ORIGINAL ARTICLES THE IDENTIFICATION OF BABESZA EQUZ IN AUSTRALIA D. F. MAHONEY*, I. G . WRIGHT*,W. M. FRERICHSt, s. GRQENENDYK~, B. M. OSULLIVANS, M. C. ROBERTS$ AND A. H. WADDELLS SUMMARY: A Babesia parasite, isolated from the blood of a horse at Bowral, New South Wales, was identified on the basis of its morphological features, host specificity and serological reactions, as Babesia egui (Laveran 1901). The case was originally reported by Churchill and Best (1976, Aust. vet. J. 52: 487) and is the first record of equine babesiosis in Australia. In preliminary studies, the organism produced only a mild disease in an intact horse, but caused the typical clinical syndrome of acute babesiosis in a splenectomised horse, which died 19 days after the intravenous inoculation of the parasites.
Introduction
Churchill and Best (1976) described a clinical condition in a horse at Bowral, New South Wales, which was characterised by mild fever, listlessness, inappetance, anaemia and jaundice. They observed organisms in the erythrocytes, which were tentatively identified on the basis of their morphology as a species of Babesia. This paper reports the identification of the species and includes some observations on the disease as it occurred in the laboratory. Materials and Methods Experimental Animals Two aged horses which had been bred in south eastern Queensland were used. One of these horses (No. 2) was splenectomised. A three-month-old pup, a six-month-old calf, and a young short-nosed bandicoot (Isoodon rnacrourus) were also splenectomised. The animals were maintained at the laboratory under conditions that precluded the accidental infestation with ticks. Smears of peripheral blood were examined for up to 2 weeks before the commencement of the experiment, for the presence of contaminating blood parasites. Haematology Thin blood films were fixed in methyl alcohol and stained with Giemsa at pH7.4. Thick blood films were prepared and stained with Giemsa according to the method of Mahoney and Saal (1961). Packed cell volume (PCV) was measured by the microhaematocrit method described by Dacie and Lewis (1968). Measurements of the size of the parasite were made on thin blood films stained with Giemsa using a Leitz 12.5 x micrometer eyepiece. Parasite multiplication rate (doubling time) was estimated by methods described by Mahoney ei a1 (1973). Histopathology Tissues were fixed in 10% buffered neutral formalin. Sections were prepared by conventional methods and stained with haematoxylin and eosin. Perl's Prussian Blue stain was used to demonstrate haemosiderin. ~~
~
~~~
~
*CSIRO, Division of Animal Health, Long Pocket Laboratories, Private Bag No. 3, P.O., Indooroopilly, Queensland, 4068 IUSDA, Parasitological Laboratory, Beltsville, Md., 20705,
Australian Veterinary Journal, Vol. 5 3 , October, 1977
Serological Tests Serum was obtained from the natural case described by Churchill and Best after clinical recovery and from the non-splenectomised horse (No. 1) immedlately before and one month after infection at the laboratory. Complement fixation tests (CF) using the technique of Frerichs et a1 (1969a) were performed by one of us (W.M.F) on these sera using known 3.caballi and B. equi antigens. The gel diffusion test for equine infectious anaemia (EIA) was carried out as described by Thomas (1975) on serum from horse No. 2. Transmission Transmission tests were performed by the intravenous inoculation of infected blood into recipient hosts.
Results
Transmission Tests Experiment I: Blood containing EDTA as an anticoagulant was obtained from the natural case on 2 August, 1976. It contained 234 organisms/ mm3. Sixty ml was inoculated into the nonsplenectomised horse No. 1, 20 ml into the splenectomised calf and 10 ml into the splenectomised dog. The calf and dog showed no evidence of illness during the period of observation and the parasite was not observed in their respective peripheral bloods. Parasites were first observed in the blood of the horse 4 days after inoculation, the parasitaemia reached a peak of approximately 2% on day 9 and then declined. Experiment 2.- Horse No. 2 (splenectomised) was inoculated intravenously with 1 x lo7 parasites from horse No. 1 on 27 August 1976. Parasites were first observed in thick films of peripheral blood on day 4 and their concentration had increased to 5 % by day 10. The horse survived until day 19 when it was destroyed in extremis by exsanguination, with a parasitaemia of approximately 40%. The bandicoot was inoculated with 0.5 ml blood from horse No. 2 containing 5% parasitaemia. It showed no signs of illness and parasites were not observed in thick films of its peripheral blood for 14 days after inoculation. 461
Parasite Doubling Time The doubling time of parasites in the blood of horse No. 1 was 8.9 hr and in the splenectomised horse No. 2) 7.2 hr. Parasite Morphology Figure 1 shows the various forms of the parasite in the erythrocytes. They appeared mostly as single, paired or “maltese cross” bodies, but other forms such as those in the process of division were occasionally observed. The single and paired organisms were either round, oval or pyriform. The singles accounted for 92% of all parasites observed, the paired forms, 5.2% and maltese crosses, 2.4%. The organisms were mainly between 1 and 2 p m in length and width, although the pyriform bodies were slightly less than 1 pm in width. These data are summarised in Table 1 .
