Mortality in horses refers to the incidence and causes of death within equine populations. Understanding mortality is important for assessing health risks, improving management practices, and guiding veterinary care. Various factors contribute to mortality in horses, including age, breed, health status, and environmental conditions. Common causes of death in horses include colic, injury, infectious diseases, and age-related conditions. Mortality rates and patterns can vary significantly depending on the population and geographic location. This page compiles peer-reviewed research studies and scholarly articles that investigate the epidemiology, causes, and trends of mortality in horses, providing insights into preventive measures and management strategies to reduce mortality rates.
Parkin TD, Clegg PD, French NP, Proudman CJ, Riggs CM, Singer ER, Webbon PM, Morgan KL.Considerable variation in the rates of equine fatality at different racecourses draws attention to probable risk factors at the level of course or race that might be partly responsible. Distal limb fractures are the most common cause of equine fatality on UK racecourses and identification of risk factors for such injuries and subsequent implementation of intervention strategies could significantly reduce the total number of racecourse fatalities. Objective: To identify race- and course-level risk factors for fatal distal limb fracture in Thoroughbreds on UK racecourses. Methods: A case-control...
Parkin TD, Clegg PD, French NP, Proudman CJ, Riggs CM, Singer ER, Webbon PM, Morgan KL.Fractures below the level of the radius or tibia (distal limb fractures) are the most common cause of equine fatality on UK racecourses; however, little is known about their epidemiology or aetiology. Identification of risk factors could enable intervention strategies to be designed to reduce the number of fatalities. Objective: To identify horse-level risk factors for fatal distal limb fracture in Thoroughbreds on UK racecourses. Methods: A case-control study design was used. Fractures in case horses were confirmed by post mortem examination and 3 matched uninjured controls were selected from...
Salazar P, Traub-Dargatz JL, Morley PS, Wilmot DD, Steffen DJ, Cunningham WE, Salman MD.To determine outcome of equids in the western United States with clinical signs of West Nile virus (WNV) infection and identify factors associated with risk of death in infected equids. Methods: Cross-sectional study. Methods: 484 equids in Nebraska and Colorado. Methods: Owners of 484 equids with laboratory-confirmed West Nile virus infection in Nebraska and Colorado were contacted by telephone, and a questionnaire was used to obtain information on signalment, management, clinical signs, date of disease onset, duration of disease, WNV vaccination status, and health status at the time of the i...
Blitvich BJ, Fernández-Salas I, Contreras-Cordero JF, Loroño-Pino MA, Marlenee NL, Díaz FJ, González-Rojas JI, Obregón-Martínez N....West Nile virus RNA was detected in brain tissue from a horse that died in June 2003 in Nuevo Leon State, Mexico. Nucleotide sequencing and phylogenetic analysis of the premembrane and envelope genes showed that the virus was most closely related to West Nile virus isolates collected in Texas in 2002.
Durand JP, Simon F, Tolou H.West Nile virus (WNV) is a common arbovirosis, transmitted by mosquitoes mainly Culex. WNV is commonly responsible for equine epizootics and epidemics in humans in sub-Saharan Africa. It has been occasionally described in Southern Europe and in some Mediterranean countries. Since 1994, WNV clinical aspects seem to change with an increase of central neurological involvement and a higher mortality, especially among people older than 50 years. In 1999, WNV reached New York, being responsible for severe clinical manifestations. It spread all over North America in less than four years Four (only fo...
Ward MP, Levy M, Thacker HL, Ash M, Norman SK, Moore GE, Webb PW.To describe an outbreak of encephalomyelitis caused by West Nile virus (WNV) in horses in northern Indiana. Methods: Case series. Methods: 170 horses. Methods: Horses with clinical signs suggestive of encephalomyelitis caused by WNV were examined. Date, age, sex, breed, and survival status were recorded. Serum samples were tested for anti-WNV antibodies, and virus isolation was attempted from samples of brain tissue. Climate data from local weather recording stations were collected. An epidemic curve was constructed, and case fatality rate was calculated. Results: The most common clinical sign...
Stephen JO, Corley KT, Johnston JK, Pfeiffer D.To determine historical, physical, and clinical factors that may affect morbidity and mortality in horses with small intestinal volvulus unrelated to other causes (e.g., incarceration, lipoma, etc.). Methods: Retrospective study. Methods: Client-owned horses (115), aged 1 month to 21 years. Methods: Data were obtained from medical records, identified by computer search and manual review. Continuous variables were compared between affected and non-affected horses with Mann-Whitney U-tests and non-continuous variables with Fisher's exact test (2 x 2 tables) or chi(2)-tests (larger tables). Stepw...
