Viral Diarrhea

Equine Rotavirus

Diarrhea is a frequently encountered medical problem of newborn foals, and rotavirus is the most common cause of foal enteritis in major breeding centers of the United States, Ireland, and England, as well as other countries.1, 2 In a recent study evaluating prevalence of infectious agents in foals with gastrointestinal (GI) disease, rotavirus was the most common agent found in the feces of sick foals (35% versus 3% in healthy foals, p = 0.0002) using polymerase chain reaction (PCR).³ Both single cases of rotaviral diarrhea and severe farm outbreaks can occur. However, with the use of a commercially available vaccine, practical farm management practices, and hygiene measures, this disease can be controlled.

Etiology

The family Reoviridae consists of five genera: Orthoreovirus, Orbivirus, Coltivirus, Aquareovirus, and Rotavirus. These are all double-stranded, ribonucleic acid (RNA), nonenveloped viruses with a diameter of about 80 nm (Fig. 18-1 ). The genus Rotavirus is subdivided into several groups (A-G) based on differences in the group-specific inner capsid protein, VP6. Equine and other animal isolates are in group A; groups B to G cause disease in humans, swine, fowl, and other animals.⁴ Further subdivision is made based on serologic assays using neutralizing antibodies to two outer capsid proteins, VP4 and VP7. Strains containing VP4 are referred to as P (protease-sensitive) serotype and strains containing VP7 as the G (glycoprotein) serotype, with many containing both.⁵ Strains are named based on the G and P genotypes. At least 7 G serotypes (G3, G5, G6, G8, G10, G13, and G14) and 6 P serotypes (P1, P3 P7, P11, P12, and P18) have been described in horses.5, 6, 7, 8, 9, 10 The G3 serotype has been identified in Kentucky and Japan and has been used in equine vaccine trials.11, 12, 13 The G3P[12] and G14P[12] serotypes are among the most prevalent in foals and are the primary strains circulating in horse populations in many countries.6, 14, 15, 16 The G3 type of equine rotavirus has two associated subtypes: G3A and G3B.

Figure 18-1.

Electron micrographs of rotavirus. A, Particles magnified at ×135,000. B, Single particle magnified at ×300,000.

(Courtesy Ms. Patricia Van Meter, University of Kentucky.)

Rotaviruses are stable within a pH range of 3 to 7 and are resistant to iodophor, quaternary ammonium, chlorine, and hypochlorite (bleach) disinfectants. Ethanol, phenols (efficacy varies with the specific product), and formalin can inactivate the virus.17, 18, 19 Peroxygen and accelerated hydrogen peroxide disinfectants are also effective against rotavirus.

Epidemiology

Equine rotaviruses have been detected from diarrheic foals in many countries, including the United States, Argentina, Britain, Ireland, Germany, Australia, New Zealand, South Africa, and Japan. In a retrospective study of foals with diarrhea presented to the University of Florida, rotavirus was the most frequently detected infectious agent (20% of cases).²⁰ Equine rotaviruses only cause clinical disease in foals and are considered primarily species specific; for example, cattle rotavirus affects cattle and not horses or humans. However, a recent report suggested that an unusual equine strain (H-1) may have originated from a porcine rotavirus strain that was transmitted to horses because at least 9 of 11 of its gene segments were found to be of porcine virus origin.²¹ No natural reservoir for equine rotavirus has been identified. Mares may shed the virus subclinically, although this is transient and not considered to be as significant in magnitude as from diarrheic foals.²² Considering the large concentrations of virus shed into the environment (10¹¹ particles/g feces) by diarrheic animals, as well as the ability of the virus to remain viable for as long as 9 months,²³ the potential for an outbreak after the first clinical case is very real.

Rotavirus is transmitted by the fecal-oral route through contaminated feces or fomites and is highly contagious. The incubation period is 12 to 24 hours. Adult horses are not clinically affected during outbreaks of foal diarrhea, but some mares with diarrheic foals will seroconvert, indicating subclinical infection.²² Studies of more than 400 adult horses performed in conjunction with rotavirus vaccination trials in Kentucky, Japan, and Argentina revealed a seroprevalence rate in broodmares approaching 100%.11, 12, 13 However, these studies were conducted in concentrated horse-breeding regions, and seroprevalence in adult horses in other geographic areas may be less.