Serology Serum samples from the naturally infected horse, taken on 9 July, 1976 and 20 July, 1976 were negative in C F tests with B. caballi antigen but reacted with B. equi antigen to a titre of 320. The pre-infection sample from horse No. 1 was negative to both antigens, but the sample taken 1 month after infection reacted to B. caballi with a titre of 80 and to B equi with a titre of 640. Serum from horse No. 2 was negative for EIA antibody when tested by the gel diffusion test. Clinical Signs The intact horse (No. 1) was not severely affected. Its temperature rose from 37.2”C to 39.4”C 9 to 10 days after infection and it was dull and anorectic. PCV fell from pre-infection level of 47% to 20% on day 12, but returned to 30% on day 13 and 42% on day 15. It then fell again to 37% at the time of the mild recrudescene of parasitaemia on day 16, but returned to the pre-infection level after day 17. Haemoglobinuria and haemoglobinaemia were not observed, but the plasma was icteric on days 9 and 10. In the several months following the recovery of this horse, the parasite was frequently detected in thick films of its peripheral blood in concentrations of up to several hundred/mm3. No significant difference was observed between the concentration of parasites in jugular and capillary (tail-tip) blood. The splenectomised horse (No. 2) “tolerated” a stationary parasitaemia of approximately 5 % from day 10 to day 17 without showing obvious signs of illness. However, during this time its PCV fell gradually from 39% to 25%. On day 17 it showed depression and inappetance and the parasitaemia commenced to increase and reached 40% on day 19. This was accompanied by a further fall in PCV to 13.5%, incoordination, extreme weakness and haemoglobinuria. Pathology (Horse N o . 2 )
Figure 1. Single, paired, dividing and maltese-cross forms of 8. equi in the blood of the splenectomised horse (Giemsa. x 1300).
462
Jaundice of the carcass was marked. The urine was dark brown and there was excessive fluid in the body cavities, particularly in the pericardial sac. Subendocaridal haemorrhage was prominent. Large yellowish (fibrin) clots were present in the vessels and were particularly obvious in the vessels of the liver. The liver was swollen and the hepatic lymph nodes were enlarged and haemorrhagic. The kidneys were pale. Australian Veterinary Journal, Vol. 5 3 , October, 1977
TABLE 1 Size and Distribution of the Various Parasite Forms Observed in 1025 infected Erythrocytes Form and
Dimension
Size ( ~ m )
Diameter Length Width Length Width
1.69 -t- 0.78 1.99 & 0.52 1.29 -t- 0.30 1.89 2 0.29 1.01 2 0.17
69.1 20.1
Shape
Single Round Oval Pyriform Pair Round/Oval
Pyriform Maltese Cross Others (Dividing, etc.)
Distribution Total)
(% of
2.9
Length Width Length Width
1.81 & 0.46 2 0.40
4.6
1.39 zk 0:33 0.97 k 0.13
0.6
Length Width
1.31 -C 0.21 0.84 -I 0.12
2.4
Histopathology (Horse No. 2 ) In all organs examined proliferation of reticuloendothelial cells was marked and parasitised erythrocytes were present in the vessels. In the liver the sinusojds and central veins were distended with macrophages containing parasitised erythrocytes and haemosiderin. Large numbers of mononuclear cells had infiltrated the portal triad areas. Organised thrombi were present in the larger vessels. Bile pigment was present in the hepatocytes. Individual hepatocyte nuclei were greatly enlarged. Kidney damage was severe, with degeneration of tubular epithelium, formation of protein and haemoglobin casts in the tubules, and accumulation of macrophages and other mononuclear cells in the interstitial tissue of the cortex and in the vessels. The sinuses of the lymph nodes were distended with parasitised erythrocytes, many of which had been phagocytosed. Syncytial accumulations of macrophages were numerous in the sinuses, engulfing parasitised erythrocytes. Adherence of erythrocytes to phagocytes was evident in the sinuses. In the lungs there was marked proliferation of macrophages with many being present in the vessels. Thrombi were seen in the larger vessels. Many haemosiderin-containing macrophages were present in the alveolar walls or attached to them. The sections of the brain examined were normal. Parasitised erythrocytes were present in the vessels but sludging in capillaries was not evident.
