What evidence is there for disease transmission to pigs from the brown rat in northern Europe?
JM Janse solis‐ID: 4105753 Supervised by: S.A. Burt
University Utrecht, Faculty of Veterinary Medicine March 2018.
Abstract
Introduction
Rodents are a common sight on farms and are often pointed at as the source of diseases. But is that really true, what are the real consequences of those rodents being there? The objective of this study was to determine whether there is any evidence for disease transmission from rodents to farm animals. To be able to make an accurate assessment on this topic the search was limited to include only the rat and specifically the brown rat Rattus norvegicus. Beside that the kind of farm was also narrowed down to only pig farms, so only Sus scrofa domesticus was considered. And finally only diseases in northern Europe were taken into account to keep the information relevant to our own environment.
There was a minimal amount of papers to be found that actually did a field study where it was tested of rats could actually transmit a disease to pigs. Most studies only found whether a certain disease was present at a farm not of it was actually transmittable from rat to pig. For example Meerburg found that the types of Campylobacter found in pigs and rats on a farm were different. (Meerburg et al. 2006, 960-962)
My findings were that though a lot of studies rightly say rats are a hygiene and health risk in and around farms, there is a lot of research to be done to be able to conclude that rats play a role in the transmission of diseases. And although for some diseases there have been studies that prove rats are part of spreading this, there are also studies that prove rats are not part of the transmission route. (Backhans, Fellström, and Lambertz 2011, 1230-1238)
My conclusion is that rats are indeed a problem on farms and proper pest control should be in place. But to say that the rat is a problem in all cases is to big a leap. The risk of rats in reference to the transmission of a disease is a matter that needs to be considered for each disease separately.
Review methodology
The research data that was collected during this study came from different search engines and sites. Most used where PubMed The National Center for Biotechnology Information (NCBI) and cab abstracts. Often the full text version could not be accessed through either of these databases. Google scholar was often used to find those specific articles.
The most used keywords were: Rattus norvegicus, Sus scrofa domesticus and Disease transmission. Because this didn’t get the wanted result a different approach was tried. First the diseases that might be of interest were sought out after which more specific terms could be used. Keywords used were: Zoonoses, Rattus norvegicus and Sus scrofa domesticus in combination with all the found diseases. Being Campylobacter, Salmonella, Listeria, E coli, Yersinia enterocolitica, Tuberculosis, Brucella, Trichinella, Echinnococcus, Toxoplasma, Rabies, Q-fever, West Nile virus, Tularaemia.
Of course not all combinations were usable or even relevant to this study and were quickly crossed off. Other methods of finding articles that could be used was by using cited articles from already found articles that could be relevant in our search.
Literature review
To be able find if rats are actually responsible for disease transmission to pigs, there are a few questions that need to be answered to get a better view of the current situation. First I needed to know which diseases are of interest in this case. To have a starting point I looked which diseases were the most reported in humans. After that the diseases mentioned in the found articles like a study on wild rat behaviour and control on a pig farm gave a whole new list of possibilities, closer to my actual searching criteria. (Akande 2008)
Secondly I wanted to know whether these diseases are also a problem in pigs (Sus scrofa domesticus) and in the brown rat (Rattus norvegicus).
For each of the diseases I looked at the transmission route to determine if the rat was indeed a factor in disease transmission.
An observation made by Backhans in a study done in 2012 is that rodents are not a reservoirs of certain pathogens, but can transmit disease within or between farms. So although they are not the source of diseases found in pigs they help spread it around once it’s there.(Backhans and Fellström 2012, 17093-9)
Which zoonotic diseases are the most reported and are found in both pigs and brown rat?
