Abstract

Babesiosis is a tick-borne disease affecting humans and many domestic and wild animals. Domestic animals showing appreciable morbidity and mortality include dogs, cats, cattle and horses. Canine babesiosis is characterised by haemolytic anaemia, icterus and haemoglobinuria and range from chronic or subclinical to peracute and fatal, depending on the virulence of the species and the susceptibility of the host. Thrombocytopenia is the most common haemostatic change reported with markedly low platelet counts, yet very few cases show any signs of haemorrhage. This review will deal with clinical and haematological findings, complications and newly identified inflammatory and prognostic biomarkers in canine babesiosis.

Epidemiology

Babesiosis is a disease of worldwide significance and was first recognised in 1888 as a cause of fever, haemolytic anaemia, haemoglobinuria and death of cattle. Canine Babesia are morphologically classified into large and small forms, both exhibiting a worldwide distribution. B. canis and another novel, as yet unnamed, Babesia sp. detected in the USA (large Babesia) and B. gibsoni and B. annae (small Babesia) have been documented to infect dogs.¹ Three subspecies of B. canis used to exist, but through improved PCR techniques have lately allowed for better definition of these parasites and they are now unique species referred to as B. rossi, B. vogeli and B. canis. These three species are antigenically distinct, transmitted by different vectors and differ widely in pathogenicity and geographic distribution. Babesia vogeli is the least pathogenic, whilst Babesia canis is of intermediate pathogenicity and Babesia rossi is ostensibly the most virulent. The smaller parasite, B. gibsoni occurs principally in the Middle East, southern Asia, Japan, North Africa, South America and is an emerging infectious disease in the USA, as well as having been detected lately in Italy and Australia. Babesia annae has been found to be endemic in dogs in northwest Spain. Various species of ticks such as Rhipicephalus sanguineus, Dermacentor spp. and Haemaphysalis appendiculatus can transmit B. canis and B. rossi, whereas B. gibsoni is transmitted by H. bispinosa and H. longicornis. B. annae is thought to be transmitted by Ixodes hexagonus. Strong circumstantial evidence exists that B. gibsoni is transmitted by dog bites, whilst transplacental transmission from dam to offspring has recently been proven as an additional mode of transmission.

Pathogenesis and Clinical Manifestations

Babesia spp. mostly cause disease in young dogs, although dogs of all ages can be affected. The incubation period of canine babesiosis varies from 10–21 days for B. canis and related species and 14–28 days for B. gibsoni. The severity of the disease depends on the species of Babesia, the presence of concurrent infections and the age and immune status of the host. The disease presentation varies widely from peracute to chronic or even subclinical. B. rossi, the dominant species found in South Africa, is very virulent and causes peracute and acute disease. Clinical signs include anorexia, depression, collapse, pale mucous membranes, tachycardia, tachypnoea, weakness, splenomegaly, icterus, red-discoloured urine and fever.¹ It is thought that the clinical signs are the result of tissue hypoxia following the anaemia and a concomitant systemic inflammatory response syndrome caused by marked cytokine release. The pathogenesis of the anaemia is incompletely understood; intravascular and extravascular haemolysis take place, but other mechanisms such as poor bone marrow response are thought to play a role as well. Some cases show additional immune-mediated breakdown of red blood cells and dogs that show in-saline-positive red blood cell agglutination have to be carefully monitored for rapid decreases in haematocrit. Mortality for Babesia spp. infections ranges from around 12% for B. rossi to approximately 1% for B. vogeli.

The severe form of the disease is characterized by marked haemolytic anaemia, severe acid-base abnormalities with frequent secondary multiple organ failure and complications such as acute renal failure (ARF), hepatopathy with marked icterus, hypoglycaemia, acute respiratory distress syndrome (ARDS), cerebral pathology and additional immune-mediated red blood cell destruction. A small subset of dogs present with relative haemoconcentration, despite vigorous haemolysis, due to presumed shifting of fluid from the intravascular to the extravascular component. These dogs are at increased risk of developing ARF or cerebral complications, as well as other organ failures. Renal damage is deemed a common, yet poorly documented, complication in canine babesiosis. Urinary immunoglobulin G and urinary C-reactive protein were measured as markers for glomerular dysfunction, while urinary retinol-binding protein was used as a marker for tubular dysfunction. Dogs with babesiosis had significantly higher concentrations of all three measured urinary markers compared to healthy dogs. The data supports the presence of both glomerular and tubular dysfunction in dogs suffering from uncomplicated B. rossi infection. These urinary markers were superior to other conventionally used biomarkers of renal dysfunction in dogs with babesiosis.²