The average age of foals with rotaviral diarrhea was 75 days, with a reported range of 2 to 155 days in one study.¹ In a retrospective study of referred cases, foals ≥1 month of age were significantly more likely to have rotavirus as compared to foals less than or equal to 1 month of age (odds ratio [OR] 13.3, 95% confidence interval [CI] = 5.3-33, p <0.00001).²⁰ In this study, the mean age of foals positive for rotavirus was 81 days (range 2-253) days. The results of these two studies are in contrast to those of a third study, which found that most diarrheic foals with rotavirus were 5 to 35 days of age, and the majority were in the younger age range.²⁴ The average duration of diarrhea in affected foals is 3 days (range, 1-9 days), with fecal shedding of rotavirus particles continuing for an average of 3 days after return of normal feces.

With appropriate supportive care, including fluid and electrolyte replacement therapy, rotaviral diarrhea has a low mortality and high morbidity in foal populations. In one study from a referral center, the survival rate of foals with rotaviral diarrhea was 94%.²⁰ During a 3-year study of rotavirus on multiple central Kentucky horse farms, no foal had a confirmed recurrence of disease after recovery. In another study, fecal shedding of virus was demonstrated in clinically normal foals on 7 of 10 farms undergoing an outbreak of foal rotaviral diarrhea.1, 25 This shedding does not continue after resolution of disease in other foals on the farm.

Pathogenesis

After rotavirus enters the GI tract, it invades and rapidly multiplies in columnar epithelial cells of the villous tips in the duodenum and jejunum.²⁴ This results in significant blunting of the villi and subsequent villous atrophy. The loss of these epithelial cells results in the absence of disaccharidases, especially lactase, causing transient lactose intolerance and a hyperosmotic solution in the intestinal lumen. This leads to malabsorption and maldigestion of nutrients and acute diarrhea. Intestinal crypt cells are not affected as they are in canine parvovirus infection. Therefore crypt cells continue to replicate, differentiate, and eventually replace the tip cells destroyed by the virus, resulting in a self-limiting disease. Chronic diarrhea (>14 days) is not typical of rotavirus infection.

Clinical Findings

The severity of rotavirus infection varies, depending on the foal's age and immune status, the virulence of the virus, and the quantity of viral inoculum. Foals less than 3 months of age are most severely affected, with neonates exhibiting the highest morbidity and mortality. Approximately 18 to 24 hours after ingestion of infective material, foals show signs of lethargy, decreased suckling, and diarrhea that may vary from “cow pie” to watery consistency. With watery diarrhea, the foals' tails may not be wet or stained with feces because of the projectile nature of the diarrhea. Fever may or may not be present. Anorexia and abdominal pain may also be present.

During farm outbreaks of rotavirus, foals may show severe diarrhea, dehydration, anorexia, fever, and lethargy within 10 days of birth. Younger foals are often more susceptible to severe disease because of their limited ability to self-correct fluid and electrolyte imbalances that accompany severe diarrhea (Fig. 18-2 ). Electrolyte imbalances may include hypochloremia, hyponatremia, hypokalemia, and acidosis. The hemogram results with rotaviral diarrhea vary and may be normal or reveal evidence of hemoconcentration (e.g., increased packed cell volume) and leukopenia.²⁶ With treatment, the mortality rate of rotaviral diarrhea is low. In one retrospective study of diarrheic foals presented to the University of Florida, 44/47 (94%) of foals with rotavirus diarrhea survived to discharge from the hospital.²⁰

Figure 18-2.

Young foals can develop life-threatening dehydration and electrolyte imbalances from rotavirus infection, requiring intensive care.

Diagnosis

Field Testing Procedures

Although electron microscopy (EM) for detection of viral particles in feces is widely considered to be the “gold standard” for rotavirus diagnosis, this test is not routinely available and is relatively expensive, and the turnaround time for results may be up to several days.

