The purpose of this study was to present the epidemiology and clinical picture of the measles outbreak in the Nylon Health District. We found that, Bonadiwoto was the most affected health area. The female gender and infants aged 9 to 59 months represented the bulk of cases. The outbreak which involved mainly unvaccinated individuals lasted 22 weeks with a peak on the 10th week after onset. The case fatality was zero.
Nylon is an urban slum with a very heterogeneous population, spontaneous habitations with a high population density and a high level of promiscuity. It is like the ghetto neighborhood of the economic capital (Douala) of Cameroon. The above conditions are known to favour the transmission of infectious diseases such as measles once the causal germ finds its way in [11
]. The district is known to be the entry point of many outbreaks or other health related problems in the cosmopolitan city of Douala just like the concomitant cholera outbreak. The district in general and Bonadiwoto health area in particular harbours most of the neighbourhoods where we find some socio-cultural groups known to be less user friendly with hospital services including childhood vaccination.
The effectiveness of the measles vaccine in reducing the number of measles cases has been established whether in routine situations or in mass campaigns as a response to an outbreak or during supplementary vaccination activities [7
]. For this to occur, a high measles vaccine coverage rate needs to be sustained so that susceptible individuals will not go beyond a threshold level for major outbreaks to occur [14
]. The Measles Containing Vaccine coverage rate of the Nylon health district was not the best over the last five years. According to unpublished data from the regional delegation of public health for the littoral region, the administrative measles vaccination coverage rate of the district dropped progressively from 80% in 2006 to 62% in 2008 and later increased to 87% and 97% in 2009 and 2010 respectively. This low measles vaccination rate over time must have led to the accumulation of a susceptible population especially amongst the under five children. This surely explains why children aged 9 to 59 months were the most affected. Coupled with the environmental condition described above, once the index case (a child from the northing region of Cameroon where there was an ongoing measles outbreak) introduced the virus into the community, the virus easily found a suitable terrain and other vulnerable persons. This is further illustrated in the epidemic curve, where we see a rapidly spreading infectious disease with a short incubation period and an inter-human transmission pattern.
Due to the quest for a better life, people leaving their villages for the city, have the tendency to settle in Douala, the economic capital of Cameroon. It becomes difficult to master the real population of the Nylon Health District hence the denominator problem as we call it. The present population (just like the previous once), which is an extrapolation from the 2005 general population census still underestimates the population of the Nylon Health District. This is illustrated during mass campaigns (such as poliomyelitis, deworming or vitamin A supplementation) activities where the district usually score an administrative coverage of more than 100% according to unpublished data in the regional EPI unit. Therefore one may be tempted to say that, with the measles vaccination coverage of over 80% in 2009 and 2010, one of the strategies adopted by the World Health Assembly to control measles (attaining at least an 80% measles coverage rate in every health district) has been met in the Nylon health district [15
]. Unfortunately, the administrative coverages of 87% and 97% are far below expectations because of the denominator problem hence leaving the population at risk of a measles outbreak.
More so, almost 80% of all the measles cases in the Nylon outbreak (or two thirds of cases aged 9 to 59 months) could be considered as unvaccinated because of unverifiable vaccination status. This brings up many questions that need to be answered in the district such as accessibility to vaccination services, the quality of vaccination and dropouts rates. Similarly, measles outbreaks with similar trends (low vaccination coverage, persons aged less than five years being the most affected) have been reported in other areas such as the Mirriah district in Niger [16
] and the Shivpuri district in India [17
]. On the other hand, bearing in mind that, a single dose of measles vaccine administered at the age of 9 months as in the EPI programme of Cameroon, is associated with an 85% efficacy [10
], some infants with card-confirmed measles vaccination status could still contract the disease. Thus there is a 15% probability that, infants normally vaccinated may not be protected. These infants added to the unvaccinated children increases the number of measles susceptible persons over time.
Almost a quarter of the cases were identified by health workers in the community through active surveillance. These were children treated by their parents using traditional herbs or over the counter drugs. Given the fact that up to a quarter of measles cases were identified in the community, it is likely that some cases must have gone unnoticed by the health system thereby leading to an underestimation of the burden of the outbreak. The parents of cases identified in the community rightly thought of measles but preferred household treatment. The case definition used was that proposed by the WHO and stated in the EPI standard operating procedures [9
Several control strategies were put in place as soon as the region got information of the outbreak. These were: re-enforcing surveillance and routine vaccination, mass vaccination campaign, information education and communication, proper management of cases and complications. The mass measles vaccination campaign was organized in the high hit health areas on the 14thepidemiological week targeting children aged 9 to 59 months. Details of the response are in a follow up paper pending publication.
We reported zero death in our results. Some cases of deaths might have gone unnoticed in the community or unreported by health facilities. Nonetheless, we reviewed hospital registers of all 28 health facilities that reported measles cases for all deaths during the period of the outbreak (4th to the 26th epidemiological week). In all, 42 deaths were registered during the period by four health facilities. The Nylon District Hospital, which is the first line referral unit in the district alone reported 36(85.7%) of the deaths. The most common cause of death was due to HIV related complications 23(54.8%), all of which occurred in the Nylon District Hospital which is also an HIV/AIDS treatment unit. The other causes of death were: still birth 6(14.3%), Severe anaemia 02(4.8%), Malaria 02(4.8%) and Metabolic disorders 02(4.8%). There was a case of death each due to Asthma, Drowning, Drug intoxication and Tuberculosis. None of these patients presented with signs that respected the WHO definition of a suspected case of measles during admission and their medical records were not in favour of measles related complications such as Gastroenteritis, Pneumonia, Sinusitis, Otitis Media, Mouth Ulcers, Upper airway obstruction, Corneal drying, Keratomalacia, Blindness, Malnutrition, Convulsions and Brain damage. Active surveillance helped us to identify 42(27.5%) cases in the community and no community death. This goes to limit misclassification, though the registers of those health facilities which did not report measles cases were not reviewed.