Haematological Changes

The blood smear findings are classically those reflecting the underlying regenerative, haemolytic anaemia and include marked anisocytosis, polychromasia, reticulocytosis, and normoblastaemia. Thrombocytopenia is a hallmark of the disease,³ regardless of the Babesia spp. involved and is often marked, but yet petechiation or epistaxis is very rarely seen, except in cases with concomitant Ehrlichia or Theileria infections. The pathophysiology of the thrombocytopenia remains undetermined, but sequestration or consumption is postulated.

Other haematological findings may include spherocytosis, in cases with secondary immune-mediated haemolytic anaemia and a left shift neutrophilia due to the marked systemic inflammatory response. Acute babesiosis can easily be misdiagnosed as primary immune-mediated haemolytic anaemia, especially if parasitaemias are low or only central venous blood is submitted.⁴

The complications associated with babesiosis are proposed to be the result of the systemic inflammatory response (SIR) present in most cases of canine babesiosis, mediated by cytokines, nitric oxide and free oxygen radicals. The SIRS can eventually progress to multiple organ failure and septic shock.⁵ Using the criteria for SIRS, 87% of complicated canine babesiosis cases, caused by B. rossi, were shown to be positive for this syndrome.⁴ Markers of the inflammatory response, such as elevated C-reactive protein (CRP), has been identified as a significant predictor of outcome in dogs infected with B. rossi, in conjunction with hypoglycemia.⁶

Coagulopathy in Babesiosis

Recent studies made use of more advanced laboratory assays, i.e., various platelet parameters, individual coagulation factor activity, AT activity, protein C activity, TEG, fibrinogen concentration and D-dimer concentration. One study reported that a consumptive coagulopathy, caused by the severe systemic inflammation present in babesiosis, and characterised by procoagulant activation (decreased clotting factor activity), inhibitor consumption (decreased protein C activity), and increased fibrinolysis (increased D-dimer concentration), does exist in dogs infected with B. rossi. The study showed that this consumptive coagulopathy was significantly more severe in those dogs that died and therefore the extent of the coagulopathy is associated with mortality.⁷ A second study looked at the use of TEG, a viscoelastic assay performed on citrated whole blood, in uncomplicated canine babesiosis. The results showed that babesiosis cases were normo- to hypercoagulable despite the low platelet count.⁸ These cases had markedly increased fibrinogen concentrations which most likely contributed to the normo- to hypercoagulability. Platelet surfaces are densely populated with receptors of which the glycoprotein IIb/IIIa complex (GPIIb/IIIa) is the most prominent. Thrombin-activated platelets bind to large amounts of fibrinogen via GPIIb/IIIa receptors, resulting in increased clot strength. A third study evaluated platelet activation as a result of the systemic inflammation and increased TF expression, in uncomplicated and complicated babesiosis using an automated analyser (ADVIA 2120, Siemens). The study reported significant differences at presentation between Babesia-infected and normal control dogs. The platelet count and mean platelet component concentration (MPC) were significantly decreased in the Babesia-infected dogs, suggesting marked platelet degranulation and activation together with the marked thrombocytopenia.⁹ The mean platelet volume (MPV), plateletcrit and mean platelet mass were significantly increased in the Babesia-infected dogs, illustrating the marked increase in size of the circulating platelets in these cases. It appears that large, degranulated platelets may play a significant procoagulant role in the lack of clinical haemorrhage, despite severe thrombocytopenia, observed in canine babesiosis. However, platelet activation as measured by MPV and MPC does not appear to be associated with outcome.