Commercial diagnostic assays are based on detection of VP6, the most abundant protein in viral particles.⁵ Because VP6 is highly conserved across rotaviruses that affect many species, human rotavirus test kits are routinely used in equine practice and veterinary diagnostic laboratories. A fecal sample (1-3 g) or fecal swabs from affected foals should be submitted for rotavirus antigen testing by latex agglutination, immunoassay (enzyme-linked immunosorbent assay [ELISA]),27, 28 or other diagnostic methods described later. Samples that can be tested within 8 hours should be held at room temperature or refrigerated. Veterinarians should contact testing laboratories for recommendations regarding storage and shipment (cooled versus frozen) of samples if testing cannot be performed within 8 hours of sample collection.

Results from latex agglutination rotavirus test kits are available (Virogen Rotatest, Wampole Laboratories, Princeton, NJ) within 10 to 15 minutes of processing the sample (Fig. 18-3 ). The Virogen Rotatest has 100% sensitivity and 96.3% specificity compared with ELISA for diagnosis of bovine rotavirus infection.²⁹ Compared with EM for identification of viral particles in pediatric stool samples, the Virogen Rotatest has a sensitivity of 86% and specificity of 95%.³⁰

Figure 18-3.

Latex agglutination test (Virogen Rotatest) showing homogenous fluid on negative sample (1, left) and agglutination of particles and clearing of fluid in rotavirus-positive sample (2, right).

The ImmunoCard STAT! rotavirus test is an immunogold-based, horizontal-flow membrane assay that yields results within 10 minutes. It had a sensitivity of 94% and specificity of 100% in one study of pediatric stool samples³¹ and has been found useful for rapid diagnosis of bovine rotavirus infection.²⁷

The Dipstick “Eiken” ROTA Dipstick immunochromatography assay was shown to have a sensitivity of 81.9% and specificity of 98.2% as compared to reverse transcription–PCR (RT-PCR) in one study.³²

In another study, there was very good agreement (kappa = 0.88, 95% CI = 0.81-0.96) between detection of rotavirus by ELISA (Sure-Vue Rota Test, SA Scientific Ltd, San Antonio, TX) and by EM, the latter widely regarded as the “gold standard” for diagnosis.²⁰ Using EM as the reference, rotavirus ELISA had a sensitivity of 91%, specificity of 98%, and accuracy of 96%.²⁰ Because none of these assays has a sensitivity of 100%, a negative result should not rule out rotavirus. If suspicion is high despite a negative result, then the test should be repeated. Alternatively, a test with higher sensitivity, such as PCR, should be used.

Polymerase chain reaction (reverse transcription; RT-PCR) of feces recently has become commercially available for use in foals.3, 33 Until more data correlating PCR results with clinical disease are available, PCR initially should be coupled with ELISA or latex agglutination during an outbreak. After the first case has been diagnosed, PCR alone can be utilized to detect additional cases and foals that are shedding virus subclinically.

Other Diagnostic Options

Other laboratory methods for diagnosis of rotaviral diarrhea include polyacrylamide gel electrophoresis³⁴ and various molecular techniques,33, 35 all using fecal samples. Rotaviruses are extremely difficult to grow in cell culture, so virus isolation is impractical for clinical use.⁴ Serology for diagnostic purposes in foals is unreliable.²²

As with testing feces for Salmonella, a single negative test result is not conclusive for the absence of rotavirus infection. In the author's experience, a minimum of three negative test results on samples properly obtained and stored before testing gives confidence in stating that a foal does not have rotaviral diarrhea.

A separate fecal sample should be obtained to rule out other causes of acute foal diarrhea, including Salmonella, Clostridium difficile or perfringens, coronavirus, Cryptosporidium, Giardia, and parasites as well as coinfection with these agents. Although concurrent infection with rotavirus and other pathogens, such as coronavirus, is possible, several studies have concluded that rotavirus infections in foals often occur without simultaneous infection with other potential pathogens.1, 2

Pathologic Findings

Mortality is low in rotavirus infections; foals less than 14 days of age are at highest risk of death. In foals that die from dehydration and electrolyte imbalances, epithelial cells at the tips of intestinal villi are destroyed in the duodenum and occasionally the jejunum. The infection produces little inflammatory response in surrounding tissue.⁴

