Waterfowl and West Nile Virus- Updated
Over the course of the last several seasons, I had experienced some
mortality among my young birds that I couldn’t explain. The deaths were
relatively sudden and involved virtually no symptoms. I suspected that the
deaths could be the result of West Nile Virus. In August of 2006 I sent one of the dead birds
off to the Pennsylvania Diagnostic Lab in New Bolton Center, PA. The lab acknowledged
receipt of the bird and gave me a preliminary report which basically ruled
out a couple of common summer problems including Coccidiosis and Enteritis.
The vet explained that West Nile was suspected but that further tests would
be needed to confirm that. The additional tests took more than two weeks but
the results were a definitive diagnosis of West Nile Virus. Since the deaths
of all of the other birds happened in an almost identical way, I proceeded on the assumption that all died from West Nile. If you are wondering about
the costs associated with the necropsy and additional tests, my costs were
about $15 to ship the bird in ice to PA and the lab charges were $35. New
Bolton Center is a Vet School so the actual costs were heavily discounted.
As most people know, West Nile Virus is transmitted to birds (and
humans) by mosquitoes that have bitten an infected bird and have become a
carrier of the virus. The fact is that West Nile can be transmitted by other
blood letting organisms including sand flies, so-called “no see ums”,
and ticks. Mosquitoes are just by far the most likely to bite multiple victims
in the time that the virus remains active in their bodies. One hundred and
thirty eight types of birds are listed by one agency as having been fatally
attacked by the virus. Of these, waterfowl are apparently among the most vulnerable.
Even among waterfowl, some species seem to be more affected than others and Mallard derivitive domestic breeds tend to be more vulnerable. Interestingly enough, chickens are evidently immune to the harmful effects
of the virus although a chicken’s body may develop antibodies if bitten
by an infected mosquito. Chickens are actually used by some agencies as “sentinel
birds” to monitor for the presence of West Nile in a given area. The
agencies take blood samples regularly to check for the presence of the antibodies.
The mosquitoes are believed to carry the most virus organisms in
the early fall, principally late August and the first part of September here
in the Midwest. Cases of West Nile Virus either in humans or birds have been
recorded in every state within the U.S. except Alaska and Hawaii. A relatively
high percentage of birds that contract West Nile seem to die from it as compared
with other organisms. In birds, perhaps 40% of the cases will prove fatal
as compared with humans and horses where the fatality rate may only be a few
percent with only about 20% even showing symptoms. The good news is there
is evidence that once an organism contracts the virus and survives, that antibodies
are produced that may confer long lasting immunity. The bad news is that at
present, no practical vaccine is available for poultry. It also seems likely
that an immune bird cannot pass that immunity on to its offspring.
My own experience jibes in many ways with the findings I have cited
above. I lost 16 young East Indies over a several week period. That number
comprised roughly 50% of the young I raised in 2006 in that breed. Once a
bird showed it was in distress, it usually was dead within about 24-48 hours.
Only one adult bird (a Mandarin) was lost during this period. The virus stopped
claiming victims as suddenly as it started and several weeks have passed with
all remaining birds seemingly quite healthy. I suspect that most if not all
of the young birds were infected but the survivor’s immune systems were
able to fight off the virus. Since there is no visual evidence left by the
virus on organs, posting a dead bird will not result in positive proof that
the bird died of West Nile. The proof can only be found in spinal cord fluid
and blood which is properly tested.
I am at a loss to point to a practical way of protecting the birds
from the mosquitoes short of housing them in screened enclosures. One obvious
thing to do is to make sure that one does not permit the mosquitoes to breed
on one’s premises. No pools of stagnant water should be allowed to form
in buckets, pots, unused feed or water containers, etc. Occasional fogging
to kill mosquitoes is probably also a good routine.
Prior to the 2007 hatching season I fully screened one rearing pen to test the effectiveness of screening in keeping the birds from being infected. None of the young East Indies or Mandarins that were reared in that pen showed symptons of West Nile Virus so I am convinced that screening is an effective (although expensive) way to protect the young birds. It remains to be seen whether young birds protected in that manner will acquire West Nile when they are yearlings and not in a screened flight pen. Stay tuned...
Update: Fall of 2008
I have two pieces of encouraging news to report. First of all, the screened rearing pen has evidently kept all birds in it from becoming infected with West Nile Virus for a second year. Second, I have not lost any of the yearling birds I protected last year in that pen even though they are no longer being kept in a screened enclosure.
My conclusions are that fully screening rearing facilities is effective and that yearling birds evidently have a fully developed immune system capable of warding off fatal onset of the virus. That is very encouraging indeed even though building fully screened rearing pens can very labor intensive and expensive.
Update Fall of 2009
I am becoming convinced that some degree of immunity is passed fom generation to generation of waterfowl once they have been exposed to West Nile Virus for several years. I have not experienced any losses from the Virus for the last two years in either young or old stock as of this writing. While I continue to keep young stock ina screened pen overnite, they are now permitted to range during the day. My old birds are not kept in a screened pen- no problems as of this writing. Interestingly, a news report hinted that the human population may also be experiencing some degree of acquired immunity. Last year in the state of Illinois, there were over 80 serious cases of West Nile Virus and a number of deaths resulted. This year, only two serious cases have been reported.
Update Fall of 2010
Our area experienced more rain this past summer than I can ever remember in the last 25 years. As a result, I was more concerned than ever that West Nile Virus would wreak havoc with my young birds in particular- it did not happen. I did have one or two deaths that I attributed to West Nile- both late hatched young East Indies. Overall, however, I had little problem with the Mosquito borne virus compared with other waterfowl breeders in my general area who lost more birds, lost large ducks, and lost both old and young instead of just young birds.
I am becoming firmly convinced that once the virus passes through an area, surviving birds do develop some degree of immunity. I did use the fully screened rearing pen to protect the young birds over night but they were given the freedom of the yard for as much as 12 hours a day even when the place was crawling with mosquitoes. Of course, the specie of mosquito most prevalent during heavy rains is the so called flood water mosquito which does not carry the Virus. As an added bonus, the ducks ate thousands of the mosquitoes they found in the grass each morning as well as many of the moths that cause the harmful (to the turf grass) Sod Web Worms. The ducks thrived on that diet and are in the best condition this fall that I have ever seen them. We are expecting a frost within the next day or two and that will end the threat of West Nile for the current year.
Originally published: 02-12-2006
Last updated: 10-03-2010