1.58 ~ .. ~
0.3
This species of Babesia has been identified as B . equi (Laveran 1901) on the following grounds:
Growth occurred only in the blood of the equine host. This excluded the possibility that the natural case was a chance cross-infection with a species of Babesia already known to infect other hosts in Australia, namely B . bovis (syn. B. argentina) and B. bigemina in cattle, B . canis in dogs and B. thylacis in the short-nosed bandicoot. On the basis of size and distribution of the various forms in the blood, the morphology agrees with the original description of B. equi given by Laveran (1901) and later by Nuttall and Strickland (19 12). A diagnostic morphological feature of B . equi is the maltese-cross form: which is produced by division of the single parasite into 4 pyriform bodies by simultaneous budding. This feature was originally considered sufficient on which to erect a new genus Nuttallia (FranGa 1909) but its validity is not recognised by modern taxonomists (Levine 1971) and Pierce (1975) has pointed out that its use for any piroplasm is invalid. B equi is considerably smaller than the larger parasite of the horse, B. caballi. The results of the CF test supported the diagnosis of B. equi, and suggested that the organism was not associated with the other species infecting equidae, B . caballi. There were no detectable autibodies to B. caballi in the serum of the naturally infected horse and the low titre to this parasite in the serum of the experimental horse was satisfactorily explained on the basis of cross-reaction. Serological cross-reaction with other Babesia species is common just after recovery from the acute phase of infection with a single species. However, the heterologous titre is lower than the homologous and remains for only a relatively short period (Mahoney 1964; Frerichs et al 1969b).
Australian Veterinary lournal, Vol. 5 3 , October, 1977
463
Discussion
A feature of the clinical syndrome in the nonsplenectomised horse was its mildness*, which is out of character with overseas descriptions. In other countries, B. equi is generally regarded as the cause of severe disease in horses, with acute onset, high fever, anaemia, parasitaemia of up to 90%, haemoglobinuria and high mortality (Levine 1973). Thus, in this country, it may yet appear in an acute form in which the clinical and pathological findings would be similar to those described for the spenectomised horse. The gross pathological changes seen in our horses were similar to those described by Theiler (1903) and Roberts et a1 (1962). However, the microscopic examination of the liver by the latter did not reveal the marked cellular response of the reticuloendothelial system seen in the splenectomised horse in this study. It is a matter for conjecture whether such a proliferation of mononuclear cells might be seen in an entire, infected horse. Similarly the effect of “active” exsanguination (or bleeding out) on the organs, particularly the cerebral capillaries, is unknown. The pathological changes seen in the tissues of fatal cases of B. bovis (syn. B. argentim) infection in cattle (Rogers 1971) are not markedly different from those seen in our splenectomised horse. The major differences seen in the horse were thrombi in the larger vessels, marked mononuclear cell proliferation in the liver and kidney, marked erythrophagocytosis in all organs examined and the presence of large syncytial accumulations in the hepatic lymph nodes. The presence of thrombi does not appear to have been previously reported in ,equine babesiosis. In a recent study, Allen et Ccl (1975) could not demonstrate large vessel thrombosis in B. caballi infections even though they did find coagulation defects. Vascular fibrin deposition has been described in strongyle infections (Drudge e l a1 1966) but whether strongyles contributed to the large vessel thrombosis observed in this study is not known. ‘There was no means of finding .out if the horse had been previously infected. Because of its age, it could have been infected in early life and then become zerologically negative with the passage o f time. However, it could have retained a degree of immuruty (Frerichs el al 1969b)
464
