form of hepatozoonosis was caused by a different organism. This conclusion was based on a comparison of gamont size, histopathologic findings, and the inability to infect Rhipicephalus sanguineus ticks (the vector host of H. canis) with the parasite. The North American isolate was subsequently designated Hepatozoon americanum. ³ American canine hepatozoonosis (ACH) is caused by H. americanum, a protozoal, apicomplexan parasite. Since the initial case reported from Texas in 1978, other cases of ACH have been reported in the Gulf coastal states (Texas, Louisiana, Mississippi, Alabama, and Florida) as well as Georgia. ²

Transmission of ACH occurs via ingestion of infected Amblyomma maculatum, the Gulf Coast tick. This hard tick serves as the definitive host of H. americanum. The Gulf Coast tick is a three-host tick that parasitizes small vertebrates in the larval and nymphal stages and large vertebrates in the adult stage of development. ⁴ Currently, this is the only tick that is thought to transmit H. americanum. ⁵ There is significant overlap between the geographic range of this tick (Gulf coastal states, Oklahoma, and Kansas) and states which have reported cases of ACH.

Life Cycle

Like other Hepatozoon species, Hepatozoon americanum has a two-host life cycle. Merogony occurs in the vertebrate host and syngamy and sporogony occur in the tick host. ⁴ The natural intermediate host of H. americanum is unknown; however, research has shown that infected dogs can easily transmit H. americanum to larval or nymphal A. maculatum ticks during feeding. It is unlikely that dogs are the natural intermediate host of H. americanum because infection often causes death if untreated. Therefore, dogs would not serve as a successful reservoir for this parasite. In addition, larval and nymphal A. maculatum ticks do not preferentially feed on dogs. Thus, dogs are unlikely to be the parasitic reservoir. ^5,6

Figure 1. Life cycle of Hepatozoon spp. (Reprinted from Trends in Parasitology, Vol. 19, Baneth G, Mathew JS, Shkap V, Macintire DK, Barta JR, Ewing SA, Canine hepatozoonosis: two disease syndromes caused by separate Hepatozoon spp., Pages 27-31, Copyright (2003), with permission from Elsevier.

As shown in Fig. 1, Gulf Coast ticks acquire Hepatozoon americanum by feeding on an infected intermediate host (such as a dog). Within the tick, the protozoa undergo gametogenesis and fertilization resulting in formation of a zygote. The zygote undergoes fission in a process called sporogony and produces oocysts that contain infective sporozoites. ⁷

When a dog ingests the tick, the sporozoites are released. It is suspected that a sporozoite invades a leukocyte host cell and is transported via the blood to skeletal or cardiac muscle. The host cell produces a multilayered, mucopolysaccharide wall and the cytoplasm becomes vacuolated (Fig. 2A), so that the cell assumes an "onion skin cyst" appearance. ⁸ The sporozoite undergoes merogony (multiple fission), producing merozoites. ^7,8 Figures 2B and 2C demonstrate the formation of merozoites within the meront cyst. As the cyst wall disintegrates, exposure of the merozoites to skeletal muscle results in myositis characterized by pyogranulomatous inflammation. A granuloma forms and the merozoites may be ingested by leukocytes which may arrive by vascular invasion of the granuloma (Figure 2D). As a naive tick feeds on the infected dog, the potential for disease transmission escalates. Furthermore, merozoites probably encyst in new tissues and continue to undergo merogony, possibly explaining the relapse of clinical signs. ⁸

Figure 2. Stages of the protozoa within canine tissues. Figure 2A demonstrates the onion-skin appearance of layers of mucopolysaccharide (MPS) surrounding the organism (arrowhead) within a canine muscle cell (arrow). Figure 2B reveals the meront (M) and surrounding mucopolysaccharide (MPS). Figure 2C shows detail of individual merozoites within a meront. Figure 2D is a granuloma containing macrophages with phagocytized organisms. Arrows denote vascular spaces. Reprinted with permission from Animal Health Research Reviews 4:27-34, 2003.

