(CNN) The Philippines has reported its first case of polio since it was declared free of the childhood disease 19 years ago, dealing a blow to the campaign to eradicate it.

The country's department of health said the disease was "re-emerging," with a case confirmed in a 3-year-old girl from Lanao del Sur, on the southern island of Mindanao, and a suspected case awaiting confirmation.

The World Health Organization said it was "very concerned" at the re-emergence of the disease in the country; UNICEF described it as "deeply disconcerting."

A global campaign to eradicate polio was launched in 1998 and cases due to the wild poliovirus have decreased by more than 99% since then, from an estimated 350,000 cases to 33 reported cases in 2018, according to WHO.

However, the disease is still present in Pakistan and Afghanistan and the emergence of new, vaccine-derived strains of polio have complicated efforts to rid the world of the disease.

The last known case of wild poliovirus in the Philippines was in 1993. The country was declared wild polio-free in 2000 along with the rest of WHO's Western Pacific Region.

The Philippines case was unexpected and the country was not on a list of at-risk countries compiled by the Polio Global Eradication Initiative.

Vaccination drive

In addition to the confirmed and suspected cases, the polio virus was detected in samples taken from sewage in the capital, Manila, and in waterways in Davao, Mindanao, the country's third-largest city, as part of the regular environmental surveillance, the department said. The samples were verified by the US Centers for Disease Control and Prevention and the Japan National Institute for Infectious Diseases.

The government said it was preparing a rapid response to the outbreak in coordination with WHO and UNICEF, with a mass polio immunization campaign for all children under 5 starting in October.

"We strongly urge parents, health workers and local governments to fully participate in the synchronized polio vaccination," Philippines Health Secretary Francisco Duque III said in a statement.

"It is the only way to stop the polio outbreak and to protect your child against this paralyzing disease."

Polio is an infectious disease which spreads rapidly. It can cause paralysis and, on rare occasions, can be fatal. There is no cure for polio -- it can only be prevented with multiple doses of polio vaccines, WHO said.

"Aside from immunization, we remind the public to practice good personal hygiene, wash their hands regularly, use toilets, drink safe water, and cook food thoroughly," Duque added.

Trust in vaccines was undermined in the Philippines after the government was forced to suspend a dengue fever vaccination program in 2017. The drug was distributed to more than 800,000 students as part of a school-based government immunization program, but was halted after clinical trial data showed that it could have unintended consequences in non-infected patients.

Wild vs vaccine-derived polio

The 3-year-old girl was found to have a vaccine-derived strain of polio virus type 2, which WHO said was of particular concern because the wild strain of this virus was eradicated in 2015.

Vaccine-derived polio happens when live strains of poliovirus that are used in the oral poliovirus vaccine mutate, spread and, in rare cases, trigger an outbreak. Most of the time the virus dies off but it can sometimes spread in an area where there is low vaccination coverage.

"If a population is not sufficiently immunized, the weakened virus can continue to circulate. The longer it is allowed to survive, the more changes it undergoes. In rare instances, the virus can change to a vaccine-derived poliovirus (VDPV), a form that has regained the ability to cause paralysis," WHO said.

"Poorly conducted immunization activities, when too few children have received the required three doses of polio vaccine, leave them susceptible to poliovirus, either from vaccine-derived or wild polioviruses. Full immunization protects them from both forms of the virus," it added.