How did the COVID pandemic affect the spread of other zoonotic diseases?

By Sam HancockReviewed by Danielle Ellis, B.Sc.Jan 12 2022

The origin of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been speculated about in different ways since it first emerged. One of the most widespread theories reported in the media suggested zoonotic transmission had occurred in a 'wet market' in Wuhan, China, where different, sometimes wild, live animals are kept in close proximity. Although there is disagreement over different theories, the general consensus is that humans were not the initial hosts of severe acute respiratory coronavirus 2 (SARS-CoV-2).

Study: The impact of the COVID-19 pandemic on the incidence and mortality of zoonotic diseases in China. Image Credit: Kateryna Kon / Shutterstock.com

The most deadly diseases are often zoonotic because neither the disease nor the host has adapted to each other. Recently, there have been suggestions that the Omicron variant is the beginning of these adaptions, as it appears to cause significantly less severe disease.

Most governments worldwide introduced restrictions to reduce the transmission of SARS-CoV-2, including mandatory face masks, social distancing policies, and even total lockdowns/stay-at-home orders. However, people's lifestyles, economics, and environmental vectors are all impacted by these difficult circumstances, as are the epidemic characteristics of other diseases.

According to some studies, this has a significant impact on respiratory conditions, particularly seasonal influenza. By controlling the spread of the coronavirus, COVID-19 cases decreased, and potential transmission of other infectious diseases was reduced. As a result, the incidence of influenza has decreased around the world. Furthermore, some studies on low- and middle-income countries have reported that strict movement restrictions may also contribute to a delay in diagnosis and treatment of diseases caused by parasites, fungi, bacteria, and viruses (such as HIV, tuberculosis, and malaria).

In new research, scientists from the Beijing University of Technology, Renmin University, Zhejiang University School of Medicine and Nanjing Jiliang Information Technology have been investigating the impact of the COVID-19 pandemic on the incidence and mortality of zoonotic diseases in China. Their work is published in the journal BMJ Global Health.

The Study

The researchers obtained data from the National Health Commission used by China on monthly cases and deaths from 10 different zoonotic diseases between January 2015 and April 2021. They removed H5N1 and Plague, as the sample sizes were too small to analyze appropriately. The eight remaining infectious diseases consisted of Asian Lineage Avian Influenza A(H7N9), hemorrhagic fever, rabies, anthrax, brucellosis, leptospirosis, hydatid disease, and schistosomiasis. Most of these are class B, with hydatid disease as class C. The National Bureau of Statistics provided population data.

The scientists had to consider the response strategies China used to minimize the spread of COVID-19. Hubei province launched a second-level public health emergency response on the 22 of January 2020, while Wuhan was on lockdown on the 23rd. All 31 provinces launched the first-level response. While different areas applied different containment and suppression strategies, the most important measures taken in the most severely affected areas included lockdowns, restricted travel, cancellations of mass gatherings, and mandatory face masks.

The researchers calculated averages for annual incidence, number of cases, mortality, number of deaths, monthly incidence and monthly mortality. They compared different groups using proportional tests and ratio Z tests. The growth rate was computed to examine the growth of incidence and mortality. The Cox-Stuart test assessed increasing/decreasing long-term trends for monthly incidence and mortality, and monthly predictive incidence was generated using Farrington surveillance algorithms to avoid the interference of the long-term trend of the disease itself.

The researchers found the three most common zoonotic diseases were brucellosis, which hovered between ~80% and ~85%, hemorrhagic fever (~7.5%-~14%), and schistosomiasis (15%). Schistosomiasis did show significantly lower incidence in 2020 and 2021. A total of 25,016 cases were detected in 2021, 63,226 in 2020, and 77,448 per year between 2015 and 2019. The fewest deaths were seen in 2021 at 77, with 244 in 2020 and 673 per year between 2015 and 2019. Rabies caused the most deaths in all years, followed by H7N9 and hemorrhagic fever.

Zoonotic diseases tend to follow long-term seasonal trends. 2015 showed the highest incidence of these diseases between 2014 and 2020. Following the routine stage in 2020, incidence rose toward the average monthly incidence between 2015 and 2019. The researchers found that the average monthly incidence between January and April 2021 was significantly higher than in both 2020 and 2015-2019. Brucellosis and hydatid disease showed substantially higher monthly incidence. They also discovered significant long-term downward trends of zoonotic diseases, although it is acknowledged that natural long-term trends of the diseases could affect this feature.

Comparing monthly incidence and monthly mortality in 2020 and 2015-2019, the researchers discovered that as the emergency routines to prevent the transmission of COVID-19 began to take effect, zoonotic diseases declined significantly. Monthly incidence reduced by 25.79% and mortality by 72.39%. These figures were also considerably lower than predicted.

Conclusion

The authors highlight that they have reported the most recent trends of these diseases successfully and with a considerable sample size. A significant decline was found in incidence rates of all the investigated diseases in 2020 compared to the previous five years. Still, this decline did not continue into 2021, with incidence rates climbing once again. This information could help inform future strategies to help reduce the transmission of these diseases.