New Zealand animal health expert Roger S. Morris has examined some of the major evolutionary factors affecting diseases in the global pork industry. He reported his observations in detail during a major presentation at this year’s International Pig Veterinary Society meeting in Ames, Iowa.
Morris, a professor of animal health and director of Massey University EpiCentre, Palmerston North, New Zealand, says “the key to understanding the evolution of diseases is to understand underlying ecological processes which cause changes in measurable features of disease in pig populations.”
The first article in this series looked at how the dynamics of swine diseases have been changing over the last several decades. This article looks closer at specific changes in known disease agents.
Morris reports that “some of the notable disease epidemics of recent times have involved evolution of a novel variant of a disease agent from a known one.” The result of this has been an old disease with modified characteristics, he says.
He cites as an example the emergence of a very highly pig-adapted strain of foot-and-mouth disease (FMD) that caused a series of epidemics in Taiwan during the 1990s. What was unusual about this was that FMD in these epidemics largely affected pigs, “thereby changing the character of the epidemic and the options for control.”
On the other hand, Morris says that the 01 Asia pandemic strain of FMD that has spread so widely in the last few years appears to involve ruminants more than pigs. “FMD is renowned for these epidemiological variations, which can produce very divergent disease patterns.”
The widespread adaptation of artificial insemination is an
Morris says that in the past there has been a tendency to equate differences in antigenic characteristics of agents with different patterns and severities of disease.
“Increasingly, such factors as quantitative variation in numbers of organisms disseminated through different routes of excretion may determine the pattern of transmission,” says Morris. It can do this by raising the challenge dose to which a susceptible host is exposed up to the point where an alternative route of infection begins to operate.
“This may substantially change the character of a disease because it may push a disease across a threshold to the point where a new mode of transmission becomes dominant and generates a new disease or new form of a disease.”
For example, the original single strain of African swine fever virus circulated in warthogs and bushpigs, according to Morris. The soft ticks of the genus Ornithodoros acted as a reservoir and a vector of infection to both wild and domestic pigs
“This strain was highly virulent for domestic pigs, but as pigs were exposed to infection by the oral route, the virus began to transmit directly and has evolved into multiple strains with a full spectrum of virulence,” he says. Further, he adds, carrier pigs, rather than ticks, have now become the most important source of virus.
Influenza type A viruses have even more complex interactions than African swine fever viruses, according to Morris. “Because of their segmented genome, these viruses undergo extensive genetic reassortment when human and avian viruses infect the same host. This leads to new reassortants which may then have markedly different pathogenicity, host range and principal routes of transmission. These factors can substantially change the character of the disease.”
Much of this reassortment takes place in the pig and has produced many new strains in the past. Morris says that most strains that shift between species are reassortants, but on much less frequent occasions a complete virus can become infectious for a second species without reassortment. “It appears that once some particular influenza viruses are circulating in pig populations, they become genetically very unstable, leading to the generation of many variants.”
Disease process changes
Changing relationships between the host and the disease agent may not only induce changes in the agent itself but may change the disease process and the transmission of infectious agents between hosts, according to Morris.
As a general rule, when exposure of the host to a particular agent occurs only rarely, the lack of both population selection for genetically determined resistance and the immunologically naive state of the host animals produces the most severe form of the disease that can occur and the lowest variation in age susceptibility.
If exposure is more common, immunological responses are activated in most animals, says Morris. Also, the age range of susceptibility to the disease narrows and the severity of the disease typically declines.
For agents with a very stable genotype, such as porcine parvovirus, the disease largely disappears in an endemically stable situation, but infection becomes ubiquitous and sporadic incidents occur in pockets of naive animals, Morris says.
For agents that show moderate potential for the evolution of new variants, a wider spectrum of milder strains that can co-exist with the population tends to emerge. This often suppresses the most pathogenic strains from clinical expression or leads them to show more cyclic patterns of occurrence as herd immunity waxes and wanes.”
African swine fever and erysipelas broadly fit this pattern, according to Morris. “Where the agent is genetically labile, such as PRRS, influenza type A, vesicular stomatitis or FMD, the agent tends to evolve new strains that have reduced immunological cross-protection with the older strains. This makes the host again susceptible to the new strains as they emerge.”
Morris concludes that such diseases have tended to be among the most troublesome to the producer because they vary capriciously in severity and pattern of occurrence. “Vaccines and other control methods may be quite variable and unpredictable in their effectiveness for these diseases.” n
Next: The evolution of swine diseases at the herd level.