American experience has shown that many similarities exist between equine babesiosis and equine infectious anaemia (EIA) and a careful differential diagnosis between these two conditions should be made (Sippell et a1 1962). The pathological findings presented here also indicate that marked histological similarities exist between the two conditions, particularly the changes in the liver. Acknowledgments We are indebted to Mr D. Best of the Bowral, New South Wales, and to Mr G. Merritt, CSIRO, McMaster Laboratory, Sydney, for the supply of material from the naturally infected horse. References Allen, P. C., Frerichs, W. M. and Holbrook, A. A. (1975)-Expl. Parasit. 37: 373. vet. Churchill, R. C. and Best, D. R. (1976)-Aust. I . 52: 487. Dacie, J. V. and Lewis, S. M. (1968b--‘‘Practical Haematology”, 4th edn., Churchill, London. Drudge. J. H., Lvons, E. T. and Szanto, J. (1966)In “Biologv of Parasites”, edited bv E. J. L. Soulsby, Academic Press, New York, p. 199. FranCa, C. (1909)-Arch. do R. Znst. Bacteriul. Camara Pestana (Lisbon) 3: 11. Frerichs, W. M., Holbrook, A. A. and Johnson, A. J. (1969a)-Am. J. vet. Res. 30: 1337. Frerichs. W. M., Holbrook, A. A. and Johnson, A. J. (1969bL-Am. I . vet. Res. 30: 697. Laveran, M. (1901)-C. R. S6anc. SOC. Biol. 53: 385. Levine, N. D. (1971)--Trans. Am. microsc. SOC. 9 0 2. Parasites of Levine, N. D. (1973b-“Protozoan Domestic Animals and of Man”, 2nd edn., Burgess Publishing, Minneamlis. Mahoney, D. F. (1964b-Aust. vet. I . 40: 369. vet J. Mahoney, D. F. and Saal, J. R. (1961)-Aust. 37: 44.
Mahoney, D. F.. Wright, I. G. and Ketterer, P. J. D. (1973)-1nt. J. Parasit 3: 209. Nuttall, G. H. F. and Strickland. C. (1912)--Parasitology 5: 65. Pierce, M. A. ( 1 9 7 5 ) - f n t .
J. Parasit. 5: 285.
Roberts. E. D.. Morehouse. L. G.. Gainer. J. H. and Am. vet.’ rned. Ass. McDaniel, H. A. (1962)-J. 141: 1323. Rogers. R. J. (1971)-dust. vet. I. 47: 242. Sippel,’ W. L.’, Cooperrider, D. E., Gainer, J. H. Allen, R. W., Mouw, I. E. B. and Teigland, M. B. (1962)-J. Am. vet. med. Ass. 141: 694. Theiler, ‘A. (1903)--1. Comp. Path. Ther. 16: 97. Thomas, R. J. (1975)-Aust. vet. I. 51: 440. (Received for publication 10 February 1977)
Australian Veterinary Journal, Vol. 53, October, 1977
Introduction
Churchill and Best (1976) described a clinical condition in a horse at Bowral, New South Wales, which was characterised by mild fever, listlessness, inappetance, anaemia and jaundice. They observed organisms in the erythrocytes, which were tentatively identified on the basis of their morphology as a species of Babesia. This paper reports the identification of the species and includes some observations on the disease as it occurred in the laboratory. Materials and Methods Experimental Animals Two aged horses which had been bred in south eastern Queensland were used. One of these horses (No. 2) was splenectomised. A three-month-old pup, a six-month-old calf, and a young short-nosed bandicoot (Isoodon rnacrourus) were also splenectomised. The animals were maintained at the laboratory under conditions that precluded the accidental infestation with ticks. Smears of peripheral blood were examined for up to 2 weeks before the commencement of the experiment, for the presence of contaminating blood parasites. Haematology Thin blood films were fixed in methyl alcohol and stained with Giemsa at pH7.4. Thick blood films were prepared and stained with Giemsa according to the method of Mahoney and Saal (1961). Packed cell volume (PCV) was measured by the microhaematocrit method described by Dacie and Lewis (1968). Measurements of the size of the parasite were made on thin blood films stained with Giemsa using a Leitz 12.5 x micrometer eyepiece. Parasite multiplication rate (doubling time) was estimated by methods described by Mahoney ei a1 (1973). Histopathology Tissues were fixed in 10% buffered neutral formalin. Sections were prepared by conventional methods and stained with haematoxylin and eosin. Perl's Prussian Blue stain was used to demonstrate haemosiderin. ~~
~
~~~
~
*CSIRO, Division of Animal Health, Long Pocket Laboratories, Private Bag No. 3, P.O., Indooroopilly, Queensland, 4068 IUSDA, Parasitological Laboratory, Beltsville, Md., 20705,
Australian Veterinary Journal, Vol. 5 3 , October, 1977
Serological Tests Serum was obtained from the natural case described by Churchill and Best after clinical recovery and from the non-splenectomised horse (No. 1) immedlately before and one month after infection at the laboratory. Complement fixation tests (CF) using the technique of Frerichs et a1 (1969a) were performed by one of us (W.M.F) on these sera using known 3.caballi and B. equi antigens. The gel diffusion test for equine infectious anaemia (EIA) was carried out as described by Thomas (1975) on serum from horse No. 2. Transmission Transmission tests were performed by the intravenous inoculation of infected blood into recipient hosts.
