Bunyavirus: La Crosse Encephalitis Virus

Group #7Randall Chan

Kamal Hamouda

General Overview Of Bunyaviruses•Negative-sense RNA virus-Class V•Enveloped, spherical•80-100 nm in diameter•Genome, composed of 3 RNA segments,

Large, Medium and Small (L,M,S respectively)▫ L encodes RNA Dependent RNA-pol▫ M encodes for viral glycoproteins, to attach to host cell▫ S encodes for nucleocapsid protein▫ ssRNA helical structure, L and M are negative sense▫ Ambisense in phlebovirus

Mode of Transmission for Bunyavirus•Usually found in arthropods or rodents, but

some do infect humans, and rarely plants.•Generally a vector-borne virus.•Transmission occurs via arthropod, with

hantavirus through deer mice feces.•Closely follows vector activity, such as

more in the summer.•Has high levels of morbidity and mortality,

BSL 4.

Genera of Bunyaviruses• Bunyavirus: includes Akabane and California

encephalitis• Phlebovirus: includes Rift Valley fever virus,• Nairovirus: includes Nairobi sheep disease and

crimean-congo hemorrhagic fever viruses• Uukuvirus: does not contain pathogenic viruses• Hantavirus: infects rodents and causes

hemorrhagic fever with renal syndrome in humans.▫Assoc. with fever, lung edema and pulmonary failure

• Orthobunyaviruses, includes Bunymwera virus

Symptoms of Bunyaviruses

Dengue

Congo-Hemmorhaggic Fever

Hantaviruses

History of La Crosse Virus•La Crosse virus was discovered in La

Crosse, Wisconsin in 1963.•Occurs in the Appalachian/Midwestern

regions of the United States.•Recent cases have also occurred in the

South East of the United States.•Suspected to have broader distribution

and higher incidence in Eastern United States, but is under-reported.

Mode of Transmission for La Crosse Virus:

•Mosquito Vector:▫Virus is maintained during the winter by

“transovarial transmission” in mosquito eggs.

▫Zoonotic pathogen: Primarily found in infected treehole mosquito Aedes triseriatus, but can also be found in Aedes canadensis, Aedes sollicitans, and Aedes vexans.

▫The virus is maintained and amplified in treehole mosquito species through transovarial and venereal transmission.

La Crosse Virus Mode of Transmission:•Other Mode of Transmission:

▫Amplification can also occur in chipmunks and squirrels by mosquito infection.

▫Humans are dead-end hosts for the virus: Do not circulate enough La Crosse virus in

the bloodstream to infect mosquito. The infection cannot be spread from person

to person.

La Crosse Virus Transmission Cycle:

Pathogenesis of La Crosse Virus• The gut of the

vector is infected initially, and after a few days/weeks the virus appears in the saliva.

• The mosquito is infected, but is not ill.

• Through mosquito bite, the infected saliva enters the small capillaries or lymphatics of the human or other vertebrate host.

Pathogenesis of La Crosse Virus•An incubation period of a few days ensues, after which the vertebrate host enters the bloodstream.•The infection is generally inapparent, but in some circumstances can cause host to become febrile.•Unless the virus spreads to the target organ, humoral antibody is able to stop the spread of the virus and host recovers.•The target organ for La Crosse virus is the brain.

Symptoms & Disease Caused by La Crosse Virus:

• Takes 5 to 15 days after the bite of an infected mosquito to develop symptoms of La Crosse virus disease.

• Symptoms for mild cases:• Nausea• Headache• Vomiting

• Symptoms for severe cases:• Seizures• Coma• Paralysis• Permanent brain damage

Symptoms & Disease Caused by La Crosse Virus:

• La Crosse encephalitis occurs as a nonspecific summertime illness.

• Severe disease generally occurs in children under the age of 16.

• Death from La Crosse encephalitis is often rare; occurring in less than 1% of clinical cases.

Group #7-Case Study:•A 15 year-old summer camp counselor in

Ohio suddenly complained of a headache, nausea, and vomiting; she had a fever and experienced a stiff neck. She was admitted to the hospital, where a spinal tap and examination of cerebrospinal fluid revealed inflammatory cells. She became lethargic over the next day but became alert again after 4 to 5 days.

Case Study Questions:1. The physician suspected La Crosse encephalitis virus

as the agent. What clues pointed to La Crosse virus?