Therapy

Isotonic fluid therapy to correct electrolyte imbalances is especially important in young foals and those that are significantly dehydrated. Adsorbents and protectants, such as bismuth subsalicylate (0.5-1 mL/kg orally [PO] 2-4 times/day), di-tri-octahedral smectite paste (0.6 mL/kg PO 2 times per/day), and activated charcoal (0.25-0.5 g/kg PO once to once daily) for a few days may help in binding toxins and firming the feces, although owners should be aware that overuse can cause constipation. In addition, smectite and activated charcoal should be staggered with other oral medications to avoid nonspecific binding. Feces of foals treated with bismuth subsalicylate may appear dark in color, similar to the color observed with proximal GI tract hemorrhage. Doses of bismuth subsalicylate should be conservative because foals with compromised GI tracts may absorb salicylate and be subject to systemic effects.

In severely affected animals, parenteral nutrition may be indicated.26, 36, 37 Parenteral nutrition allows for “bowel rest” by preventing foals from nursing for a 12- to 24-hour period if diarrhea is severe.

Because of the blunting effects of rotavirus on the small intestinal villi, exogenous lactase should be supplemented in foals with rotaviral diarrhea. A dose of 6000 Food Chemicals Codex U/50-kg foal (120 U/kg), PO every 3 to 8 hours has been recommended.

For uncomplicated rotavirus diarrhea in older foals (>30 days), antibiotics may not be indicated in all cases unless the animal is highly compromised or is in danger of septicemia, in which case broad-spectrum parenteral antibiotics should be used. The presence of neutropenia or fever warrant antibiotic use in these cases. Foals less than 30 days of age routinely should be treated with broad-spectrum antibiotics because of a high prevalence of bacteremia in foals with diarrhea/enteritis (up to 50% at admission in one study).³⁸ Foals with failure of passive transfer or with hypoproteinemia secondary to diarrhea may benefit from replacement with intravenous (IV) plasma transfusion.³⁶

Foals with rotavirus diarrhea may present with mild signs of colic. Pain may be controlled with appropriate doses of nonsteroidal antiinflammatory drugs (NSAIDs) such as flunixin meglumine (e.g., 0.25 mg/kg every 12 hours [q12h] or 0.5-1 mg/kg IV q24h) or butorphanol (0.02-0.05 mg/kg intramuscular [IM] q4-6h). Respiratory rate and character should be monitored if butorphanol is used, as depression may be a side effect. Frequent or prolonged administration or inappropriately high doses of flunixin meglumine increase the risk for gastric ulceration and renal disease in foals.³⁷ Serum creatinine and albumin or total protein concentrations should be monitored in foals being administered flunixin meglumine.

Diarrhea is a significant risk factor for the development of gastric ulcers in foals.³⁹ Prophylactic treatment with proton-pump blockers (e.g., omeprazole 2-4 mg/kg PO q24h or pantoprazole 1.5 mg/kg IV diluted q24h) is recommended for most affected foals. Omeprazole oral paste (GastroGard, Merial, Duluth, GA) facilitates healing of equine gastric ulcers and is approved for treatment of foals greater than or equal to 4 weeks of age but has been studied and can be used in younger foals.40, 41 Other antiulcer medications include H2 antagonists such as ranitidine (6.6 mg/kg PO q8h), famotidine (2.8 mg/kg PO q12h), or cimetidine (12-20 mg/kg PO q8h). However, ranitidine has exhibited a blunted duration of alkalinizing response in clinically ill neonatal foals.⁴² Ranitidine has some prokinetic properties and should be used with caution in foals with dysmotility. Sucralfate can also be used (20 mg/kg PO q6-8h) to potentially promote ulcer healing. Because sucralfate may inhibit the absorption of other oral medications, it should not be administered at the same time as other drugs.⁴³

Supportive care and hygiene are critically important for foals with rotaviral diarrhea to avoid pressure ulcers, scalded perianal skin, and secondary infections. Foals recumbent in soiled surroundings are predisposed to secondary problems such as infected skin wounds and pneumonia. Stalls should be kept as dry and clean as possible and should be heavily bedded. To prevent dermatitis between the hind legs and in the perianal area, baby oil, baby powder, zinc oxide ointment, or commercial diaper rash ointments may be applied. Should the skin become compromised, the area should be cleaned and an antibiotic ointment applied.