Results
Transmission Tests Experiment I: Blood containing EDTA as an anticoagulant was obtained from the natural case on 2 August, 1976. It contained 234 organisms/ mm3. Sixty ml was inoculated into the nonsplenectomised horse No. 1, 20 ml into the splenectomised calf and 10 ml into the splenectomised dog. The calf and dog showed no evidence of illness during the period of observation and the parasite was not observed in their respective peripheral bloods. Parasites were first observed in the blood of the horse 4 days after inoculation, the parasitaemia reached a peak of approximately 2% on day 9 and then declined. Experiment 2.- Horse No. 2 (splenectomised) was inoculated intravenously with 1 x lo7 parasites from horse No. 1 on 27 August 1976. Parasites were first observed in thick films of peripheral blood on day 4 and their concentration had increased to 5 % by day 10. The horse survived until day 19 when it was destroyed in extremis by exsanguination, with a parasitaemia of approximately 40%. The bandicoot was inoculated with 0.5 ml blood from horse No. 2 containing 5% parasitaemia. It showed no signs of illness and parasites were not observed in thick films of its peripheral blood for 14 days after inoculation. 461
Parasite Doubling Time The doubling time of parasites in the blood of horse No. 1 was 8.9 hr and in the splenectomised horse No. 2) 7.2 hr. Parasite Morphology Figure 1 shows the various forms of the parasite in the erythrocytes. They appeared mostly as single, paired or “maltese cross” bodies, but other forms such as those in the process of division were occasionally observed. The single and paired organisms were either round, oval or pyriform. The singles accounted for 92% of all parasites observed, the paired forms, 5.2% and maltese crosses, 2.4%. The organisms were mainly between 1 and 2 p m in length and width, although the pyriform bodies were slightly less than 1 pm in width. These data are summarised in Table 1 .
Serology Serum samples from the naturally infected horse, taken on 9 July, 1976 and 20 July, 1976 were negative in C F tests with B. caballi antigen but reacted with B. equi antigen to a titre of 320. The pre-infection sample from horse No. 1 was negative to both antigens, but the sample taken 1 month after infection reacted to B. caballi with a titre of 80 and to B equi with a titre of 640. Serum from horse No. 2 was negative for EIA antibody when tested by the gel diffusion test. Clinical Signs The intact horse (No. 1) was not severely affected. Its temperature rose from 37.2”C to 39.4”C 9 to 10 days after infection and it was dull and anorectic. PCV fell from pre-infection level of 47% to 20% on day 12, but returned to 30% on day 13 and 42% on day 15. It then fell again to 37% at the time of the mild recrudescene of parasitaemia on day 16, but returned to the pre-infection level after day 17. Haemoglobinuria and haemoglobinaemia were not observed, but the plasma was icteric on days 9 and 10. In the several months following the recovery of this horse, the parasite was frequently detected in thick films of its peripheral blood in concentrations of up to several hundred/mm3. No significant difference was observed between the concentration of parasites in jugular and capillary (tail-tip) blood. The splenectomised horse (No. 2) “tolerated” a stationary parasitaemia of approximately 5 % from day 10 to day 17 without showing obvious signs of illness. However, during this time its PCV fell gradually from 39% to 25%. On day 17 it showed depression and inappetance and the parasitaemia commenced to increase and reached 40% on day 19. This was accompanied by a further fall in PCV to 13.5%, incoordination, extreme weakness and haemoglobinuria. Pathology (Horse N o . 2 )
Figure 1. Single, paired, dividing and maltese-cross forms of 8. equi in the blood of the splenectomised horse (Giemsa. x 1300).
