Why we need to cure the in­fec­tion in cattle first

Down to Earth - - FRONT PAGE - KUN­DAN PANDEY |

FOR A world free of tu­ber­cu­lo­sis, the World Health Or­ga­ni­za­tion has set a few tar­gets: re­duc­tion of in­ci­dence rate by 90 per cent and deaths by 95 per cent by 2035 from the 2015 lev­els. To com­ply with the tar­gets In­dia—that ac­counts for 23 per cent of the global TB bur­den—has adopted newer strate­gies and tools. It is also cop­ing with chal­lenges like emer­gence of drug-re­sis­tant TB. It may still fall short of the tar­gets be­cause the strate­gies ig­nore a cru­cial fac­tor—trans­mis­sion of TB from an­i­mals, es­pe­cially cattle.

My­cobac­terium tu­ber­cu­lo­sis is a pathogen that pri­mar­ily causes TB in hu­mans. Oc­ca­sion­ally peo­ple also get in­fected with M bo­vis, the pri­mary pathogen of bovine TB. But this bur­den of zoonotic TB (trans­mit­ted from an­i­mals to hu­mans) re­mains un­der­es­ti­mated, says a re­view of stud­ies pub­lished in The Lancet In­fec­tious Dis­eases in Septem­ber. Au­thors of the re­view call for a crit­i­cal eval­u­a­tion of zoonotic TB, es­pe­cially in ar­eas where TB is en­demic and where peo­ple live in close prox­im­ity to in­fected an­i­mals or con­sume raw dairy prod­ucts. Ear­lier in 2012, the In­ter­na­tional Live­stock Re­search In­sti­tute had es­ti­mated that bovine TB could be re­spon­si­ble for up to 10 per cent of hu­man TB cases.

These stud­ies are alarm­ing for In­dia, which ac­counts for 52 per cent of the global cattle pop­u­la­tion and where cattle are an in­te­gral part of ru­ral house­holds. In the ab­sence of a na­tional agency to doc­u­ment the preva­lence of bovine TB, let alone study its trans­mis­sion to hu­mans, ex­ist­ing re­search paints a dis­turb­ing pic­ture.

A study pub­lished in Vet­eri­nary World in 2010 shows high preva­lence of bovine TB in dairies in Hi­machal Pradesh. The re­searchers from Dr G C Negi Col­lege of Vet­eri­nary & An­i­mal Sci­ences, Palam­pur, screened 440 cows from six dairies and found 63, or 14.3 per cent, of them suf­fer­ing

from the dis­ease. In one farm, 34 per cent of the cows had TB. While the study does not spec­ify the bac­te­ria that caused TB in the an­i­mals, another re­search, pub­lished in Trans­bound­ary and Emerg­ing Dis­eases in 2013, says M tu­ber­cu­lo­sis is also ca­pa­ble of caus­ing bovine TB. The re­searchers ex­am­ined au­topsy lung tis­sue sam­ples of 30 TB sus­pected an­i­mals from a dairy in North In­dia, and found that eight of them were in­fected with M tu­ber­cu­lo­sis. This sug­gests pos­si­ble adap­ta­tion of the bac­te­ria in cattle and sub­se­quent cattle-cattle and cattle hu­man trans­mis­sion, con­cludes the study.

Re­searchers have also found mixed in­fec­tion with the bac­te­ria duo in hu­mans as well as cattle. A 2005 study pub­lished in Tu­ber­cu­lo­sis found mixed in­fec­tion in 8 per cent of hu­man sam­ples and 35 per cent of cattle sam­ples. This shows po­ten­tial trans­mis­sion of these pathogens from hu­mans to cattle and vice versa, the study con­cludes.

Such bi-di­rec­tional trans­mis­sion ca­pa­bil­ity of the bac­te­ria duo poses a se­ri­ous chal­lenge to the erad­i­ca­tion of tu­ber­cu­lo­sis. Ex­perts say even if the dis­ease is erad­i­cated, it can stage a come­back from an­i­mals ei­ther through M bo­vis or M tu­ber­cu­lo­sis. How­ever, there is lit­tle aware­ness of how the bac­te­ria in­fect sev­eral host species.