Nitazoxanide (NTZ) has been studied for use in human children with rotaviral diarrhea. It is labeled for treatment of diarrhea caused by Cryptosporidium and Giardia infections in humans older than 1 year of age. A randomized clinical trial showed that NTZ decreased the duration of rotaviral diarrhea in hospitalized pediatric human patients.⁴⁴ However, diarrhea and colitis are risks associated with NTZ use in horses, and it should therefore be used with extreme caution until further studied.

Prevention

Vaccination

Since 1996, the Equine Rotavirus Vaccine (Pfizer Animal Health, Madison, NJ) has been marketed under conditional license (U.S. Department of Agriculture [USDA]). The vaccine contains an inactivated strain of a G3 equine rotavirus serotype (H2 strain or G3P[12]) in a metabolizable oil-in-water adjuvant.¹¹ This vaccine is administered intramuscularly to pregnant mares at 8, 9, and 10 months of gestation to heighten colostral immunity. Each pregnancy requires revaccination with 3 doses. This vaccine has been utilized extensively in equine intensive locales in central Kentucky, Florida, Newmarket in the United Kingdom (UK), and other major breeding centers in the United States, UK, and Ireland. Antirotaviral antibodies are concentrated in colostrum and absorbed by suckling foals.11, 12 In one study, foals of 100 vaccinated and 65 unvaccinated mares were studied. Vaccinated mares had received an inactivated rotavirus (G3P2, G3P12, and G6P1) vaccine at 9 and 10 months of gestation. The morbidity of diarrhea was 30% in foals born to vaccinated mares, in contrast to 80% in foals from unvaccinated mares. Duration of diarrhea was shorter in foals in the vaccinated mare group (1.8 days versus 7.3 days). Importantly, there was 0% shedding detected in the diarrheic foals born to vaccinated mares, as opposed to 80% shedding in the diarrheic foals of unvaccinated mares.⁴⁵ In a second study of 627 mares over 2 years, the vaccine was administered at 8, 9, and 10 months of gestation in a randomized controlled trial. The vaccine was determined to be safe, and antibody titers were significantly higher in foals born to vaccinated mares for up to 90 days postpartum. The incidence of rotaviral diarrhea was decreased in the foals of the study group as compared to controls; however, this was not statistically different. Although beneficial, vaccination should be considered an adjunctive to farm hygiene and meticulous husbandry practices, rather than a replacement for them.

Husbandry/Infection Control

Because rotaviruses are excreted from diarrheic foals in large concentrations (up to 10¹¹ particles/g), overcrowding is a significant risk factor for outbreaks of foal diarrhea. Rotavirus can survive for months in the environment.⁴ Manure and bedding from stalls of affected foals should be considered a biosecurity threat to unaffected foals. This material should not be spread on pastures but rather composted in an area isolated from horses or disposed of according to local ordinances.

Hygiene of foaling areas is critical for disease prevention. Affected foals should be isolated and released when fecal shedding stops (through repeat fecal PCR testing). Because rotavirus is a nonenveloped virus, it has natural resistance to disinfectants that primarily disrupt the viral lipid envelope. Prevention and control of outbreaks of equine infectious diseases are discussed in Chapter 63. Peroxygen, accelerated hydrogen peroxide, and some phenolic disinfectants have efficacy against rotaviruses.

Public Health Considerations

Rotavirus is not regarded as a zoonotic disease because of species specificity. However, standard barrier precautions of disposable gloves, disinfectant foot baths, use of dedicated equipment, and hand washing should be taken with any equine diarrheic disease.

