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Friday, February 22, 2013

Vetmed subject: West nile

Zoetis talk at the vet school today - presenters Rocky Bigbie and Jeff Hall, both DVMs.

Below I'm going to summarize my notes from the talk, and insert some other learning points from my classes. Call it AERC convention practice.....

West Nile

Single stranded enveloped RNA virus (Flaviviridae, arbovirus). What does that mean? Typically enveloped viruses are not as hardy in the environment - viruses known as being especially persistent in the environment (polio, parvo) typically don't have an just from this information, I know that WNV probably is a reservoir in a host somewhere NOT in the environment.

Transmitted primarily by mosquitos belonging to Culex sp. (this is important - sometimes the only reason there isn't a certain disease in a certain country is because the vector isn't present. )

First isolated in Uganda (West nile district) in 1937. Noted in Egypt and france in early 1960s. Entered the western hemisphere in 1999. First discovered at the Bronx zoo - lots of birds were dying. Public health and CDC officials running all over.......thought it was japenese encephalitis and the zoo vet is the one that confirmed west nile!!!!!!!! (that's because vets are the coolest... :))

Life cycle: mosquito to birds to mosquitos to birds. 99% of the virus on the planet is in the blood of birds --> There can be so much virus in the blood of birds that it can actually be like sludge.

Horses and humans are incidental infections and dead-end hosts - virus levels stay low enough that it cannot be passed on, even to pasturemates. Dogs and cats DO get west nile - however usually the disease is of no-consequence unless the animal is already sick.

WNV mutated when it came to north america. The new mutant has a shorter incubation period (by 2 to 4 days) in mosquitoes compared to the original strain - this allowed for more efficient transmission within the mosquito population and selection pressure for the newer strain. HOWEVER, unlike the influenza mutations you might be familiar with, this mutation did not affect the immunity any of us (or our vaccinated horses) have, even if we were infected with the "old strain" (because of where the mutation is in the virus). - The mutation affected how it amplified in mosquitos and is the reason why WNV spread so much faster than the experts initially predicted.

Clinical signs in horses - (BTW - mules/donkeys get WNV just like horses - the rumor that they didn't has turned out to be false)

1. muscle fasciculations (60%). A very unique feature of this disease - looks like nothing else.
2. Recumbency (35%)
3. Depression (51%)
4. Mortality rate (30-38% overall, 70% for those that become recumbent).
5. Weakness (100%)
6. Ataxia (100%)
7. Fever
*****Major rule out (alternative diagnosis) is EPM (which I'm also learning about in school, and that is a sucky disease too...)

*Remember that some horses won't present for any signs at all, get bit, get the virus, get over it and have titers and you won't even know they've had it. A bunch of us have west nile titers too --> got bit by a mosquito, got a headache, took care of it.

Treatment: nursing care, symptomatic. KEEP THEM UP, keep them hydrated, anti-inflammatory. Average cost over $1,000/day if recumbent. (about $2,200 per horse in one survey - $350-8,500 range)

Prevention: eliminate standing water - tires are the worst offenders. In a single tire, 10,000 mosquito larvae can be harvested. Get rid of the tire piles. This is probably NOT a good fencing alternative if you are concerned about west nile....

****4 days required for mosquito breeding!

Add fish to any tanks or ornamental pools.

Aerial spraying may be of limited value (even though the adults die there are TON of larvae and egg stages that aren't affected).

Topical insect control on the horse

Stalling horses from dusk to dawn with a fan is an excellent preventative.

Vaccinate: Disease kills, vaccine works, adverse reactions are rare (keeping in mind that this presentation was by a vaccine company). Case control study showed almost 97% vaccine efficacy. Vaccinated horse are 31x less likely to get the disease.

What about bird control? no vaccine labeled for use in birds (however there have been people who have given 1000's of doses to birds). Zoos are experimenting with using WNV vaccines in birds - response is species dependent and it's important individually test the efficacy in each species. Adverse reactions almost non-existant. (FYI - the California condors have been vaccinated.......I think the paper is on science direct if you want to google and read it).

Other random facts of interest

FYI September is the WORST month for west nile infections (for us horse owners that are vaccinating --> best to do so prior to the vector season and make sure that September is included if you are using a 6 month protocol.

2012 worst year for West Nile (human infections)--> just not talking about it anymore in the nightly news......The horses aren't falling over dead because the vaccination protocols have largely worked.

Important to remember that even though we don't get nightly updates from the news on the Risk from the virus is still out there!

Compared west nile to another disease - Parvo. Once epidemic, now endemic. Was shown some data that when states stopped vaccinating because the thought was the prevalence was low and perhaps it wasn't necessary to vaccinate any more, the disease rates shot up.


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  2. BTW - WNV is a vaccine that I am careful to keep current. Farley lives on the bank of a river.....and there are a TON of mosquitos with mild weather (thank you CA) so yeah.....This along with rabies/tetanus is something I consider part of Farley's core.

  3. Lots of WNV outbreaks: I think that there are more vaccine reactions that don't get reported, or that go unnoticed. I am super careful about vaccinating.

  4. We live in a swamp. We vaccinate!

  5. Karen - checking out that outbreak link now and it's really cool! signed up for alerts. Rabies and west nile in my area (of course) with a human case reported about half way between where I board and where I go to school.

  6. Btw - zoetis is the new name for the Pfizer company :)


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