In mares with placentitis does the duration of antibiotic treatment affect foal outcome?

a Knowledge Summary by

Elizabeth Barter BVSc (Hons) MVM ^1*

¹Scone Equine Hospital, 106 Liverpool Street, Scone, NSW 2337, Australia
^*Corresponding Author (Elizabeth.Barter@sconeequine.com.au)

Clinical scenario

Placentitis in mares is typically characterised by premature udder development or vaginal discharge from mid to late gestation and carries a high risk of abortion or birth of a compromised foal (Macpherson et al., 2013; and Waldridge & Pugh, 2001). The cause of placentitis and agent/s involved however are often unknown and treatment, regardless of cause, usually involves antibiotics, anti-inflammatories, and progesterone (Cummins et al., 2008; and Waldridge & Pugh, 2001). Microbial agents thought to be involved in placentitis include bacterial species such as Streptococcus equi subsp. zooepidemicus,(S. zooepidemicus) and Escherica coli (Canisso et al., 2015; and Curcio et al., 2017). Fungal infections have also been reported including Aspergillosis (Canisso et al., 2015). When mares are diagnosed with placentitis antibiotics may be prescribed either for the remaining length of pregnancy or in a pulsatile manner (LeBlanc, 2010; Christensen et al., 2010; Curcio et al., 2017; Bailey et al., 2010; and Ryan et al., 2008). Pulsatile therapy frequently recommends mares to receive antibiotics for 10 consecutive days a month starting at 7 months gestation (LeBlanc, 2010). No scientific data is currently available to confirm the efficacy of this treatment protocol (LeBlanc, 2010). Some evidence suggests that prolonged treatment is unable to eliminate bacteria and intermittent use may support suppression of bacterial growth (Bailey et al., 2010). Clinicians are reliant on experimental models and anecdotal evidence in making appropriate decisions regarding antibiotic choice and antibiotic length. There are currently a lack of studies comparing short- and long-term antibiotic use as a direct measurement of foal survival from mares with placentitis.

Abbreviation

CCFA – ceftiofur crystalline free acid

CFU – colony forming units

CTUP – combined thickness of the utero-placental unit

IV – intravenous administration

PO – oral administration

TMS – trimethoprim/sulphonamide

IM – intramuscular

The evidence

The literature search identified six publications that included antibiotic length of treatment and foetal outcome. The publications consisted of four non-randomised non-blinded controlled trials of level 4 (lower quality controlled trials) (OCEBM Levels of Evidence Working Group) (Macpherson et al., 2013; Christensen et al., 2010; Murchie et al., 2006; and Ryan et al., 2008). This included one pilot study (Ryan et al., 2008) and one short report presented at a conference (Christensen et al., 2010). Two publications were randomised non-blinded controlled trials of level 2b (lower quality randomised controlled trials) (Bailey et al., 2010; and Curcio et al., 2017). All studies involved experimental induction of ascending placentitis with S. zooepidemicus. Pony mares were used in 4/6 studies with one study involving light bred horses (Christensen et al., 2010) and one study using Criollo type mares (Curcio et al., 2017). All horses were in late gestation either unspecified gestational days (Christensen et al., 2010) or between 269 to 300 days gestation (Macpherson et al., 2013); 280–295 days; (Ryan et al., 2008) 290 days; (Bailey et al., 2010) 280–295; (Murchie et al., 2006) 269–288; and (Curcio et al., 2017) 300 days. The number of mares included in the studies were low ranging from five (Murchie et al., 2006) to 33 (Christensen et al., 2010) with number of mares per treatment group ranging from two (Murchie et al., 2006) to 12 (Bailey et al., 2010) limiting clinical extrapolation.

The outcome consistently measured between studies was ‘foal viability’, however the parameters determining viability varied between studies. Foal viability was described by Macpherson et al. (2013) and Bailey et al. (2010) as a foal able to right themselves, breathe without assistance, respond to stimuli and suckle. Low risk foals were determined by Curcio et al. (2017) as those able to breathe without assistance in less than 2 minutes, attain sternal recumbency in less than 5 minutes, a normal suckle reflex in 20 minutes, and able to stand with little or no assistance. Foal viability parameters were not defined in two studies (Christensen et al., 2010; and Murchie et al., 2008) and live foal rate was used in a single study (Ryan et al., 2008).

