13. reoviridae & general arboviruses

steps in rabies pathogenesis

1. entry = skin via bite wound

2. local replication

   • can be blocked by antibodies via vaccination pre- or post-exposure

3. spread via nerves → NO viremia

   • replicate in brain neurons

4. spread via nerves to salivary glands

5. shed in saliva

in dogs and cats, what is the duration from rabies infection to clinical signs?

typically 6 months → quarantine to determine if animals are rabies-free

in dogs and cats, how long is the time between shedding in saliva & clinical signs? can this duration be used in other animals?

1. 10 days → quarantine (after bite) to determine if animal was infectious at time of exposure

2. NOT predictable for other animals

what type of genome do reoviridae viruses have?

segmented RNA → reassortment can occur

are reoviridae naked or enveloped?

naked → very stable in environment

what 3 reoviridae genera have veterinary importance?

• orthoreoviruses = respiratory, enteric, systemic disease

• rotaviruses = enteritis in young

• orbiviruses = arboviruses → systemic disease

rotaviruses are a very important cause of what pathologic condition? what age group is most severely affected by rotavirus spp.?

diarrhea in young animals

• usually < 2 months

• most severe in neonates

rotavirus host range

• many species, including humans, foals, calves, lambs, piglets, rabbits, etc.

• many serotypes — host specific

rotavirus transmission

fecal-oral

• virus shed at high levels in feces

• very stable in environment

rotavirus pathogenesis

• from lumen, rotavirus infects epithelial cells at the tips of the villi in small intestine → villous atrophy → malabsorption diarrhea

• reduced levels of lactase in gut & impaired glucose-dependent sodium transport → osmotic/maldigestion diarrhea

  • undigested lactose promotes secondary bacterial infection

• viral protein (NSP4) acts as enterotoxin → secretory diarrhea

rotavirus clinical signs

• 1-24 hour incubation period → rapid onset

• voluminous soft/liquid diarrhea ± mucus

  • “milk or white scours”

• distended abdomen

• severe diarrhea → dehydration → possible death

what makes an arthropod a biologic vector?

• virus replicates in the arthropod & is secreted in saliva when it feeds on new host

• arboviruses

what are examples of virus families that contain arboviruses? (from handout)

• togaviridae

• flaviviridae

• bunyaviridae

• reoviridae

• rhabdoviridae

• asfarviridae

what makes an arthropod a mechanical vector?

virus does not replicate in the arthropod & is transmitted on arthropod’s mouth parts during arthropod biting & feeding

examples of viruses that are transmitted via mechanical vectors

• swinepox virus

• myxoma virus

• equine infectious anemia virus

• lumpyskin disease virus

what is an enzootic cycle?

• transmission that maintains the virus in nature

• involves amplifying host and arthropod vector

what is an epizootic cycle?

• transmission that results in disease outbreak

• often due to “spill-over” into dead-end hosts

• virus itself, amplifying host, and mosquito species may differ from the enzootic cycle

steps in viral replication within biological vector

1. virus imbibed in blood

2. virus replicates in intestine

3. virus spreads to other tissues and replicates

4. virus replicates in salivary gland

steps in viral replication in amplifying vertebrate host

1. virus inoculated by mosquito bite

2. virus replicates in tissues

3. virus in blood

4. virus transmitted to naive mosquito in blood

amplifies the virus

virus infection in incidental or dead-end host

1. virus inoculated by mosquito bite

2. virus replicates in tissues

3. little or no virus in blood (not enough to infect naive arthropod vector)

important examples of orbiviruses

• bluetongue virus

• epizootic hemorrhagic disease virus

• african horse sickness virus (exotic to US)

geographical distribution of bluetongue virus

worldwide distribution

• emerging into new regions of US

• spreading in Europe

bluetongue virus vector

culicoides spp. (biting midges)

bluetongue virus transmission cycle

enzootic cycle

• amplifying hosts: ruminants

• arthropod vector: culicoides spp.

bluetongue virus alternate transmission

• vertical transmission to fetus can occur → abortion or hydrocephalus of newborn (common during outbreaks)

• rare events:

  • via infected meat to carnivores in zoo

  • via contaminated canine vaccines

orbivirus pathogenesis

1. virus deposited in skin by biting midge

2. spread to draining lymph node → viremia

3. replication in macrophages, dendritic cells, and endothelial cells

4. cytokine response & endothelial cell damage → vascular leakage, hemorrhage, infarctions

what are the 4 “E’s” (clinical signs) that can be observed during orbivirus infection?

