WEST NILE VIRUS

We have received numerous inquiries regarding how to prepare for West Nile Virus (WNV) in 2004 and specifically, how to approach vaccination from the standpoint of safety, efficacy, and economics. These are difficult questions to answer as there is limited information that specifically addresses disease risk and vaccination in llamas and alpacas. Based on the many reports from private practitioners and our experience at the CSU Veterinary Teaching Hospital, we know that both llamas and alpacas are susceptible to WNV infection and disease. While there is no specific data available, the reported frequency of confirmed and suspected cases in alpacas seems to be higher than llamas. Thus, it could be inferred that alpacas are more susceptible than llamas to clinical disease. The first Colorado case (human) of WNV in 2003 was in early June. The first alpaca case of WNV seen at CSU in 2003 was in mid August and the last confirmed case at CSU was near the end of September. In the near future, we will be sending out a questionnaire to llama and alpaca owners to try to ascertain more detailed information relative to WNV disease in Colorado during 2003.

As with other vector borne infections, WNV disease is best prevented by control of the vector, in this case mosquitoes, and stimulating immunity in susceptible animals. With most contagious diseases, establishing 70% to 80% herd immunity will successfully limit clinical disease from spreading among susceptible animals. However, in the case of WNV, individual immunity is much more important than population immunity because the reservoir hosts are birds and mosquitoes. Because of this, a susceptible non-immune animal is just as likely to become infected whether 0% or 99% of your herd is immune. Therefore, when we are looking at WNV vaccination programs we want to do our best to ensure that EVERY animal has sufficient immunity during the critical period of seasonal exposure.

There are currently two commercial vaccines licensed for use in horses. One is a killed virus vaccine (Innovator, Fort Dodge Animal Health) and the other is a recombinant canarypox DNA vaccine (Recombitek, Merial). The Fort Dodge Animal Health (Innovator) vaccine has been administered to llamas and alpacas but use of this vaccine must still be considered with appropriate caution. Studies by Dr. David Anderson at Ohio State University and Dr. Michelle Kutzler at Oregon State University suggest that this vaccine is safe in llamas and alpacas and will stimulate a humoral antibody response following three intramuscular injections given 30 days apart. The Merial canarypox DNA vaccine incorporates WNV membrane and envelope proteins into the canarypox DNA and expresses these proteins following administration. The method of antigen expression with the canarypox vaccine would suggest stimulation of both humoral (antibody) responses and cytotoxic T-cell responses believed to be important in both the prevention and clearance of WNV infection. No safety or efficacy studies of the Merial canarypox DNA vaccine have been performed in llamas or alpacas. We are working with Merial and the Colorado llama and alpaca industry to perform a safety and seroconversion study with the canarypox DNA vaccine prior to Spring 2004. Efficacy studies in horses show that both vaccines provide effective prevention of viremia following experimental challenge. While challenge studies have not been performed in llamas or alpacas, it is believed that the immunity induced by these vaccines will help reduce the incidence and severity of disease in these species.

Last year, anaphylactic-like reactions were described in two alpacas from Colorado that were vaccinated with the Fort Dodge Inovator vaccine. While both animals survived, this observation highlights the potential for adverse reactions when administering any vaccine to any animal. Because of the importance of monitoring adverse vaccine reactions, owners and veterinarians are strongly encouraged to report any adverse vaccine reactions to Fort Dodge Animal Health and the USDA Center for Veterinary Biologics by visiting the Adverse Event Reporting web site at http://www.aphis.usda.gov/vs/cvb/ic/adverseeventreport.htm or by calling 800-752-6255 . You may also send reports of WNV vaccine reactions to me by email (rcallan@colostate.edu) so that we can keep all Colorado camelid producers informed as further information is reported.

While there is significant latitude in how you administer any vaccination program, the following guidelines should help those of you planning to vaccinate your animals for WNV this year.

o Begin vaccination in March or April so that the animals will have antibody titers by the start of WNV season.

o As with any vaccine, inflammatory responses induced by vaccination can adversely affect pregnancy. If possible, avoid vaccinating breeding females within 60 days of breeding or within 30 days of parturition.

o At this time, there is no indication that animals will need an additional booster in the middle of the WNV season. However, some owners and veterinarians may want to consider an additional booster in August to provide higher immunity during the peak WNV months of September and October.

o If the dam was vaccinated or has a confirmed WNV titer, start vaccination after 3 months of age or later depending on the season.

o If the dam was not vaccinated and/or has a negative WNV titer, start vaccination at 1 month of age or later depending on the season.

Due to the expense of the WNV vaccine, some veterinarians and producers have suggested screening the herd for antibody titers prior to vaccinating and only giving the three dose vaccination regimen to animals with negative titers. WNV titers can be determined by collecting a blood sample from the animal and sending it for a WNV Microtiter Serum Neutralization Test (Cornell Veterinary Diagnostic Laboratory) or a Plaque Reduction Neutralization Test (National Veterinary Services Laboratory). This is a reasonable strategy but it must be emphasized that even if a large proportion of a herd has antibody titers, the seronegative animals still need to receive a full three dose series in order to be protected. Otherwise, you will still have individual animals that are susceptible to infection and disease.

The most complete screening and vaccination recommendation is to test a portion of your herd (i.e. 10 animals or 10%) to determine an estimate of the prevalence of seronegative animals. From that information, you can evaluate the economics of whether you should just vaccinate all animals with three doses, or test all animals and vaccinate the seronegative ones with three doses and the seropositive ones with just a single dose. We have received several inquires as to how to apply this process and at what seropositive prevalence does one decide whether or not to vaccinate all animals with three doses or test the remaining herd and vaccinate based on titer. Basically, this is an economic decision where you need to decide which is less expensive, to test and differentially vaccinate or to simply vaccinate all animals with three doses. The basic economic model indicates that one should simply vaccinate all animals with 3 doses of vaccine if the prevalence of seropositive animals is less than ½ the cost of testing divided by the cost of a single vaccination dose.

