Three emerging vector-borne diseases in dogs and cats in the United States
Several vector-borne diseases in dogs and cats appear to be emerging in the United States, including babesiosis, cytauxzoonosis, bartonellosis, leishmaniasis, hepatozoonosis, and feline ehrlichiosis. This article focuses on babesiosis, cytauxzoonosis, and bartonellosis, which have been reported with increased frequency in the United States over the past decade.1-20
It is not always clear whether the prevalence of these infections is truly increased or whether the more frequent diagnoses are due to increased awareness and improved diagnostic testing. In some cases of emerging diseases, such as with bartonellosis, the reason the disease is being diagnosed more often may simply be related to the discovery of its existence in dogs and our increased awareness. In other cases, such as with babesiosis and cytauxzoonosis, the reasons behind the emergence are not fully elucidated, but international travels, possible genetic mutations of the host or pathogen, and increased clinician awareness are suspected causes. Regardless of the reasons for their emergence, practitioners need to consider these infections in their patients. This article reviews the clinical signs and current recommendations for diagnosing and treating these three infections.
CANINE BABESIOSISBovine babesiosis was the first infectious disease for which tick transmission was documented. After this discovery in 1893, bovine babesiosis was eventually eradicated in the United States. Canine babesiosis was first described in South Africa in the late 1800s and was first reported in the United States in 1934.21 An increased number of cases of canine babesiosis have been reported in the United States since the late 1990s.1-4,22
It appears that breed predispositions for canine babesiosis exist in the United States. Most Babesia canis infections reported in the United States have occurred in greyhounds, and most Babesia gibsoni cases have occurred in American pit bull terriers.1,4,23,24 The underlying causes for these predispositions are unknown, but lifestyle and housing conditions that lead to an increased risk of exposure are thought to be more likely than a genetic predisposition for infections. Besides ticks, perinatal transmission, direct dog-to-dog transmission by bite wounds, and mechanical transmission are the predominant modes thought to be involved.25
Babesia species are intraerythrocytic protozoan parasites that can be transmitted by ticks. Babesia species have been classified historically based on their size and the mammalian species they infect. Large Babesia species are 3 to 6 µm in length, and small Babesia species measure 1 to 3 µm. In dogs, B. canis is the most commonly identified large Babesia species. Babesia canis includes three subspecies: B. canis vogeli, B. canis canis, and B. canis rossi. These three subspecies are genetically distinct, are transmitted by different vectors, and have different geographic distributions and varying degrees of pathogenicity.26-28 Babesia canis vogeli has a worldwide distribution, is transmitted by Rhipicephalus sanguineus, and is considered to be mildly to moderately pathogenic.23,24 Babesia canis canis is found primarily in Europe, is transmitted by Dermacentor reticulatus, and is moderately pathogenic.29-32 Babesia canis rossi is endemic to Africa, is transmitted by Haemaphysalis leachi, and is a virulent subspecies.27,33,34 At least one other species of large Babesia has been identified in a dog with babesiosis.35
At least three genetically distinct small Babesia species can infect dogs.36-38 Babesia gibsoni has a worldwide distribution, is transmitted by Haemaphysalis species ticks, and has variable degrees of virulence ranging from subclinical infections to severe life-threatening disease.39,40 A second small Babesia species was identified in southern California in 1991 and was also referred to as B. gibsoni.41 However, genetic data suggest that this organism is more closely related to some Theileria species than it is to Babesia species.36,37 The official phylogenic description of this organism is yet to be reported and is referred to as the western piroplasm in this article. The western piroplasm has only been reported in Southern California, the vector is unknown, and it is a virulent pathogen. The third small piroplasm is tentatively named Theileria annae.38 Theileria annae is endemic to Spain. The vector is suspected to be Ixodes hexagonus, and it appears to be a virulent pathogen.42-44