• erythema of skin and mucosa

• erosions of mucosa, skin, teats

• edema, especially of head & neck

• effusions (pleural & pericardial)

bluetongue disease affects what species?

• sheep primarily

• also deer, pronghorn antelope, and bighorn sheep

• usually subclinical in cattle (but can cause clinical disease)

• rare disease in carnivores

• NOT zoonotic

clinical signs of bluetongue disease (in sheep)

• “sore muzzle disease”

• erythema, edema (can rarely cause cyanosis or “blue tongue”), erosions, effusion

• coronitis → lameness

• abortion or congenital defects (e.g. “dummy lamb” due to hydrocephaly")

• hyperemia of skin → “wool breaks” in survivors

clinical bluetongue in cattle

• erythema & discharge — eye & nose

• erosion, erythema & discharge — nares

• erosions, erythema — teats

bluetongue virus control

• vector control

• immunity is serotype-specific

• vaccine used in areas with disease

• import restrictions to prevent introduction of exotic serotypes

• reportable in wisconsin

are animals with bluetongue virus directly contagious?

NO → no shedding from lesions or mucosal membranes

epizootic hemorrhagic disease (EHD) is one of the most important diseases of what species?

white-tailed deer

• very similar to bluetongue disease

geographical distribution of EHD

• disease emerging into new regions of US with some disease in cattle

• disease outbreaks common

  • annual outbreaks in southeastern US

  • sporadic outbreaks in other regions

epizootic hemorrhagic disease vector

culicoides spp. (biting midges)

EHD host range

• primarily disease of white-tailed deer

• NOT sheep

• disease in cattle can occur (during outbreaks in deer)

• NOT zoonotic

EHD important clinical signs in deer

• erosions in mouth

• disseminated intravascular coagulation (DIC) → not usually seen with BTV

• mortality ~33%

EHD important clinical signs in cattle

• subclinical disease most common

• erosions on tongue, mouth, and teats

• coronitis & lameness

• death uncommon

EHD control

• vector control

• reportable in wisconsin

erosion

• destruction of surface layer of skin (epidermis) or mucosa (epithelium)

• basement membrane is intact

ulcer

• destruction into deeper layers of skin (dermis) or mucosa (submucosa)

• deeper than basement membrane

vesicle

• fluid filled sac within skin epidermis or mucosal epithelium

• blister-like

what is an important thing to consider when evaluating BTV/EHDV lesions?

• BTV and EHD cause erosions in mouth and coronary band

• in ruminants, must R/O foot-and-mouth disease (vesicular lesions; foreign animal disease)

  • ruptured vesicle without “top” flap of tissue looks like erosion

why would you not want to use a mucosal tissue sample when diagnosing bluetongue virus/EHD?

virus does not replicate in mucosa → infarction/lack of blood supply secondary to severe edema kills the mucosa, NOT viral replication

african horse sickness (AHS) geographical distribution

• **EXOTIC to US**

• enzootic in sub-saharan Africa

• outbreaks in europe, middle east

african horse sickness vector

culicoides spp. (biting midges)

how does african horse sickness compare to bluetongue virus?

pathogenesis is very similar to bluetongue virus except no erosions observed

african horse sickness host range

• primarily in equines

• horse > mule > donkey » zebra

• zebras are important reservoir

• fatal disease in dogs can occur (eating infected horse meat)

• NOT zoonotic

AHS major clinical signs 

• supraorbital edema

• frothy discharge due to fluid leakage in lower respiratory tract (pulmonary form)

• erythema and hemorrhage from eye

• high mortality (50-95% in horses)

AHS gross pathology

• pulmonary edema

• froth in airways

• pleural & pericardial effusions

• petechial hemorrhages

is AHS reportable?

YES

• exotic disease

• reportable within 1 day

AHS control

• surveillance/quarantine for horses imported from enzootic regions

• vector control

• vaccines in enzootic regions

  • no cross protection between serotypes