Peter J. Irwin, BVetMed, PhD, FACVSc, MRCVS
Babesiosis (also referred to as piroplasmosis) is caused by tick-borne intraerythrocytic protozoan parasites of the genus Babesia, is one of the most common infections of animals worldwide, and is gaining interest as an emerging zoonosis in humans. In dogs and cats, babesiosis was originally viewed as a tropical and subtropical disease, but in recent times it has been recognised with increasing frequency in temperate regions of the world.
Babesia spp. are placed in the phylum Apicomplexa, however the traditional methods of classification are gradually being replaced by molecular biological techniques, as morphological features by themselves are not sufficient for species differentiation. Molecular phylogenetic analysis has been useful not only for defining the relationships between individual Babesia spp. but also for further elucidating the association between Babesia and closely related parasites such as Theileria and Cytauxzoon.
Babesia parasites of dogs and cats
Babesia parasites have been grouped informally into 'small' Babesia and 'large' Babesia within their vertebrate hosts. The larger piroplasm in dogs "Babesia canis" is recognised to represent at least three (sub-) species: Babesia canis canis, B. canis rossi and B. canis vogeli, which are transmitted by different tick vectors. Until recently it was assumed that Babesia gibsoni was the only small piroplasm to exist in dogs. It is considered to be widespread and endemic throughout Asia. The full geographical range of B. gibsoni, as with the other canine piroplasms, has yet to be elucidated in detail but the organism has been found in dogs in the Middle East, parts of Africa, North America, Europe and most recently in Australia. Reports of B. gibsoni from countries outside Asia are sporadic and probably reflect its introduction as a consequence of pets traveling across international boundaries, however it may well become locally established if the vector is present.
A number of new canine piroplasms have been described and there is growing evidence that some of these should be classified as Theileria spp. rather than within the Babesiidae family. A small piroplasm genetically distinct from B. gibsoni has been reported to occur in dogs in southern California. A third small canine piroplasm has recently been found to be endemic in northern Spain, transmitted by Ixodes hexagonus. This piroplasm (originally referred to as Theileria annae) closely aligns with B. microti, a parasite more commonly associated with rodent babesiosis.
Feline babesiosis appears to be less common and is not as well researched as the disease in dogs. The classification of Babesia in cats is far from clear at the present time and is based largely on morphological studies of parasites obtained from a variety of wild and domestic hosts. Babesia felis is currently recognised as the cause of babesiosis in domestic cats in parts of Africa.
All Babesia parasites described to date are transmitted to their vertebrate hosts by the 'hard' ticks (Ixodidae). Infective sporozoites are injected into the dog or cat within saliva during engorgement of the tick. These organisms then invade, feed and divide by binary fission within, and rupture erythrocytes during repeated phases of asexual reproduction, releasing merozoites that find and invade other erythrocytes. Transmission to the vector may occur at any time that a parasitaemia exists. Whilst tick transmission is the major source of infection, babesiosis may also occur after transfusion of infected blood and in neonates after transplacental transfer. A higher prevalence B. felis infection has been observed in Siamese and Oriental cats in South Africa and there appears to be a curious overrepresentation of Bull Terrier type in reports of B. gibsoni infections outside Asia, prompting the suggestion of vertical transmission within this breed and the possibility of inoculation of infected blood during fights. Babesiosis is considered to be an emerging disease in many parts of the world. Veterinarians should retain a high degree of clinical suspicion when faced with cases of haemolytic anaemia and should include questions relating to the pet's travel history during consultation. An increasing number of cases of canine babesiosis are reported in regions where the disease was not previously known to exist (e.g., northern Europe). Concurrent infection with other haemoparasites, notably Ehrlichia spp., Mycoplasma (Haemobartonella) spp., Hepatozoon spp. and other species of Babesia appears to be a common occurrence in endemic regions and potentially complicates the diagnosis and management of individuals by the veterinarian. Multiple coinfections are difficult to diagnose without highly sensitive tests such as PCR.
