At a Maryland country fair in 2017, the prize pigs were not looking their best. Farmers reported feverish hogs with inflamed eyes and running snouts. But while fair officials worried about the pigs, the Maryland department of health was concerned about a group of sick fairgoers. Some had pet the pigs, while others had merely been near their barns; but soon, 40 of these attendees would be diagnosed with swine flu. More often than not, sick animals don’t infect humans. But when they do, these cross-species infections, or viral host jumps, have the potential to produce deadly epidemics. So how can pathogens from one species infect another, and what makes host jumps so dangerous?
Na Marylandskom sajmu 2017. godine nagrađene svinje nisu izgledale najbolje. Farmeri su se žalili kako svinje imaju vrućicu, upaljene oči te im curi iz njuške. Dok su se službenici sajma zabrinuli za svinje, Ministarstvo zdravstva Marylanda se zabrinulo za grupu bolesnih posjetitelja. Neki su dirali svinje, dok su drugi samo viđeni pored štala, ali ubrzo je 40 ljudi bilo dijagnosticirano sa svinjskom gripom. Najčešće bolesne životinje ne prenose bolest na ljude. Ali kada se to desi, takav prijenos između vrsta, skok na novog domaćina, može dovesti do smrtonosnih epidemija. Kako patogeni jedne vrste mogu zaraziti drugu i što skakanje na novu vrstu čini opasnim?
Viruses are a type of organic parasite infecting nearly all forms of life. To survive and reproduce, they must move through three stages: contact with a susceptible host, infection and replication, and transmission to other individuals. As an example, let’s look at human influenza. First, the flu virus encounters a new host and makes its way into their respiratory tract. This isn’t so difficult, but to survive in this new body, the virus must mount a successful infection before it’s caught and broken down by an immune response. To accomplish this task,
Virusi su vrsta parazita koji se nalaze u gotovo svim oblicima života. Kako bi preživjeli i razmnožili se, prolaze kroz tri faze: kontakt s prijemčivim domaćinom, zarazu i razmnožavanje, te prijenos na druge pojedince. Uzmimo za primjer gripu u ljudi. Prvo virus gripe dođe do novog domaćina i dospije u njegov dišni sustav. To nije teško, ali kako bi preživio u novom tijelu, virus mora uspješno zaraziti domaćina prije nego što je uhvaćen i razgrađen od strane imunosnog sustava. Kako bi to postigli,
viruses have evolved specific interactions with their host species. Human flu viruses are covered in proteins adapted to bind with matching receptors on human respiratory cells. Once inside a cell, the virus employs additional adaptations to hijack the host cell’s reproductive machinery and replicate its own genetic material. Now the virus only needs to suppress or evade the host’s immune system long enough to replicate to sufficient levels and infect more cells. At this point, the flu can be passed on to its next victim via any transmission of infected bodily fluid.
virusi su razvili specifične odnose s njihovim domaćinima. Virusi gripe su prekriveni proteinima koji su se prilagodili za spajanje s receptorima na ljudskim stanicama. Jednom kad je u stanici, virus vrši daljnje promjene kako bi preuzeo mehanizam za razmnožavanje stanice i umnožio svoj genetski materijal. Virus mora utišati i izbjegavati imunosni sustav domaćina dok se ne umnoži dovoljno i ne zarazi još stanica. U tom trenu gripa se može prenijeti na druge osobe bilo kojim putem kroz zaražene tjelesne tekućine.
However, this simple sneeze also brings the virus in contact with pets, plants, or even your lunch. Viruses are constantly encountering new species and attempting to infect them. More often than not, this ends in failure. In most cases, the genetic dissimilarity between the two hosts is too great. For a virus adapted to infect humans, a lettuce cell would be a foreign and inhospitable landscape. But there are a staggering number of viruses circulating in the environment, all with the potential to encounter new hosts. And because viruses rapidly reproduce by the millions, they can quickly develop random mutations. Most mutations will have no effect, or even prove detrimental; but a small proportion may enable the pathogen to better infect a new species. The odds of winning this destructive genetic lottery increase over time, or if the new species is closely related to the virus’ usual host. For a virus adapted to another mammal, infecting a human might just take a few lucky mutations. And a virus adapted to chimpanzees, one of our closest genetic relatives, might barely require any changes at all.
Obično kihanje također raspršuje virus i na vaše ljubimce, biljke, pa čak i na hranu. Virusi se stalno susreću s novim vrstama i pokušavaju ih zaraziti. U većini slučajeva ne uspiju. U većini slučajeva, genetska raznolikost je prevelika između dva domaćina. Za virus koji se razvio za napad na ljude, stanica zelene salate je nepoznata i neprijateljska površina. No, u okolišu postoji mnogo virusa i svi imaju potencijal da se susretnu s novim domaćinom. Kako se virusi izrazito brzo razmnožavaju, govorimo o milijunima, brzo može doći do nasumičnih mutacija. Većina mutacija nema učinka ili se čak pokažu štetnima, ali, mali udio možda dovede do boljeg primanja virusa na novu vrstu. Vjerojatnost za dobitak na ovoj genetskoj lutriji se povećava s vremenom, pogotovo ako je nova vrsta genetski bliska s dosadašnjim domaćinom virusa. Za virus koji napada druge sisavce, potrebno mu je možda nekoliko mutacija kako bi mogao zaraziti i ljude. A virusi koji napadaju čimpanze, najbliže rođake što se genoma tiče, jedva da trebaju ikakvu promjenu.
It takes more than time and genetic similarity for a host jump to be successful. Some viruses come equipped to easily infect a new host’s cells, but are then unable to evade an immune response. Others might have a difficult time transmitting to new hosts. For example, they might make the host’s blood contagious, but not their saliva. However, once a host jump reaches the transmission stage, the virus becomes much more dangerous. Now gestating within two hosts, the pathogen has twice the odds of mutating into a more successful virus. And each new host increases the potential for a full-blown epidemic.
No potrebno je više od vremena i genetske sličnosti kako bi skok na novog domaćina uspio. Neki virusi imaju oruđe kojim lako napadnu stanice novog domaćina, ali se ne mogu sakriti od imunosnog odgovora. Drugi možda imaju problem kako doći do novog domaćina, na primjer, ako je virus zarazan u krvi, ali ne i u slini. No jednom kada virus uspije doći do novog domaćina, on postaje opasan. Sad kad se virus razvija u dva domaćina, dvostruko su veće šanse za uspješnu mutaciju. I svaka nova zaražena osoba povećava šansu za nastanak epidemije.
Virologists are constantly looking for mutations that might make viruses such as influenza more likely to jump. However, predicting the next potential epidemic is a major challenge.
Virolozi stalno tragaju za mutacijama koje bi mogle dovesti do skoka virusa poput virusa gripe. Predviđanje sljedećih potencijalnih epidemija je veliki izazov.
There’s a huge diversity of viruses that we’re only just beginning to uncover. Researchers are tirelessly studying the biology of these pathogens. And by monitoring populations to quickly identify new outbreaks, they can develop vaccines and containment protocols to stop these deadly diseases.
Postoji velika raznolikost među virusima koju tek sad otkrivamo. Znanstvenici neumorno proučavaju biologiju tih patogena. Prateći populaciju kako bi otkrili potencijalnu epidemiju, mogu razviti cjepiva i metode sprječavanja širenja smrtonosnih zaraza.