Recently Identified Prognostic Factors

The prognosis for infected dogs is generally quite good, with approximately 85–90% of cases surviving the illness, depending on the level of care and the Babesia spp. involved. The use of blood transfusions has a major impact on survival in severely anaemic animals. Cases with haemoconcentrated babesiosis and cases developing acute renal failure, acute respiratory distress syndrome or cerebral babesiosis have the worst prognosis and mortality can be greater than 50% - in some cases approaching 100%, despite intensive, technically advanced interventions. The prognosis (at least in B. rossi infections) is statistically worse in dogs with high parasitaemias,⁴ although exceptions can occur.

Lately, several important prognostic factors have been described:

 Serum lactate concentrations
Admission lactate > 5 mmol/l is associated with increased mortality, but lactate > 2.5 mmol/l and/or increase after admission and/or failure to decrease to < 50% of admission value, is strongly predictive of death. Lactate persistently > 4.4 mmol/l strongly predicts death and lactate > 4.4 mmol/l at 24 hours after admission correctly predicts death in every case. Hyperlactataemia is present in approximately half of hospitalised cases. Extremely high values (> 10 mmol/l) are not uncommon. Lactate correlates negatively with glucose. Hyperlactatemia is positively associated with clinical collapse.¹⁰

 Clinical collapse
Hypotension is common in babesiosis and is associated with clinical collapse. Clinical collapse is also associated with hypoglycaemia. This appears to be a very simple and useful prognostic factor.⁴

 Hypoglycaemia
Hypoglycaemia (glucose < 3.3 mmol/l) is present in c. 20% of cases. Hypoglycaemia is associated with stupor, collapse and miosis, which disappear after dextrose infusion. Dogs that are collapsed (non-ambulatory) at presentation have an 18-fold increased risk for hypoglycaemia. Pups less than 6 months of age are at risk of developing hypoglycaemia. The risk is slightly increased in dogs with severe anaemia, icterus or vomiting.¹⁰ Response to treatment with intravenous dextrose is sometimes dramatic. Hypoglycaemia is an important differential diagnosis for cerebral babesiosis. When glycaemic status is taken into account, the incidence of true cerebral babesiosis is very low.

 Capillary and venous parasitaemia
Timing, source of blood (venous versus capillary) and counting method cause large variability in parasite counts. A significant positive correlation between high parasitemia and mortality has recently been demonstrated. There is also a significant positive correlation between capillary and venous parasitaemia, with capillary parasitaemias being consistently higher than venous ones, making the capillary the preferential site for obtaining diagnostic smears.⁴

 Serum cortisol and thyroxine concentrations
Dogs that died had a significantly higher median cortisol concentration (482 nmol/l) than other admitted dogs that survived (150 nmol/l). Conversely, the total and free thyroxine concentrations in all but one of the dogs that died were below the limit of detection and significantly lower (2.7 nmol/l and 0.12 pmol/l, respectively) than in other admitted dogs that survived (7.4 nmol/l and 0.4 pmol/l, respectively).^11,12

In summary, it is clear that the disease caused by Babesia is caused by an exuberant and ineffective immune response that results in sometimes lethal collateral organ damage. The result is a severe, often lethal, blood-borne multisystem disease that may cause death through complete organ failure. It seems clear that disruption in normal haemostasis may be crucially involved in this complex pathogenesis.

References

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8.  Liebenberg C, Goddard A, Wiinberg B, Kjelgaard Hansen M, Schoeman JP. Thromboelastographic evaluation of hemostatic abnormalities in uncomplicated canine babesiosis caused by Babesia rossi. Congress of the European College of Veterinary Internal Medicine, Toulouse, France (abstract); 2010.

9.  Goddard A, Leisewitz A, Schoeman JP. Platelet indices in virulent canine babesiosis and their association with outcome. Biennial Congress of the International Society for Animal Clinical Pathology & Conference of the European Society of Veterinary Clinical Pathology, Ljubljana, Slovenia (abstract); 2012.

10. Keller N, Jacobson LS, Nel M, De Clerq M, Thompson PN, Schoeman JP. Prevalence and risk factors of hypoglycemia in virulent canine babesiosis. J Vet Intern Med. 2004;18:265–270.

11. Schoeman JP, Herrtage ME. Adrenal response to the low dose ACTH stimulation test and the cortisol-to-adrenocorticotrophic hormone ratio in canine babesiosis. Vet Parasitol. 2008;154:205–213.

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