Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, South Africa
The piroplasms Babesia and Theileria result in significant disease burdens in dogs across the globe and in some cases they are responsible for emerging zoonotic diseases.1 They are all transmitted by ixodid ticks although blood transfusions and transplacental transmission have been reported. In the case of B. gibsoni, there is clear evidence that dog-bite wounds also result in transmission. Widespread travel now means that these parasites are likely to have rapidly growing distributions and veterinarians need to be aware of infections that may have been hitherto undiagnosed in their regions. The spectrum of clinical disease caused varies widely from mild non-life-threatening to severe and sometimes fatal disease. The types of clinical signs seen generally include lethargy, weakness, anorexia and pale mucous membranes. Typical also are splenomegaly, haemolytic anaemia, haemoglobinuria, icterus, and thrombocytopaenia.
In years gone by life seemed simple as blood parasites were identified morphologically under a light microscope. The size of the intraerythrocytic piroplasms has traditionally been used to classify organisms into small (0.5–2.5 µm) and large (3–5 µm) parasite groups. With the use of PCR in diagnosis and DNA sequencing, the truth has become significantly more complicated (and interesting) as we have become aware of the vast array of previously unidentified and even unrecognized haemoprotozoal parasites carried by dogs. Blood smear is still very useful to diagnose and distinguish the large from the small parasites. The small parasites do however appear to be able to cause clinical disease at parasite densities below the level of light microscope detection and in these cases a clinical suspicion needs to be present to prompt the clinician to seek molecular diagnosis. The large Babesia organisms are represented by 3 species (B. canis, B. vogeli, B. rossi).
The small parasites were initially thought to be represented by B. gibsoni alone. The Theileria parasites fall within the group of small parasites and will form the basis for further discussion here. Babesia gibsoni is found on 5 continents (including Europe but not Africa). A new addition to this group is B. conradae - a small parasite identified from California. There has been some controversy around the identification of a B. microti-like parasite in dogs that has been named Theileria annae which was initially identified in northern Spain but has subsequently been identified in the USA and Croatia. This parasite causes typical babesial disease. It is phylogenetically more closely related to Theileria than B. gibsoni. The reason why the use of the genus Theileria has been questioned by some is because no one has identified a pre-erythrocytic or lymphocytic stage of the parasite nor is there evidence of transovarial transmission of the parasite within the tick vector.2 In addition to causing the disease typical of Babesia, T. annae has also been known to cause azotaemia, proteinuria with hypoalbuminaemia and hypercholesterolaemia with a urine sediment typical of a protein-losing nephropathy (hyaline casting). These are all signs typical of a glomerulonephritis. This is not usual for the disease caused by other canine piroplasmids. Other Theileria parasites that have been diagnosed in dogs include T. annulata and T. equi. Theileria annulata was diagnosed in an asymptomatic dog, whilst T. equi was found in one sick dog and 3 healthy ones.
In South Africa specifically, where we only get large Babesia spp. (B. rossi and B. vogeli) and no small B. gibsoni-like parasites in dogs, Theileria species have been identified by molecular techniques. Visualization of these parasites by light microscopy is not routine. The first report of this was by Matjila et al. (2008) who identified Theileria in two populations of dogs. In the Pietermaritzburg area, 79 of 192 blood samples were positive and in the Onderstepoort area 3 of 1137 samples were positive.3 The parasite DNA was identified by reverse line blot and then subsequently sequenced and the parasite identified was most closely related to a Theileria found in the wild sable antelope. A subsequent South African study focussed on the clinical presentation of 6 dogs with Theileria infections (Rosa et al. 2014, in press). These authors did not focus on molecular aspects of the parasite (although parasite DNA was demonstrated by reverse line blot technology, the DNA was no sequenced). Four of these 6 dogs had a Theileria sp. identified and 2 had T. equi present. No other blood parasites were demonstrated concurrently in these dogs (using the same methods of identification as for the Theileria). Five of the 6 dogs were anaemic and interestingly, these same 5 dogs showed a clinically obvious bleeding tendency consistent with a thrombocytopaenia. This was clinically the most significant finding and is a finding never previously attributed to clinical disease caused by Theileria in dogs. All were thrombocytopaenic. Two dogs in which bone marrow biopsies were collected demonstrated myelofibrosis. The sick dogs were treated with imidocarb dipropionate and those that showed an immune-mediated haematological disorder were also given prednisolone and azathioprine. Five dogs achieved clinical cure and in the three of these dogs that were tested, there was no PCR evidence of parasite persistence.
The longer and harder you look using molecular tools, the more you find and the more interesting and complex the blood parasite load of dogs becomes. It is clear that Theileria causes significant clinical disease in dogs and is not an infection that can be ignored. Additional research is required to better define the parasite phylogenetically, to identify the tick vector and to optimise treatment and prophylactic strategies.
1. Chomel B. Tick-borne infections in dogs - an emerging infectious threat. Veterinary Parasitology. 2011;179(4):294–301.
2. Goethert HK, Telford SR 3rd. What is Babesia microti? Parasitology. 2003;127(Pt 4):301–309.
3. Matjila PT, Leisewitz AL, Oosthuizen MC, Jongejan F, Penzhorn BL. Detection of a Theileria species in dogs in South Africa. Veterinary Parasitology. 2008;157(1–2):34–40.