PHE 321 Module 5 Project One Example

Project One: Counting the Unseen. An Estimate of Chagas Disease in Three South Texas Counties

[Student Name]

Southern New Hampshire University

PHE 321: Biological Concepts for Public Health

Project One

[Instructor Name]

[Date]

The organization, setting and figures below are a composite written as a model document. No real employer, client, colleague or patient is described.

Project One: Counting the Unseen. An Estimate of Chagas Disease in Three South Texas Counties

Nobody in the three counties served by the district has been systematically tested for Trypanosoma cruzi, and almost no one with the infection knows it. Yet planning a response requires some idea of how many people might be infected and where they are. This project estimates the burden of Chagas disease in the district's population of about 48,000 using indirect methods, shows every assumption and checks the result against the only local data available, blood donor screening. It ends with what the estimate does and does not tell the district.

The Method

National estimates of Chagas disease in the United States rely on a simple idea. Because most infections in the country were acquired in Latin America, the number of infected residents can be approximated by multiplying the number of people born in endemic countries by the prevalence of infection in those countries. Manne-Goehler et al. (2016) used this approach with census data and international prevalence figures and arrived at roughly 238,000 infected people across the country, placing Texas in the small group of states above 10,000. The method has known weaknesses. It assumes that immigrants have the same infection rate as their home country, though many came from cities where transmission was lower. It misses people not counted in surveys. And it ignores infections acquired in the United States. This project adopts the method but expresses the result as a range.

Inputs

Census data for the three counties show about 7,800 residents born in Mexico or Central America, most from northern Mexican states. Published prevalence estimates for Mexico are below 1%, and northern states generally report less transmission than southern ones. To reflect that uncertainty, the project uses a low prevalence of 0.5% and a high prevalence of 1.0% for the foreign-born population. For people born in the United States, the project assumes no infection for the main estimate, then considers local transmission separately.

The Estimate

Multiplying 7,800 residents by 0.5% gives about 39 infected people; by 1.0%, about 78. The district's best estimate is therefore that between roughly 40 and 80 residents carry the parasite, almost all unaware of it. Applying the long-term outcomes described by Rassi et al. (2010), in which 30% to 40% of infected people develop heart or digestive disease, between about 12 and 31 of them may already have or will develop complications. Because the infection is silent until damage appears, most of these complications would not be recognized as Chagas disease unless someone thought to test for it.

Checking Against Donor Data

The regional blood center reported five confirmed positive donors from the three counties over two years, among about 6,000 donations from district residents. Blood donors are not representative of the population; they are healthier, often younger and more likely to be U.S.-born. Still, finding five positives among donors is consistent with dozens of infections in the wider population, and the donor results make the estimate's range plausible rather than exaggerated. They also show that infected people are present and identifiable now, not only in theory.

Expected Congenital Cases

About 250 babies a year are born in the district to mothers born in endemic countries. If 0.5% to 1.0% of those mothers are infected, one to three infected pregnancies would be expected each year. Pooled studies put mother-to-baby transmission just under one birth in twenty (Howard et al., 2014). That suggests roughly one congenitally infected baby every eight to sixteen years in the district. The number is small, but each such infection is highly treatable in infancy and, if missed, may become a lifelong condition, and the mother's other children could also be infected.

Local Insect-Borne Risk

Local transmission is the hardest piece to estimate. Bern et al. (2011) report only a small number of documented locally acquired human infections in the United States, though many are thought to go unrecognized. Garcia et al. (2015) found five such cases among positive blood donors in a pilot study in southeast Texas. In the district, kissing bugs and infected dogs have been documented, so local transmission is possible, but it likely adds only a handful of cases to the estimate. The project therefore treats local risk as real but small, warranting awareness and veterinary surveillance rather than large vector control programs.

Sensitivity of the Estimate

Because the inputs are uncertain, it helps to ask which ones matter most. The prevalence assumption dominates: doubling it doubles the estimate, while a 10% error in the census count changes the result by only a few people. The estimate would also rise if undocumented residents are undercounted in census data; Manne-Goehler et al. (2016) noted that national figures excluded undocumented immigrants who might add substantially to the total, and the same gap applies locally. It would fall if most foreign-born residents came from northern Mexican cities with little transmission, which census data cannot show. Testing a few hundred women in prenatal care would replace the assumed prevalence with a measured one, narrowing the range far more than any other step. For now, the range of 40 to 80 should be read as a reasonable order of magnitude, not a count, and plans should work across that range rather than depend on its midpoint.

What the Estimate Means

The estimate is uncertain, but it is useful. It tells the district that dozens of residents, not thousands, are likely infected, that most acquired the infection long ago and abroad and that the most valuable actions are finding and evaluating those people, especially women of childbearing age. It also identifies what would improve the estimate: testing in prenatal care would produce local prevalence data, and follow-up of positive donors would show how many reach care.

Conclusion

Using a national method adapted to local data, the district can expect roughly 40 to 80 residents with Chagas disease, a dozen or more with present or future complications, an infected newborn every decade or so and a small amount of local insect-borne risk. Project Two will propose how to find and help them.

References

Bern, C., Kjos, S., Yabsley, M. J., & Montgomery, S. P. (2011). Trypanosoma cruzi and Chagas' disease in the United States. Clinical Microbiology Reviews, 24(4), 655-681. https://doi.org/10.1128/CMR.00005-11

Garcia, M. N., Aguilar, D., Gorchakov, R., Rossmann, S. N., Montgomery, S. P., Rivera, H., Woc-Colburn, L., Hotez, P. J., & Murray, K. O. (2015). Evidence of autochthonous Chagas disease in southeastern Texas. American Journal of Tropical Medicine and Hygiene, 92(2), 325-330. https://doi.org/10.4269/ajtmh.14-0238

Howard, E., Xiong, X., Carlier, Y., Sosa-Estani, S., & Buekens, P. (2014). Frequency of the congenital transmission of Trypanosoma cruzi: A systematic review and meta-analysis. BJOG: An International Journal of Obstetrics and Gynaecology, 121(1), 22-33. https://doi.org/10.1111/1471-0528.12396

Manne-Goehler, J., Umeh, C. A., Montgomery, S. P., & Wirtz, V. J. (2016). Estimating the burden of Chagas disease in the United States. PLOS Neglected Tropical Diseases, 10(11), Article e0005033. https://doi.org/10.1371/journal.pntd.0005033

Rassi, A., Jr., Rassi, A., & Marin-Neto, J. A. (2010). Chagas disease. The Lancet, 375(9723), 1388-1402. https://doi.org/10.1016/S0140-6736(10)60061-X