West NileVirus Vaccination: IgM and IgG responses in horses after injection in Different Muscles & The Prevalence of MRSA in healthy horses in the Netherlands: a follow-up study.

Abstract

West Nile Virus Vaccination: West Nile Virus (WNV) may cause severe symptoms in birds, horses and humans. WNV is not (yet) present in the Netherlands, but it is steadily approaching from south-eastern Europe. Recently, a WNV-vaccine (Duvaxyn®-WNV) became available in Europe. It is claimed that vaccination results solely in an IgG response making it possible to differentiate acutely-infected horses from vaccinated horses by using an IgM-based ELISA. The aims of the study were to investigate this claim and to evaluate whether different intramuscular injection sites influence the immunological responses and whether any local or systemic adverse reactions would occur. Twenty horses, 3 to 21 years old, were vaccinated twice (day 0 and 21) in different muscle groups. Weekly blood samples were collected over a period of 42 days and tested for Flavivirus IgM antibodies using a WNV-IgM ELISA and for Flavivirus Ig antibodies using a WNV blocking ELISA. None of the horses tested positive for WNV antibodies on Day 0. No side effects were found following any of the intramuscular injections. Location of the intramuscular injections showed no significant effect on the immunogenic response. All horses showed a clear Ig (total antibody) response to the WNV vaccination, but in two horses this response was limited. Surprisingly, ten horses gave also a (limited) positive IgM response. This indicates that an IgM capture ELISA is not reliable to diagnose acute WNV infections in recently vaccinated horses.

MRSA: Methicillin resistant Staphylococcus aureus (MRSA) is considered to be an increasing problem in hospitalized horses in the Netherlands. In 2008, 24 of 259 horses admitted to a veterinary hospital tested positive (1), while in a study in 2005, 200 randomly selected ostensibly healthy horses all tested negative for MRSA (2). This raised the question as to whether the high percentage of MRSA-positive horses reflected an overall increase in the population or was a product of the selection of the referral horses. The aim of the present study was to perform a follow-up screening of the 200 horses tested previously in the 2005 study. Of the horses screened in 2005, only 48 were still accessible (11 different premises). These horses were sampled with an additional 54 horses selected randomly at 6 other premises. Two nasal swabs were collected from each horse and incubated in two different selective media: one method similar to the study in 2005 (2) and one more sensitive method that is now used as a standard and has been used in the study of the referral horses (1). All 102 horses tested negative for MRSA using both isolation methods. No MRSA was found in the 102 healthy horses tested in the Netherlands. Further studies are needed to establish whether specific risk factors for testing MRSA-positive can be identified in the group of referred horses.