Early pathogenesis of Venezuelan equine encephalitis virus infection in horses

Abstract

In horses, it is not known which tissues support Venezuelan equine encephalitis virus (VEEV) replication during the early stages of disease, nor how the virus gains access to the CNS. My research focused on the early viral titers in tissues, the interferon response in horses and diagnosis of VEEV viremia by a quantitative RT-PCR. Two horses each were challenged with the virulent Trinidad donkey strain of VEEV, and two sacrificed at 24, 48, 96 and 144 hours post challenge. Viremic titers peaked at 103.8 to 104.5 PFU/mL between 60 and 72 hours after challenge. VEEV titers peaked in the draining lymph nodes (103.9 to 106.1 PFU/g tissue) at 24 to 48 hours. Highest tissue titers were noted in the bone marrow (106.2 to 108.3 PFU/g) at 48 and 96 hours, the olfactory bulb (106.5 PFU/g) and tract (106.8 PFU/g) at 96 hours post challenge. Virus was detected in the dental pulp (102.8 to 105.1 PFU/g), trigeminal nerve (102.9 to 103.6 PFU/g), and olfactory tract and bulb, prior to detection of virus in the cerebrum. This data indicates the virus replicates in the lymph nodes and bone marrow, prior to entry into the CNS through the olfactory and trigeminal nerves. The serum interferon (IFN) responses were assayed in horses challenged with four strains of VEEV, the TrD strain, a non-virulent IE strain, a virulent IE strain, and the V3526 vaccine strain. IFN response was greatest (320-1280 IU/mL) for the TrD strain challenge. The initiation, peak, and length of serum IFN titers in horses challenged with TrD correlated with the initiation, peak and length of viremic titers. Virulent IE VEEV challenge of three horses resulted in no detectable serum IFN response, although clinical illness was severe in the horses. The avirulent IE and V3526 VEEV strains resulted in a low, transient but detectable IFN response. Results of the quantitative RT-PCR indicate viral antigen can be detected in horse sera at titers as low as 2.0 PFU/mL.