Clinical Signs of Babesiosis
The most consistent finding on physical examination is pallor of the mucous membranes, with a variable occurrence of fever, hepatosplenomegaly, icterus, dehydration and shock. Severe intravascular haemolysis associated with virulent babesiosis leads to haemoglobinuria ('red water') and affected dogs develop signs that reflect widespread organ dysfunction associated with hypotension, hypoxaemia and extensive tissue damage, such as anuria or oliguria, neurological dysfunction, coagulopathies and acute respiratory distress. Petechial and ecchymotic haemorrhages may be observed on the gums or ventral abdomen in some dogs, consistent with the presence of concurrent thrombocytopenia or thrombocytopathy. Dogs that survive the initial infection become life-long carriers of the parasite despite appropriate treatment and resolution of the original signs. Many dogs remain subclinical, despite intermittent parasitaemias and recrudescence of parasites into the bloodstream may occur following immunosuppressive therapy or concurrent disease. Feline babesiosis tends to manifest as an afebrile, chronic, low-grade disease. Anorexia, depression and pallor were the clinical signs attributed to feline babesiosis most commonly in one study, with weight loss, icterus, constipation and pica recorded less frequently.
The definitive diagnosis of babesiosis currently requires visualisation of the parasite during light microscopic examination of a blood smear. Identification of the Babesia species, or at least a distinction between 'small' and 'large' babesial organisms, is important with regard to the choice of therapeutic agent. Higher parasitaemias can be demonstrated in blood samples collected from the ear tip and toenail. Routine laboratory tests generally yield non-specific results in babesiosis, but provide important information about organ function, electrolyte status and acid: base balance that is invaluable in managing the case. The IFAT seems to be the most reliable serological assay, offered by commercial diagnostic laboratories in the USA and Europe, but its use in the clinical setting is somewhat limited. Amplification of Babesia DNA using PCR is a highly sensitive technique that is gaining wider acceptance as a diagnostic test, although it will be some years before PCR becomes commercially available on a wider scale. Its particular value is likely to be for the detection of subclinical carriers in specific situations, such as before importation into Babesia-free regions and for screening potential blood donors.
Treatment and Prevention
Imidocarb (5mg/kg IM or SC once, repeated 14 days), diminazene (3.5mg/kg IM once), phenamidine (15mg/kg SC once or repeat 24h) and trypan blue (10mg/kg IV once as a 1% solution) are the preferred choices for canine babesiosis, and primaquine (0.5mg/kg PO once)is used for treating B. felis infections. In dogs with severe anaemia, normalising circulatory status is best accomplished by blood transfusion. Blood transfusion is generally indicated in dogs with a PCV <0.20L/L and cats with a PCV < 0.15L/L. Fresh whole blood transfusions are preferred for severe babesiosis but packed red cells are adequate in other cases. Crystalline fluid therapy should be given with caution in anaemic patients in order to avoid further haemodilution or exacerbating respiratory distress. Oxygen therapy does not alleviate the hypoxia in anaemic states, but is indicated for the therapy of pulmonary oedema in complicated babesiosis. Glucocorticoids, including dexamethasone and prednisolone (or prednisone) have been recommended by some authors but their benefits in babesiosis are currently unproven.
Removing all possibility of exposure to the vector is the best way to prevent babesiosis. However, this is rarely achievable in endemic areas despite attentive ectoparasite control. Regular spraying, dipping or bathing with topical ascaricidal preparations in accordance with the manufacturers' instructions should be practiced in regions where tick challenge is continual. Owners should be encouraged to search their pets daily for ticks and once found to physically remove and dispose of them. A vaccine made from cell culture-derived antigens PIRODOG (Merial) provides relatively good protection against challenge by homologous strains, but is non-protective against heterologous challenge, thus restricting its use to Europe.
References are available upon request.