Cat-to-Human Transmission of Mycobacterium bovis, United Kingdom

Human infection with Mycobacterium bovis is reported infrequently in the United Kingdom. Most cases involve previous consumption of unpasteurized milk. We report a rare occurrence of 2 incidents of cat-to-human transmission of M. bovis during a cluster of infection in cats.

I n the United Kingdom, Mycobacterium bovis infection in humans is relatively rare (1), and most cases involve previous exposure to well-recognized risk factors, such as unpasteurized milk (2). However, with >4,500 new cases The potential for cat-to-human transmission of M. bovis has always been recognized. Although concurrent infection in cats and humans in the same household has been reported (6), and reports of potential transmission exist (7), documented transmission events have not been clearly described. We report a rare occurrence of microbiologically and genetically confirmed cat-to-human transmission of M. bovis.

The Study
During December 2012-March 2013, a veterinary practice in Berkshire, England, diagnosed 7 confirmed (culture from lesions or wounds) and 2 suspected (clinically compatible) cases of M. bovis disease in domestic cats. One additional suspected case was identified after an interview with an affected household. No samples were available from any of the suspected cases for confirmation for this study. The 10 cats belonged to 9 separate households, of which 6 were <250 m from each other. All cats had severe systemic infection, including discharging lymph nodes, nonhealing or discharging infected wounds, and radiographic pulmonary signs. Isolates from the culture-confirmed cases were of the same genotype (10:u), were similar by whole-genome sequencing, and separated into 2 clusters by a single informative polymorphism (8). Veterinary investigations did not determine the source of infection, but the source was believed to be infected wildlife, most likely rodents or badgers, for at least some of the cats. Further information on the investigation into this cluster of infection in cats has been reported (8).
The unusual size and severity of the cluster of feline M. bovis cases led to the decision that TB screening (9) would be offered to all human household members and others who had close contact with the infected cats. Local Health Protection Teams of Public Health England identified 39 human contacts; 24 accepted TB screening. Three persons (person A, 13 years of age; person B, 18 years of age; and person C, 39 years of age) were positive for latent TB infection (LTBI) by a combination of interferon-γ release assays and Mantoux screening tests; none showed evidence of active disease.

Conclusions
Before this incident, the absence of confirmed reports of human cases of M. bovis infection acquired from pet cats led us to believe that the risk for cat-to-human transmission was negligible. Thus, no public health action was warranted. However, with the evidence of transmission from 1 cat to these 2 patients, the risk for spread of M. bovis from cats to their human contacts was increased from negligible to low (11). Cats with clinical signs compatible with disseminated disease are believed to have the greatest risk to humans, most likely by ingestion from a contaminated environment, following handling of discharges from exudative tuberculous lesions, or by aerosols from cats with respiratory signs or aerosolgenerating procedures.
Public Health England now advises that all close contacts of household companion animals with confirmed M. bovis infections should be assessed by a public health professional and receive guidance on how to best minimize zoonotic transmission (12,13). In addition, as part of an enhanced surveillance system in England and Wales, newly diagnosed human case-patients with M. bovis infection are now also asked explicitly about contact with pets with suspected or confirmed M. bovis disease (14).
In summary, M. bovis disease in companion animals, particularly cats with severe systemic features including exudative lesions, can no longer be regarded as posing a negligible public health risk. Guidance should be provided to minimize the risk for transmission to human contacts.

Figure.
Whole-genome sequencing phylogenetic relationship of genotype 10:u isolates of Mycobacterium bovis from 1 human, 7 cats, 11 cattle, and 1 alpaca, and 10:a isolates from 3 cattle (maximum-likelihood tree of all single-nucleotide polymorphisms [SNPs]). Cat and human isolates are indicated by solid arrowheads. Heat-killed cultures were sequenced by using a MiSeq Sequencer (Illumina, https://www.illumina.com), and reads were mapped by using reference strain AF2122. The average coverage ranged from 23.9-fold to 88.8-fold. The human and their household cat contact (cat 5) isolates were indistinguishable in their genome sequences. Scale bar indicates SNPs.