Prevalence of infections with a new type of influenza virus – influenza D virus in humans and animals
Authors: Markowska-Daniel Iwona , Mickiewicz Marcin , Witkowski Lucjan , Kita Jerzy 
Published: October 2016

In April 2011, in the USA, a new influenza virus type D (IDV) was isolated for the first time from pigs and then from cows. In the paper the data about the prevalence of this virus in humans and animals are described. It was evidenced that HE protein of IDV could bind to cellular receptors in the epithelium of the human trachea. Moreover, the new virus was shown to infect and spread in ferrets. These findings make it possible to conclude that IDV seems to be able to infect humans and have zoonotic potential. Up to now the percent of human seroconverted to IDV has been low, about 1.3%. The susceptibility of pigs to IDV has been shown in natural and experimental conditions. Virus specific antibodies were detected in 9.5% of pigs. Additionally the isolates from pigs were obtained in the USA and Italy. This suggests that pigs are not the natural reservoir of this virus and the infections are uncommon in pig populations. Several studies demonstrated that the new virus is widely spread in cattle. It was isolated in the USA, Canada, China, France and Italy, which proved that the intercontinental transmission of IDV had occurred. The highest percentage of infected cows, mainly animals manifesting BRDC symptoms, was found in Minnesota, Oklahoma and Mississippi, and it varied from 4.8 to 29.1%. Additionally the virus was detected in 2.4% of healthy animals. Seroconversion was demonstrated in 32.7 to 87.5% of the bovine herds tested in Mississippi and Minnesota, respectively. Retrospective analysis revealed that antibodies specific to IDV had been detected in bovine samples since 2004. This suggests that the new virus has circulated in cattle at least since 2004. The analysis of the relationship between the seroconversion and the age of animals showed that over 50% of cows aged 1–14 years were seropositive. Also more than 90% of newborn calves had maternal antibodies to IDV. The spread of IDV was also linked to the type of farm, with the lowest percentage of animals seroconverting in closed herds and the highest in order-buying facilities. Therefore cattle seem to be the natural host of IDV. In the USA and Canada the infections caused by IDV were also found in sheep and goats. Herd-level and individual-level seroprevalence in sheep was 15.3 and 6.1%, respectively, and these figures were even higher in goats: 20% and 25.9%, respectively. This shows that small ruminants are susceptible to IDV infection. Moreover, retrospective analysis confirmed that one serum from a goat taken in Massachusetts in 2002 was positive to IDV. This suggests that the new virus may have circulated among goats as early as in 2002. Serological studies of chicken and turkey sera from the herds in Minnesota, located near the positive bovine herds, yielded negative results. This might suggest that birds are resistant to IDV infection; however, the number of birds tested is insufficient to substantiate such a statement. The studies aimed at evaluation of zoonotic potential of the new virus proved that it replicated efficiently in the upper respiratory tract of ferrets. This suggests that the new virus may have an influence on public health. In experimental conditions the susceptibility of guinea pigs to IDV infection was also evidenced. In conclusion, all the studies demonstrated that cattle were the natural reservoir of the new virus, despite the fact that humans as well as various animal species could also be infected. Furthermore, cattle are likely to be a potential source of infection for both animals and humans.