Search

Intravenous injection of pregnant mice with St. Louis encephalitis (SLE) virus at 8 days of gestation resulted in infection of the fetus. Progeny developed no antibody or tolerance to SLE virus since the viral antigen was cleared by maternal antibody before antibody-forming competence developed in the young. Temporary growth retardation was observed in a number of young at 3 weeks of age. After the initial setback the growth rate increased, indicating that early runting was due to an inability to adjust adequately to extrauterine life, which was subsequently overcome. In most other young there were no significant effects on growth, reproduction, or life expectancy. A few young died at or shortly after birth; in these, neurological changes ranging from gross defects such as encephaloceles and hydrocephalus to histological evidence of necrosis and congestion were observed. Neurologically related behavioral changes were detected by using the open field test and the rotating-rod test, which indicated neurological damage and memory impairment in the surviving intrauterinely infected animals.

In the summer of 1966, an epidemic of St. Louis encephalitis occurred in Corpus Christi, Texas, coincident with one occurring in Dallas about 563 km to the north. Among the 76 cases confirmed in Corpus Christi, there were two deaths; the attack rate was 41.0 per 100,000. In contrast with a concurrent outbreak in Dallas and the 1964 outbreak in Houston, attack rates were much higher in populations of the upper socioeconomic districts. This distribution may have resulted from the combined effects of an unusual concentration of vector mosquito breeding sites in storm sewers in the upper socioeconomic districts and a higher degree of residual immunity in the residents of the lower socioeconomic areas.

Albino Swiss mice, immunized with any of several types and strains of dengue viruses, were afforded substantial protection against peripheral Japanese B encephalitis or St. Louis encephalitis virus challenge. Dengue-2 (New Guinea “C”)-immunized mice showed, 10 and 20 weeks after immunization, undiminished resistance with concomitant cyclophosphamide treatment and virus challenge. Examination of the effects of immunization on Japanese B encephalitis virus pathogenesis, after virus challenge with concomitant cyclophosphamide treatment, indicated that protection was associated with decreased viremia and virtually no virus replication in the brain as compared with controls. These effects could be demonstrated before detection of any neutralizing antibody to the challenge virus. From the applied aspect, the data support the hypothesis, based on epidemiological evidence and experiments in hamsters, that prior exposure of man to dengue viruses can confer some degree of protection against Japanese B encephalitis or St. Louis encephalitis disease.

Summary St. Louis encephalitis virus was isolated on 23 occasions from mosquitoes collected between September 7 and October 25 at widely separated localities of the four-county Tampa Bay area during the St. Louis encephalitis epidemic of 1962. Twenty-two of these isolations were obtained from 485 pools of Culex nigripalpus; the 23rd was from 91 pools of Melanoconion, a subgenus of Culex. The mosquitoes were collected alive in battery-powered light traps and traps baited with ducklings or chicks. These isolations indicate that infected vectors were prevalent over a vast area exposing the human population to infection throughout the region.

The present experimental results concern primarily the question, whether or not mosquitoes feeding on chickens having viremia, as a result of the bite of infected mites, can acquire the virus of St. Louis encephalitis and whether or not mosquitoes thus infected, can transmit the virus to chickens and hamsters. During the course of the investigation, 7 species of mosquitoes of 3 genera were infected with the virus in one or two or all of three ways: by feeding on a suspension of infected mouse brain tissue, by feeding on chickens in which viremia had been produced by subcutaneous inoculation of virus, and by feeding on chickens having viremia as a result of the bite of infected mites. These mosquitoes transmitted the virus to chickens at periods varying from 5 to 33 days after the infective meal. The virus of St. Louis encephalitis was transmitted to hamsters by Culex pipiens at periods varying from 4 to 27 days after feeding on chickens having viremia as a result of the bite of infected mites. While viremia was demonstrated readily in hamsters, signs of encephalitis did not develop. In all transmission experiments the method of chorioallantoic passage proved necessary for the demonstration of viremia. A concept of the epidemiology of St. Louis encephalitis is presented: two blood-sucking vectors may be involved, one an arachnid, the mite, maintaining the virus in nature by transovarial passage, and the other, an insect, the mosquito, which carries the infection from birds to other vertebrates including man.

