An Alternate Point of View on West Nile Vaccination
Scott and I are having a difference of opinion. Speaking from experience, it’s certainly difficult to win a debate with him most days (that might be a huge understatement), but I’ll weigh in with my two cents on this one. If Scott were to vaccinate a horse for West Nile virus (WNV), he’s stated that he wouldn’t do so until July, based on when the peak incidence of disease and therefore presumably the period of highest risk is. I would still vaccinate a southern-Ontario horse in April. Here’s why:
West Nile vaccines are labeled to provide protection for 12 months. Before they can be marketed with such a label, these vaccines have to be tested to prove that they still offer some protection for the animal for at least that long. For some vaccines, like rabies, protection likely lasts much longer than the label claim, but until recently no one’s bothered to study most vaccines beyond one year. I have no doubt that the protective immunity does decrease with time – the protective effects of vaccine are likely highest (as Scott said) about 30 days post vaccination, and lowest at the end of the 12 months. But there is no evidence that the immunity drops off so fast that after 4-6 months the vaccine would require a booster to be adequately, if not maximally, effective. There are vaccines, like herpesvirus and influenza, for which we recommend boosters for horses semi-annually, but this is for animals that are at ongoing high-risk for exposure to these diseases, which are very common. Six months after mid-April is mid-October, and in this part of the world there are very few mosquitoes still flying around at that point.
Scott pointed out that the first part of the summer is likely lower risk in terms of WNV transmission, at least for horses. But there is a time of year when the risk is even lower – October to April, when (as I just said) there are virtually no mosquitoes. If you vaccinate a horse in July, its immunity will be lowest over the first three months of the summer, when there are still birds and mosquitoes around that are carrying WNV. If you vaccinate a horse in April, its immunity is lowest in the late winter, when there’s almost no risk of transmission, so it doesn’t matter! I would rather have a horse protected for the entire mosquito season (May-September), and in the vast majority of cases the animal’s immunity will still be quite adequate come peak season in August, even without an extra booster in July. (Although we want to protect our animals from infectious disease, we also don’t want to give them any more vaccines than we have to.)
I don’t think there’s a right or wrong answer in this case. The best thing to do is talk to your veterinarian about the pros and cons of doing things either way, while taking into consideration the conditions in your specific region in terms of vector populations, disease prevalence, and the health management priorities for your own animals.
Photo: Transmission electron micrograph (TEM) of the West Nile virus (WNV). (Credit: Cynthia Goldsmith, CDC Public Health Image Library ID#10701)
Dr. Anderson,your idea, is the correct one in this case ,to get better protection for a longer period of time. My opinion. I really like the topics that you both talk about. keep up the excellent work. bw.
Good points. But...how long does protection actually last after vaccination. The AAEP recommends that for high risk animals "more frequent vaccination or appropriately timed revaccination is recommended in order to induce protective immunity during periods of likely exposure." That's not a ringing endorsement for full year protection. Like Maureen said, there's not a clear answer. I like to focus on risk-based methods and I think while you can argue either approach, I'll stick with my idea until I get good evidence to the contrary.