Gawrylash SK.Thyroid hyperplasia and musculoskeletal deformity, described in foals in western Canada, has not been recognized as a common cause of neonatal foal mortality in Ontario. A case is reported from Ontario, with clinical and histopathological findings consistent with the syndrome described in western Canada. Résumé — Hyperplasie thyroïdienne et déformation musculosquelettique chez une pouliche Standardbred. L’hyperplasie de la thyroïde et la déformation musculosquelettique, décrites chez les poulains dans l’ouest du Canada, n’a pas été reconnue comme cause fréquente de mortalité...
Woods LW, Filigenzi MS, Booth MC, Rodger LD, Arnold JS, Puschner B.Three horses died as a result of eating grass hay containing summer pheasant's eye (Adonis aestivalis L.), a plant containing cardenolides similar to oleander and foxglove. A 9-year-old thoroughbred gelding, a 20-year-old appaloosa gelding, and a 5-year-old quarter horse gelding initially presented with signs of colic 24-48 hours after first exposure to the hay. Gastrointestinal gaseous distension was the primary finding on clinical examination of all three horses. Two horses became moribund and were euthanatized 1 day after first showing clinical signs, and the third horse was euthanatized af...
Parkin TD, Clegg PD, French NP, Proudman CJ, Riggs CM, Singer ER, Webbon PM, Morgan KL.The risk of fatal distal limb fractures in thoroughbreds racing in the UK was calculated and shown to vary considerably between the different types of race. Flat turf racing was associated with the lowest risk (0.4 per 1000 starts) and national hunt flat racing was associated with the highest risk (2.2 per 1000 starts). The types of fracture were classified by detailed radiographic and postmortem examinations of all the cases recorded over two years, and the distribution of the different types of fracture in the five main types of racing was examined. Overall, lateral condylar fractures of the...
Borst GH, van der Weij PJ, Vos JH.Acute idiopathic gastric rupture resulted in sudden death of a 4.5-months-old healthy Friesian foal. No morphological lesions were present which could explain the preceding gastric impaction. In this case hypodypsia may have been (partially) responsible for the impaction as only a mechanical drinking apparatus, unsuitably for horses, was available.
Smith GA, Scherzer DJ, Buckley JW, Haley KJ, Shields BJ.Ninety-six children were admitted during a 9-year period to a pediatric level 1 trauma center for treatment of farm-related injuries. The age range was from 6 weeks to 17 years (median, 7.5 years; mean, 7.6 years; standard deviation, 4.4). Thirty-nine patients (40.6%) had an animal-related injury, including 36 children (37.5%) who had an injury associated with a horse. Amish children had an increased risk of horse-related injury when compared with non-Amish children (p=0.04; RR=2.09, 95% CI: 1.18<RR<3.70). Thirty-seven patients (38.5%) had farm machinery-related injuries, including 14 ch...
Hughes KJ, Dowling BA, Matthews SA, Dart AJ.To report on the outcome of surgical treatment of acute abdominal crises in miniature breed horses. Methods: Retrospective case series of miniature horses presented to the University Veterinary Centre, Camden with an acute abdominal crisis. Methods: Hospital records of all miniature horses that underwent ventral midline laparotomy for acute abdominal crisis between 1997 and 2001 were reviewed. The signalment, history, clinical signs, results of ancillary diagnostic procedures, location and type of intestinal lesion, treatment and outcome were retrieved from each case record. Long-term survival...
Marr CM.Cardiac disease and dysfunction can occur as a primary disorder(ie, with pathology situated in one or more of the cardiac structures) or can be classified as a secondary problem when it occurs in patients with another primary problem that has affected the heart either directly or indirectly. Primary cardiac problems are encountered in horses presented to emergency clinics; however,this occurs much less frequently in equine critical patients than cardiac problems arising secondary to other conditions. Nevertheless,if primary or secondary cardiac problems are not identified and addressed, they c...
