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February 24, 2017

Yellow Fever Hemorrhagic Disease is just around the corner (1)

Yellow fever is an acute haemorraghic fever caused by the yellow fever virus, a 40- to 50-nm-wide enveloped RNA virus, belonging to the family Flaviviridae and genus Flavivirus. Approximately 200,000 cases of yellow fever occur annually, worldwide, resulting in about 30,000 deaths, with nearly 90% of these occurring in tropical and subtropical areas of South America and Africa. Yellow fever manifests as a range of clinical diseases rather than one distinct condition. In areas where the virus is endemic, some infections may be entirely asymptomatic while in some cases it could constitute a severe, life-threatening illness. Overall, however, the case-fatality rate is high: approximately 20% among populations where the disease is endemic, and possibly as high as 90% among persons from non-endemic areas (travelers).

Yellow fever is endemically transmitted in forests and savannahs (jungle and savannah yellow fever) of Africa and South America by Aedesafricanus, and Hemagogusjanthinomys respectively, periodically emerging from enzootic (animal) cycles to cause epidemics (urban yellow fever) of hemorrhagic fever transmitted by Aedesaegypti, with reported high fatality rates due to two principal syndromes: yellow-fever associated neurologic disease characterized by encephalitis, myelitis or myelo-encephalitis; and yellow-fever associated viscerotropic disease, which usually involves multi-organ failure especially liver, renal and circulatory failure. In severe cases, death could occur between the seventh and tenth days after the onset of the first symptoms.

The adult of A.aegypti, a small to medium-sized mosquito, approximately 4 to 7mm, is recognized by white scales on the top surface of its thorax. Each segment of the hind legs possesses white basal bands in form of stripes. The abdomen is generally dark brown to black, but also may possess white scales. Aedes spp.,are container-inhabiting mosquitoes, often breeding in unused flowerpots, spare tyres, untreated swimming pools, and drainage ditches. They thrive in urbanized areas, in close contact with people. They are extremely common in areas lacking pipe-borne water, and depend greatly on water stored in open containers for breeding. Male and female adultsfeed on nectar of plants; but the females rely mostly on human blood in order to produce and nurture their eggs. Once infected with the virus, Aedesspp remain infectious throughout their lifetime of about two to three months, and although the mosquitoes could be killed by extremes of heat and cold, the virus can survive from season to season in their eggs, thereby making Yellow Fever eradication a difficult task to accomplish. Both A. aegypti and A. africanus which are also responsible for the transmission of Chikunguya, Dengue and Zika viruses are well distributed in Nigeria. Aedes spp. are active during daylight hours, and bite from dawn to dusk. Yellow fever virus has also been reported to be transmitted by blood transfusion as well as breast feeding (vaccine strain)

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Phylogenetic analyses indicate that the yellow fever virus originated from East or Central Africa, with transmission between monkeys and humans, and spread from there to West Africa. The virus as well as its vector A.aegypti, were probably brought to the western hemisphere and the Americas by slave trade ships from Africa after the first European exploration in 1429. The virus was discovered to be the first insect-transmitted virus (Arbovirus) in 1900 by Walter Reed, an American Army Doctor and his colleagues.

Yellow fever is endemic in 32 countries of Africa and it is a major threat in Nigeria. The most recent outbreak of yellow fever infection in Africa occurred in Viana municipality, Luanda province, Angola, on 5th December, 2015 and by 23rd July, 2016, about 3,625 people had become infected and 357 deaths had been recorded. The Angolan outbreak have been reported to spread to Democratic Republic of Congo, infecting about 1,798 people and killing about 85 of them; China (11 cases), Kenya (3 cases and 1 death); and Namibia (10 cases). The Chinese case was the first time Yellow Fever was appearing in that country and indeed all Asia although the mosquito vector is well distributed in that continent. There is also an ongoing autochtonous (indigenous) outbreak of Yellow Fever in Uganda, which, as at 23rd July, 2016, had recorded 68 cases and 7 deaths; Ethiopia (22 cases and 5 deaths), Ghana (4 cases), and Peru (55 cases). Other countries that have reported autochtonous outbreak of Yellow Fever since the beginning of 2016 include Brazil, Chad, and Colombia. The Chinese have reacted positively to the outbreak in their country by sending a team of health professionals to Angola to vaccinate most of their citizens living and working in that country as well as raising health teams at all the entry ports in their country to monitor the temperature of all visitors arriving in China.

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The first outbreak of urban yellow fever in Nigeria occurred in Lagos in 1864. The disease was again reported in Lagos in 1894, 1905, 1906, 1925 and 1926. In 1913 urban yellow fever was reported again in Abeokuta, Forcados, Warri, Onitsha, and Calabar. In 1950 and 1952, the disease struck again in many parts of southern and central Nigeria with an estimated 12,000 cases per individual year. It was not however, until 1969 in Jos, that the disease wreaked its first major havoc in the country, infecting about 100,000 people and killing about 40,000 of them. In 1976, another outbreak of Yellow Fever which occurred for the first time in Uyo, Itu and Abak divisions in the then Cross River State had a mortality rate of about 56% among hospitalized patients. Furthermore, between 1986 and the year 2000, Yellow Fever outbreaks plagued various parts of Nigeria including Jos (1986) with approximately 120,000 cases and 200 deaths, Oyo state(1993) with 152 cases, Orsu, Imo state (1994) with 1,227 cases and 415 deaths, and Ogbomosho (1997)with 7 cases. The last recorded case of Yellow Fever in Nigeria was in Kano in May, 2000 with 2 cases. The 1986 outbreak spread to larger African urban centers, and by the end of 1987, 116,000 people had become infected and 24,000 deaths had been recorded. The Imo State outbreak was an offshoot of the disease which began in Ihiala, Anambra State in early 1994 and spread to Orsu, Imo State by August of the same year.

 

By Robert Obi

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