These Dogs May Save the Citrus Industry From a Devastating Pandemic

CC BY 4.0. Gottwald et al / ARS / PNAS

Researchers find that dogs can be trained to sniff out the bacteria that causes citrus greening, with 99+ percent accuracy.

Sometime over the course of the last few centuries, a bacterium known as Candidatus Liberibacter asiaticus (CLas) hopped over from the animal to plant kingdom, where it has been thriving ever since. The bacterium gives rise to a disease called huanglongbing, doing business as "citrus greening disease." Researchers from the USDA's Agricultural Research Service (ARS) call huanglongbing "one of the most severe pandemics in modern times."

The disease is proving treacherous to the citrus industry across the globe, and doing so with extra oomph in the Western Hemisphere. In the past decade in the United States, huanglongbing (HLB) has caused around a 21 percent decrease in the fresh citrus fruit market and about a 72 percent decline in the production of oranges used for juice and other products. Florida is being hit especially hard; if not curtailed, the sunshine state's citrus industry could be destroyed.

Now how hard can it really be to fight a little bacterium? I mean, we have figured out how to do things like hurtle ourselves through outer space and put all of human knowledge in a little box in our pocket – can we really not stop a bug from killing all our orange trees?

Well apparently, it's hard. But now researchers from ARS have struck upon a brilliant possible solution: Bring in the dogs.

"We turned to detector dogs, an ancient technology, which can rapidly survey large plantings without laborious sample collection or laboratory processing," write the authors of a study describing their findings. Yay for ancient technologies!

Plant diseases are in fact particularly tricky. For HLB, there is no post-infection therapy and since plants don't have an immune system (don't tell them that), they can not be vaccinated. Thus, early detection and response are essential.

But, "human visual assessment is insufficiently sensitive to detect new plant infections in a responsive timeframe, and molecular assays are expensive and not easily deployable over large crop landscapes," notes the study. But the dogs? They detected infections with more than 99 percent accuracy, weeks to years prior to visual survey and molecular methods. They were highly specific and able to sniff out the target pathogens from other pathogens.

"We found that, once trained, these dogs were able to identify infected trees within two weeks of the trees being inoculated," said ARS plant epidemiologist Timothy R. Gottwald. "The dogs also were able to distinguish the citrus greening pathogen from a variety of other citrus bacterial, viral, fungal, and spiroplasma pathogens, including closely related Liberibacter species."

They started with 20 dogs, comprised of Belgian Malinois, German shepherds, hybrids of the two, and springer spaniels. The authors explain that the breeds were selected based on “'drive,' the instinct to hunt by odor, large stature to enable the canine to traverse long distances, and endurance." Ten of the dogs were used during the first year of the study and 10 more during the second year in anticipation of more dogs being needed for commercial deployment.

During the study, the superhero detector dogs had a total of 4 to 15 false alerts on 950 to 1,000 trees per dog. Sometimes they alerted on clean trees ... but as it turns out, clean trees that were in the same spot where an inoculated tree had been placed in previous tests.

Compare that to the only USDA-approved method for confirming the presence of CLas: A DNA-based assay test that detected less than three percent of infections at two months. Also note that the DNA tests are labor-intensive, take a lot of time, and are expensive.

"When we ran epidemiological models, we found canine detection combined with infected tree removal would allow the citrus industry to remain economically sustainable over a 10-year period, compared to using molecular assays or visual inspection combined with tree removal, which failed to suppress the spread of infection," Gottwald explained.

The training is similar to that for dogs who detect explosives – though I am guessing orange groves are preferable to airports. Once they find the scent they are trained to find, they sit down near the source – and by the looks of this one, they seem pretty happy about it. The dogs are rewarded with play time and a toy ... and an orange tree gets a new lease on life. Let's hear it for ancient technologies! And of course, the very very good dogs.

The paper, Canine olfactory detection of a vectored phytobacterial pathogen, Liberibacter asiaticus, and integration with disease control, was published in the Proceedings of the National Academy of Sciences.