Why not Concentrate on Safe Protection of Citizens from Mosquito-Borne Diseases?

Jim Northup
    The Sacramento-Yolo Mosquito Vector Control District (SYMVCD, or the District) is not utilizing the full compliment of safe and effective biological-control agents for mosquito-borne diseases.  While the SYMVCD grows its own mosquito fish, we have queried officials about why they have not resurrected their program of culturing Romanomermis culicivorax, a mosquito-parasitic nematode that has achieved much higher kill rates and has been well studied here in California (Brown, Platzer and Hughes 1977; Peterson 1977; Peterson, Chapman and Willis 1979). This organism is an obligate parasite of larval Culicidae, mosquitoes, and infects nothing else.  That is, it presents no risk to either human health or the environment.  Roughly 20 years ago both the Sac-Yolo and Sutter-Yuba mosquito-abatement districts cultured their own Romanomermis in vivo, but there has been no mention of re-establishing such a culture program or developing one for additional bio-control agents.

    The efficacy of Romanomermis in combating mosquito-borne diseases is well established.  For example, in 1983, responding to a desperate appeal of the Health Ministry of Colombia, the Sutter-Yuba Mosquito Abatement District furnished one hundred million eggs of Romanomermis to be utilized in an isolated region of Choco Province, Colombia. A 95% reduction of Anopheles albimanus was achieved, and the human epidemic of malaria was monitored for two years and showed a reduction in the prevalence of the disease from 23% to near zero in humans.  This reduced rate persisted for nearly two years on a single inundative release (Rojas et al, 1987).

    The answer from the SYMVCD is that supplies of Romanomermis are not commercially available.  This is intentionally misleading.  It is true that this agent cannot be cultured artificially in a way that retains its infectivity, and for this reason it is not being produced commercially.  However, with the expertise of the District programs to culture this agent in vivo should easily be devised.  The mosquito-pathogenic fungus, Lagenidium giganteum, has exhibited similar efficacy of 95% mosquito mortality (Lasko and Washino 1983), is even more specifically effective against the putative vectors of WNV, and has been shown effective in areas where mosquito fish can’t be used.  Lagenidium could also be cultured in vivo.  Both of these agents establish ongoing populations and remain effective at controlling mosquito populations over a number of years.

    My complaints to the Governor’s office about the spray program drew the reply from CDHS that these safe and effective agents were “not operationally viable.”  The claim seems bewildering since the product of a 200 cubic meter facility at UCD produced enough viable and infectious inoculums of Lagenidium every 2-3 months to wipe out the Culex mosquitoes in rice plots more sizable than the permanent ponds around Davis. This material was cultured in the local strain of Culex tarsalis, the vector species amplifying the WNV in birds in California. That 95% reductions of larval Culex would be “operationally viable” in suppressing WNV transmission is patently obvious. Expanding horizons out, a facility some twenty times larger at Sutter-Yuba M.A.D. produced enough Romanomermis in three months (in Culex pipiens , another WNV vector species) to wipe out the Anopheles mosquitoes in a 400 square kilometer area of Colombia. These kinds of results would seem to prove the feasibility rather than dismiss it.

    In addition, such a mass culture facility could also produce Bacillus spaericus (Goldberg et al 1974) and Bacillus thuringiensis ser.H14 (Goldberg and Margalit 1977), both of which have efficacy in reducing mosquito populations.  The District currently purchases commercial formulations of both of these microbes and could save ongoing expenditures on acquiring these materials.  There are a host of other mosquito-specific pathogens that could be cultured.  Many of these have shown efficacy in field trials as well as a long list of others that infect mosquitoes in laboratory situations but haven’t yet been employed in field trials.  On the short list are the fungal genera Coelomomyces (Couch 1972; Chapman and Glen 1972) and Culicinomyces  (Sweeney and Panter 1977).  Another fungus, Tolypocladium cylindricosporium, has the property of transtadial transmission, so that the infected larva that hasn’t died from its disease will become a carrier adult and infect its own eggs with the fungus. 

    Of course mosquitoes also suffer from protozoal and viral infections.  These include Nuclear Polyhedrosis Virus, Cytoplasmic Polyhedrosis Virus, Mosquito Iridescent Virus, Turquoise Mosquito Iridescent Virus among the viruses, and Amblyospora californica and Nozema algerae among the protozoans.  None of these is easy to culture, even for the experts, however.

    All of the agents that can reasonably be cultured can be used in concert with each other without diminishing the efficacy of the others.  The one difficulty with them all is the requirement of growing them in mosquitoes.  Since this is a labor-intensive process, it is perhaps not the best opportunity for entrepreneurs to make a handsome profit in the same way profits are available in manufacturing and dispersing chemical agents.  Might we conclude that there is insufficient profit in safely protecting citizens from mosquito-borne diseases?  It seems to us that the vector-control districts are charged with the protection of the citizens of California, and any profit motives should be shelved.  The ample existing budgets from our tax dollars and the large sums of money that are being spent on aerial spraying could easily fund an excellent facility.  Such a facility would be a valuable resource to all of California, since this renewable resource could provide the seed culture for many similar programs around the state, and the products would be effective tools in preventing all mosquito-borne diseases.

    Additional evidence of feasibility and cost-efficacy of such programs is given by Dr. Edward Platzer of UC Riverside.  He collaborates with Dr. Rafael Pacheco Pérez, the director of a mass culture program in Oaxaca City, Mexico, producing several Mermithid species on a large scale.  In a recent letter he notes that while some hybridization efforts have not yet reached fruition, the mass culture efforts have been very successful and that there have been improvements in the culture that have cut costs (see Platzer letter).  Dr. Platzer indicates that he would be pleased to assist Sac-Yolo in setting up a facility and staffing it.

    It seems almost ridiculous for SYMVCD to consider multiple millions of dollars in expenditures on insecticidal agents and not spend a dime on these safe and effective biological agents.  The risk-benefit comparisons should be obvious.  On the one hand we have a material with dubious efficacy and a guaranteed universal exposure to an incompletely assessed risk; and on the other we have a proven safe and effective set of biological controls, which will be safe and effective tools in an arsenal to combat general mosquito-borne illnesses into the future, well past the problems presented by West Nile Virus.

    When cost questions are introduced the comparison is even more exaggerated.  The added labor required to culture the District’s own biological alternatives would end up both a benefit to community employment and substantially less costly for the District’s budget than procuring poisons and aircraft delivery systems.  That the District is choosing ongoing outlays for 100% depreciable investment in distributing poison as opposed to investing in a permanent facility for production of a renewable resource is beyond any reasonable comprehension.

References:

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