Dallap BL.Disseminated intravascular coagulation (DIC) is a clinicopathologic syndrome resulting from a multitude of underlying causes that manifests itself clinically as hemostatic/fibrinolytic failure. There is much debate on the definition, diagnosis, and treatment of DIC, a situation that is most likely the result of the multifaceted clinical presentation of the syndrome and the fact that patient outcome is often influenced by the underlying disease process. The fact that DIC increases morbidity and mortality in critical care patients is well established, but the exact mechanism of what specifically...
Southwood LL.Horses with large colon volvulus (LCV) often require intensive postoperative medical management. Although early referral and surgical intervention are critical for minimal postoperative complications and a successful outcome, this is often not possible for many reasons. Endotoxemia, extensive mucosal damage with subsequent severe hypoproteinemia, and focal ischemic necrosis are the major causes of postoperative morbidity and mortality. Although fluid therapy, flunixin meglumine, and antimicrobials are still the mainstay of perioperative management, synthetic colloids are being used to maintain...
Allison N, Moeller RB, Duncan R.A teratocarcinoma was diagnosed in the amnion of a 5-year-old Arabian mare that delivered a healthy, full-term foal. The foal died at 2.5 months of age as a result of metastasis of an undifferentiated component of the mass. This case is unique because it is the first reported case of placental teratocarcinoma in animals and the malignant component apparently metastasized to the foal resulting in its death.
Kirkpatrick JF, Turner A.Despite a large body of safety data, concern exists that porcine zonae pellucidae (PZP) immunocontraception--used to manage wild horse populations--may cause out-of-season births with resulting foal mortality. Our study at Assateague, Maryland indicated the effects of immunocontraception on season of birth and foal survival between 1990 and 2002 on wild horses from Assateague Island. Among 91 mares never treated, 69 (75.8%) of foals were born in April, May, and June (in season). Among 77 treated mares, 50 (64.9%) were born in season. Of 29 mares foaling within 1 year after treatment (contracep...
Johnston GM, Eastment JK, Taylor PM, Wood JL.Approximately 1 in 100 horses suffer unexpectedly from anaesthetic-related death. Identification and use of the safest anaesthetic drugs should support this aim. Experimental evidence has suggested that isoflurane should be a safer maintenance agent in equine anaesthesia than halothane. Objective: The death rate would be reduced in horses being maintained with isoflurane compared to halothane. Methods: A multicentre randomised controlled trial was undertaken to compare the effects of isoflurane and halothane for maintenance of equine anaesthesia for all types of operation. Data were analysed f...
Blikslager AT.Ischemic injury is one of the most important causes of mortality in equine veterinary medicine. Although treatment of reperfusion injury has been attempted in a number of experimental trials to reduce the level of injury subsequent to an ischemic episode, this research has not resulted in the development of useful clinical treatments. Nevertheless, recent studies assessing intraluminal application of solutions containing antioxidants, nutrients, and vasodilators are promising. Furthermore, focusing on improving mucosal recovery after an ischemic event may provide an alternative method of reduc...
Davis JL.In summary, peritonitis in the horse is a potentially life-threatening disease that must be treated promptly and aggressively. Therapy should be aimed at reducing systemic shock and hypovolemia, correction of the primary cause, antibiotic and anti-inflammatory therapy, and abdominal drainage and lavage. The prognosis depends on the ability to diagnose and treat the underlying cause and prevent the development of complications. Mortality rates can be as high as 59.7%, with horses developing postoperative peritonitis having a 56% mortality rate. Long-term complications like adhesion formation or...
Stephen JO, White NA, McCormick WH, Cowles RR, Corley KT.To identify the types of injuries sustained by horses that competed in steeplechase races and determine the prevalence of and risk factors for those injuries. Methods: Prospective study. Methods: 2,680 horses that competed in various types of steeplechase races from 1996 through 2000. Methods: Data regarding races; environment; equipment problems; the number of horses that entered, started, and finished races; the number of riders that fell; and the number of horses that were slowed or stopped by the rider, ran off the course, fell, and sustained injuries or physical abnormalities during races...
Johansson AM, Gardner SY, Jones SL, Fuquay LR, Reagan VH, Levine JF.This study was initiated to identify the signalment and clinical variables potentially associated with hypomagnesemia in horses evaluated at the North Carolina State University, College of Veterinary Medicine (NCSU-CVM) veterinary teaching hospital between January 1999 and May 2001. A nested case reference study (nested case-control study) was conducted to examine the potential relationship between hypomagnesemia and signalment, serum chemistry panel analyses, number of hospitalization days, discharge status, and diagnosis. A series of independent and multivariable logistic regression models w...