Analysis of purified Saint Louis encephalitis (SLE) virus by acrylamide gel electrophoresis revealed that the virions contained three structural proteins designated SP-1, SP-2, and SP-3 which had molecular weights of 63,000, 18,000, and 8,500, respectively. The envelope contained proteins SP-1 and SP-3 which were removed from the nucleocapsid by nonionic detergent treatment. Nucleocapsids prepared by deoxycholate treatment of complete virions had a density of 1.301 in potassium tartrate and contained SP-2 and SP-3. Brij-58-prepared SLE nucleocapsids had a density of 1.321 and contained only SP-2. Cycloheximide treatment for 1 hr in the presence of actinomycin irreversibly inhibited BHK cellular protein synthesis and reversibly inhibited the synthesis of SLE viral protein and ribonucleic acid. Three structural proteins and five virus-specific nonstructural proteins were detectable in SLE virus-infected BHK cells treated with actinomycin and pulse-inhibited with cycloheximide. Formation of each individual viral structural protein was detectable within 30 min after cycloheximide removal and continued with only minor changes from 12 to 18 hr after infection. Late in the infection cycle, synthesis of the nucleocapsid structural protein SP-2 and SP-3, the small envelope protein, was no longer detectable.

Summary Experience with twenty-five cases of acute encephalitis believed to be of viral origin has been analyzed. There is serological evidence that three of these cases were due to St. Louis virus. An unknown virus was isolated from the serum of one patient. In most of the cases, onset of illness was sudden. Fever, headache and mental change were the main symptoms. There was a mortality rate of 16 percent. It is significant that in about 35 percent of the cases, there was some residual defect after illness.

Anesthesia with diethyl ether significantly alters the course and outcome of experimental infections with the equine encephalomyelitis virus (Eastern or Western type) or with the St. Louis encephalitis virus. No comparable effect is observed in experimental infections produced with rabies or poliomyelitis (Lansing) viruses. The neurotropic virus infections altered by ether anesthesia are those caused by viruses which are destroyed in vitro by this anesthetic, and those infections not affected by ether anesthesia are caused by viruses which apparently are not destroyed by ether in vitro. Another striking difference between these two groups of viruses is their pathogenesis in the animal host; those which are inhibited in vivo by ether anesthesia tend to infect cells of the cortex, basal ganglia, and only occasionally the cervical region of the cord. On the other hand, those which are not inhibited in vivo by ether anesthesia tend to involve cells of the lower central nervous system and in the case of rabies, peripheral nerves. This difference is of considerable importance in view of the fact that anesthetics affect cells of the lower central nervous system only in very high concentrations. It is obvious from the complexity of the problem that no clear-cut statement can be made at this point as to the mechanism of the observed effect of ether anesthesia in reducing the mortality rate in certain of the experimental neurotropic virus infections. Important possibilities include a direct specific effect of diethyl ether upon the virus and a less direct effect of the anesthetic upon the virus through its alteration of the metabolism of the host cell.

Summary The transovarian passage of hemagglutination-inhibition (HI) antibodies to St. Louis encephalitis (SLE) virus was studied in 18 naturally infected hens and 51 of their chicks. HI antibody was readily detected in the 24-hour-old chick in quantitative relationship to maternal antibody titers. The loss of this inhibitor was an exponential function of time, and was below detectable levels at the 4th week. Serum-neutralizing antibody studies on the hens indicated the SLE-HI inhibitor was specific antibody. In contrast, the Eastern encephalitis-HI inhibitor appeared to be a non-specific substance related to the ovulatory activity of the bird. The interpretation of SLE-HI antibodies in chicks under 4 weeks of age requires consideration of the presence of maternal antibody as a contributing cause.