Ways to ex­change vic­tims

Sar­man Singh, who heads the divi­sion of clin­i­cal mi­cro­bi­ol­ogy and molec­u­lar medicine at the All In­dia In­sti­tute of Med­i­cal Sci­ences in Delhi, says M bo­vis can in­fect hu­mans liv­ing in close prox­im­ity to in­fected cattle or con­sume con­tam­i­nated, un­pas­teurised dairy prod­ucts. In­creas­ing con­sump­tion of raw milk is lead­ing to in­creas­ing cases of in­testi­nal TB, and most of these cases are caused by M bo­vis, he adds.

Though direct trans­mis­sion of the bac­te­ria from an­i­mal to hu­mans through air is thought to be rare, it can spread from per­son to per­son when peo­ple in­fected with the dis­ease in their lungs cough or sneeze.

An­i­mals usu­ally get in­fected with M tu­ber­cu­lo­sis while feed­ing on con­tam­i­nated graz­ing area, shows a re­search pub­lished in the In­dian Jour­nal of Med­i­cal Re­search in 2008. The re­searchers an­a­lysed 768 spec­i­mens from 161 cattle from a farm of Meerut Can­ton­ment, Ut­tar Pradesh, and found that over 28.5 per cent of the milk sam­ples and seven per cent of pha­ryn­geal swabs taken from the an­i­mals tested pos­i­tive for M tu­ber­cu­lo­sis. Maybe, the an­i­mal han­dlers had TB, and they passed on the dis­ease to the cattle by in­dis­crim­i­nately spit­ting and uri­nat­ing in the graz­ing area, note the au­thors. This was demon­strated in a study con­ducted for a decade by J M Grange, a sci­en­tist from the Cen­tre for In­fec­tious Dis­eases and In­ter­na­tional Health, Windeyer In­sti­tute for Med­i­cal Sci­ence, Lon­don. It showed that a farmer in­fected 48 healthy cattle in four herds by uri­nat­ing in their shed.

Singh says re­search sug­gests that the preva­lence of bovine TB in hu­mans in In­dia is around 1 per cent, where as 6-7 per cent cap­tive an­i­mals (this in­cludes live­stock, an­i­mals in zoo and pets) are in­fected with hu­man TB. And this calls for an ur­gent con­trol and preven­tion strat­egy.

Needed, a mixed strat­egy

Un­til the 1920s, bovine TB was re­garded as a ma­jor dis­ease of do­mes­tic an­i­mals in de­vel­op­ing coun­tries. They have man­aged to con­tain its spread through sys­tem­atic im­ple­men­ta­tion of preven­tion and con­trol mea­sures that in­clude tu­ber­culin skin test and sub­se­quent slaugh­ter of in­fected an­i­mals, and fol­lowed up with san­i­tary sur­veil­lance in slaugh­ter­houses. Singh says hy­giene is the most cru­cial con­trol­ling mea­sure. In many Euro­pean coun­tries, the en­tire herd is culled even if one an­i­mal is de­tected with TB. While In­dia needs to learn from in­ter­na­tional ex­pe­ri­ences, it also needs to over­come sev­eral chal­lenges. For in­stance, there is no pro­to­col to test and slaugh­ter in­fected an­i­mals. Even the di­ag­nos­tic tests are ex­pen­sive. In the ab­sence of suf­fi­cient vet­eri­nary ex­perts and re­search on the epi­demi­ol­ogy of zoonotic TB, the dis­ease re­mains scot-free.

Mitesh Mit­tal, who was part of the 2013 study and works with Se­cun­der­abad unit of the De­fence In­sti­tute of Bio-En­ergy Re­search, says that it is time In­dia had a pro­to­col for di­ag­nos­ing the My­cobac­terium strains and en­cour­aged re­search to de­velop the vac­cine against bovine TB.

Bovine TB could be re­spon­si­ble for up to 10 per cent of TB cases in hu­mans

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