Could treatment of pregnant mares prevent abortions due to equine piroplasmosis ?

INTRODUCTION Piroplasmosis is regularly diagnosed in South Africa in new-born foals and aborted foetuses. In a recent survey of abortions in Thorougbred mares in South Africa, it was found that 11 % were due to Theileria equi. Babesia caballi, the other causative organism of equine piroplasmosis, is rarely incriminated. Prevention of abortion or neonatal mortality due to piroplasmosis is of significant economic importance. Abortions, usually during the last trimester of gestation, stillbirths and peracute neonatal piroplasmosis, which is almost invariably fatal, are rarely accompanied by clinical disease in the mare. Some Thoroughbred stud managers administer imidocarb dipropionate (Forray-65, Hoechst Roussel Vet) to brood mares 6–8 weeks before the expected foaling date in an attempt to prevent abortions due to piroplasmosis (personal observations). There are no published data available on whether imidocarb crosses the equine placenta. It is therefore possible that this chemotherapy is ineffective due simply to the imidocarb not reaching the foetus. A recent study demonstrated the presence of T. equi in healthy equine foetuses as early as the 120th day of gestation, whereas most abortions occur much later. This implies that the most appropriate time for prevention or treatment of transplacental infection is not known. There are also no published data available regarding the serum clearance times for imidocarb in horses. In sheep, peak plasma concentrations are reached within 4 h of administration, decreasing rapidly between 4 and 6 h postadministration and then decaying slowly by 1st-order kinetics over a 3-week period. It has been suggested that the peak plasma concentration in horses would also be reached between 1 and 4 h post injection (M Allen, Hoechst Roussel Vet, South Africa, pers. comm., 1998).


INTRODUCTION
Piroplasmosis is regularly diagnosed in South Africa in new-born foals and aborted foetuses 4,7 .In a recent survey of abortions in Thorougbred mares in South Africa, it was found that 11 % were due to Theileria equi 2 ‡ .Babesia caballi, the other causative organism of equine piroplasmosis, is rarely incriminated 9 .Prevention of abortion or neonatal mortality due to piroplasmosis is of significant economic importance.
Abortions, usually during the last trimester of gestation, stillbirths and peracute neonatal piroplasmosis, which is almost invariably fatal, are rarely accompanied by clinical disease in the mare 4,7 .Some Thoroughbred stud managers administer imidocarb dipropionate (Forray-65, Hoechst Roussel Vet) to brood mares 6-8 weeks before the expected foaling date in an attempt to prevent abortions due to piroplasmosis (personal observations).
There are no published data available on whether imidocarb crosses the equine placenta.It is therefore possible that this chemotherapy is ineffective due simply to the imidocarb not reaching the foetus.A recent study 6 demonstrated the presence of T. equi in healthy equine foetuses as early as the 120th day of gestation, whereas most abortions occur much later.This implies that the most appropriate time for prevention or treatment of transplacental infection is not known.
There are also no published data available regarding the serum clearance times for imidocarb in horses.In sheep, peak plasma concentrations are reached within 4 h of administration, decreasing rapidly between 4 and 6 h postadministration and then decaying slowly by 1st-order kinetics over a 3-week period 1 .It has been suggested that the peak plasma concentration in horses would also be reached between 1 and 4 h post injection (M Allen, Hoechst Roussel Vet, South Africa, pers.comm., 1998).

MATERIALS AND METHODS
Three Nooitgedacht mares were used in this trial.Mares V90 and V26 were 7 months pregnant and mare V127 was 5 months pregnant.Indirect fluorescent antibody test (IFAT) and DNA probe analysis of the blood of all 3 mares confirmed them to be T. equi carriers.All 3 mares were injected intramuscularly with imidocarb at a dosage of 3 mg/kg and mechanical abortion 10 , resulting in the delivery of a live foetus, was performed after 4 h in mare V26, 9 h in mare V90 and 48 h in mare V127.
One hour before the abortion, a 10 m blood sample was collected from each mare into heparinised vacutainer tubes.Immediately after the foetus was delivered, at least 7 m of blood were collected from its umbilical vein into a heparinised vacutainer tube, after which the foetus was euthanased by intra-cardiac injection of pentobarbitone sodium (Euthanaze, Centaur-Bayer Animal Health).
The heparinised blood samples were centrifuged at 3000 rpm for 15 min; the plasma was collected and stored at -15 °C.A spectrophotometric assay was conducted to compare the relative concentrations of imidocarb in the plasma of each mare and her foetus 1 .

RESULTS
The plasma samples from mare V127 and her foetus became cloudy when thawed and it was impossible to get absorbency readings from either sample.The imidocarb concentrations in the plasma samples of the other 2 mares and their respective foetuses were very similar.Plasma from mare V26 contained 4.54 µg/m imidocarb and that of her foetus 5.16 µg/m .Plasma from mare V90 contained 2.35 µg/m imidocarb and that of her foetus 2.75 µg/m .

DISCUSSION
The results clearly show that in the 2 cases tested, imidocarb did cross the equine placenta and was detectable in the foetal circulation at a level similar to that found in the dam's blood.A sample size of 2 mares and their foetuses is very small, however, and care should be taken when extrapolating these results to an entire population.
The presence of imidocarb in foetal blood suggests that administration of the compound during pregnancy could help to prevent abortions due to piroplasmosis.As imidocarb does not sterilise T. equi infections at the normal therapeutic dose 3,5 , it would only be of value if administered at the time when the parasites were multiplying, and before the parasitaemia in the foetus had reached a point likely to cause abortion.As there is currently no practical way of determining the likelihood or time of an individual mare aborting, it is impossible to know when to administer imidocarb to achieve the desired effect.Therefore, although administering imidocarb during pregnancy could potentially help to reduce the incidence of abortions due to piroplasmosis, not knowing when to treat the pregnant mare significantly reduces the value of such treatments.Although abortion storms due to T. equi are very rare, it may be useful to apply the treatment to all late-pregnant mares in a herd where more than 1 mare has aborted owing to T. equi infection in a short time interval.
This report emanates from project 36.5.168, approved by the Ethics Commit-tee of the Faculty of Veterinary Science, University of Pretoria.