Unlike the Anopheles mosquito, which causes malaria and breeds mostly in stagnant water around homes, Aedes mosquitoes either breed around houses, in the jungle or in both habitats.
The yellow fever virus, carried in the insect, is transmitted when an infected mosquito bites and sucks blood from an uninfected person.
The symptoms are not very specific but are similar to those found in malaria and hepatitis, says Dr Henry Akpan, chief consultant epidemiologist at the federal health ministry.
“Usually you could have somebody presenting with fever, body pains, headache, symptoms that not very specific but usually with yellowness of the sclera of the eyes,” he explains.
The sclera is an opaque outer layer covering the eye, and the yellowing of this cover is responsible for the word “yellow” in the name of the disease.
Medically, it is known as a viral haemorrhagic fever—one caused by a virus and causing bleeding from the gums, skin and in stool.
The disease is zoonotic—meaning it can be passed from animals to humans.
One kind, known as jungle yellow fever, is transmitted by aedes mosquitoes from monkeys to humans. A second kind, urban yellow fever is transmitted among humans.
At risk
More than 900 million people across 45 countries in Africa and Latin America are at risk of yellow fever, according to the World Health Organisation.
Some 508 million are in 32 African countries alone.
At least 200, 000 cases of yellow fever are recorded worldwide, causing 30,000 deaths.
Countries considered free of yellow fever report a few cases due to international travel.
The disease is difficult to diagnose in the early stages, since it is easily confused with “severe malaria, dengue hemorrhagic fever, leptospirosis, viral hepatitis (especially the fulminating forms of hepatitis B and D), other hemorrhagic fevers (Bolivian, Argentine, Venezuelan hemorrhagic fevers and others flavivirus as West Nile, Zika virus etc) and other diseases, as well as poisoning,” says the WHO.
The virus incubates for up to a week in the body, followed by infection most times in two phases.
The first phase usually causes fever, muscle pain with prominent backache, headache, shivers, loss of appetite, and nausea or vomiting.
“Most patients improve and their symptoms disappear after 3 to 4 days,” says WHO.
However, 15% of patients enter a second, more toxic phase within 24 hours of the initial remission, according to the global health body.
“High fever returns and several body systems are affected. The patient rapidly develops jaundice and complains of abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach.
“Once this happens, blood appears in the vomit and faeces. Kidney function deteriorates. Half of the patients who enter the toxic phase die within 10 to 14 days, the rest recover without significant organ damage.
Historicity
Apart from supportive care to treat the fever and dehydration that results from yellow fever infection, there is no known specific treatment for the disease.
But global health authorities recommend states to implement vaccination, control mosquitoes and be prepared for any epidemic.
International Health Regulations set forth in 2005 by the World Health Organisation requires countries where risk of exposure to yellow fever virus is high to carry evidence of vaccination against the disease.
Known as the International Certificate of Vaccination or Prophylaxis—or a yellow card—it is issued by health authorities of individual states.
Port health services, which issue yellow cards, are supervised by the federal health ministry at five airports, 10 seaports and 14 land borders.
The vaccine protection lasts 10 years from date of immunization.
The row between Nigeria and South Africa centres on requirements for international travels to guard against imported diseases, but Akpan says Nigeria “has not had any outbreak [of yellow fever] since 1995.”
Health minister Onyebuchi Chukwu said Nigerians had a duty “to ensure they get vaccinated and get the Yellow Card before travelling to countries that require the Yellow Card.”
“In Nigeria, the approved vaccination centres have been issuing Yellow Cards and we want to advise travellers that where there is any doubt about the authenticity of the Yellow Cards given to them, they should report to the Federal Ministry of Health,” he said at press briefing in response to South Africa’s deportation of 125 Nigerians on March 3.