462
Jaundice of the carcass was marked. The urine was dark brown and there was excessive fluid in the body cavities, particularly in the pericardial sac. Subendocaridal haemorrhage was prominent. Large yellowish (fibrin) clots were present in the vessels and were particularly obvious in the vessels of the liver. The liver was swollen and the hepatic lymph nodes were enlarged and haemorrhagic. The kidneys were pale. Australian Veterinary Journal, Vol. 5 3 , October, 1977
TABLE 1 Size and Distribution of the Various Parasite Forms Observed in 1025 infected Erythrocytes Form and
Dimension
Size ( ~ m )
Diameter Length Width Length Width
1.69 -t- 0.78 1.99 & 0.52 1.29 -t- 0.30 1.89 2 0.29 1.01 2 0.17
69.1 20.1
Shape
Single Round Oval Pyriform Pair Round/Oval
Pyriform Maltese Cross Others (Dividing, etc.)
Distribution Total)
(% of
2.9
Length Width Length Width
1.81 & 0.46 2 0.40
4.6
1.39 zk 0:33 0.97 k 0.13
0.6
Length Width
1.31 -C 0.21 0.84 -I 0.12
2.4
Histopathology (Horse No. 2 ) In all organs examined proliferation of reticuloendothelial cells was marked and parasitised erythrocytes were present in the vessels. In the liver the sinusojds and central veins were distended with macrophages containing parasitised erythrocytes and haemosiderin. Large numbers of mononuclear cells had infiltrated the portal triad areas. Organised thrombi were present in the larger vessels. Bile pigment was present in the hepatocytes. Individual hepatocyte nuclei were greatly enlarged. Kidney damage was severe, with degeneration of tubular epithelium, formation of protein and haemoglobin casts in the tubules, and accumulation of macrophages and other mononuclear cells in the interstitial tissue of the cortex and in the vessels. The sinuses of the lymph nodes were distended with parasitised erythrocytes, many of which had been phagocytosed. Syncytial accumulations of macrophages were numerous in the sinuses, engulfing parasitised erythrocytes. Adherence of erythrocytes to phagocytes was evident in the sinuses. In the lungs there was marked proliferation of macrophages with many being present in the vessels. Thrombi were seen in the larger vessels. Many haemosiderin-containing macrophages were present in the alveolar walls or attached to them. The sections of the brain examined were normal. Parasitised erythrocytes were present in the vessels but sludging in capillaries was not evident.
1.58 ~ .. ~
0.3
This species of Babesia has been identified as B . equi (Laveran 1901) on the following grounds:
Growth occurred only in the blood of the equine host. This excluded the possibility that the natural case was a chance cross-infection with a species of Babesia already known to infect other hosts in Australia, namely B . bovis (syn. B. argentina) and B. bigemina in cattle, B . canis in dogs and B. thylacis in the short-nosed bandicoot. On the basis of size and distribution of the various forms in the blood, the morphology agrees with the original description of B. equi given by Laveran (1901) and later by Nuttall and Strickland (19 12). A diagnostic morphological feature of B . equi is the maltese-cross form: which is produced by division of the single parasite into 4 pyriform bodies by simultaneous budding. This feature was originally considered sufficient on which to erect a new genus Nuttallia (FranGa 1909) but its validity is not recognised by modern taxonomists (Levine 1971) and Pierce (1975) has pointed out that its use for any piroplasm is invalid. B equi is considerably smaller than the larger parasite of the horse, B. caballi. The results of the CF test supported the diagnosis of B. equi, and suggested that the organism was not associated with the other species infecting equidae, B . caballi. There were no detectable autibodies to B. caballi in the serum of the naturally infected horse and the low titre to this parasite in the serum of the experimental horse was satisfactorily explained on the basis of cross-reaction. Serological cross-reaction with other Babesia species is common just after recovery from the acute phase of infection with a single species. However, the heterologous titre is lower than the homologous and remains for only a relatively short period (Mahoney 1964; Frerichs et al 1969b).