Wilkins PA, Palmer JE."Shaker foal" disease, toxicoinfectious botulism of foals, was 1st described as a clinical entity in 1967. The reported mortality rate was 90%, with death occurring within 24-72 hours of the onset of the characteristic clinical signs. The mortality rate decreased when equine-origin botulism antitoxin became available; however, a certain percentage of foals continued to die of respiratory failure. Mechanical ventilation is an important part of the treatment of infant botulism and is essential to the survival of many affected infants. We report a retrospective study of 9 foals with toxicoinfecti...
Wilkins PA, Palmer JE.Botulism has been recognized as a clinical entity in foals since the 1960s. Also known as "Shaker foal" disease, the toxicoinfectious form of botulism affects foals, with the highest incidence in the United States seen in Kentucky and the mid-Atlantic region. The disease is characterized by progressive muscular weakness caused by the action of botulism neurotoxin at cholinergic neuromuscular junctions. Increased number of episodes and duration of recumbency, muscular trembling, and dysphagia are seen in affected foals. Left untreated, the disease can be rapidly fatal, with death occuring secon...
Welland LM.A 6-year-old, primiparous standardbred mare was presented with a history of intestinal protrusion from the rectum shortly after parturition. A subacute grade IV rectal tear was diagnosed. The unusual nature of the tear led to speculation of a possible owner palpation accident. A grave prognosis was given and the mare was euthanized.
Durham AE, Smitht KC, Newton JR, Hillyer MH, Hillyer LL, Smith MR, Marr CM.The study was designed to investigate the prognostic value of liver biopsy during investigation of cases of suspected liver disease in mature horses. Objective: That liver biopsy is the most prognostically useful technique in common usage in the investigation of suspected liver disease. Methods: This study examined the prognostic value of liver biopsy during the investigation of suspected liver disease in 73 mature horses. Histopathological variables comprising fibrosis, irreversible cytopathology, inflammatory infiltration, haemosiderin accumulation and biliary hyperplasia were found to be si...
Northey G.The aims of this study were to investigate the extent of equestrian injuries in New Zealand and provide a range of prevention interventions. Methods: An examination of New Zealand Health Information Service (NZHIS) morbidity data for 1993 to 2001 and mortality data for 1993 to 1999 was undertaken. Recent studies on equestrian injuries were evaluated. Results: NZHIS data indicated that as a result of horse-related injuries 5613 people were hospitalised between 1993 and 2001 and there were 16 fatalities between 1993 and 1999. Horse-related injuries were most prevalent in young females aged 10 to...
Perryman LE, McGuire TC, Banks KL.This research explores X-linked agammaglobulinemia in horses, a severe immune deficiency found in various horse breeds, leading to clinical signs like pneumonia and arthritis. Lymphoid tissues show an absence of […]
Byard RW.A search was undertaken of the Forensic Science South Australia, Australia, autopsy database over a 25-year period from 1995 to 2019 with the aim of understanding more about the role that recreational horse use not involving racing may have in mortality. Eight cases were found with an age range of 8-73 years (mean 47 years) and a male:female ratio of 5:3. The major category was a fall while riding (N = 7) with the remaining case (N = 1) sustaining a lethal kicking. Blunt craniocerebral trauma was responsible for death in 4 cases with injuries following falls including acute subdural and ...
Ruggerone B, Paltrinieri S, Giordano A, Scavone D, Nocera I, Rinnovati R, Spadari A, Scacco L, Pratelli P, Sgorbini M.In several species, paraoxonase-1 (PON-1) decreases during inflammation, because of the presence of oxidative stress; its measurement recently has been validated in horses, but its role as a clinical biomarker is unknown. Objective: To evaluate sensitivity, specificity and likelihood ratio of PON-1 activity to identify systemic inflammatory response syndrome (SIRS)-positive horses or horses with a poor prognosis. Methods: One hundred seventy-two blood samples from 58 sick horses from 3 different veterinary hospitals. Methods: In a cross-sectional study, PON-1 activity was measured upon admissi...
Giles C, Ndi O, Barton MD, Vanniasinkam T.Rhodococcus equi is a respiratory pathogen which primarily infects foals and is endemic on farms around the world with 50% mortality and 80% morbidity in affected foals. Unless detected early and treated appropriately the disease can be fatal. Currently, there is no vaccine available to prevent this disease. For decades researchers have endeavoured to develop an effective vaccine to no avail. In this study a novel human adenoviral vector vaccine for R. equi was developed and tested in the mouse model. This vaccine generated a strong antibody and cytokine response and clearance of R. equi was d...