Equine Coronavirus

Coronaviruses cause a variety of GI and respiratory diseases in a number of veterinary species. Although coronavirus-induced enteritis has been suspected in foals with diarrhea, direct pathogenicity of equine coronavirus (ECoV) in equids has not been definitively demonstrated until recently.46, 47 Traditionally regarded as a pathogen of foals only, investigators have now documented the presence of ECoV in adult horses with fever and enteric disease.⁴⁸

Etiology

Coronaviruses are members of the Coronaviridae family, order Nidovirales, all of which are positive-sense RNA viruses.49, 50, 51, 52 The family Coronaviridae has two subfamilies: Coronavirinae and Torovirinae. The subfamily Coronavirinae contains three genera: Alphacoronavirus, Betacoronavirus, and Gammacoronavirus. Equine coronavirus is a Betacoronavirus similar to human coronavirus OC43, human enteric coronavirus, bovine coronavirus, and canine respiratory coronavirus.⁵³ The coronaviruses were so named because the unusually large, club-shaped peplomers projecting from the envelope give the viral particle the appearance of a solar corona.50, 52, 54 The tubular nucleocapsid is composed of a phosphorylated nucleoprotein and appears to be connected directly to a transmembrane protein, M, which spans the lipid bilayer three times. Only a small fraction of its mass is exposed to the external environment.50, 52, 54, 55 Two types of prominent spikes line the outside of the virion. The long spikes, which consist of the S (spike) glycoprotein, are present on all coronaviruses and give them their characteristic “corona” appearance. The short spikes, which consist of the hemagglutinin-esterase (HE) glycoprotein, are present in only some coronaviruses.50, 52, 53, 55 The other subfamily under family Coronaviridae is Torovirinae, and within that family is the genus Torovirus. Toroviruses are established agents of gastroenteritis in animals, and the type species of the genus is Berne virus (BEV), a chance isolate from a diarrheic horse in 1972.51, 55, 56, 57, 58 Torovirus is discussed in more detail later in this chapter.

Epidemiology and Clinical Findings

Coronaviruses are a common cause of disease in humans and domestic animals.⁵² They have been identified in mice, rats, chickens, turkeys, swine, dogs, cats, rabbits, horses, cattle, camelids, and humans.46, 50, 59, 60 They cause respiratory, GI, neurologic, and generalized infections.⁵⁵ In horses, it is believed that coronavirus spreads through fecal-oral transmission; however, other routes of transmission, such as respiratory and mechanical, may also be possible but so far unknown.55, 61 Most coronaviruses infect only cells of their natural host species and a few closely related species.⁵² In their natural host species, coronaviruses exhibit marked tissue tropism. Virus replication in vivo can be either disseminated, causing systemic infections, or restricted to a few cell types, often the epithelial cells of the respiratory or enteric tract and macrophages, causing localized infections. Coronavirus replication takes place in the cytoplasm of infected cells.⁵² Like rotavirus, coronavirus causes damage to the villi of the intestinal mucosa, resulting in loss of absorptive capacity, leading to a malabsorptive diarrhea. The course of disease is typically initiated in the proximal small intestine.

Anzai et al⁶¹ investigated the effect of long-distance transport of 29 racehorses (age 2 years) on serologic evidence of infection with potential respiratory pathogens, including coronavirus. Serum antibody titers to coronavirus were evaluated by serum neutralization (SN) test using bovine coronavirus (BCV), which is closely related antigenically to ECoV.53, 61, 62 Two horses were seropositive for BCV 1 month before transportation (titers 10 and 40). These horses were transported in the same vehicle as four horses that were seronegative to coronavirus. The four seronegative horses seroconverted after transportation (titers between 10 and 20 within 1 month), but none developed clinical signs, and a direct relationship between disease and coronavirus infection could not be confirmed.⁶¹ This study suggested that ECoV could possibly spread among horses while they are stabled together or during transport, although this requires confirmation. This hypothesis is consistent with serologic evidence that BCV or its related virus is widely prevalent in horses in Japan.61, 62

Coronavirus-like particles have been observed by negative-contrast EM in fecal samples from healthy and diarrheic foals,63, 64, 65, 66, 67, 68 from one foal with combined immunodeficiency syndrome and diarrhea,⁶⁹ and from adult horses concurrently diagnosed with Neorickettsia risticii, the causative agent of Potomac horse fever.⁷⁰ Concurrent infections with rotavirus65, 66 and Cryptosporidium ⁶⁹ have also been reported in foals.