Australian Veterinary lournal, Vol. 5 3 , October, 1977
463
Discussion
A feature of the clinical syndrome in the nonsplenectomised horse was its mildness*, which is out of character with overseas descriptions. In other countries, B. equi is generally regarded as the cause of severe disease in horses, with acute onset, high fever, anaemia, parasitaemia of up to 90%, haemoglobinuria and high mortality (Levine 1973). Thus, in this country, it may yet appear in an acute form in which the clinical and pathological findings would be similar to those described for the spenectomised horse. The gross pathological changes seen in our horses were similar to those described by Theiler (1903) and Roberts et a1 (1962). However, the microscopic examination of the liver by the latter did not reveal the marked cellular response of the reticuloendothelial system seen in the splenectomised horse in this study. It is a matter for conjecture whether such a proliferation of mononuclear cells might be seen in an entire, infected horse. Similarly the effect of “active” exsanguination (or bleeding out) on the organs, particularly the cerebral capillaries, is unknown. The pathological changes seen in the tissues of fatal cases of B. bovis (syn. B. argentim) infection in cattle (Rogers 1971) are not markedly different from those seen in our splenectomised horse. The major differences seen in the horse were thrombi in the larger vessels, marked mononuclear cell proliferation in the liver and kidney, marked erythrophagocytosis in all organs examined and the presence of large syncytial accumulations in the hepatic lymph nodes. The presence of thrombi does not appear to have been previously reported in ,equine babesiosis. In a recent study, Allen et Ccl (1975) could not demonstrate large vessel thrombosis in B. caballi infections even though they did find coagulation defects. Vascular fibrin deposition has been described in strongyle infections (Drudge e l a1 1966) but whether strongyles contributed to the large vessel thrombosis observed in this study is not known. ‘There was no means of finding .out if the horse had been previously infected. Because of its age, it could have been infected in early life and then become zerologically negative with the passage o f time. However, it could have retained a degree of immuruty (Frerichs el al 1969b)
464
American experience has shown that many similarities exist between equine babesiosis and equine infectious anaemia (EIA) and a careful differential diagnosis between these two conditions should be made (Sippell et a1 1962). The pathological findings presented here also indicate that marked histological similarities exist between the two conditions, particularly the changes in the liver. Acknowledgments We are indebted to Mr D. Best of the Bowral, New South Wales, and to Mr G. Merritt, CSIRO, McMaster Laboratory, Sydney, for the supply of material from the naturally infected horse. References Allen, P. C., Frerichs, W. M. and Holbrook, A. A. (1975)-Expl. Parasit. 37: 373. vet. Churchill, R. C. and Best, D. R. (1976)-Aust. I . 52: 487. Dacie, J. V. and Lewis, S. M. (1968b--‘‘Practical Haematology”, 4th edn., Churchill, London. Drudge. J. H., Lvons, E. T. and Szanto, J. (1966)In “Biologv of Parasites”, edited bv E. J. L. Soulsby, Academic Press, New York, p. 199. FranCa, C. (1909)-Arch. do R. Znst. Bacteriul. Camara Pestana (Lisbon) 3: 11. Frerichs, W. M., Holbrook, A. A. and Johnson, A. J. (1969a)-Am. J. vet. Res. 30: 1337. Frerichs. W. M., Holbrook, A. A. and Johnson, A. J. (1969bL-Am. I . vet. Res. 30: 697. Laveran, M. (1901)-C. R. S6anc. SOC. Biol. 53: 385. Levine, N. D. (1971)--Trans. Am. microsc. SOC. 9 0 2. Parasites of Levine, N. D. (1973b-“Protozoan Domestic Animals and of Man”, 2nd edn., Burgess Publishing, Minneamlis. Mahoney, D. F. (1964b-Aust. vet. I . 40: 369. vet J. Mahoney, D. F. and Saal, J. R. (1961)-Aust. 37: 44.
Mahoney, D. F.. Wright, I. G. and Ketterer, P. J. D. (1973)-1nt. J. Parasit 3: 209. Nuttall, G. H. F. and Strickland. C. (1912)--Parasitology 5: 65. Pierce, M. A. ( 1 9 7 5 ) - f n t .
J. Parasit. 5: 285.
Roberts. E. D.. Morehouse. L. G.. Gainer. J. H. and Am. vet.’ rned. Ass. McDaniel, H. A. (1962)-J. 141: 1323. Rogers. R. J. (1971)-dust. vet. I. 47: 242. Sippel,’ W. L.’, Cooperrider, D. E., Gainer, J. H. Allen, R. W., Mouw, I. E. B. and Teigland, M. B. (1962)-J. Am. vet. med. Ass. 141: 694. Theiler, ‘A. (1903)--1. Comp. Path. Ther. 16: 97. Thomas, R. J. (1975)-Aust. vet. I. 51: 440. (Received for publication 10 February 1977)
Australian Veterinary Journal, Vol. 53, October, 1977
