Rabies transmission via vampire bats identified in Brazil

In a recent study published in the journal PLOS Neglected Tropical Diseases, researchers evaluated the concerns regarding the hematophagous bat-borne rabies in Brazil.

Background

Human rabies is an overlooked reemerging viral disease with a significant impact on public health in low- and middle-income communities and countries. Rabies has an annual incidence of roughly 59,000 cases in 150 nations, with a fatality rate of nearly 100%.

It is spread through exposure to the infected animal's saliva and is completely preventable with vaccination. Vaccinating more dogs (canines) in a location creates herd immunity and lowers the chances of rabies transmission. However, wild animals, especially chiropterans, sustain the rabies virus (RABV) lineages in various separate epidemiologic loops via sylvatic cycle throughout the Americas.

About the study

In the present research, the scientists evaluated the alteration in the epidemiological pattern of rabies in Brazil. They also determined how this affects monitoring of rabies and disease management efforts.

Findings and discussions

Latest studies indicate a dramatic decrease in rabies cases in Brazil in recent decades. While 72 rabies cases were documented in the Brazilian Ministry of Health in 1990, just one case of rabies by bat transmission was reported in 2020. 

A shift in the way of rabies transmission was also seen over the decades. In 1990, 50 rabies cases were transmitted by dog bites, 11 by bat bites, and two were transmitted by feline bites. In 2017, none of the six documented cases were linked to a dog bite. The five reported rabies cases were linked to bat bites, and one was linked to a feline bite. Bats were responsible for 11 of the reported cases in 2018. The single rabies case reported in 2019 was caused by the bite of a feline infected with the antigenic variant AgV-3 from bats.

These findings indicate that controlling rabies in Brazil will be more challenging if strategies to reduce the bat population are not implemented. Hence, bat-transmitted rabies must be included in the scope of monitoring health services in Brazil. Once a report is verified, resources for a prompt reaction, including control vaccinations among cases of outbreaks and broad public concern, must be mobilized and managed.

The shift in the Brazilian epidemiological profile of rabies puts bats at a crucial point in the present rabies transmission chain. Although various bat species were rabies positive, the vampire bat/Desmondus rotundus (D. rotundus) was primarily responsible for the RABV transmission to humans. Among 150 rabies cases confirmed in humans from 2000 to 2017 in Brazil, the most frequent viral variant was from the D. rotundus bats, AgV3. Further, it was also reported in three rabies transmissions by felines. Rabies variants such as AgV2 and AgV1 from species Canis lupus and AgVnC from Callithrix jacchus were documented during that period. These inferences confirmed the role of vampire bats in rabies transmission in Brazil.

The hematophagous bats were responsible for transmitting several emerging and zoonotic illnesses additional to rabies. The sanguivorous mammals usually prey on dogs, cattle, horses, and even humans at night. Vampire bats were more likely to be seen in areas where the abundance of their natural prey falls. This inference was typically associated with alterations in land-use patterns by humans. Further, small villages in rural Amazonia settlements demonstrate difficulty in access to healthcare facilities. Thus, the probabilities for post-exposure prophylaxis were lower in these villages and were also associated with higher chances of blood-eating bats predation. In Brazil, power outages and being under 17 years of age were the risk factors for an outbreak of rabies in humans in 2009, caused by an attack of D. rotundus in regions where bat bites were thought to be normal. 

Although canine rabies has been completely under control in Brazil, bovine rabies remains widespread in many parts of the nation. Rabies is a major zoonotic viral illness in cattle in Brazil, with an estimated yearly loss of 850,000 animals worth $17 million, and was caused often by the presence of the D. rotundus. The National Program for Rabies Control in Herbivores has recently been successful in lowering disease prevalence in domestic herbivores. The bovine rabies cases declined from 555 to 317 during 2017 and 2019. Nevertheless, this data might be skewed by the probable underreporting of infections due to the absence of regular sample collection for laboratory analyses.

The transportation of cattle during the dry season to save the animals from drought and starvation resulted in human rabies outbreaks in the past decades in the Northeastern region of Brazil. This observation was because the D. rotundus were forced to find an alternative food source, which included humans. Vampire bats were becoming more frequent in areas with large domestic cattle populations, and hence, the bovine acts as sentinels in regions where the rabies virus is spreading.

