Breaking the Transmission Cycle?

    Each year when SYMVCD sprays adulticides over populated areas officials make some variation of this statement to the press: "When a certain threshold of the virus is reached we have to spray to break the transmission cycle." Another phrase we heard this year is "tamp down the virus." While both are catchy phrases, which make it sound as if they are actually accomplishing something and saving people from a serious threat, we urge people to insist on answers to the following questions:

1.  Now that the previous trigger has been abandoned, which involved the virus in mosquitoes, birds, humans, horses, and sentinel chickens, what exactly is the formula or mechanism for determining that the spray threshold has been reached?

2.  What is the scientific justification for this threshold?

3.  What exactly does "break the transmission cycle" or "tamp down the virus" mean?

4.  What is the scientific evidence that this spray program does that, whatever it means?

    We have queried officials over the years about this trigger, and the responses have always been very murky.  Based on this experience we suggest that the actual answers to the above four questions are as follows:

1.  The formula or mechanism must be "spraying by the seat of their pants."  A perusal of the SYMVCD website reveals no trace of the previous index, for which we have never been given any scientific basis, and the current discussion makes it difficult to tell how this threshold is now determined.  It appears that birds and mosquitoes may have replaced humans in the definition of epidemic.

2.  There is none – no scientific justification has ever been supplied, for what they did before or what they are doing now.

3.  This means nothing.  As mathematical modeling and data collection show, a two or three day spray program knocks down the numbers of mosquitoes temporarily, but the numbers quickly rebound to their previous levels.  No effect on status of the virus in the birds has been reported, and the birds are a critical component in the spread of the virus.  This lack of good scientific evaluation about the level of infection in the birds makes the assessment of efficacy of any kind of control program almost impossible to justify scientifically because the most important data is absent.  The mathematical modeling suggests that a small, 6%, increase in mortality in larval mosquito populations early in the season has the same effect as a 90% reduction of the adult population in July. This modeling would justify an early treatment date for larvaciding but not insecticides applied against the adult mosquitoes.

4.  There is none – no cycle of any kind is broken by this.  There is neither scientific nor anecdotal evidence to support suppression of the virus long enough to actually halt transmission of the virus to humans.  Short of actually breaking the transmission cycle, effective control can be obtained if at least 90% of the adults can be killed, as Cornell entomologist David Pimentel notes.  However, local officials report cumulative kill of only about 50% of the important vector, Culex tarsalis, during three days of spray in the sprayed area. The current two day protocol could be expected to kill less than 40% of the adult mosquito population.  And, both mosquitoes and birds will move within and into the sprayed region to help boost the virus back to pre-spray levels. 

    It seems critical to insist on answers to the four questions in any discussions with vector control officials.  Many people seem to assume that the spray program is effective and then concern themselves with the tradeoff of putting more toxins in the environment for protection from a disease.  Since the spray program is ineffective, the only thing accomplished is to add toxins to the environment – we are assuming a certain level of risk for zero gain – spraying adulticides puts us in a lose-lose situation.

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June 26, 2012 Note:  After we recently posted the above analysis, a belated 2012 West Nile virus Activity page appeared on the SYMVCD website.

    Observe that the totals for the year include the components used in risk assessment in previous years: humans, horses, sentinel animals, mosquitoes, and birds.  However, a perusal of the cited West Nile Risk Assessment for 6/22/2012 reveals the use of different components and not humans: environmental conditions, mosquito abundance, virus infection in mosquitoes, sentinel chickens, and birds.  Also, the assessment appears to be a district-wide one, so it is unclear how a specific spray area can be targeted.  We still have been given no scientific justification for any of this, and it thus seems to remain "seat of the pants."  Perhaps it is time to begin to press officials for the reason they spray somewhere in the Sac/Yolo district at least once a year – is it out of a genuine concern for the public health or is it out of concern for maximizing upcoming SYMVCD budgets by giving a highly visible illusion that something is being accomplished?  Whatever the motivation, the complete lack of evidence of efficacy for the spray program means that SYMVCD is indeed synonymous with Seat of the Pants Vector Control.