In studies with positive control (infected and not treated) any treatment improved foal outcome compared to no treatment at all (Bailey et al., 2010; and Curcio et al., 2017). The exception is Macpherson et al. (2013) that showed no difference in foal viability in untreated mares and mares receiving CCFA. There was no evidence to support that continuous antibiotics improved foal viability with similar foal viabilities seen in mares treated with TMS for 10 days in Curcio et al. (2017) at 93% (26 viable foals from 28 infected mares that received antibiotics) compared to antibiotics prescribed until abortion or parturition in Ryan et al. (2008) 64% (nine viable foals from 14 mares infected and received antibiotics), Christiansen et al. (2010) 71% (17 viable foals from 24 infected and treated pregnancies), and Bailey et al. (2010) at 83% of viable foals (10 viable foals from 12 infected and treated pregnancies). These findings support the need for ongoing research into identifying mares at risk of placentitis and once diagnosed monitoring clinical progression.

Summary of the evidence

Appraisal, application and reflection

Placentitis is a common condition estimated to affect 3–5% of Thoroughbred pregnancies and can be challenging for the clinician to diagnose and treat (Canisso et al., 2015). Current treatment protocols recommend a multifactorial approach involving antibiotics, anti-inflammatories and progesterone (Murchie et al., 2006; and Waldridge & Pugh, 2001). Two different antibiotic regimes have been proposed in the treatment of placentits; 1) pulse antibiotics involving 10 consecutive days each month until parturition; and 2) continuous antibiotics until parturition. The purpose of this Knowledge Summary was to evaluate the evidence comparing the treatment length of antibiotics and foal viability as no studies reported the use of pulse antibiotic therapy.

Six studies met the inclusion criteria and all the studies involved experimental induction of ascending placentitis using S. zooepidemicus. Two studies Curcio et al. (2017) and Bailey et al. (2010) provide the most comparable in terms of length of antibiotic treatment. Curcio et al. (2017) treated for 10 days with an average time from inoculation to parturition of treated mares 23.7 days and foal survival of 93% (26/28). Bailey et al. (2010) treated until abortion or delivery with average time from inoculation to parturition 31 days with 83% (10/12) of treated mares producing viable foals. Treatment commenced later in the study by Curcio et al. (2017) with the average gestation of mares approximately 300 days, whilst mares in the study by Bailey et al. (2010) were 280–295 days gestation. Viability of the foals and mean gestational lengths were similar between the studies, regardless of duration of antibiotics (both over 320 days gestation). Clinically it is recognised that chronic insidious placentitis carriers a better prognosis compared to acute disease as the foal has time to mature. The aim of placentitis treatment may be to delay parturition and allow foetal maturation as opposed to eliminating bacteria.

All the studies used supraphysiological doses of S. zooepidemicus to initiate placentitis. The amount of colony forming units were unspecified in two studies (Murchie et al., 2006; and Bailey et al., 2010) whilst the remaining studies varied. Ryan et al. (2008) inoculated 2 x 10⁶ CFU, whilst Christiansen et al. (2010) had a variable dose between 2–10 x 10⁶ CFU, Macpherson et al. (2013) and Curcio et al. (2017) used a higher dose of 1 x 10⁷ CFU. Clinical ascending placentitis is thought to be insidious in onset and reflective of mare perineal or cervical conformation (Waldridge & Pugh, 2001) and experimental inoculation may limit clinical extrapolation of results. The strain used was tested sensitive to the antibiotics prescribed in vitro (TMS) in Ryan et al. (2008) and Bailey et al. (2010), however was not specified if it was sensitive to the antibiotics prescribed in the other studies. No studies commented on culture and sensitivity of vaginal discharge post inoculation or uterine fluid post foaling/aborting.

