It’s hard to argue with the weather, and it is getting even harder to argue with climate change. April 2016 was the 12th consecutive month that Earth has recorded its warmest respective month on record, meaning that it has been one hot year. Furthermore, May clouds threatened to drop the most precipitation on record, and June is just starting to heat up.
But climate change is not just going to rain on your parade – it will also affect human health. Climatic conditions strongly affect vector-borne diseases, including Lyme disease, malaria, and the recently infamous Zika virus. All three illnesses are transmitted by disease vectors, such as ticks, fleas, and the dreaded mosquito. Vectors pick up infectious pathogens from animals in the form of viruses, bacteria, and protozoa and carry them to human hosts. Changes in temperature and precipitation can lengthen the transmission seasons of vector-borne diseases and increase their geographic range.
The geographic range of the deer ticks that carry the Lyme disease spirochete Borrelia burgdorferi is currently limited by temperature. However, as global temperatures rise, these ticks are likely to become active earlier in the season, and their range is likely to continue to extend northward.
The number of cases is already expanding. In 2013 the Centers for Disease Control reported that their previous estimate of 30,000 annual Lyme disease cases in the U.S. was actually closer to 300,000. They attributed the majority of the discrepancy to the result of inconclusive diagnoses – the symptoms of Lyme disease often imitate those of other diseases, such as multiple sclerosis. On the other hand, Jennifer Reid, founder of the Lyme Disease Task Force in Ridgefield, Connecticut, blames climate change and deforestation for the influx of Lyme into suburban communities.
Regardless, it is clear that a reliable diagnostic method is needed to treat the growing numbers of people infected with Lyme disease. To do so, researchers must uncover more about the source of the disease and its symptoms, as well as about the deceitful spirochete that causes it. To quote Dr. Joseph Breen, the program officer for the Lyme Disease Research Program at the National Institutes of Health (NIH), “The better we understand the disease, the better we understand Borrelia burgdorferi, and the better we understand a response of the host to Borrelia burgdorferi then we should be able to better design diagnostics, better treatments and potentially prevent the disease using vaccine.”
As Lyme disease affects the entire body, animal research is the most reliable method of studying the effects of the disease and testing possible cures. Only testing with animals can effectively mimic the disease and thus confirm a vaccine or treatment. Previous studies in rodents and rhesus macaque monkeys have provided valuable information regarding the behavior of Borrelia burgdorferi and the host immune response. However, more research is needed, and animal research will be essential.
Similar to deer ticks, mosquitos thrive only in certain climate conditions – warm temperature, rain, and high humidity are all essential to mosquito survival. Warmer global temperatures as well as increased global rainfall will expand their breeding period, population size, and location.
As mosquitos move outward from the tropics, the diseases they carry come with them. Malaria, for example, is transported by Anopheles mosquitos. It is caused by only a single parasite, Plasmodium, and yet, it kills approximately 600,000 people annually, the majority of which are young children in Africa. The World Health Organization predicts a global temperature rise of two degrees Celsius would put seven percent more people at risk of malaria. World leaders agreed at the Paris Climate Conference last year that the global temperature will increase by at least two degrees Celsius in the coming decades, threatening several million people globally.
Although control measures are aimed at both the mosquito vector as well as the parasite inside its human host, and even though $2.5 billion are spent annually fighting the disease, new safe and effective drugs are desperately needed. The parasite has become resistant to most of the currently used drugs, with the exception of the most recently developed drug artemisinine and its derivatives. Additionally, many treatments try unsuccessfully to target the parasite in the blood during its quick invasion stages and only a few kill it during its replication phase in the liver.
Scientists hope to develop a new drug to combat malaria and to defend uninfected regions as Anopheles’ range expands in a fight for eventual eradication. To reach this goal, animal research will play an essential role, as it is the only accurate way to study the results of treatments on a whole living system. Numerous early studies in rodents have already provided essential information about potential immune protective mechanisms. Trials with goats have also been helpful, and recent studies with African thicket rats from the Congo by researchers from the Laboratory of Malaria Immunology and Vaccinology at the US National Institute of Allergy and Infectious Diseases are poised to shed more light on the dark and gloomy subject of malaria.
Another mosquito-borne disease, Zika, has been dominating the news lately. It is spread by the Aedes mosquito, which was noted by researchers last year to be more widely distributed globally than ever before. Heavy rains and high temperatures in South America have helped the mosquito to thrive and Zika to spread.
Despite widespread news coverage, there is little scientific research on Zika. As researchers scramble to put together a treatment plan, animal research will be paramount to preventing the spread of Zika in the future. Dr. Francis Collins, director of NIH agrees that, “It is now critically important to confirm, through careful epidemiological and animal studies, whether or not a causal link exists between Zika virus infections in pregnant women and microcephaly in their newborn babies.” Currently, studies with mice and rhesus macaque monkeys are being used to confirm that Zika does cause microcephaly in fetuses – information that will be vital to controlling the virus. Additionally, animal research will be the foundation of vaccine and treatment development, as soon as scientists understand enough about the Zika virus to begin developing a cure.
The renewed global concern with infectious disease is an early testament to the effect climate change will have on human health. Warmer temperatures and extra rainfall have both played a role in the recent outbreaks of Lyme disease, malaria, and the Zika virus. Although the spread of climate-sensitive diseases depends on both climate and non-climate factors – such as land use, socioeconomic conditions, and access to health care – there is no doubt that climate change will continue to profoundly shape medicine and human health. As it does, animal research is often the best tool for researchers to study new diseases and treatments. Thus, it is crucial researchers remain able to perform humane and responsible research with animals, given the increasingly dire threat infectious diseases pose to millions of people globally.