In a single case report, a coronavirus antigenically related to BCV was identified in a 5-day-old Quarter Horse foal with enterocolitis.⁴⁷ This foal developed severe diarrhea starting on the second day of life, indicating a short incubation period, and had multiple secondary complications, including fungal pneumonia, marked limb edema, and hypoproteinemia. A complete blood count revealed an inflammatory leukogram with a regenerative left shift, anemia, and thrombocytopenia. The foal developed vascular pathology in the distal limbs and eventually had detachment of the hoof wall from the laminar structures and was euthanized. Bacterial cultures from feces were negative for enteric pathogens, and viral particles were not observed on EM. Coronavirus was identified in intestinal tissues of the foal by immunohistochemistry using BCV-specific monoclonal antibodies and in feces using an antigen-capture ELISA designed for BCV detection, indicating a lack of sensitivity of EM in this case.47, 53 The foal's serum antibody titer to BCV increased over an 8-day period from 1 : 25 to 1 : 100.²

Despite the reports of probable coronavirus infection in foals, there were no definitive descriptions of ECoV isolation from sick horses before 2000.47, 63, 64, 69, 70 The first isolation and characterization was described in 2000 from the feces of a 2-week-old Arabian foal.53, 75 The foal had diarrhea and fever, which persisted for 6 days, after which the foal recovered with medical treatment.

A third case report described coronaviral diarrhea in a 3-day-old Thoroughbred foal that developed diarrhea on the second day of life.⁷¹ This foal had mild anemia, leukopenia with a left shift and lymphopenia, and hyperfibrinogenemia. Interestingly, this foal developed marked hyperglycemia (257-500 mg/dL) and transient diabetes mellitus associated with low insulin concentrations. Coronavirus particles were observed on EM. The foal survived with intensive treatment, including a 26-day course of insulin therapy. The authors speculated whether the coronaviral infection could have played a role in pancreatic injury, resulting in transient diabetes mellitus. Coronavirus has been associated with pancreatic damage in other species.72, 73

Coronavirus in Adult Horses

Recently coronavirus has been associated with febrile and enteric disease in adult horses.⁴⁸ Previously, there was suggestion of a coronavirus-like agent in horses with Potomac horse fever.⁷⁰ Aside from that, coronavirus was thought to be an intermittent pathogen of only young foals. In the recent study from Japan,⁴⁸ 132/600 draft racehorses that were 2 to 4 years old developed increased rectal temperature and in some, diarrhea of 2 to 4 days' duration. Coronavirus infection was documented through RT-PCR, virus isolation, indirect immunofluorescence, and viral genome sequencing, as well as serology of affected horses.

In 2011-2012, a number of outbreaks of coronavirus disease in adult horses occurred in the United States, including California, Texas, Wisconsin, and Massachusetts.⁷⁴ These were diagnosed through RT-PCR of feces, and the virus was sequenced to have 98% to 99% homology to the NC99 strain described by Zhang et al.⁷⁵ Eighty six percent of sick horses tested by fecal PCR (38/44) were positive for ECoV. Seven percent (7/96) of clinically healthy horses on the premises were fecal positive.⁷⁴ The most common clinical signs were anorexia (88%), lethargy (78%), and fever (73%).⁷⁴ Other signs included changes in fecal character (soft-formed to watery) and colic, but these were in a minority of horses. The morbidity ranged from 20% to 57% on affected premises. Clinical signs resolved in most horses within 1 to 4 days. Approximately 6.8% of horses were euthanized because of the severity of clinical signs, including acute neurologic disease or marked endotoxemia. Common hematologic changes included leukopenia with neutropenia and/or lymphopenia. Shedding in feces generally occurred for a few days. In this report, follow-up fecal samples were available for seven horses, and PCR detection of ECoV persisted for 3-9 days.⁷⁴

A recent coronavirus outbreak occurred in Japan, involving 204/650 (31%) horses.⁷⁶ Fever was found in 96% of horses, and only 13% developed diarrhea and/or colic. Most horses recovered in 2-4 days, except for those with watery diarrhea which took 5-10 days. Two horses (0.98%) with watery diarrhea and stomatitis died. Two strains were identified, which represented antigenic shift from prior strains.⁷⁶