Robinson B, Gummow B.A disease outbreak investigation was conducted in western Queensland to investigate a rare suspected outbreak of pyrrolizidine alkaloid (PA) toxicosis in horses. Thirty five of 132 horses depastured on five properties on the Mitchell grass plains of western Queensland died in the first six months of 2010. Clinical-pathological findings were consistent with PA toxicosis. A local variety of Crotalaria medicaginea was the only hepatotoxic plant found growing on affected properties. Pathology reports and departure and arrival dates of two brood mares provided evidence of a pre wet season exposure ...
Davis DM, McClure JR, Bertone AL, Cazayoux CA, Vice JD.A 16-year-old stallion was presented to the Louisiana State University Veterinary Teaching Hospital for evaluation of acute abdominal pain. Physical examination and diagnostic procedures indicated a strangulating obstruction of the small intestine. At exploratory celiotomy, a strangulating incarceration of the jejunum through the epiploic foramen was found. The incarcerated small intestine was reduced, then resection of the nonviable bowel and anastomosis performed. After surgery, the horse exhibited clinical signs and laboratory findings associated with hypoglycemia and died in spite of emerg...
Studdert MJ.Set in a context of immunodeficiency diseases in general this paper provides a brief, illustrated review of a primary, severe, combined immunodeficiency (PSCID) disease of Arabian foals. Affected foals are clinically normal at birth but beginning at about 10 days of age they develop a range of clinical signs particularly bronchopneumonia and diarrhoea with which adenoviruses are peculiarly associated. Despite intensive therapy foals invariably die by about 3 months of age. Affected foals are profoundly lymphopagenic (greater than 1000 lymphcoytes per mm3). There is thymic and lymph node hypopl...
van Galen G, Delguste C, Sandersen C, Verwilghen D, Grulke S, Amory H.Few studies exist about factors affecting the outcome of horses with tetanus. Methods: 31 equids (30 horses and 1 donkey) with a clinical diagnosis of tetanus admitted to the Equine Clinic of the University of Liege between 1991 and 2006. The cases were divided into two groups according to the outcome (survivors and non-survivors). Methods: The clinical data of survivors and non-survivors were compared using an ANOVA (continuous data) or a Fisher's test (discrete data). Results: The survival rate was 32%. Most animals were 5 years or younger, and none had been appropriately vaccinated. The non...
Dorman DC, Haschek WM.Toxicosis attributable to propylene glycol (1,2-propanediol) was suspected in an 8-year-old 450- to 500-kg male Quarter Horse. Clinical signs of toxicosis developed within 15 minutes of the accidental iatrogenic oral administration of 3.8 L of propylene glycol. Clinical signs of toxicosis included salivation, sweating, ataxia, and signs of pain. Additionally, at 24 hours after propylene glycol ingestion, the horse became increasingly atactic, had an abnormal breath odor, developed rapid shallow breathing, and was cyanotic. The horse died of apparent respiratory arrest 28 hours after the propyl...
Smanik LE, Moser DK, Rothers KP, Hackett ES.Prospective clinical study of blood lactate concentration in horses undergoing colic surgery is needed to determine utility in outcome prediction. Objective: To evaluate venous lactate measurements in horses following colic surgery, including immediately after anesthetic recovery and daily throughout hospitalization, as well as to determine if lactate concentrations were significantly higher in horses that developed postoperative complications or did not survive to hospital discharge. Methods: Horses > 1 year of age undergoing surgery for colic and recovered from general anesthesia were sample...
Gitari A, Nguhiu J, Varma V, Mogoa E.The aim of this study was to determine the treatments and their outcomes in horses with colic in Nairobi County, Kenya. Methods: This is a retrospective study to determine the occurrence, treatments, pain management, and outcomes of colic in horses in Nairobi County. Association between pain management protocols and the outcomes of colic with regard to recovery or death was also determined. Data collected from four equine practitioners were organized manually and given numerical codes as appropriate to facilitate entry into the computer. The coded data were entered into Microsoft Excel 2010 an...