Vaccinate All Animals with 3 Vaccine Doses IF:

P< T/2V where:

P = Estimated prevalence of seropositive animals based on your herd screening

T = Test Cost per animal including supplies, veterinary charges, labor, and postage

V = Vaccine Cost per dose including vaccine, supplies, veterinary charges, and labor

You should work with your veterinarian to determine the specific costs of testing and vaccination in your herd but in general, it is less expensive to simply vaccinate all animals unless the prevalence of seropositive animals in the herd is expected to be greater than 75%. Because of the need to test at least 10 animals to get an accurate estimate of the prevalence of seropositive animals, the break even prevalence becomes higher for herds with less than 100 animals. Even after last year, it is unlikely that many herds have a seroprevalence higher than 75% unless they were in a high endemic area. Thus, it seems that simply vaccinating all animals with three doses is the most economical protocol to adopt for the average herd.

The last consideration is the point where potential economic loss from disease becomes less than the cost to vaccinate. Based on very limited information, our best estimate at this time is that approximately 10% of infected non-vaccinated alpacas become ill with a case mortality rate approaching 50%. In an endemic area, the infection rate may approach 50% and you might expect 1 in 20 animals to become sick and perhaps 1 in 40 animals to die from WNV. Very few areas of Colorado saw this level of infection last year. Assuming a 50% infection rate in an endemic area, the estimated cost of WNV is 2.5% (0.5 infected x 0.1 cases per infection x 0.5 case mortality) the average value of the animals plus veterinary expenses. Some producers may take the approach that aggressive vaccination of the entire herd is not warranted and will accept some losses from this disease. In general, vaccination of animals is economically warranted if the cost of vaccination is less than the potential savings from preventing the disease through vaccination. For example, if we make the assumption that the vaccine will prevent disease in 100% of infected animals, then the average return on vaccination is the average cost of disease per susceptible animal. It is estimated that in an endemic area the average cost of disease per susceptible animal in a herd is $275 based on conservative figures relative to the value of individual animals and the cost of treatment. In this case, you can see that three doses of vaccine and/or annual boosters are economically prudent. The vaccination break even point occurs when the economic value of the animal is $1400 in an endemic area. This break even point decreases if the cost of vaccination, the value of an individual animal, the cost of treating a sick animal, or the incidence of WNV infection is lower. Thus vaccination may not be economically warranted under many circumstances.

Hopefully this discussion will give you some useful information to help decide how to approach WNV vaccination. You should work closely with your veterinarian to determine if WNV vaccination is appropriate for your herd. If you have further questions, please consult with your local veterinarian or feel free to contact the veterinarians at CSU.

As many of you know, the high desert, western and mountain areas of the United
States are currently associated with a growing West Nile virus outbreak. In
addition, there are other spotty outbreaks. Hundreds of human cases have occurred
with many hospitalizations and some deaths. Network television recently
reported "significant alpaca deaths due to West Nile virus" in Colorado. This
has fueled misinformation both regarding susceptibility of alpacas to the disease
and has unnecessarily caused concern that alpacas may be "spreading the disease."
Both are incorrect.

However, veterinarians in the heavily afflicted areas are frantic with calls from
concerned camelid owners and are incredibly busy treating actual cases and
potential infections. Due to the workload, they have not had the time to
research the Internet or to call other colleagues to discuss promising clinical
treatments, recent developments and other relevant information.

To this end, we are posting website, telephone, fax, and e-mail for contacting
academic veterinarians who have significant experience with this disease. We are
not listing the many superb non-academic veterinarians with West Nile experience
as we recognize that many of these field clinicians are so busy treating actual
cases. The contact information may be passed on to your local veterinarian.

We will soon be posting a web-site URL so that veterinarians and other interested
individuals can access the latest information through the International Camelid
Institute (ICI). The URL for the ICI site is https://icinfo.vet.ohio-state.edu/ (double click
on the blue text to automatically go there). It will soon have links to the
camelid emergency medical alert and notification system.

In addition, the Alpaca Research Foundation (ARF) has updates http://www.alpacaresearchfoundation.org as well as the Rocky Mountain Llama & Alpaca Association (RMLA) which has very recent communications on outbreaks.

Ohio State University:
Dr. Miesner, DVM and International Camelid Institute
Emergency Cell Number: (614) 403-1016
Fax (614) 292-3530
Dr. David Anderson Emergency Contact Number: (614) 403-1017
e-mail for Dr. Anderson anderson.670@osu.edu
Veterinary Hospital (24 hour): (614) 292-6661, ext 10 (large animal)

Colorado State University
Dr. Van Metre, DVM
Dr. Callan, DVM
Telephone: 970-221-4535
Fax: (970) 491-4100

Oregon State University
Telephone: (541) 737-2858
Dr. Chris Cebra
Telephone: (541) 737-2858
E-mail: christopher.cebra@oregonstate.edu

Websites for more information:
Your State Department of Health: (search for West Nile)
Your StateDepartment of Agriculture
Centers for Disease Control and Prevention: "http://www.cdc.gov/" (search
for West Nile)
United States Geological Survey

David E Anderson, DVM, MS
The Ohio State University
https://icinfo.vet.ohio-state.edu/