1. Beginning at 24 hours after intravenous injection of about 10 million intracerebral LD50 of virus there was evidence of simultaneous, progressive multiplication in the brain and intestinal tract. 2. When the virus was introduced directly into the brain or the nasal cavities and mouth, none was found in the intestinal tract until there was general centrifugal spread from the central nervous system during the last stages of the infection at 96 or 120 hours after inoculation when the virus in the entire brain had reached a concentration of about 3 billion LD50. 3. Centrifugal spread began when the virus in the brain reached a concentration of about 400 million LD50 and virus appeared in the pharynx, tongue, and adrenals before it was demonstrable in the intestinal tract, blood, or viscera such as the spleen, liver, and kidneys. 4. Despite the high concentrations of virus which developed in the intestinal tract following intravenous inoculation, it was not demonstrable in the stools, differing in this respect from Theiler's virus in mice and poliomyelitis virus in human beings and monkeys. 5. No antiviral agent was found in the stools, but the urine of normal mice having a pH of 5.6, inactivated large amounts of St. Louis encephalitis virus. 6. There was no evidence of multiplication in the nasal mucosa of mice which succumbed with encephalitis following nasal instillation of the virus, the course of events being comparable in this respect to the behavior of the M.V. poliomyelitis virus in rhesus monkeys. 7. At the terminal stage of infection the virus content per milligram of tissue was as great in the leg muscles as in the sciatic nerves. Since this was also true for the urinary bladder, heart, lungs, and tongue among other tissues, and since the amount in the blood was too negligible to account for it, it would appear that the virus either accumulated in these tissues by diffusion from the nerve fibers, along which it was spreading from the central nervous system, or that it multiplied in some constituent other than the nerve fibers.

Summary The present report concerns the susceptibility of three species of insectivorous bats (Tadarida brasiliensis mexicana, Myotis lucifugus lucifugus, Eptesicus fuscus fuscus) to experimental infection with several strains of Japanese B encephalitis (JBE) and St. Louis encephalitis viruses. All three bat species proved to be susceptible to subcutaneous inoculation with one or more virus strains, the intensity of the infection depending on the mouse-passage level and origin of the strain. In general, high mouse-passage strains were less infective for bats than recent isolates. The characteristics of experimental arbovirus infection in bats include a persistent viremia (15–30 days) and viral invasion and multiplication in interscapular brown adipose tissue and, to a lesser degree, in brain and kidney tissue. Intracerebral inoculation of JBE virus produced a more intense and widespread infection in the big brown bat (Eptesicus f. fuscus) than subcutaneous inoculation, yet evidence of viral pathology was not observed in sections of infected brain or brown fat. None of the infected bats showed evidence of disease, indicating that these viruses are capable of replicating in various tissues of the bat without noticeably damaging cells or producing overt signs of encephalitis. Reasons why bats might be effective reservoirs of arboviruses in various phases of the year-round transmission cycles are discussed and the application of these experimental studies in the planning of field work is indicated.

Summary Laboratory studies were conducted in attempts to infect three common species of bird mites, Dermanyssus gallinae, Ornithonyssus bursa and O. sylviarum, with the virus of St. Louis encephalitis and to transmit infection by the bite of these mites to newly hatched chickens. The virus tests were performed in three-week-old white Swiss mice, inoculated intracerebrally. Detectable virus persisted less than 2 days in the mites, and no transmissions by bite were shown. These negative results support those previously reported by Reeves et al. (1955) and indicate a lack of importance of bird mites in transmission of active St. Louis encephalitis infection.

SummaryThe large plaque variant of the P-15 strain of SLE virus which grows to a titer of 108 to 109 TCID50/0.1 ml in HKC was inactivated by formalin and tested for antigenicity. The virus was harvested in medium 199 containing 2% H.AL, and was readily and apparently predictably inactivated by 1:4000 formalin at 37°C. Loss by nitration prior to activation was in the expected range of 0.3 log. The inactivated preparation when tested for the minimal immunogenic dose (MID) in mice gave a low value, indicative of excellent immunogenicity. The MID for a preparation with a preinactivation titer of 10-7-3/0.1 ml was 0.0079 ml, whereas that with a titer of 10-8-7/0.1 ml furnished an MID of

Summary A method is described for collecting mosquito saliva in oil and measuring its volume and virus content. Preliminary results indicate that Culex pipiens may discharge 104.5 cubic microns of saliva per minute and that St. Louis encephalitis virus reaches a concentration of 109.9 suckling mouse LD50 per ml in the saliva. The concentration of virus was observed to change significantly in periods of time as short as one hour.