When and how antibiotics were prescribed varied between studies. Two studies used TMS at 30 mg/kg body weight PO every 12 hours (Christensen et al., 2010; and Bailey et al., 2010). Curcio et al. (2017) used the same dose rate, however delivered the TMS IV every 12 hours and Ryan et al. (2008) did not specify dose rate or administration route of TMS. A single study used penicillin G (22,000 u/Kg), and gentamicin (6.6 mg/kg) (Murchie et al., 2006) and a single study evaluated CCFA the active metabolite of ceftiofur at 6.6 mg/kg IM repeated after 96 hours (Macpherson et al., 2013). Four studies specified that antibiotics were commenced when clinical signs of placentitis occurred which varied between 36 hours (Ryan et al., 2008) to 48 hours (Christensen et al., 2010) or was not specified (Macpherson et al., 2013; and Bailey et al., 2010). Clinical signs attributed to placentitis included vaginal discharge (Christensen et al., 2010; and Ryan et al., 2008) or elevated CTUP (Christensen et al., 2010).  Murchie et al. (2006) initiated antibiotics 4 days post inoculation regardless of clinical signs observed, and Curcio et al. (2017) initiated antibiotics 48 hours after inoculation regardless of clinical signs observed. The length of antibiotic treatment varied amongst the studies with four of the studies continuing antibiotics until parturition (Christensen et al., 2010, Bailey et al., 2010, Macpherson et al., 2017; and Ryan et al., 2008), whilst Murchie et al. (2006) treated for 7 days and Curcio et al. (2017) treated for 10 days. No studies evaluated examined physical, chemical, or ultrasonographic parameters of mares or foals whilst the mares were on antibiotic treatment.

The study by Ryan et al. (2008) additionally evaluated the use of antibiotics in mares with placentitis after abortion or foaling. Post mortem investigation of the non-viable foals found evidence S. zooepidemicus in foal lungs from mares that were treated with antibiotics and those that were not. The majority of foals in the study by Ryan et al. (2008) born from treated mares had negative blood cultures (10/12 foals treated with antibiotics had negative blood cultures). Uterine cultures taken from mares immediately post foaling or aborting that were on antibiotics were less predictable, with two thirds of mares (8/12 mares) returning a positive uterine culture to S. zooepidemicus despite the administration of antibiotics for over 4 weeks. It was proposed by Ryan et al. (2008) that early initiation of treatment was able to inhibit bacterial growth and subsequent inflammation. As treatment was initiated at onset of clinical signs, delayed antibiotic administration may only suppress bacterial growth. Further research is required into when to start antibiotic treatment and improve detection of mares with suspected placentitis.

Methods to monitor foetal well-being to aid in determining when to treat placentitis is limited. Curcio et al. (2017) noted that 4/10 mares that were inoculated failed to develop vaginal discharge and 3/10 mares failed to show an increase in CTUP or evidence of placental separation on rectal ultrasound. Further studies into the correlation of foetal circulation including heart rate, cord pressure (Vincze et al., 2019) and thickness of the placenta measured both abdominally and rectally may aid in earlier identification of mares with placentitis (Curcio et al., 2017). Decreasing progesterone and rising oestrogen levels have been correlated with poor prognosis of foetal survival (Curcio et al., 2017). Allantocentesis samples have also been used to assess foetal health, however further investigation is warranted into the technique (Murchie et al., 2008).

All studies involved a multi-model approach to treatment with combined therapeutic regimes. 5/6 studies included altrenogest (Macpherson et al., 2013; Christensen et al., 2010; Curcio et al., 2017; Bailey et al., 2010; and Ryan et al., 2008), single studies evaluated flunixin (Murchie et al., 2006), acetylcystine (Christensen et al., 2010), and estradiol cypionate (Curcio et al., 2017). Two studies examined pentoxifylline (Bailey et al., 2010; and Macpherson et al., 2017), and two studies examined dexamethasone (Christensen et al., 2010; and Ryan et al., 2008). The evaluation of these agents into the treatment of placentitis is beyond the scope of this article.

The use of antibiotics in a pulsatile manner has been explored in dogs with chronic pyoderma (Carlotti et al., 2004). In the canine model cephalexin (15 mg/kg twice a day) was prescribed for 2 days of the week and was compared against a placebo. The study found that pulse ‘weekend’ therapy was effective in reducing the time between relapses in canine idiopathic pyoderma and no resistance was noted in the 12 month period. It has been proposed that bacterial resistance to antibiotics may be limited in pulse antibiotic therapy as it minimises the time microbes are exposed to antibiotics and the selection of resistance (Baker et al., 2018). It was noted that in pulse therapy a bactericidal drug is advised with high concentrations to minimise pathogen abundance (Baker et al., 2018; and Carlotti et al., 2004). More research is required into what drugs and clinical scenarios pulse antibiotic therapy may be implemented.

