Yellow Fever Surveillance System and their Effectiveness

Abstract

Yellow Fever is a viral hemorrhagic fever which strikes an estimated 200,000 persons worldwide each year and causes an estimated 30,000 deaths 1. Yellow fever virus is the prototype of the family Flaviviridae, which currently contains over 70 viruses, of which most are arthropod- borne, including dengue viruses 2-3.there are three different epidemiological patterns of yellow fever virus transmission. The sylvatic or forest pattern, the Aedes aegypti- borne urban cycle 4.and an intermediate cycle that bridges these two patterns. The different epidemiological patterns of transmission lead to the same clinical disease 5

The main vector of yellow fever within village and urban settlements is female Aedes. (stegomyia) aegypti (only females feed on blood to obtain protein for egg production) The virus is transmitted when a mosquito bites an infected human and then, after an extrinsic (in the mosquito) incubation period of 12-21 days bites a susceptible human. Ae, aegypti breeds readily in all types of domestic and Peridomestic collection of fresh water, including flower vases, water drums, tin cans, broken coconut shells, old tyres and gutters 6.In the forest pattern of yellow fever monkeys are the primary host, and man is an accidental host ( In South America yellow fever is an occupational disease of people cutting down the forest)7.Humans become infected with yellow fever virus when bitten by the primary mosquito vector, Ae, Africans, Ae, bromeliad or one of the several other mosquito species. Most of these mosquito breed and live in holes and cracks in the upper party of three in the forest. Intermediate epidemics are a mixture of man - to -man transmission, and are often characterized by focal outbreaks separated by areas without human cases.8.In some surveys ,it has been possible to estimate an annual incidence of infection of susceptible humans of at least 1%,so that ,by adulthood, immunity rates of 50% or more are not unusual.9. An attack of yellow fever is followed by a solid long - lasting immunity against reinfection 10. The incubation period in human is generally an infected mosquito. The Patient is only infectious to mosquito for the first three to four days after onset of symptoms. The disease of fever, headache, backache, general muscles pain, nausea, and vomiting11. Milder Cases of yellow fever may not present with jaundice. There is a characteristics bradycardia in relation to the temperature 12.About 15% of those infected develop a serious illness with several phases, an acute phase of about three days with sudden onset of fever headache, myalgia, nausea, and vomiting remission for up to 24 hours( characteristics saddle -back fever) and a toxic phase with jaundice and vomiting ( black vomitus) in which haemorrhagic signs ( bleeding of gums, nose and haematuria), albuminuria, and oliguria ( reduction of urine production) may occur. The patient may suffer from hiccups, diarrhea, progressive tachycardia, and shock. Examination of the abdomen reveals intense epigastric tenderness 13. At least half of the individuals who reach the toxic phase do not survive. Death usually occurs between the seventh and tenth day after onset.14. The possibility of yellow fever should not be dismissed in the absence of jaundice or of albuminuria. Malaria and yellow fever may coexist in a region,15 and malaria usually shows clinical symptoms almost identical with those of the early stages of yellow fever, sudden onset, headache, generalised aches, and vomiting, Even with the finding of malaria parasite in the blood smear, the possibility of yellow fever is not ruled out.16 .In the beginning of an infection, there is a little to distinguish the illness from a number of other febrile conditions. Typhoid fever, rickettsial infection, influenza, leptospirosis, viral hepatitis, infectious mononucleosis, and other arboviral fever, like dengue, Lassa fever and chikungunya may all resemble anicteric yellow fever. The definitive diagnosis of yellow fever is made by serology or virus isolation, which requires special reagents and techniques as well as expertise in the interpretation of the test results. The histopathological diagnosis is based on eosinophilic degeneration of the hepatocytes leading to the formation of councilman bodies in 1930s a viscerotome program was instituted in South America. All individuals who died after a short- term febrile illness had a liver punch specimen taken by health officials and sent to specially trained pathologist 17. Liver biopsies are not done in living patients because of the risk of severe haemorrhage. A viscerotome service has not been instituted in Africa.

Yellow fever is endemic in 34 countries of Africa with a combined population of 468 million. Yellow fever vaccine, one of the earliest viral vaccines to be developed, has proved safe and efficacious. The vaccine is transported and stored frozen. The development of new protective additives have increased the thermostability of the vaccine. The shelf life at - 20 or 40 o C is now up to two years, and the estimated half-life at a room temperature is 10 months, However once a vial is opened, the vial must be kept cold and used within one immunization session and it must be discarded after then. One dose of yellow fever vaccine provides protection for at least 10 years and possibly life long.18. A single dose will confer immunity in 95% of persons vaccinated. Four strategies have the potential to bring yellow fever fully under control in Africa epidemic control, mass immunization and surveillance.19.In Africa, epidemic control often suffers from delays of the two months or their between the onset of epidemics and their recognition, partly due to the occurrence of the first cases in remote areas with few medical services and the unfamiliarity of medical personnel with the disease .Responses to a possible outbreak include collection and testing of specimens. epidemic investigation emergency vaccination takes place as soon as an outbreak has been confirmed, In an attempt to limit the spread of infection by immunizing all person in the focus regardless of their former immune status. Good Surveillance is essential in all at - risk countries for the early detection of cases which will allow fast action to control an outbreak. It has often proved difficult to identify early, isolated cases before they trigger an epidemic because of the difficulties of distinguishing yellow fever from disease with similar symptoms (e.g malaria) Other potential problem with emergency campaigns include difficulty in obtaining the large supply of vaccine, syringes and needles, and sudden deployment at short notice of large numbers of health workers. Another disadvantage is that immunity does not appear until seven days after immunization.