Medical Mystery

Editor’s Note: This article originally appeared in Entomology Today, a project of the Entomological Society of America with the goal of reporting interesting discoveries in the world of insect science and news from various entomological societies. To learn more, visit www.entomologytoday.org.

Lesions of the cornea that cause white discolorations of the eye, called leukomas or corneal opacities, have been discovered for decades in humans, domestic animals and wild animals. Sometimes called West Indian punctate keratopathy, Florida spots or Rice’s keratopathy, this condition was a mystery to the medical community. But a recent paper in the Journal of Medical Entomology reports that the cause of these lesions has been known for a long time by indigenous peoples in Colombia. What is the cause? Wasmannia auropunctata, also known as the little fire ant.

Diego Rosselli, Ph.D., of the Pontificia Universidad Javeriana in Colombia and James Wetterer, Ph.D., of Florida Atlantic University co-authored the report, in which they gather knowledge on the condition from disparate sources to better inform the medical community.

The corneal lesions were first reported in the medical literature in 1968 by Noel Rice, who noticed it in immigrants from the West Indies in London. Rice came to the conclusion that the condition was caused by trauma to the eye. The lesions were then reported in cats in Florida in 1979 and in dogs in Brazil in 1997. In 2004, corneal lesions were reported in cats and dogs in Martinique, but they were only found in animals that lived outdoors.

Other potential explanations for the corneal lesions were proposed in the literature, including viral infections, bacterial infections and inflammation resulting from exposure to the microfilariae of the parasitic worm Onchocerca volvulus, the worm that causes river blindness.

None of these researchers were aware of the true cause of the lesions — but indigenous cultures in Colombia have known the cause for a long time — stings from the little fire ant, Wasmannia auropunctata. The lesions could arise from other causes, but, as Rosselli and Wetterer write in their paper, if there was no trauma to the eye and there was no infection, W. auropunctata is the cause.

The little fire ant originated in the lowlands of Central and South America. Carried by human trade, it has spread to Florida, Africa, Israel, Australia and many Pacific islands, including Papua New Guinea, Hawaii, the Solomon Islands and the Galapagos. The ant creates a toxin that, when it stings the eye, can cause corneal lesions. The stings often occur when humans or animals blink in response to the presence of the ant, and the blinking triggers the stinging.

“Indigenous knowledge, which is normally neglected, has observed the association for many years. Thus, scientists should try to be aware of discoveries in fields different from their own,” says Rosselli.

Asked what the biggest surprise of the study was, Rosselli says, “Ants represent a large proportion of all insects (and therefore of all animals), but their association to human diseases has been scarcely explored.”

Eli Sarnat, Bugwood.org

WHAT’S NEXT? Regarding next steps in this research, Rosselli says research on the chemistry of little fire ant toxins could be valuable: “I was surprised to know that despite the impact of this ant in agriculture, in animal diseases, and, with our paper, in human disease, very little is known about the toxins produced by the little fire ant (as compared with its larger relative the big red fire ant).”

As for possibilities for control of the little fire ant, Rosselli says results of studies have been mixed. In Florida, for example, insecticides were very effective in reducing populations of the little fire ant. But, when the insecticide use is scaled back, the ants tend to return. Physical methods such as applying scalding water to colonies, or excavating and removing colonies, also can be effective. Biological control methods may offer the greatest promise. The little fire ant is more destructive in regions where it has invaded than in its native habitats, which might be a result of biological control from species present in its native habitats. Introduction of such biological control species might be effective in helping to control outbreaks in other regions. To prevent eliminating biological control species, insecticide baits are most effective if they focus on mounds with a colony of the little fire ant, rather than being broadcast to a wider area where other species could be affected.

Agricultural workers are sometimes exposed to the ants when harvesting products, in settings such as citrus orchards, cacao orchards and palm tree plantations. Domestic animals are sometimes exposed when the ants are in their food bowls. Rosselli mentioned that goggles might be used by workers in some agricultural settings, such as palm tree plantations, and insecticides might be applied to animal food bowls to keep the ants away from domestic animals.

One characteristic that makes control of the little fire ant so problematic is that the ant is a “tramp species,” spreading throughout the world via human commerce. Increased inspections of agricultural products, and quarantines of products infested with W. auropunctata, could be useful components of control.

John P. Roche, Ph.D., is an author, biologist, and science writer dedicated to making rigorous science clear and accessible. He has a Ph.D. in biology and has published more than 180 articles. For more information, visit http://authorjohnproche.com.