Clinical Signs

In a report of 22 dogs from Alabama and Georgia with ACH, the most common clinical signs were pyrexia (86%), which ranged from 102.7 to 105.6°F, weight loss (82%), muscle atrophy (64%), mucopurulent ocular discharge (77%), and pain on palpation (64%). Many owners reported that infected dogs had a generalized stiff gait and hindlimb paresis.¹⁰ Dogs with ACH may display waxing and waning signs of disease, which may eventually result in a chronic wasting condition.² Evidence suggests that dogs may become re-infected through the asexual reproduction of the merozoites, which may be associated with the waxing and waning signs.

Clinical Laboratory, Radiologic, and Pathologic Findings

Based on the study of 22 cases conducted by Macintire et. al., the most consistent abnormality detected on the complete blood cell count was leukocytosis (100%) characterized by a mature neutrophilia with or without a left shift. In addition, 63% of dogs had a normocytic, normochromic, nonregenerative anemia (most likely due to anemia of chronic/inflammatory disease). Platelet counts were variable. ¹⁰

The serum biochemical analyses showed increased alkaline phosphatase activity in all dogs. Other common biochemical abnormalities were hypoglycemia (90%), and hypoalbuminemia (86%). Clinical signs associated with hypoglycemia were not evident, and may have been due to increased glucose metabolism in the serum tube by the leukocytes and/or parasites. Some dogs also had hyperphosphatemia and low BUN concentration. Although a decreased total calcium concentration was reported in 14 of the dogs, most were normocalcemic after correction for hypoalbuminemia (ionized calcium concentrations were not measured). Renal insufficiency appeared to be a common finding in dogs with prolonged disease (27% of infected dogs). ¹⁰

In a separate study conducted by Panciera et al., examination of blood smears revealed the parasite within leukocytes in only 31% of neutrophilic samples.⁸ Parasitemia occurred at irregular intervals, and organisms were found in less than 1% of leukocytes when present. The parasites in peripheral blood neutrophils were found in nucleated and anucleate forms, which are demonstrated in Figures 3 and 4.

Figure 3	Figure 4
Arrows indicate nucleate (Figure 3) and anucleate (Figure 4) H. americanum in peripheral blood segmented neutrophils. (Panciera RJ, Ewing SA, Mathew JS, Cummings CA, Kocan AA, Breshears MA, Fox JC: Observations on tissue stages of Hepatozoon americanum in 19 naturally infected dogs. Reprinted with permission from Vet Parsitol 78:265-276, 1998)

Radiographically, 82% of infected dogs had stimulation of perisoteal new bone formation affecting the long bones, ilium, and numerous vertebral bodies. ¹⁰ The pathogenesis of this lesion is unknown.

Findings on necropsy included pyogranulomatous inflammation and myositis in 68% of affected dogs. Meronts were most commonly found in cardiac and skeletal muscle, but also were observed in lymph nodes, spleen, pancreas, and tongue. Granulomas were noted in pancreas, skeletal muscle, and cardiac muscle. ¹⁰

Pathogenesis

Dogs are infected by ingesting ticks that contain sporulated oocysts, ^5,6 but not by infected ticks taking a blood meal from a dog. Dogs may be infected by ingesting ticks during grooming behavior or eating prey that has attached ticks.

In experimentally infected dogs, clinical signs of hepatozoonosis appeared 4 to 5 weeks after exposure to H. americanum. Histologic lesions were visible on skeletal muscle biopsies within 3 weeks post-exposure. Within 4-7 weeks the parasite could be observed in peripheral blood leukocytes. ¹¹ The onset of clinical signs was associated with the inflammatory response caused by the cysts in tissue. Likewise, the relapse and return of clinical signs was associated with repeated release of merozoites and subsequent tissue cyst formation.

Diagnosis

Presumptive diagnosis of ACH is based upon:

• A history of tick exposure

• Clinical signs of chronic wasting, fever, ocular discharge, and pain on palpation of long bones

• Neutrophilic leukocytosis

• Radiographic evidence of symmetrical periosteal bone proliferation. ⁹

The definitive diagnosis of H. americanum infection is dependent upon demonstration of the cysts, meronts, or granulomas in tissue biopsies. It may also be possible to observe infected leukocytes on a peripheral blood smear, but this is an infrequent occurrence.^8,9

Treatment

The currently recommended therapy for hepatozoonosis is to administer trimethoprim-sulfadiazine, clindamycin, and pyrimethamine (TCP) for two weeks to destroy extracellular protozoa. ² A typical initial treatment regimen is as follows:

• Trimethoprim-sulfadiazine 15 mg/kg PO q 12 h; ²

• Clindamycin 10 mg/kg PO q 8 h²

• Pyrimethamine 0.25 mg/kg PO q 24 h²

Follow-up therapy with daily administration of decoquinate for two years is recommended to prevent asexual reproduction of the parasite which may lead to an exacerbation of clinical signs of disease. ² Decoquinate is a coccidiostat that prevents sporozoite development.¹²

• Decoquinate 10 to 20 mg/kg PO q 12 h mixed in food²

Treatment of hepatozoonosis may be difficult if not instituted early. Treatment has not been shown to eliminate the organism completely; however, experimental studies have shown that a relatively good quality of life can be provided by treatment with TCP and decoquinate. To date, no treatment has successfully eliminated the intracellular merozoites. Therefore, dogs with ACH often relapse. ²

In experimental treatment protocols, dogs that did not receive long-term decoquinate therapy initially responded well to antiprotozoal treatment; however, disease relapse occurred in 88% of dogs within 6 months. These dogs were treated again with an antiprotozoal treatment; but the intervals between relapse became shorter each time. The median survival time of H. americanum-infected dogs was 12 months. Mean survival times with decoquinate administration were not reported because most dogs were still alive at the termination of the study. ²

References

1. Craig TM, Smallwood JE, Knauer KW, McGrath JP: Hepatozoon canis infection in dogs: Clinical, radiographic and hematologic finding. J Am Vet Med Assoc 173:967-972, 1978.

2. Macintire DK, Vincent-Johnson NA, Kane CK, Lindsay DS, Blagburn BL, Dillon AR: Treatment of dogs infected with Hepatozoon americanum: 53 cases (1989-1998). J Am Vet Med Assoc 218:77-82, 2001.

3. Vincent-Johnson NA, Macintire DK, Lindsay DS, Lenz SD, Baneth G, Shkap V, Blagburn BL: A new Hepatozoon species from dogs: Description of the causative agent of canine hepatozoonosis in North America. J Parasitol 83:1165-1172, 1997.

4. Bledsoe D: Ticks and tick-borne diseases of dogs: A brief overview for veterinarians. Bayer Advantage Scientific Report 3, 1999.

5. Ewing SA, Mathew JS, Panciera RJ: Transmission of Hepatozoon americanum (Apicomplexa: Adeleorina) by Ixodids (Acari: Ixodidae). J Med Entomol 39:631-634, 2002.

6. Ewing S, DuBois JG, Mathew JS, Panciera RJ: Larval Gulf Coast ticks (Amblyomma maculatum) [Acari: Ixodidae] as host for Hepatozoon americanum [Apicomplexa: Adeleorina]. Vet Parasitol 103:43-51, 2002.

7. Baneth G, Mathew JS, Shkap V, Macintire DK, Barta JR, Ewing SA: Canine hepatozoonosis: Two disease syndromes caused by separate Hepatozoon spp. Trends in Parasitol 19:27-31, 2003.

8. Panciera RJ, Ewing SA, Mathew JS, Cummings CA, Kocan AA, Breshears MA, Fox JC: Observations on tissue stages of Hepatozoon americanum in 19 naturally infected dogs. Vet Parsitol 78:265-276, 1998.

9. Panciera RJ, Ewing SA: American canine hepatozoonosis. Animal Health Res Rev 4:27-34, 2003.

10. Macintire DK, Vincent-Johnson N, Dillon AR, Blagburn B, Lindsay D, Whitley EM, Banfield C: Hepatozoonosis in dogs: 22 cases (1989-1994). J Am Vet Med Assoc 210:916-922, 1997.

11. Panciera RJ, Ewing SA, Mathew JS, Lehenbauer TW, Cummings CA, Woods JP: Canine hepatozoonosis: Comparison of lesions and parasites in skeletal muscle of dogs experimentally or naturally infected with Hepatozoon americanum. Vet Parasitol 82:261-272, 1999.

12. Plumb, DC. Veterinary Drug Handbook, 4th Ed. Iowa State Press, Ames, 2002, pp. 241-242.

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