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No to frogs, fish vs. dengue mosquitoes


Text by Henrylito D. Tacio

Photo credits: Wikipedia

Although the coronavirus disease 2010 (COVID-19) is still around, the cases are not as alarming as they used to be. It seems that it seems to be back to normal now. Some people are no longer wearing face masks. Social distancing is no longer practiced. Washing hands with water and soap or alcohol is still being followed by others.

But there’s a disease that is still with us. It becomes more apparent during the rainy season. It’s called dengue, and it is caused by a virus that comes in four types. If you’re lucky to survive the first two types, you may succumb to the other two types.

Actually, dengue is not really that deadly. It is just like an ordinary fever. But it becomes lethal when it turns into dengue hemorrhagic fever (DHF). Doctors cannot explain how a dengue becomes DHF.

That’s why if you have dengue, you better go to a hospital. It’s better safe than sorry, so goes a saying.

From January to July this year, the City Health Office of Davao City reported 969 cases of dengue with five deaths. The figure is higher compared to last year’s cases during the same period which was 928.

Both dengue fever and DHF are caused by the same viruses. “You either have a milder dengue fever or a more serious DHF,” explains Dr. Enrique Tayag, one of the country’s dengue experts. “Dengue fever does not develop into DHF!”

Studies show that up to five percent of DHF cases are fatal; without proper treatment, the rate rises to 15 percent. Most of those who die are children under the age of 15, although adults are also at risk.

Dengue fever is a severe, flu-like illness that affects infants, young children, and adults. “Dengue should be suspected when a high fever is accompanied by two of the following symptoms: severe headache, pain behind the eyes, muscle and joint pains, nausea, vomiting, swollen glands or rash,” explains the World Health Organization (WHO).

Symptoms usually last for 2–7 days, after an incubation period of 4–10 days after the bite from an infected mosquito.

DHF is a potentially deadly complication due to plasma leaking, fluid accumulation, respiratory distress, severe bleeding, or organ impairment.

“Warning signs occur 3–7 days after the first symptoms in conjunction with a decrease in temperature and include: severe abdominal pain, persistent vomiting, rapid breathing, bleeding gums, fatigue, restlessness, blood in vomit,” the WHO points out. “The next 24–48 hours of the critical stage can be lethal; proper medical care is needed to avoid complications and risk of death.”

Dengue fever occurs following the bite of an infected mosquito Aedes aegypti and Aalbopictus. Only the female Aedes bites. It bites during the day time throughout the day. However, they bite the most about 2 hours after sunrise and before sunset.

When the Aedes mosquito gorges on the blood of an infected human, the virus enters the insect’s salivary gland, where it incubates for eight to ten days. After that, the mosquito can pass the virus on to the next person it bites.

The Aedes mosquitoes breed readily in standing water, such as found in storage containers, flower pots, and discarded tires. One of the best ways to beat dengue is to control those dengue-carrying mosquitoes.

Some people think that by releasing frogs and fish in areas where mosquitoes thrive the most, they can control the population of mosquitoes. But the Biodiversity Management Bureau (BMB) discourages that method.

BMD Director Natividad Bernardino said releasing frogs and fish can disrupt the ecological balance of the surrounding environment.

Putting frogs and fish in swamps and stagnant water is not an effective solution to eliminate dengue-causing mosquitoes as they have a “diverse diet from plant materials to small invertebrates.”

There are reports that some local government units – not necessarily from Davao – released the cane toad (Rhinella marina) to combat dengue. What these officials don’t know is that those frogs are “one of the worst invasive alien species (IAS) in the world.”

“When introduced to a new environment, non-native species of frogs and fishes may become invasive and alter the biodiversity of the areas,” Dr. Bernardino pointed out.

Some years back, the Bureau of Fisheries and Aquatic Research (BFAR) released thousands of “mosquito fish” in canals as a method to help stop the spread of dengue fever in the country. The fish, known in the science world as Gambusia affinis, was given the such name because its diet sometimes consists of large amounts of mosquito larvae. 

It caught the attention of Dr. Theresa Mundita Lim, then BMB director. In her letter to the BFAR director, she requested “that BFAR exercise caution in the release of these fish species. Any release should be administered in a controlled condition and covered by environmental safeguards.”

The letter was posted in Dr. Lim’s Facebook account. “We have prior information that this species is a true alien invasive, given that it was introduced in the Philippines in the early 1900s and has been reported to compete with the native and endemic species for food and niche in areas where they have been collected,” she wrote.

AIS, as described by the United States Department of Agriculture (USDA), are plants, animals, and microbes not native to a region which, when introduced either accidentally or intentionally, out-compete native species for available resources, reproduce prolifically, and dominate regions and ecosystems.

AIS have been classified into microorganisms (avian malaria, banana bunchy top virus, rinderpest virus), aquatic plants (water hyacinth), land plant (cogon, mimosa, African tulip tree), aquatic invertebrates (green crab, marine clam), land invertebrate (common malaria mosquito, golden apple snail, common wasp), amphibian (bullfrog, cane toad), fish (carp, Mozambique tilapia), bird (Indian myna bird, red-vested bulbul), reptile (brown tree snake), and mammal (domestic cat, goat, mouse, pig, rabbit, and red fox).

One good example about IAS in the country was that of the “golden kuhol” that wreaked havoc in rice farms during the 80s.

Like the common rice field snail, the “golden kuhol” was then recognized as a delicacy. Because it was good food and an equally good alternative source of income, enterprising farmers started raising “golden kuhol” in their backyards.

But barely three years after its introduction to the Philippines, the snail, which is said to have originated from the Amazon River in South America, was practically everywhere. The snail multiplies rapidly, a characteristic that transformed them from being a rare delicacy to a dreaded pest.

“Golden kuhol” are very prolific; a female snail lays 200 to 500 eggs at a time and between 1,000 to 1,200 eggs during one month. They proliferate rapidly as their eggs and hatchlings are transported by rivers and streams. They are dispersed to the rice fields through irrigation water.

“The introduced snail has caused one of the worst biological disasters ever to have affected Philippine agriculture with its invasion of irrigated rice fields,” said Dr. Rafael D. Guerrero III, an academician at the National Academy of Science and Technology (NAST). “Because of its prolific breeding and voracious breeding habits, the snail is highly destructive to newly-planted rice seedlings.”

“Not all that is good for other countries is good for us,” reminded Dr. Guerrero, who was formerly the executive director of the Laguna-based Philippine Council for Aquatic and Marine Research and Development. “In fact, it can be a big problem.”

Dr. Bernardino agreed. Invasive species are not the solution to the dengue problem. In fact, they can negatively affect human health by directly infecting humans with new diseases, serving as vectors for certain diseases, or causing wounds through bites, stings, allergens, or other toxins.

She believed the proliferation of mosquitoes is largely attributed to environmental conditions that encourage the reproduction of disease vectors. These conditions include dirty surroundings, stagnant man-made canals and interference with natural water flows, and a decline in the quality of wetlands such as streams, creeks, rivers, swamps, and marshes due to solid wastes, invasive plants, and structures.

“The existence of natural predators in these wetland ecosystems, given that they are kept in their natural state or properly maintained, should also help control population of mosquitoes and invasive alien species should never be an option.”

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