Summary A total of 25,240 mosquitoes was captured in the Tampa Bay area during September and October 1962, using miniature, battery-operated light traps. These were tested in suckling mice in 1,114 pools. A total of 19 isolations of St. Louis encephalitis virus was made, 18 from Culex nigripalpus and a single one from Anopheles crucians. Twenty-nine other virus isolations were made. Twenty-eight were from An. crucians and have been identified as a new virus related to Cache Valley virus of the Bunyamwera arbovirus group; the other was an unidentified virus of the California arbovirus group from Aedes atlanticustormentor.

Summary Seventeen percent of 238 wild birds and 36 percent of 162 domestic birds at Caymanas, Jamaica, were found to be positive for St. Louis encephalitis virus infection by hemagglutination-inhibition and neutralization tests in 1962. Conversions detected in local and sentinel chickens suggest that between March and August there probably was an increase in the transmission rate of SLE virus. This increased rate coincided with the breeding period of birds in the area. An isolation of St. Louis encephalitis virus, confirmed by the Trinidad Regional Virus Laboratories, was made from a mockingbird nestling in August. The isolate seems to be closely related to Tr 9464 and Ja 7532 viruses, which are both closely related to the Parton strain of SLE virus.

An epidemic of St. Louis encephalitis (SLE), involving at least 14 human cases, occurred in Danville, Kentucky in early autumn, 1964. Thirty-five of 241 wild birds tested and 37 of 122 chickens had SLE antibiodes. Three isolations of SLE virus were made from Culex pipiens pipiens Linnaeus, the apparent vector. In addition, this species also yielded 2 isolations of a Flanders-related virus.

Summary A total of 287 sera collected from children under 15 years (104) and adults residing in eight communities in Monagas State, Venezuela, was examined for presence of antibodies to arthropodborne viruses by hemagglutination-inhibition and neutralization tests. No immunity to eastern and western equine encephalitis viruses was encountered, but, with the exception of Maturin, each community had some adults who were immune to Venezuelan equine encephalitis virus. Only three sera from adults were found to neutralize Mayaro. No evidence of recent activity of yellow fever and dengue type 2 viruses was discovered. Eight sera neutralized Ilheus but no other group B agent used, and two sera neutralized St. Louis encephalitis virus only. Neutralization test results with a Caraparu-like agent, TRVL 34053-1, indicate that infections with this or a closely related agent have occurred occasionally. In each community there were some sera that neutralized Cache Valley virus. Limited tests with a Manzanilla antigen gave negative results.

Rats ranging from 8–21 days of age were inoculated in the right cerebral hemisphere with St Louis encephalitis virus, Japanese B encephalitis virus, Murray Valley encephalitis virus. Mumps virus however, was injected at age one day. When very young rats were injected with these viral preparations, fatal encephalitis usually developed. Rats injected at ages of more than two weeks showed little or no clinical signs of infection. When inoculated at ages between these extremes, clinical disease developed in most rats but many survived. Physically most of these surviving animals appeared to be normal, but when tested at maturity in a rather complex maze, they showed a marked reduction in learning ability. Tilting-cage-type activity measurements taken overnight with the Murray Valley encephalitis group also indicated that most of the postinfectious rats were hyperactive. These data imply that such a laboratory model might be related to similar hyperactivity and learning disabilities seen in certain types of minimally brain damaged children. A viral-sex interaction often seen in earlier studies is also noted in our animals injected at younger ages.

SummaryIn a survey for endemic diseases of the wildlife of West Central Utah, 9,100 serum samples were examined for the presence of complement fixing antibodies against 4 arboviruses; eastern (EEE), western (WEE), Venezuelan (VEE) and St. Louis (SLE) equine encephalomyelitis. Antibody titers of 1:16 or greater, with no cross reaction evident, were found in 6 specimens from 4 animal species against EEE, 57 from 9 species against VEE and 29 from 7 species and 89 from 10 species against WEE and SLE respectively. In addition to these reactors, 162 other serum samples were found to be positive with two or more of these arbo-virus antigens. Only WEE and SLE are known to be present in this region based upon known outbreaks and previous studies of others.We express gratitude to Peter Olsen and Everett Morse for collecting and processing the samples and to Doris Dickson for technical assistance.

