a recent article noted the northward spread of a tick-borne disease, Heartland virus (HRTV), a consequence of climate change (see Heartland virus in eastern US EmergID2022 in dropbox or DOI: https://doi.org/10.3201/eid2804.211540)
Details:
-- there was a report of a human case of HRTV in Georgia, engendering a search for its source
-- 26 areas were evaluated at sites where seropositive white-tailed deer (Odocoiles virginianus) were captured, during samplings from April-October 2018
-- they evaluated the Amblyomma americanum ticks (aka the lone star tick), since these are the ones most associated with human tick bites in Georgia
-- 9,294 Amblyomma americanum ticks were processed in large sample pools, and the 3 pools with positive virus isolation of HRTV had whole-genome sequencing
Results:
-- the positive pools for HRTV had a minimum infection rate of 0.46/1000 ticks
-- genome sequencing of 3 HRTV isolates found from samplings from different areas in Georgia had almost 100% similarities between themselves but marked difference from other HRTV isolates from elsewhere (from one patient in Tennessee in 2013 and one in New York in 2018)
Commentary:
-- the Heartland virus was first identified in 2009 as a cause of human illness: 2 cases were found in Missouri after exposure to the lone star ticks
-- this Heartland virus is part of the Phenuiviridae family and Bandavirus genus, and is closely related to the Severe Fever with Thrombocytopenia Syndrome Virus (SFTSV), though that one is transmitted by the longhorned ticks: Haemaphysalis longicornis
-- SFTSV was been found in Southeast and Central Asia. Over 100 cases have been identified. symptoms include high fever, fatigue, GI symptoms, neurologic symptoms, thrombocytopenia, leukopenia, and multiorgan failure, and has a mortality rate of 27% (see
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7706950/-- viral DNA of HRTV has been found in both immature and mature stages of the lone star tick, suggesting overwintering of these ticks (and the potential for increased viral spread)
-- 40 human cases of HRTV virus has now been identified in the US: Missouri, Kansas, Oklahoma, Arkansas, Iowa, Illinois, Tennessee, Indiana, Georgia, and South Carolina (many in people who had underlying conditions, and most illnesses were severe or fatal [perhaps only the severe cases were assessed for this unusual virus???]
-- only Alabama has conclusively identified HRTV in lone star ticks [though it seems that other areas have not been investigated....; the only reference is for Alabama]
-- and, in case you were wondering how the lone star tick got its name:
-- the lone star tick is now really common in many parts of the US (southeastern, eastern, and midwestern US, but now spreading into Iowa, Indiana, Ohio, and Pennsylvania, New Jersey, Rhode Island, New York, Connecticut and possibly Maine, and is the major vector for a few tick-borne illnesses, including Ehrlichiosis, Southern tick-associated rash illness, and Tuleremia (see https://www.cdc.gov/ticks/diseases/index.html#:~:text=Ehrlichiosis%20is%20transmitted%20to%20humans,ticks%20can%20transmit%20the%20virus. ).
Here is the current distribution of the Amblyomma americanum tick in the US
-- I do want to point out that the issue of climate change and its effects on human health, and in particular the changes in geography of many vectors of human disease, has been highlighted for a long time. Dr Paul Epstein, an early investigator raising the alarm about human disease, did a 2 or 3 part series in the 1980s (i believe in NEJM, but my PubMed searches don't go back that far), subsequently noting in 1996 that "mosquito-borne diseases, including malaria, dengue, and viral encephalitides, are among those diseases most sensitive to climate ... by shifting the vector's geographic range and increasing reproductive and biting rates and by shortening the pathogen incubation period". In fact, malaria was newly found in higher altitudes in Papua New Guinea where glaciers were receding in the late 1950s, and then documented in Kenya and several other areas in Africa that had been malaria-free. Water-borne infections/toxins (cholera, shellfish poisoning) will also likely increase with higher water temperatures. And "human susceptibility to infections might be further compounded by malnutrition due to climate stress on agriculture" and possible changes in our immune system by ultraviolet light (see https://pubmed.ncbi.nlm.nih.gov/8604175/). and of all that with the understanding then that the estimate of global temperature increase would be "an unprecedented risk of 2.0 degrees C by the year 2100". rather prescient...... and the 2.0 degree change is already in the rearview mirror and receding.
-- so, the findings a few decades ago raised real concerns about what had happened (migration of many "tropical" diseases into southern US, such as dengue) because of large scale vector spread (mosquitoes, tics, etc) from pretty small increments in average temperatures in those regions. The future spread was likely (and has) continued further north with increasing temperatures. And, holistically, there will likely be a combination of increased infectious disease prevalence (viral, rickettsial, bacterial.... ) associated with potentially increased human susceptibility (changes in UV light, nutrition). And the slope of the temperature changes currently seen far outstrips the prior, and quite concerning, mathematical modeling.
Limitations:
-- the pickup of HRTV in lone star ticks was quite low, which only confirms that some lone star ticks have the virus. But this finding cannot conclusively show that this was the mechanism of human transmission of HRTV. this finding in ticks certainly suggests that there is a viable reservoir of HRTV there, though it could well be than human transmission is through an intermediary, perhaps a vertebrate running around (as was found in the Japanese study of SFTSV cases, where cat bites were associated with transmission to humans)
-- they focused on the lone star tick; it is also very possible that other ticks (eg, the longhorned tick) may also be vectors. and longhorned ticks are also spreading quickly to other areas in the US
-- the number of transmitted cases of HRTV (and also SFTSV) is likely under-reported, since (likely) only severe cases, perhaps with diagnostic uncertainty, were tested. Mild and asymptomatic cases likely were not on their radar screen
so, as we journey now into spring, a recent NPR segment noted that we in New England are in for a very warm spring, which translates into a huge tick infestation early and lasting 6+ weeks. We, as a global population, really really do need to do all the things that are needed to decrease the slope of climate change, especially by the major polluters, past and present. it is notable that even small changes in average weather can lead to huge changes in the geographical reach of many disease vectors. And all of this is already happening and at a rapid pace.
-- the science of climate change has been clear for decades, and it has been documented in the medical literature increasingly so (though often not in the most read medical journals). And climate change is clearly a global existential issue...
-- also, this report reinforces that we should start to include novel infectious diseases in our differential diagnosis of unusual clinical scenarios, since several are potentially treatable and associated with potentially really bad clinical outcomes if untreated.....
geoff
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