### Validity of results of mathematical modelling for a hepatitis E outbreak

#### Posted by ra1302 on 14 Sep 2012 at 04:10 GMT

I have read with interest the recent paper by Nannyonga et al1 regarding dynamics of an outbreak of hepatitis E in Uganda. Since data on dynamics of hepatitis E outbreaks are extremely limited, this paper is a useful addition to the available literature on this disease, interest in which has shown an increase over the last few years. However, the results of mathematical models depend heavily on the assumptions made. In this context, some of the assumptions made in the above paper do not appear to be valid, as discussed below, casting a shadow on the authors’ conclusions.

First, the model assumed that every person infected with hepatitis E virus developed clinical disease. Asymptomatic infection with hepatitis E virus is common in disease endemic areas. The authors’ failure to take this into account would have led to an inaccurate estimation of transmission parameters of the infection in their population.

Second, the authors used a parameter l, i.e. the proportion of households that had access to latrines. For this, they assumed a value of 0.037, assuming that each existing latrine served only one household. This may not be true since more than one household could have shared a latrine.

Finally and most importantly, the authors assumed a value of 2 years for a parameter D, i.e. the average duration of disability for a patient with hepatitis E, in their calculation of disability-adjusted life years (DALYs). This is clearly invalid. Since most patients with acute hepatitis E recover by the end of 6 weeks, 2 a typical patient with hepatitis E is disabled for only a few weeks. A recalculation using values for ‘D’ of 4/52 (four weeks) or 6/52 years (six weeks) reveals years lived with disability of only 6.14 or 13.81 years, as against the authors’ estimate of 4035.29 years. This major discrepancy implies that the authors grossly overestimated the burden of morbidity during the outbreak, with consequent downstream effects on their calculation of cost-effectiveness of various disease control measures.

Rakesh Aggarwal
Department of Gastroenterology
Sanjay Gandhi Postgraduate Institute of Medical Sciences
Lucknow 226014, India

References

1. Nannyonga B, Sumpter DJ, Mugisha JY, Luboobi LS. The dynamics, causes and possible prevention of hepatitis E outbreaks. PLoS One. 2012;7(7):e41135.

2. Aggarwal R. Clinical presentation of hepatitis E. Virus Res. 2011;161:15-22.

No competing interests declared.

### RE: Validity of results of mathematical modelling for a hepatitis E outbreak

#### betty1 replied to ra1302 on 14 Sep 2012 at 11:35 GMT

Dear
Prof Rakesh Aggarwal

I have the following responses:

The data used were for the reported clinical cases. That is, all the people that were reported to have displayed clinical symptoms. So those who were exposed and did not display clinical symptoms were not included in the incidence term.

The value of l (0.037) was not assumed. It was obtained from a report by the Uganda Red Cross that out of the 20485 people (6039 house holds) in the IDC camp, the latrine coverage was as low as 3.7%. See the WHO Report 2009 (Uganda). This meant 1 latrine for 27 people or about 1 latrine fr 6 households. Therefore the latrines were indeed shared among households. (This was stated in the text).

The 2 year disability (D) was not assumed. It was actually the duration of the epidemic in the area 2007-2009 (for the entire population) not per individual. Therefore, in general for the entire period of 2 years, the population of 28045 suffered the total disability stated. Calculating on individual basis, will indeed yield different results.

Betty Nannyonga.
bnk@math.mak.ac.ug

No competing interests declared.