Findley JA, Salem S, Burgess R, Archer DC.Relaparotomy may be required to investigate and manage complications that occur following surgical management of colic. Objective: To report factors associated with survival following relaparotomy. Methods: Retrospective cohort study. Methods: Records of horses that had undergone exploratory laparotomy for treatment of colic over a 10-year period (2002-2012) and had undergone relaparotomy <8 weeks following the initial surgery were reviewed. Descriptive data were generated and association with survival time was modelled using Cox proportional hazards models. Results: Relaparotomy was perfor...
Gift LJ, Gaughan EM, DeBowes RM, Pintchuk PA, Nickels FA, Foreman JH.Medical records of 11 adult horses with jejunal intussusception examined at 5 veterinary teaching hospitals between 1981 and 1991 were reviewed. Nine of 11 horses had signs of acute abdominal discomfort for < 24 hours, whereas 2 horses had a history of chronic signs. Five of 11 horses had an intraluminal or intramural mass associated with the jejunal intussusception. Two horses died or were euthanatized prior to surgery. Partial jejunal resection and jejunojejunal anastomosis were performed in 9 horses. One horse died during surgery and 2 were euthanatized prior to hospital discharge because o...
Littlejohn A.The mortality following operations for intestinal obstructions in horses decreased from 80 per cent during the period 1849-1913 to 22 per cent during the period 1934-1964. Mortality following operations for high intestinal obstructions was greater than for low intestinal obstructions, and the mortality following operations for strangulating obstructions was more than twice as great as the mortality following simple obstructions. The most frequent cause of death following operations was failure to correct the causal condition, or subsequent impaction.
The anaesthesia techniques most frequently...
Nielsen IL, Jacobs KA, Huntington PJ, Chapman CB, Lloyd KC.Adverse reactions to intramuscular injections of procaine penicillin G are reported in 11 horses, five of which died. The clinical findings are presented and suggest central nervous involvement in most cases. Post mortem findings in one horse were consistent with anaphylaxis whereas in other cases the clinical findings, duration of treatment, speed of onset and subsequent completion of treatment supports diagnosis of an acute procaine toxicity syndrome.
Bentz KJ, Burgess BA, Lohmann KL, Shahriar F.A 4-year-old Belgian mare was presented with a 1-week history of fever, suspected of being caused by peritonitis. The mare died before the diagnostic procedures had been completed; postmortem examination revealed torsion of the left medial lobe of the liver, resulting in diffuse necrosis of liver tissue and severe peritoneal effusion. Une jument belge de 4 ans est présentée avec une anamnèse de fièvre qui persiste depuis 1 semaine, que l’on soupçonne causée par une péritonite. La jument est morte avant l’achèvement des procédures diagnostiques; l’autopsie a révélé une torsio...
Mair TS, de Westerlaken LV, Cripps PJ, Love S.Samples of faeces and blood were obtained from 66 adult horses with diarrhoea. The results of routine bacteriological, parasitological, haematological and biochemical tests were correlated with the outcome of the cases. Twenty-two (33 per cent) of the horses died or were destroyed as a consequence of the diarrhoea. A diagnosis was reached in only 23 cases (35 per cent), and in nine of them only at post mortem examination. Salmonella typhimurium was isolated from five cases. Statistical analysis revealed significant differences between the horses which survived and those which died in their pac...
Scherrer NM, Lassaline-Utter M, McKenna BC.To determine the histologic findings associated with masses in the nictitating membranes (third eyelids) of horses and to identify prognostic factors associated with recurrence and the outcome. Methods: Retrospective case series. Methods: 50 horses with masses in a third eyelid. Methods: Medical records of horses with partial or complete excision of a third eyelid and submission for histologic evaluation between September 1998 and November 2012 were reviewed. Horses were included if follow-up information for at least 1 year after surgery was available. Information regarding signalment and trea...
Duarte PC, Conrad PA, Barr BC, Wilson WD, Ferraro GL, Packham AE, Carpenter TE, Gardner IA.The study objective was to assess the risk of transplacental transmission of Sarcocystis neurona and Neospora hughesi in foals from 4 California farms during 3 foaling seasons. Serum of presuckle foals and serum and colostrum of periparturient mares were tested using indirect fluorescent antibody tests for S. neurona and N. hughesi. Serum antibody titers were < or =10 in 366 presuckle foals tested. There was no serologic or histologic evidence of either parasite in aborted fetuses or placentas examined. Positivity for S. neurona and N. hughesi in mares increased with age. Mares < or =9 y...