2. What other agents would also be considered in the differential diagnosis?

3. How was the patient infected?

4. How would the transmission of this agent be prevented?

5. How could the local Public Health Department determine the prevalence of La Crosse virus in the environment of the summer camp? What samples would they obtain, and how would they test them?

1. The physician suspected La Crosse encephalitis virus as the agent. What clues pointed to La Crosse virus?

•Symptoms▫Headaches, vomiting, fever, neck stiffness▫Inflammatory cells in CSF▫-General Encephalitis▫4-5 days of incubation

•Location▫Forested Area in the summertime

•Patient fits the category▫Usually found in younger cohorts

2. What other agents would also be considered in the differential diagnosis?

•Meningitis distinguishes it from other viral, fungal or bacterial agents▫Cannot be distinguished from other tick-

borne encephalitis•Other bunyaviruses are fairly rare in the

United States▫Others could include Dengue, Eastern

Equine Encephalitis•Midwestern United States

▫Most likely La Crosse Virus

3. How was the patient infected?

•Patient was infected through the bite of the vector, mosquito▫Maintained in nature through vertical

transmission in trans-ovarial transmission▫Can be horizontally transmitted to other

vertebrates who act as amplifiers▫Humans are considered dead-end hosts

•Mosquito most likely belongs to Aedes triseriatus

4. How would the transmission of this agent be prevented?

•No vaccination or preventative medication discovered for La Crosse virus infection.

•Supportive therapy: includes hospitalization, respiratory support, IV fluids, and prevention from other infections.

• Preventative measures: greatly reduces infection of La Crosse virus.

Preventative Measures:•Insect repellent: applying Deet,

picaridin, IR3535 or oil of lemon eucalyptus (PMD) on exposed skin or clothing.

•Clothing: wearing long sleeves, thick pants, and socks will also reduce the chances of a mosquito bite.▫Can be problematic because of infecting

cycle of La Crosse virus occurs during spring/summer.

Preventative Measures:•Eliminating mosquito breeding site:

▫ Aedes triseriatus are tree hole breeding mosquitoes that live in woodland habitat.

▫Prevent these mosquitoes breeding site by emptying water from containers, barrels, flower pots, etc…

▫Mosquito breeding sites are generally tree holes, filling the tree holes around the environment of breeding sites with soil.

5. How could the local Public Health Department determine the prevalence of La Crosse virus in the environment of the summer camp? What samples would they obtain, and how would they test them?

Prevalence of La Crosse Virus:•Collect samples of possible carriers or

hosts (mosquitoes, chipmunks/tree squirrels, humans) of the La Crosse virus in the summer camp environment.

•Samples should be obtained as either CSF (cerebral spinal fluid) or serum.

Prevalence of La Crosse Virus:•Reverse transcriptase polymerase

chain reaction (RT-PCR):▫Can be used if virus is isolated.

•Serological Tests Includes:▫Complement fixation test▫ Immunofluorescent antibody tests▫ The enzyme-linked immunosorbent assay

(ELISA)▫ Hemagglutination inhibition test▫Neutralization test

Prevalence of La Crosse Virus:•Serological tests allow detection of either

virus-specified antibody or antigen to the virus in the samples.

•Testing for immunoglobulin M antibodies (IgM) and neutralizing antibodies to La Crosse virus and many other viruses with similar classification is necessary in order to find out if La Crosse virus is the causative agent.

Prevalence of La Crosse Virus:•Samples that are tested positive for LACV

should be sent to the Centers for Disease Control and Prevention (CDC) in order to affirm that the samples contain LACV.

•The samples should indicate whether there is a high or low prevalence of the surrounding summer camp environment.

•Local Public Health Department can take the appropriate actions depending on the prevalence of La Crosse virus in that environment.

References• Shope RE. Bunyaviruses. In: Baron S, editor. Medical Microbiology. 4th edition.

Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 56. Available from: http://www.ncbi.nlm.nih.gov/books/NBK8004/

• La Crosse Encephalitis Virus Disease. Ohio Department of Health Fact Sheet[Internet]. 2012 Jan [cited 2012 Oct 27] Available from: http://www.odh.ohio.gov/pdf/idcm/lac.pdf

• “La Crosse Encephalitis.”Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 16 Oct. 2009. 27 Oct. 2012. http://www.cdc.gov/lac/gen/qa.html