SummaryA virus recently isolated from the salivary glands of Mexican free-tailed bats (Tadarida mexicana) appears to be a new member of the JE-WN-SLE-MVE group of viruses. On the basis of the results of complement fixation tests it appears that the salivary gland virus of bats is more closely related to the West Nile virus than to other members of the group.

SummaryA summary of data relating to the initial isolation and identification of St. Louis encephalitis (SLE) virus from Culex mosquitoes in Florida has been presented. The agent PI5 was isolated from mosquitoes collected in November 1961 and was used to prepare HA and CF antigens for serological studies of the 1962 epidemic in Florida.

Cerebrospinal fluid determinations of immunoglobulin M were made by immunoelectrophoresis in persons with arbovirus encephalitis, other nervous system infections, and noninfectious diseases. CSF IgM was present in 24 of 40 persons with infections and 9 of 56 without (p

An epidemic of St. Louis encephalitis (SLE) occurred in Dallas, Texas, in the summer of 1966. A total of 545 suspected cases within Dallas city and county were reported, of which 145 were laboratory-confirmed as SLE virus infection. The greatest concentration of cases occurred in lower socioeconomic areas of the central part of the city in black populations. The attack rate and mortality rate increased markedly with age. The overall attack rate was 15.2 per 100,000, with a case fatality rate of 9.7%. During the course of the epidemic, most of the county was sprayed aerially with an ultra-low volume (ULV), high-concentration malathion mist. The effects of this treatment cannot be adequately assessed from the human epidemiologic aspect alone, but the spraying clearly reduced the number and infection rate of the vector mosquitoes.

Active surveillance for dengue and yellow fever is needed because of a potential risk of introduction into the southern United States. Research on the biology and vector competence of Aedes aegypti and a contingency plan should dengue and yellow fever appear in epidemic form, is recommended. Surveillance and mosquito control remain as basic strategy to check arbovirus encephalitis. Rabies in man is rare in the United States; wildlife rabies constitutes the major exposure. Studies of the epidemiology and pathogenesis of rabies infection in wildlife, and development of effective vaccines for man and better seroprophylaxis are needed.

A difference was observed in susceptibility of adult male and female mice to St. Louis encephalitis (SLE) virus as measured by the death rate after intravenous challenge. Female mice that had susceptibility similar to that of males at 2 months of age had increased resistance to SLE virus at 3 and 4 months of age. The increased resistance occurred after sexual maturity, indicating that the resistance factor possibly was related to an aging process in the female. The susceptibility of male mice remained unchanged over the 2- to 4-month period. Neither pregnancy nor castration had any effect on resistance of adult mice to St. Louis encephalitis virus.

A night heron, a green heron, a white ibis, and a cattle egret were individually exposed overnight in 8-ft cages to densities of Culex nigripalpus ranging from 25 to 1,200 per cage. Surviving mosquitoes were examined the following morning and engorgement rates were determined as a measure of effect of antimosquito activity. In addition, the behavior of each bird was recorded for 2-hr periods at three different mosquito densities. Results indicated an inverse relationship between the density of mosquitoes and the proportion that successfully obtained blood. This relationship was ascribed to the increase in defensive activity displayed by the birds (differing in degree according to species) when exposed to increases in mosquito density. The frequency of interrupted feeding (= partial blood meals) was also related to host behavior and mosquito density.

Summary The usefulness of intracerebral neutralization, intraperitoneal neutralization, and complement-fixation tests for the diagnosis of human cases of western equine encephalomyelitis and St. Louis encephalitis was compared. In cases of western equine encephalomyelitis it was found that the use of all three tests assisted in making the diagnosis, but that each method failed to demonstrate a significant antibody increase in more than one-fifth of the cases. In cases of St. Louis encephalitis the intraperitoneal neutralization test was the most useful method. The intracerebral neutralization test showed an increase in antibody in only 7 of 20 cases. Complement-fixation tests were valuable in the diagnosis of both western equine encephalomyelitis and St. Louis encephalitis.

Summary Two isolations of St. Louis encephalitis virus were made from homing pigeons in central Florida. One of the isolates was obtained from the brain of a dead pigeon and the second from the blood of an apparently healthy one. Twelve of 32 pigeons in the domestic flock from which the isolates were obtained died.