Equine Pediatric Medicine (AAEP Convention 2001)
Investigating Respiratory Disease
Debra Sellon, DVM, PhD, of Washington State University's College of Veterinary Medicine, presented guidelines for systematically investigating and treating foal respiratory disease outbreaks. "While many outbreaks of respiratory disease in foals are infectious in nature, this is not always the case," she began. "Environmental factors can have a profound effect."
Sellon suggested six steps for investigating/controlling outbreaks:
- Evaluate farm and individual history. Look into prior infections on the farm, deworming status, parasite surveillance, vaccination status, exposure to transient horses, and ages of affected foals.
- Inspection of premises. Check barn ventilation, hay and straw storage, bedding, hygiene practices, sanitation, pasture access, pasture condition (vegetation, soil, population density), and population characteristics (ages, group sizes, transient vs. permanent horses, etc.).
- Examination of individuals. Do a complete physical examination, rebreathing exam, and laboratory exam, and examine other foals (which might have subclinical disease that you can stop early).
- Figure out which organism is responsible for infectious disease. The veterinarian should now be able to develop a list of differential diagnoses ranked in order of likelihood for that farm. Ancillary diagnostic tests such as transtracheal wash or nasopharyngeal swabbing might help identify the responsible pathogen(s).
- Separate sick foals from exposed but not sick foals, and separate unexposed, healthy horses from both groups. Control of human and animal traffic through the quarantine area is essential.
- Treat affected foals. Use appropriate antimicrobials for the diagnosed cause and suggest any environmental changes that might help (cleaning stalls when horses are outside so they are not exposed to all that dust, improving ventilation, etc.).
Sellon added that corticosteroids might help stop the cycle of inflammation in selected difficult, chronic cases (if active bacterial infection is no longer thought to be the cause).
"A logical, systematic approach to identification of specific primary pathogens and improvement of environmental conditions will significantly decrease morbidity and mortality," she concluded.
Overo Lethal White Syndrome
When breeding Paint horses, breeders must be cognizant of the genetics behind each mating. The condition known as overo lethal white syndrome (OLWS) can be a problem for the breeder who loses a foal and for the veterinarian who is helpless to prevent the death, said Paul Vrotsos, BS, CVT, of Peterson Smith Equine Hospital in Ocala, Fla., while presenting "The Impact of the Mutation Causing Overo Lethal White Syndrome on White Patterning in Horses."
OLWS foals are born completely or mostly white and die from an inherited, incurable intestinal problem. It was recently found that this condition results from a mutation in the endothelin receptor B (EDNRB) gene. The substitution of lysine for isoleucine at residue 118 of EDNRB is responsible for OLWS. Vrotsos emphasized that breeders should determine through DNA testing whether the pair to be bred carries the EDNRB genotype. "If carrier-to-carrier matings are prevented, breeders can eliminate the production of lethal white foals," he said.
During a study supported by the Minnesota Agriculture Experiment Station,
Vrotsos and colleagues determined that the "Ile188Lys EDNRB mutation" causes OLWS when a foal inherits a copy from each parent. Through phenotyping and genotyping of 1,000 horses (with greater than 90% being registered Paint horses), it was determined that all foals with OLWS were homozygous for the Ile188Lys EDNRB mutation. White patterning was strongly associated with EDNRB genotype, with 96% of frame and frame-blend overos possessing the Ile188Lys EDNRB mutation. It was also found that calico overos with a large amount of white coloring had 100% incidence of the Lys188 allele, indicating that largely white calicos are actually frame blends. Mating these heterozygous horses to other heterozygous horses might produce an OLWS foal.
In addition, Vrotsos said that even though there is a low incidence (10%) of the mutation in tobianos, mating a heterozygous tobiano to a heterozygous overo has a 13.3% chance of producing an OLWS foal.