Editor’s note: This is the second part of a four-part series of articles on the evolution of pig diseases. The information was presented at the International Pig Veterinary Society meeting in Ames, Iowa in June.
Swine disease transmission mechanisms
The routes of excretion of disease agents, or portals of exit, and the quantitative scale of excretion by each route comprise another important element in the evolution of swine diseases, says New Zealand animal health expert Roger Morris.
Most agents can be excreted by more than one route, but frequently one route predominates under typical circumstances. For example, an agent may principally be excreted by the fecal route, but there may be a low level of respiratory route excretion. Changes in the antigenic structure may change this so that the volume of infectious material excreted as an aerosol may increase and change the pattern of disease, says Morris.
Respiratory coronavirus is an example where a relatively minor change in the transmissible gastroenteritis (TGE) virus produced a variant agent. This agent was transmitted principally by the respiratory route in place of the fecal route. The agent was much less pathogenic, according to Morris.
Influenza A viruses are a good example of an agent where different strains show varied mixes of aerosol and fecal/fomite transmission. Different patterns are determined by quite small differences in the hemagglutinin and neuraminidase genes.
The application of some control measures or management changes may reduce one transmission mechanism substantially while leaving another unaltered, Morris says. This might reduce the disease overall, but favor the selection of strains that are more capable of aerosol transmission, he says. The disease may rebound in incidence while becoming resistant to control measures that are ineffective against this route of transmission.
The dynamics of aerosol transmission
Whether particular agents have the capacity to form infectious aerosols is an influential factor in evolutionary processes for some diseases where respiratory transmission is important, says international animal health expert Roger Morris.
Some agents, such as foot-and-mouth disease (FMD) virus, are capable of forming infectious aerosols that may be carried for long distances over land and substantially further over water.
For unknown reasons, there are large differences in the capacity of FMD strains to be transmitted by aerosol, Morris says. Among other factors, there appear to be strain differences in capacity to form aerosols. This may reflect the biological properties of the virus — whether it readily forms infectious droplets with water vapor, whether virus particles coalesce to form infectious packets that exceed the minimum infectious dose and/or the distance that droplets formed by the agent can travel as an aerosol.
“Part of the continuing debate over whether some diseases, such as PRRS, can be transmitted on wind may result from strain variation, with some strains being effectively transmitted between farms while others are not,” says Morris. “Over time there is likely to be a selection in favor of strains of an agent which can more readily be transmitted by aerosol since they will spread more widely and be more difficult to control. This can lead to dominant strains in the overall pig population.”
There are also issues about the distance that various agents can travel and still cause infection.
A range of bacteria can be transmitted by aerosol, says Morris. In general, bacteria are transmitted in dried down particles of proteinaceous material that can float in the air for considerable time, allowing transmission without immediate contact.
How readily such organisms establish in a susceptible host depends on the aerodynamics of organisms within the respiratory tract and the size of the infectious dose required to establish infection through various entry points such as tonsil or alveolus, according to Morris.
“This can be further complicated by interactions among organisms,” he remarks. “For example, prior infection by another agent or exposure to mildly toxic airborne chemicals such as ammonia may reduce the level of activity of respiratory tract cilia and allow small numbers of bacteria to establish an infection in a host that would normally control such challenges.
“Some viruses and mycoplasmas can be carried in aerosols within a production system and even to its immediately surrounding area under most conditions, according to Morris. “A more limited range of viruses can be carried very long distances if meteorological conditions are favorable for creation of a virus plume, which can be transmitted downwind and may infect pigs or other species”
This has been extensively documented for FMD virus, says Morris. “With this virus, pigs typically are the source and ruminants are the recipients because of their susceptibility to respiratory route exposure. Because of this, what was earlier attributed to carriage of virus by birds is now recognized to be long-distance airborne spread.”
There is anecdotal and some research evidence that pseudorabies, PRRS and Mycoplasma hyopneumoniae can be transmitted by aerosol between and within farms. Even so, the epidemiological features of transmission remain unclear.
Industry practices and disease evolution
Changes in industry practices have affected the evolution of swine diseases, says Roger Morris.
He cites the widespread adaptation of artificial insemination.
Classical swine fever is a disease for which the epidemiology had been well characterized, yet transmission in semen had never been identified as a significant route, says Morris.
However, according to Morris, a 1997-98 epidemic in The Netherlands demonstrated the potential for disease spread from semen. In this case, semen had been distributed from an artificial insemination center. This epidemic showed that semen could be a potent new way of seeding infection into a range of herds over a very short period, says Morris.