Ramblemuse Touch Points

Last July (2012), at the Society for Mathematical Biology (SMB) conference in Knoxville TN, I attended two talks on modeling the spread of Huanglongbing (HLB), commonly known as citrus greening disease. The two related talks were given by Jillian Stupiansky and Karly Jacobsen, both from the University of Florida, Gainesville. Stupiansky and Jacobsen note that HLB is “a vector-transmitted bacterial disease that is significantly impacting the citrus industry in Florida and poses a great risk to the remaining citrus-producing regions of the United States.”

An insect known as the Asian citrus psyllid, Diaphorina citri, carries the organism that causes citrus greening, Candidatus Liberibacter asiaticus (Las). An infected psyllid carries the bacteria in its saliva and infects a healthy tree when it feeds. Similarly, a healthy psyllid can get the bacteria from a diseased tree. Observations suggest that once a tree becomes infected, it may remain asymptomatic for six months to six years, contributing to the difficulty in controlling the disease. While an adult psyllid can usually only fly a mile, citrus greening has been able to spread throughout almost the entire state of Florida due to the transportation of infected citrus stock by discount stores. Recent findings also support the idea that the disease is transmitted transovarially and sexually amongst the psyllid population. This would help to explain why the disease has spread so rapidly.

Based on a control strategy of “roguing” (removal and burning of infected trees), Stupiansky and Jacobsen discussed their disease transmission model, aiming to estimate the reproduction number, R₀, obtainable by conventional strategies.

The tree population is divided into susceptible, infectious and asymptomatic, infectious and symptomatic, and removed (considered to be dead) compartments. Roguing of symptomatic and dead trees occurs with a positive probability of the replanted tree directly entering the infectious state. Prior work also included calculation of the basic reproduction number, R₀, and determining a condition for the existence of an endemic equilibrium.

With the talks by Stupiansky and Jacobsen as my prior background, I was extremely interested in Amy Harmon’s recent article: A Race to Save the Orange by Altering Its DNA. Following the science-writing precept of “write a story, not just an essay about a topic”, Harmon crafts a narrative around Ricke Kress, president of Southern Gardens Citrus, and his battle to save the orange. Harmon well-captures the race to create a resistant plant before production collapses, the lack of existing resistant species, and thus the decision to turn to DNA modification. She delves into the search for a gene that will convey resistance while providing both scientific background and the angst of potential public nonacceptance. A story well told and well-balanced.

While the story of citrus greening and the U.S. citrus industry is the economically largest example, the too often human-assisted spread of a blight and the search for a resistant variant is not unique. The helplessness of watching and waiting until the word comes down that “It’s here!” has been with us for over a century. A recent article in The Economist opens with the plight of the American Chestnut.

Once upon a time, according to folklore, a squirrel could travel through America’s chestnut forests from Maine to Florida without ever touching the ground. The chestnut population of North America was reckoned then to have been about 4 billion trees. No longer. Axes and chainsaws must take a share of the blame. But the principal culprit is Cryphonectria parasitica, the fungus that causes chestnut blight. In the late 19th century, some infected saplings from Asia brought C. parasitica to North America. By 1950 the chestnut was little more than a memory in most parts of the continent.

The article continues on describing recent efforts, including genetic manipulation, to create a chestnut that is sufficiently American yet also resistant to the fungus. The article briefly notes that the Chestnut projects, if successful, could provide a model for restoring the American Elm. Bruce Carley in Saving the American Elm describes the spread of Dutch Elm Disease.

Many of us remember how painful it was for our communities to witness the tragedy that recurred throughout the eastern states during the 1950’s, 1960’s, and 1970’s. Many remember watching helplessly as countless main streets, parks, historic sites, and neighborhoods that had been so handsomely graced with fine elms were transformed within a few years into barren, urban-looking landscapes devoid of trees, the result of a frighteningly efficient epidemic that had appeared suddenly. We can imagine the profound dismay of the citizens of Portland and New Haven as each “City of Elms” was transformed rapidly into a “City of Firewood,” necessitating almost phenomenal removal expenses. Some may recall marveling at the futility of the “cut and burn campaigns” which were initiated to halt the spread of an epidemic that was killing trees literally by the millions each year.

Turning to another threatened crop, in Transgenic Papaya in Hawaii and Beyond Dennis Gonsalves describes the spread of papaya ringspot virus (PRSV) in Hawaii and the development of the resistant rainbow papaya — a development that he helped bring about. Without the rainbow papaya, Hawaiian papaya production would have collapsed much as orange production is threatened now. Ironically, the herd immunity provided by the rainbow papaya (getting R₀ less than one) is helping to sustain some production of nonresistant varieties. As rapid spreaders of blights across the world, we humans either come up with new ways of developing resistant strains or sit by and watch favorite species get decimated.

Lest we become too smug about our own genetic purity, it’s revealing to discover that we ourselves are genetically modified organisms. Carl Zimmer has written some fascinating articles on the bits of virus-injected DNA hiding and, at times, put to use within our own genome. See: The Lurker: How A Virus Hid In Our Genome For Six Million Years, Mammals Made by Viruses, We are Viral from the Beginning, and Hunting Fossil Viruses in Human DNA. Let they who have no inserted DNA cast the first stone.