In Brazil, vaccination campaigns for domestic animals have resulted in a significant drop in human rabies cases transmitted by canines, with no cases of the canine variant reported since 2015. Nevertheless, certain municipalities have canceled or postponed animal rabies vaccination campaigns due to the lack of vaccines delivered by the Brazilian Ministry of Health to state administrations. Some municipalities deferred animal rabies vaccination campaign in 2020 so as not to disrupt the physical and social distancing measures associated with the coronavirus disease 2019 (COVID-19) pandemic.

The shift in the epidemiological pattern of rabies in Brazil coincided with the alteration in the administered anti-rabies serum doses. A total of 11,000; 112,000; 91,000; and 11,000 doses were administered in 2001, 2010, 2011, and 2019, respectively. Nonetheless, in recent years, the number of administered rabies vaccine doses has declined. About 1 million; 583,000; 443,000; 335,000; 260,000; and 127,000 doses were administered from 2015 to 2020, respectively. The factors posing a threat to Brazil's effective programs for the elimination and control of dog-borne rabies include the possibility of a stock shortage of anti-rabies serum and rabies vaccine and inefficient treatment protocols for post-rabies exposure.

In 2019, a woman aged 58 years died of rabies following a cat bite in a rural region of the Gravatal municipality, State of Santa Catarina, southern Brazil. Nevertheless, the last case of rabies in humans was reported in 1981, and that in dogs and cats were in 2006 and 2016, respectively, in Santa Catarina. These incidents should serve as red flags to health officials since they happened in places where rabies had not been detected in years. Further, the Rio de Janeiro State Health Department documented a new rabies-related death from the rural region of Angra dos Reis city in 2020. This rabies-associated death was the first since 2006 transmitted by a bat in Angra dos Reis city. These inferences may or may not be linked to alterations in natural landscapes or climate change.

Climate change might impact bat population dynamics and, as a result, rabies outbreaks. Several investigations on the spatiotemporal distribution of RABV in bats and other mammalian species have found that variables like rainfall, temperature, and the El Nio Southern Oscillation (ELNSO) impact the prevalence of rabies outbreaks in certain places, seasons, and during the year. The shift in regional and local climates because of global warming might change areas abundant with bats and lead to incidences of human rabies in regions where the disease has not been previously reported. Further, deforestation, changes in land-use patterns, and the replacement of natural ecosystems with meadows when livestock increases in number also lead to a shift in the habitat of bat species.

Some researchers predicted that certain locations in southern Texas might become favorable for sanguivorous bats by 2070 via modeling future scenarios for these bats along the US-Mexico border. Nevertheless, due to a high level of uncertainty, the conclusions derived from climate models for future scenarios were not unanimous. Thus, more investigations are warranted to estimate the spread of rabies under various degrees of climate change and global warming scenarios.

Conclusions

The study findings implied that human rabies spread by bats became a significant problem in Brazil over the last few decades. According to recent research, the rabies virus might spread to Brazilian locations where the disease has never been seen before.

In Brazil, the number of human rabies cases caused by dogs has decreased in recent years, from 15 in 2001 to zero in 2021. This decrease is due to effective pet vaccination, public health monitoring, and the availability of rabies post-exposure prophylaxis. Nonetheless, habitat fragmentation, bat range changes that corresponded with temperature rises, and the increasing availability of cattle as a feeding supply for vampire bat species might have promoted rabies transmission among bat populations in recent years in Brazil.

Written by

Shanet Susan Alex

Shanet Susan Alex, a medical writer, based in Kerala, India, is a Doctor of Pharmacy graduate from Kerala University of Health Sciences. Her academic background is in clinical pharmacy and research, and she is passionate about medical writing. Shanet has published papers in the International Journal of Medical Science and Current Research (IJMSCR), the International Journal of Pharmacy (IJP), and the International Journal of Medical Science and Applied Research (IJMSAR). Apart from work, she enjoys listening to music and watching movies.