Bats, not just for rabies anymore

A timely post about bats during the Halloween season. This paper is a well written review by the CDC and Emory University of the emerging infectious diseases (EIDs) from bats. While long, at 17 pages (13 with 4 pages of references), it is quite interesting with regards to the various viruses bats carry, some of which have infected humans.

Obviously, the first topic they discuss is bat rabies. If you weren’t aware, the rabies virus (RABV) is carried worldwide by multiple carnivores, but the only bats that carry it live in the Americas. However, there are 11 other lyssaviruses in the Old World that cause the human disease known as rabies. Ten of those 11 are known to be carried by bats, and the 11th is suspected but not confirmed to have a bat vector. These other viruses carry colorful names such as: Lagos bat virus, Duvenhage virus, European bat lyssavirus, and others.There remain questions as to whether bats can asymptomatically shed the virus or not. The old teaching was that they could, but further research showed that bats do exhibit rabies symptoms, and those without symptoms were not infectious. More recent research has shown that perhaps they do shed while asymptomatic, and the authors recommend further studies before making a definite statement.

Another interesting fact is that big brown bats are 90% of identified rabid bats in the United States, but do not make up the majority of human rabies cases. These come from silver-haired and eastern tri-colored bats, which together make up 60% of human cases. This is more paradoxical when you consider that big brown bats have much more contact with humans, as they live in human structures, where the the other two do not. Explanations include greater pathogenicity of the viral variants in the silver-haired and tri-colored bats. Mexican free-tailed bats cause several human cases as well.

Old World bat rabies, as mentioned above, isn’t from RABV, but from the other lyssaviruses. Unfortunately, surveillance is limited to Western Europe and Australia, except during outbreaks, so data is limited in the geographic locations it is most important. Frighteningly, only some of the lyssaviruses are covered by current biologic treatments. West caucasian, Shimoni, Lagos bat viruses and Mokola virus are not susceptible to rabies vaccine and human rabies immune globulin. Thankfully, no human cases are referenced in the paper, but many animal infections have occurred both in the lab and in nature.

The paper makes a point about the prevalence of bat rabies in Africa by noting a case report of a Dutch tourist who was attacked by a bat while vacationing in Kenya. He went to get treatment, but was told that bat rabies does not exist in Kenya and was not given post exposure prophylaxis. He, like nearly all other untreated rabies cases, died later. Similarly, Australian bat lyssavirus (ABLV) was discovered in 1996 in previously “rabies free” Australia. Shortly after, 2 patients contracted a clinical disease consistent with rabies later found to be ABLV. Both later died.

As if rabies wasn’t scary enough, bats carry other virus. Comically, the filoviruses Marburg and Ebola get lesser billing in this paper due to the fact that they aren’t as lethal as rabies. These were finally found to be carried by bats between 2001-2005, even though Marburg virus was first identified in 1967. Using the retrospectoscope, they found that the majority of Marburg infections were linked to cave or mine visitation. More recently, a number of people died of Marburg after visiting caves in Uganda. What is interesting is that these viruses seem to be carried by fruit-eating bats. Seroprevalence is low, so there is always the possibility of an as yet unidentified third vector.

Another viral family the paper discusses is the paramyxoviruses. These include Hendra, Nipah, Tioman, Pulau, and Menagle viruses. They cause respiratory infections as well as encephalitis, and human to human transmission has been observed. Case fatality rates approach 90% in these patients.

The last viral family discussed in the paper is the coronaviruses (CoV). It isn’t a timely paper if they don’t discuss SARS. During the surveillance period after the outbreak in China, they discovered SARS-like coronaviruses in horseshoe bats. Genetically they’re different enough that they likely aren’t the direct source of the virus to people. Once discovered in bats in China, other bats worldwide were tested and found to carry CoV on every continent except Antarctica.

So what makes bats different? A fair amount of the paper tries to find this answer, and is left wanting. Is it that they fly, thus allowing them to spread virus over large areas? Or is it that they fly, and thus have hollow bones, lacking normal bone marrow B cells and immune systems? There does seem to be a link to bats and the RNA viruses, and this may be related to their lymphocyte types. Certainly, the increase in human infections is due to encroachment into native habitats. Vampire bats will switch from wild animals to domestic animals when they encounter them. The viral infections seem to persist in bats (and colonies) without significant changes in the colony mortality.

The paper does discuss treatment and post-exposure prophylaxis, as well as pre-exposure prophylaxis in high risk groups. These include veterinarians, animal handlers, rabies and other lab researchers, and travelers to endemic areas with poor health resources. Rabies vaccination has worked in domestic animals in the US, but does not appear to affect wild animal rabies. Other interesting control ideas include anticoagulating livestock, as vampire bats can only digest coagulated blood (who knew?).

In the end, it all comes down to this. Bats carry a plethora of viruses, and the paper discusses that they probably carry just as many bacterial (bartonella!) and fungal (histoplasmosis!) pathogens. Treatment should always be aggressive once bat exposure has been documented. Surveillance for EIDs is of utmost importance, and prevention and control modalities need to be created to avert epidemics.

Kuzmin IV, Bozick B, Guagliardo SA, Kunkel R, Shak JR, Tong S, Rupprecht CE Bats, emerging infectious diseases, and the rabies paradigm revisited. Emerg Health Threats J. 2011 Jun 20;4:7159 [PMC3168224]