Pathogenesis of Venezuelan Equine Encephalomyelitis Virus Infections in the Developing Chick Embryo

The effect of Tembusu virus infection in different week-old Cherry Valley breeding ducks

To study the effect of Tembusu virus (TMUV) infection on Cherry Valley Breeding ducks of different ages, 350 five-week-old ducks were divided into 14 groups. Ducks in seven experimental group were respectively infected with 1.265×10(5) mean embryo lethal dose (ELD50) of TMUV-AHQY strain (in 4.2mL) by intravenous route. Ducks in control groups were inoculated with Phosphate-buffered Saline (PBS) in the same way. Clinical symptoms, gross and microscopic lesions, viral loads and serum antibodies were detected and recorded for 20days after infection. Some ducks infected at 7 and 21 week s of age showed severe clinical symptoms including depression and inappetence, and no obvious clinical symptoms were seen in other week-old infected ducks. Severe gross lesions including hepatomegaly, meningeal congestion, myocardial hemorrhage, intestinal, myocardial and pulmonary edema were observed in ducks infected at 7, 18 and 21 weeks of age. No or mild gross lesions were observed in ducks infected at 14 and 16 weeks of age. The main microscopic lesions including hyperaemia, degeneration and necrosis of different cells and inflammatory cellular infiltration mainly consisting of mononuclear cells or lymphocytes were observed in ducks infected at 7 and 21 week of age. But relatively intact structures and rare lymphocytic infiltration were presented in ducks infected at 14 and 16 weeks of age. Viral antigen was more frequently observed in organ slices collected from 7 week-old infected ducks and few positive staining was found in 14 and 16 week-old infected ducks. Less viral loads in different tissues and swabs were detected by a quantitative real-time PCR assay. The level of viral loads in the tissues of ducks infected at 14 and 16 weeks of age was very lower than that of ducks infected at 7 and 21 weeks of age. Meanwhile, less viral copy numbers were detected in swab samples collected from 14 and 16 week-old infected ducks. Ducks infected at 14-week-old developed significantly higher serum neutralizing antibody titers than those infected at other week of age. These results indicated that the effect of TMUV infection on Cherry Valley ducks is partly related to weeks of age. 7-10 week-old and 18-21 week-old ducks were more susceptible to TMUV infection, but 14-16 week-old ducks were more resistant to this disease.

Studies on Newcastle disease virus; behavior of the virus in the embryo

The virus of Newcastle disease of chickens resembles those of the encephalitis group in its ability to spread throughout the developing egg and embryo, but it is similar to influenza virus in the high concentration of it found in the allantoic fluid before death. No effect of the size of the inoculum on the final titer of virus in the allantoic fluid was detected. Good growth occurred at temperatures from 35° to 41°C., apparently more rapid at 40°C. than at 35°C. No appreciable development of virus capable of agglutinating red cells but of low embryo infectivity was found. Although virus multiplication was not immediately perceptible after inoculation, this cannot on present evidence be attributed to a real lag phase.

ESTIMATION OF TITER OF VENEZUELAN EQUINE ENCEPHALOMYELITIS VIRUS PREPARATIONS FROM A SINGLE-DILUTION ASSAY

Riley, Jean M. (U.S. Army Biological Laboratories, Fort Detrick, Frederick, Md.), William C. Patrick, III, and William E. Campbell, Jr. Estimation of titer of Venezuelan equine encephalomyelitis virus preparations from a single-dilution assay. J. Bacteriol. 85: 1256–1260. 1963.—When suspensions of Venezuelan equine encephalomyelitis (VEE) virus were injected intracerebrally into groups of mice, a nearly linear relationship was observed between the concentration of the virus injected and the mean reciprocal time-to-death of the mice. A total of 91 VEE preparations were assayed in duplicate, and, by plotting the relationship between the reciprocal time-to-death for mice given the 10 −6 dilution of virus and the micld 50 (mouse intracerebral challenge, ld 50 response) values for the virus preparations, a reference curve was established. Using this reference curve, it was possible to estimate directly the ld 50 values of virus suspensions of unknown concentration from the mean reciprocal time-to-death of a group of mice injected with a single dilution. In this work, the number of mice used was reduced by 62.5%, the titrations were complete in 3 to 5 days compared with the usual 10 to 14 days, three to four times as many assays could be done in a day, and no assays had to be repeated since end points were not missed. The precision of the single-dilution assay compared favorably with that of the ld 50 titration.

INTERFERENCE BETWEEN VIRUSES IN TISSUE CULTURE

The influence of one virus on the growth of another in tissue culture was investigated. The 17DD High strain of yellow fever virus was found capable of completely suppressing the growth of both the Asibi strain of the same virus and of the heterologous West Nile virus, even when these were added to the cultures in large amounts. The 17DD High strain of yellow fever virus and the West Nile virus produced either partial or complete suppression of growth of the Venezuelan equine encephalomyelitis virus, depending upon the quantity of the latter inoculated into the cultures. Owing to lack of methods for the detection of interference except in a single direction, reciprocal interference with these viruses could not be investigated. The 17DD High strain of yellow fever virus and the West Nile virus were able to suppress completely, or almost completely, the growth of influenza A virus added to the infected cultures in maximal amounts. Interference in the reverse direction, even with the use of small amounts of the neurotropic viruses, was not demonstrable. Cultures infected with the 17DD High strain of yellow fever virus were examined for the presence of neutralizing antibodies and non-specific antiviral substances; neither was found present.