Multiple types of placentitis have been described in the literature including ascending, focal mucoid (nocardioform), diffuse (haematogenous) and multifocal (Canisso et al., 2015). All the above studies involved an experimental model to induce ascending placentitis and the extrapolation to other forms is limited. Placentitis treatment remains frustrating for the clinician to treat with limited ability to perform culture and sensitivity against the potential organism/s involved. Current literature supports the use of antibiotics in the combined treatment of placentitis but does not provide evidence of the length of time they should be prescribed for. Further investigation of placentitis may involve correlating foetal and placental well-being to foal survival to be used as a measure of ceasing or altering treatment regimes.

Methodology Section

Search Strategy
Databases searched and dates covered:	Search terms were applied in PubMed Central accessed on NCBI website (1910–2019), CAB Abstracts database accessed on OVID platform (1973–2019)
Search strategy:	CAB Abstracts: (equine* or horse* or equus or equid* or mare or mares or broodmare or broodmares or 'brood mare' or 'brood mares' or pony or ponies).mp. or exp equidae/ or exp equus/ or exp horses/ or exp mares/) (Placentitis or placenta or pregnancy or fetus or foetus).mp. or exp placentitis/ or exp pregnancy/) (antibiotic or antibiotics or antimicrobial or antimicrobials or anti-microbial or anti-microbials or antibacterial or antibacterials or 'antiinfective agent' or 'antiinfective agents' or 'anti-infective agent' or 'anti-infective agents' or penicillin or gentamicin or ceftiofur or excede).mp. or exp antibacterial agents/ or exp antibiotics/ or exp antiinfective agents/) 1 and 2 and 3 PubMed: (equine OR horse OR mare OR mares OR broodmare OR brood mare OR pony OR ponies) (Placentitis or placenta or pregnancy or fetus or foetus) (antibiotic or antibiotics or antimicrobial or antimicrobials or anti-microbial or anti-microbials or antibacterial or antibacterials or 'antiinfective agent' or 'antiinfective agents' or 'anti-infective agent' or 'anti-infective agents' or penicillin or gentamicin or ceftiofur or excede) 1 and 2 and 3
Dates searches performed:	9 August 2019

Exclusion / Inclusion Criteria
Exclusion:	Non-English language, narrative or non-systematic review articles, unpublished data, pharmacokinetic, or in vitro experimental studies
Inclusion:	Any reported use of antibiotics in the treatment of placentitis in mares

Search Outcome
Database	Number of results	Excluded – non- English language publication	Excluded – systematic review or non-equine	Excluded – did not state duration of antibiotic	Excluded – single case report	Total relevant papers
CAB Abstracts	285	18	214	4	38	11
PubMed	263	10	216	2	30	5
Total relevant papers when duplicates removed						6

The author declares no conflicts of interest.

