Copepods are sometimes found in public main water supplies, especially systems where the water is not mechanically filtered, such as New York City, Boston, and San Francisco. This is not usually a problem in treated water supplies. In some tropical countries, such as Peru and Bangladesh, a correlation has been found between copepods presence and cholera in untreated water, because the cholera bacteria attach to the surfaces of planktonic animals. The larvae of the guinea worm must develop within a copepods digestive tract before being transmitted to humans. The risk of infection with these diseases can be reduced by filtering out the copepods (and other matter), for example with a cloth filter.
Copepods have been used successfully in Vietnam to control disease-bearing mosquitoes such as Aedes aegypti that transmit dengue fever and other human parasitic diseases.
The copepods can be added to water-storage containers where the mosquitoes breed. Copepods, primarily of the genera Mesocyclops and Macrocyclops (such as Macrocyclops albidus), can survive for periods of months in the containers, if the containers are not completely drained by their users. They attack, kill, and eat the younger first- and second-instar larvae of the mosquitoes. This biological control method is complemented by community trash removal and recycling to eliminate other possible mosquito-breeding sites. Because the water in these containers is drawn from uncontaminated sources such as rainfall, the risk of contamination by cholera bacteria is small, and in fact no cases of cholera have been linked to copepods introduced into water-storage containers. Trials using copepods to control container-breeding mosquitoes are underway in several other countries, including Thailand and the southern United States. The method, though, would be very ill-advised in areas where the guinea worm is endemic.
The presence of copepods in the New York City water supply system has caused problems for some Jewish people who observe kashrut. Copepods, being crustaceans, are not kosher, nor are they quite small enough to be ignored as nonfood microscopic organisms, since some specimens can be seen with the naked eye. Hence, large specimens are certainly non-Kosher. However, some species are visible to the naked eye, but are small enough that they only appear as little white specks. These are problematic, as it is a question as to whether they are considered visible enough to be non-Kosher.
When a group of rabbis in Brooklyn, New York, discovered these copepods in the summer of 2004, they triggered such debate in rabbinic circles that some observant Jews felt compelled to buy and install filters for their water. The water was ruled kosher by posek Yisrael Belsky, chief posek of the OU and one of the most scientifically literate poskim of his time. Meanwhile, Rabbi Dovid Feinstein, based on the ruling of Rabbi Yosef Shalom Elyashiv - the two widely considered to be the greatest poskim of their time - ruled it was not kosher until filtered. Several major kashrus organizations (e.g OU Kashrus and Star-K) require tap water to have filters.