48. Equine encephalomyelitis caused by togaviruses (Zoon.). Flashcards
1
Q
Occurrence?
A
Eastern-, Western & Venezuelan Equine Encephalomyelitis ʹ Notifiable, Zoonotic
- Acute neurological disease of equine, only AMERICAs, transmit by mosquitoes, affects several species ʹ
- including Humans (birds maintain the virus)
Occurrence
- Eastern: Canada ʹ USA east coast, Central America
- Western: Canada ʹ USA west coast ʹ Mexico ʹ S. America
- Venezuelan: USA southern states ʹ Central & S. America
- Rarely in other continents in rodents & in birds detected ʹ without disease
2
Q
Causative agents?
A
Causative agents
- Togaviridae family, Alphavirus genus
- +ssRNA, icosahedral capsid, enveloped
- Weak resistance in the environment (heat, pH, drying out, detergent ʹ lipid layer)
- so the virus needs help to avoid the environmental conditions
- Inactivate above 37c,
- can handle only neutral pH
- Arbovirus ʹ mosquito vectors (biological vectors, virus replications after blood ʹ transovarial
- transmission too vertical)
- Euryxen: horse (dead-end host unless ʹ epizootic Venezuelan, high titre virus can infect others),
- donkey, mule, rodents, birds, humans (dead-end hosts)
- Rodents/birds maintain outside of mosquito season
- Antigenic diversity (types, subtypes)
- vaccines
3
Q
Epizootiology?
A
Epizootiology
- Eastern:
- Bird ʹ mosquito cycle (Culiseta melanura)
- Long-distance transmission ~ bird migration/maintenance host
- Seasonal epidemics ~ mosquito gradation ʹ warm, rainy summer & fall; wetlands
- Horse, turkey, pheasant, quail, duck, human (Eq + Ho: dead-end host)
- Western
- Bird ʹ mosquito (Culex tarsalis) ʹ rodent cycle
- Host preference of mosquitoes (birds vs mammals)
- Horse, human ʹ dead-end hosts
- Venezuelan
- Horse/rodent ʹ mosquito ʹ horse/rodent cycle
- Summer/rainy season
- Enzootic (rodent)
- Less pathogenic
- Epizootic (Eq)
- More apt to Eq: clinical signs and death, higher virulence!!
4
Q
Pathogenesis?
A
Pathogenesis
- Mosquito bite -> regional LN>1st viraemia-> visceral organs (br & s.c, haemorrhage or skin rash,
- fever, edema, if cross BBB Æ encephalitis!)
- 2nd viraemia (2 -5 days)
- Clinical signs, infection of the CNS
- Transplacental infection congenital
- embryo developmental issues (hypoplasia,
- hydrocephaly)
- Course of age & dose-dependent
- Abortive infection:
- only the first fever (immunocomp & low dose, vaccine -)
- Unapparent infection: no fever, low titre viraemia
- Abortion , damage placental vessels
- Mortality
- Eastern: 75-98%, Western: 10-50%, Venezuelan: 32-86%
- Virulence-variants!
- Convalescence: Life-long immunity, Partial cross-protection
5
Q
Clinical signs?
A
Clinical signs
- Incubation time: 1-3 weeks
- Peracute form: fever (>41oC), shock, death, within 12 hours
- Acute, subacute form:
- Biphasic fever
- At start, excitement, increased sensitivity
- Later apathy, low of appetite, icterus
- Signs of encephalitis (visual disturbances, head convulsions, paralysis)
- Signs of myelitis (lameness, convulsions, paralysis)
- terminal stage -> death
- Venezuelan: colic, diarrhoea too damage visceral organs
- Chronic form: rare, 1-2 weeks
- Recovery : permanent damages
6
Q
Pathology, histopathology?
A
Pathology, histopathology
- Macroscopically unspecific (e.g. icterus)
- Histopath:
- Encephalitis & myelitis
- Both grey & white matter affected
- Lymphatic cells, neutrophil granulocytes
- BV endothelial damage, thrombosis
- Chronic ʹ degenerative & reparative processes
- Diagnosis
- Clinical signs, seasonality (Late spread), geographic origin (America)
- Lab investigations
- Sample: blood (PBMC), brain
- Virus isolation: suckling mouse brain, embryonated egg, cell cultures
- RT-PCR, IHC; serology: HAI, VN, CF
7
Q
Differential diagnosis?
A
Differential diagnosis
- Infectious diseases (with CNS signs)
- Rabies ʹ course, lab
- Rhinopneumonitis ʹ mainly myelitis, abortions
- West Nile fever, Borna, Getah ʹ lab tests
- St. Louis encephalitis, Equine encephalosis ʹ occurrence, lab tests
- Listeriosis ʹ histopathology, rare in horse
- Botulism ʹ signs, histopath, bacteriology
- Non-infectious diseases
- Heavy metal poisoning (no encephalitis, labs)
- Plant poisons (same)
8
Q
Treatment, prevention and control?
A
Treatment, prevention & control
- In enzootic countries (Americas)
‣ Supportive treatment
‣ Immunisation
๏ Inactivated, trivalent vaccine
๏ 2x basic immunisation + yearly repetitions
๏ Venezuelan: strain TC 83 ➝ attenuated
➡ To block an outbreak
➡ Mild residual virulence, transmission into vectors
‣ Mosquito (and other vector) control - All 3 are OIE-listed diseases and notifiable in EU
- In EEV/WEEV/VEEV free countries
9
Q
Public health considerations?
A
Public health considerations
- Zoonosis! All 3 are OIE-listed 2020 & notifiable in EU
- Simultaneously with horse cases (same vectors)
- Infection of pregnant mother in the 2nd-3rd trimester abortion, newborn cerebral necrosis
- Children are more sensitive, more frequent CNS symptoms
- Fever, headache, nausea, vomiting
- Head, leg oedema, cyanosis
- Convulsions, paralysis, high mortality
- Permanent defects after recovery
- Central & S. America: extensive epidemics
- Human vaccine is available
10
Q
Further alphavirus infection?
A
Further alphavirus infections
- Chikungya
- Africa, India, Indonesia: great epidemics in 2006-2007
- Human: joint pains, fever, rash, haemorrhage
- Vector : Aedes albopictus (Asian tiger mosquito)
- Intro into Italy in 1990, with used car tyres
- Intro of the virus into Italy in 2007 - ~200 autochthonous cases, 1 lethal
- Further autochthonous cases in France & in Italy in 2015 & 2017
- Global warming ʹ establishing Asian mosquito population!
- O-nyong-nyong, Igbo Ora (in Africa) ʹ human: fever, rash
- Ross River fever (Australia, Ne-Guinea, Fiji-islands) ʹ human: fever, arthritis, rash
- Getah virus (S. E. Asia) ʹ horse encephalitis (in Japan)
- Sindbis virus (Africa, Europe) ʹ human: Ockelbo, Pogosta disease, Carelian fever (Scandinavia)
- Semliki forest virus (Africa)