1. Bailey, C. S., Macpherson, M. L., Pozor, M. A., Troedsson, M. H., Benson, S., Giguere, S., Sanchez, L. C., LeBlanc, M. M. & Vickroy, T. W. (2010). Treatment efficacy of trimethoprim sulfamethoxazole, pentoxifylline and altrenogest in experimentally induced equine placentitis. Theriogenology, 74(3), 402–12. DOI: http://dx.doi.org/10.1016/j.theriogenology.2010.02.023
2. Baker, C. M., Ferrari, M. J. & Shea, K. (2018). Beyond dose: Pulsed antibiotic treatment schedules can maintain individual benefit while reducing resistance. Scientific Reports, 8(1). DOI: http://dx.doi.org/10.1038/s41598-018-24006-w
3. Canisso, I. F., Ball, B. A., Cray, C., Squires, E. L. & Troedsson, M. H. (2015). Use of a Qualitative Horse-Side Test to Measure Serum Amyloid A in Mares With Experimentally Induced Ascending Placentitis. Journal of Equine Veterinary Science, 35(1), 54–59. DOI: http://dx.doi.org/10.1016/j.jevs.2014.11.007
4. Carlotti, D. N., Jasmin, P., Gardey, L. & Ssanquer, A. (2004). Evaluation of cephalexin intermittent therapy (weekend therapy) in the control of recurrent idiopathic pyoderma in dogs: a randomized, double-blinded, placebo-controlled study. Veterinary Dermatology, 15(s1), 1–19. DOI: http://dx.doi.org/10.1111/j.1365-3164.2004.00410_2-4.x
5. Christensen, D. L., Moulton, K., Hopper, R., Walters, F. K., Cooley, A. J., LeBlanc, M. & Ryan, P. (2010). Evidence-based medicine approach to develop efficacious therapies for late-gestation mares presenting with uterine infections using and experimentally induced placentitis model. Animal Reproduction Science, 121, 345–346.
6. Cummins, C., Carrington, S., Fitzpatrick, E. & Duggan, V. (2008). Ascending placentitis in the mare: a review. Irish Veterinary Journal, 61(5), 307–313. DOI: http://dx.doi.org/10.1186/2046-0481-61-5-307
7. Curcio, B. R., canisso, I. F., Pazinato, F. M., Borba, L. A., Feijo, L. S., Muller, V., Finger, I. S., Toribio, R. E. & Nogueira, C. E. W. (2017). Estradiol cypionate aided treatment for experimentally induced ascending placentitis in mares. Special Issue: The David E. Bartlett Award for Lifetime Achievement in Theriogenology., 9, 454. DOI: http://dx.doi.org/10.1016/j.theriogenology.2017.03.010
8. LeBlanc, M. M. (2010). Ascending placentitis in the mare: an update. Reproduction in Domestic Animals, 45, 28–34. DOI: http://dx.doi.org/10.1111/j.1439-0531.2010.01633.x
9. Macpherson, M. L., Giguere, S., Hatzel, J. N., Pozor, M., Benson, S., Diaw, M., Sanchez, L. C., Vickroy, T. W., Tell, L., Wetzlich, S. & Sims, J. (2013). Disposition of desfuroylceftiofur acetamide in serum, placental tissue, fetal fluids, and fetal tissues after administration of ceftiofur crystalline free acid (CCFA) to pony mares with placentitis. Journal of Veterinary Pharmacology Therapeutics, 36(1), 59–67. DOI: http://dx.doi.org/10.1111/j.1365-2885.2012.01392.x
10. Macpherson, M. L., Giguere, S., Pozor, M. A., Runcan, E., Vickroy, T. W., Benson, S. A., Troedsson, M. H. T., Hatzel, J. N., Larson, J., Vanden Berg, E., Kelleman, A. A., Sanchez, L. C. & LeBlanc, M. M. (2017). Pharmacokinetics of ceftiofur sodium in equine pregnancy. Journal of Veterinary Pharmacology Therapeutics, 40(6), 656–662. DOI: http://dx.doi.org/10.1111/jvp.12399
11. Murchie, T. A., Macpherson, M. L., LeBlanc, M. M., Luznar, S. & Vickroy, T. W. (2006). Continuous monitoring of penicillin G and gentamicin in allantoic fluid of pregnant pony mares by in vivo microdialysis. Equine Veterinary Journal, 38(6), 520–525. DOI: http://dx.doi.org/10.2746/042516406X156136
12. Murchie, T. A., Macpherson, M. L., LeBlanc, M. M., Luznar, S. & Vickroy, T. W. (2008). Detection of gentamicin and penicillin concentrations in allantoic fluid of pregnant pony mares by in vivo microdialysis. Havemeyer Foundation Monograph Series, 33.
13. Ryan, P., Crouch, J., Sykes, D., Moulton, K., Christiansen, D., Hopper, R., Read, R., Bennett, W. & LeBlanc, M. M. (2008). Experimentally induced placentitis in late gestation mares with Streptococcus equi zooepidemicus: prevention of pre-term birth. Havemeyer Foundation Monograph Series, 35–36.
14. Vincze, B., Baska, F., Papp, M. & Szenci, O. (2019). Introduction of a new fetal examination protocol for on-field and clinical equine practice. Theriogenology, 125, 210–215. DOI: http://dx.doi.org/10.1016/j.theriogenology.2018.11.004
15. Waldridge, B. M. & Pugh, D. G. (2001). Equine placentitis. Compendium on Continuing Education for the Practicing Veterinarian, 23, 573–575.

RCVS Knowledge was supported in producing this Knowledge Summary by an educational grant from Petplan Charitable Trust.