Diagnosis

Coronavirus infection may be suspected if other etiologic agents of diarrhea in foals or adults have been ruled out, especially in outbreak situations. It can also occur as a coinfection with other enteric pathogens. Coronaviruses are difficult to isolate and propagate in cell culture. The diagnostic method of choice is direct demonstration of coronavirus antigens in biologic samples.⁹ Current practical and sensitive means of diagnosis is through fecal RT-PCR.3, 77

Negative-stain EM can also be used to identify coronavirus-like particles in feces; however, it is not as sensitive as PCR.53, 64, 65, 66, 69, 70, 78 If viral particles are not present in sufficient numbers, EM examination may require considerable searching or may be unrewarding.2, 16 Fecal ELISA may also be used, although its sensitivity and specificity for use in horse feces remains to be determined.

Because of the cross-reactivity between BCV and ECoV, detection of neutralizing antibody to BCV in horses provides presumptive evidence of exposure to ECV.47, 53, 61, 62, 64 Because the presence of SN antibodies against BCV in equine sera may be a common finding, acute and convalescent samples should be examined for evidence of increasing titer.47, 61, 62 Convalescent serum samples from horses with suspected ECoV infection may be evaluated approximately 10 days after the onset of disease. In human patients, a fourfold increase in titer to coronavirus is indicative of recent active infection. An antemortem diagnostic panel for ECoV can include assay for serum antibody titer to BCV and fecal capture ELISA evaluation for coronavirus antigen.⁴⁷ However, RT-PCR of fecal samples is currently more practical (and rapid) for use by practitioners.

Additional studies are needed to determine the prevalence of ECoV infection in healthy and sick horses, the occurrence of mixed infections of coronavirus and other enteric pathogens, and the relative importance of ECoV as a cause of enteric disease in horses.⁵³

Husbandry/Infection Control

Horses infected with ECoV should be isolated due to potential for fecal-oral spread. Fortunately, most horses appear to shed the virus only for a few to several days.⁷⁴ Optimally horses should be tested negative on fecal PCR prior to being released from isolation. Coronaviruses are enveloped viruses and are therefore susceptible to common disinfectants including sodium hypochlorite, ethyl acohol, povidone iodine, phenols, accelerated hydrogen peroxide, and peroxygen based disinfectants. However, based on work with human coronaviruses, quaternary ammonium compounds may have variable to relatively poor activity against coronavirus. The presence of organic matter should be a consideration in choice of disinfectant.

Equine Torovirus

Etiology

Equine torovirus (Berne virus) was originally isolated from a rectal swab of a horse with hepatic and GI disease in Berne, Switzerland, in 1972.⁷⁹ It is currently classified in the subfamily Torovirinae with bovine, human, and porcine toroviruses, within the family Coronaviridae and order Nidovirales.80, 81, 82 The enveloped virions are pleomorphic with large protein spikes on the surface, resembling the peplomers of coronaviruses. The nucleocapsid has a tubular appearance with helical symmetry. The positive-sense RNA genome is estimated to be 20 to 25 kilobases in length with six open reading frames (ORFs). Four structural proteins have been identified: spike (S), membrane (M), hemagglutinin-esterase (HE), and nucleocapsid (N) proteins.

Epidemiology

Although originally isolated from a horse with GI disease, a causal link between Berne virus and equine disease has not been established. Limited seroepidemiologic studies indicate that the virus is present in Europe and the United States. Neutralizing antibody is also found in the sera of other ungulates (cattle, sheep, goats, pigs), laboratory rabbits, and at least two species of wild mice (Clethrionomys glareolus and Apodemus sylvaticus).⁸³

Clinical Findings

Despite widespread evidence of exposure to Berne virus, no evidence indicates that this virus is associated with clinical disease in horses. Inoculation of the virus into two foals induced neutralizing antibody without associated clinical signs.⁸³ Bovine torovirus has been associated with gastroenteritis in calves and possibly pneumonia in older cattle.82, 84 Human and porcine toroviruses are associated with gastroenteritis in people and pigs, respectively.85, 86, 87

The complete reference list is available online at www.equineinfectiousdiseases.com.