Diagnosis of West Nile virus infection in horses.

The research tries to establish optimal strategies for diagnosing the West Nile Virus (WNV) in horses. They compared the efficacy of different diagnostic methods, and found that the IgM-capture enzyme linked immunosorbent assay had higher sensitivity compared to reverse transcriptase-polymerase chain reaction (RT-PCR) done on blood samples. A significant subset of horses seemingly “seroconverted” without showing symptoms. The researchers also established that the highest concentration of WNV RNA was found in the medulla of the horses’ neural tissue.

Study on West Nile Virus in Horses

• The research was initiated with the realization that the West Nile Virus (WNV), an epidemic which began in North America in 1999, resulted in significant morbidity and mortality in horses. The inconsistent development of encephalitis in horses experimentally infected urged the researchers to investigate natural cases to better understand the disease.
• Various diagnostic strategies were evaluated, and their sensitivity and specificity in diagnosing WNV infection in horses were compared and optimized.

Findings from the Investigation

• It was found that although WNV RNA could be detected by RT-PCR performed on whole blood collected from both clinically affected horses and unaffected herd mates, the diagnostic sensitivity of this approach was low compared to the IgM-capture enzyme-linked immunosorbent assay (ELISA).
• The study revealed that 18.5% of the herd mates of clinically ill horses seroconverted to WNV but manifested no overt clinical signs of WNV encephalitis. This implies that these horses developed immunity to the virus without showing any external symptoms.
• WNV RNA was found in the neural tissue of 46 out of 64 deceased horses suspected to have had WNV encephalitis, indicating that the virus had infiltrated their nervous systems.

Additional Outcomes

• An alternative cause of death other than WNV encephalitis was identified in 15 out of the 64 cases. However, the final diagnosis for 3 of these cases could not be determined.
• Quantitative RT-PCR analysis of neural tissue from WNV RNA-positive horses showed that the medulla contained the highest average concentration of viral RNA and that WNV RNA could be detected in samples extracted from formalin-fixed neural tissue.
• Different WNV RT-PCR amplification strategies were compared and it was found that nested RT-PCR slightly improved diagnostic sensitivity over a single round of amplification. On the other hand, a quantitative (TaqMan) assay showed equivalent sensitivity and specificity to those of nested amplification.

Implications of the Study

• The study’s findings have important implications for diagnostic procedures. Understanding that an IgM-capture ELISA has higher sensitivity compared to a RT-PCR performed on blood samples can affect the strategy used to diagnose WNV infection in horses.
• The information regarding “silent” seroconversion can be critical in disease control efforts, since these horses could potentially carry and transmit the virus without showing symptoms.
• The discovery that the medulla contains the highest mean concentration of WNV RNA provides useful information for researchers in designing more accurate and effective diagnostic procedures.