Caution With Quinolones in Foals
Quinolones, broad-spectrum antibiotics, have been shown to cause articular cartilage damage and lameness in young animals of many species, including dogs, rodents, horses, and rats. In a study presented by Sally Vivrette, DVM, PhD, Dipl. ACVIM, of Triangle Equine in Cary, N.C., on "Quinolone-Induced Arthropathy in Foals," it was determined that joint damage in foals is likely with quinolones. Vivrette recommended that the administration of quinolones (such as enrofloxicin) be reserved for cases where other antibiotics failed, and when antimicrobial sensitivity results indicate quinolones are the only option.
During the North Carolina State University study, four foals were given enrofloxicin intravenously at the accepted dose (5mg/kg). The foals were turned out during the day and monitored for clinical signs of lameness and joint distention. At the end of the experiment, synovial fluid was analyzed, and the foals were euthanized so that a necropsy could be performed.
Three of the four foals were lame, and all showed articular cartilage damage. There was moderate to severe joint distention. Synovial fluid cytology revealed a decrease in viscosity, suppurative to chronic inflammation, and mildly elevated protein concentrations. The articular damage included cartilage roughening and superficial to full thickness erosions of the tibial-tarsal, femoral-tibial, or the humeral-radial joints. Lesions were found in all foals with mild chondrocyte necrosis, cleft and vesicle formation, and the loss of proteoglycans.
Rhodococcus equi Pneumonia
"Rhodococcus equi...is the most devastating cause of pneumonia in foals between three weeks and five months of age," began Steeve Giguï¿½re, DVM, PhD, Dipl. ACVIM, of the University of Florida's College of Veterinary Medicine in his presentation.
R. equi is present on almost any farm, though not all strains are virulent (disease-causing). One study Giguï¿½re mentioned evaluated 31 horse farms and found various strains of R. equi on all of them. Virulent R. equi was found in 23% of the samples (in 24 of 31 farms). Pneumonic foals are a major source of environmental contamination with virulent R. equi.
Control of R. equi on enzootic (peculiar to or constantly present in a locality) farms should include:
- Reduction of the size of infective challenge--by rotating pastures, removing manure, and isolating infected foals.
- Identifying infected foals early--with weekly physical exams, ultrasonography or radiography, and/or laboratory tests. One study found that twice weekly physical examination including listening to the lungs helped promote early diagnosis and prevent mortality.
- Using hyperimmune plasma (HIP)--is one of the best means of preventing severe, fatal R. equi. It is not always enough to prevent infection on its own, and does not replace the need for early screening of foals for disease. Also, the efficacy of one HIP product can't be extrapolated to another that is prepared in a different fashion.
As for treatment, research has found that azithromycin and clarithromycin appear to be suitable alternatives to erythromycin, though more research on safety and efficacy needs to be conducted. Until then, erythromycin and rifampin should remain the treatments of choice.
When dealing with an infectious disease outbreak, especially one involving foal diarrhea, knowing the cause of the disease and the way it works can keep other horses from being infected. "Implementing an infectious disease program revolves around vaccination, disinfection, and management," said Roberta Dwyer, DVM, MS, Dipl. ACVPM, of the University of Kentucky Gluck Equine Research Center, during her presentation, "Control and Prevention of Foal Diarrhea Outbreaks." She emphasized that a
diarrheic foal should be considered contagious until proven otherwise, except in the case of foal heat diarrhea.
Dwyer had several recommendations for control of a disease outbreak, such as:
- Isolate all sick animals and those that have been in contact with the sick animal;
- Clean all surfaces of organic matter with a detergent and rinse them prior to disinfection;
- To prevent cross-contamination between animals, use separate equipment or disinfect equipment with a chemical which is effective against the pathogen, can be used in the presence of organic matter, and is effective on all surfaces involved;
- Avoid spreading manure from sick horses onto fields;
- Quarantine new animals for at least 14 days (21 days is recommended if a horse might have been exposed to strangles);
- Disinfect barns periodically to prevent pathogen build-up; and
- Keep horses in one stall as much as possible rather than rotating them between stalls.
More on disease control can be found on page 39 of the AAEP Wrap-Up.
POLL: Enriching Your Horse's Environment