The European Centre for Disease Prevention and Control and the European Food Safety Authority monitors zoonoses. The following diseases where the most reported in humans in 2015. Campylobacter, Salmonella, Listeria, E coli (shiga toxin-producing), Yersinia enterocolitica, Tuberculosis due to Mycobacterium bovis, Brucella, Trichinella, Echinnococcus, Toxoplasma, Rabies, Q-fever, West Nile virus and Tularaemia. (European Food Safety Authority and European Centre for Disease Prevention and Control 2017, n/a)
Some of these, for example Tularaemia, West Nile virus, Rabies, M tuberculosis and Brucella, where eliminated from the search list. This was done because there was no relevant information to be found about its occurrence in the brown rat or pig, or it simply isn’t a disease normally found in one of these animals. Other relevant diseases that were found by reading through articles are Staphylococcus aureus, Lawsonia intracellularis, Brachyspira and Bordetella bronchiseptica.
I was able to find the most information about Salmonella. A study in 2002 found that fresh rat faeces contained more salmonella than older faeces. But Salmonella could be found in the droppings for a significant amount of days. Which could be a potential risk for spreading salmonella while there is no active infection.(Hilton, Willis, and Hickie 2002, 163-168)
However researchers in Denmark found that if there hadn’t been an salmonella infection on the farm the wildlife, including rats, didn’t carry salmonella. Only when de production animals were going through an infection salmonella was found in the other animals living on the farm. So transmission of salmonella is a possibility but from the pigs to the rat. (Skov et al. 2008, 1558-1568)
Is there evidence for transmission from the brown rat to pigs?
A study done by Battersby states that rodents can be vectors. By getting pathogens on their fur or feet when they walk through a contaminated environment, think of sewers, they can take an infectious agent from one place to the next. Thereby spreading diseases from farm to farm. They can also be carriers of diseases, this means that they are infected with a disease but show no clinical signs of this. But they can pass these diseases on for example through with faeces and urine infected food or water sources. (Battersby 2015, 81-100)
A lot of articles that were found say that rodents are carriers of diseases and play an important role in spreading diseases. This is for the most part true, but the distinction between being carriers and being part of the transmission route to a certain animal isn’t made. An article that makes that distinction quite clearly is a study from le Moine. In his article it’s clearly stated that they only found the presence of infectious agents but that conclusions about the transmission route could not be made. For some diseases like the rotavirus they weren’t able to say if it came from the pigs or it had a different origin. On the other hand they found that Bordetella bronchiseptica, a pathogen found in both pigs and rat, could possibly be transmitted to each other. (Le Moine, Vannier, and Jestin 1987, 399-408)
A study done on rodents as carriers of potential pathogens found that a lot of diversity in the genus Brachyspira could be isolated in rats. They concluded that because of this diversity it could be possible for the rat to transmit Brachyspira to another species. (Backhans 2011) a different study found that the brown rat and the house mouse are a potential vector for Brachyspira and are able to transmit this to pigs. They also found studies that detected the same strains of Brachyspira in both pigs and rats on the same farm.(Alvarez-Ordóñez et al. 2013, 1927-1947)
Are any infection studies reported?
A study done in Belgium that investigated the presence of MRSA and how it spreads. Although this study is more focussed on humans and other animals on pig farms they also had a few rats. In total there were 17 rats and mice caught. The rats that where caught are black rats (Rattus rattus) but still worth mentioning is the spa- and SCCmec-types they found in rats are the types they found most in pigs. They concluded that animal to animal and environment to animal transmission a possibility is.(Pletinckx et al. 2013, 293-303)
In Sweden researchers found that rodents were not a reservoir for Yersinia. Although they are not a reservoir they still form a risk for pig farms. The rat is a carrier of this disease and therefor helps the transmission of Yersinia between pigs within a herd. (Backhans et al. 2011, 1230-1238)
Another pathogen that could be transmitted by rats is Lawsonia intracellularis. The transmission of Lawsonia intracellularis is through ingestion of contaminated faeces. After challenging rats and mice with Lawsonia large amounts of lawsonia intracellularis were shed in their faeces. Collins found that for lawsonia to be transmitted to a pig it needed to ingest less than a gram of rat faeces.(Collins et al. 2011, 384-388)
A study that found that the rat is not a reservoir but a carrier for a certain pathogen is a study done in Croatia. This study states that Trichinella spiralis was only found in rats living on pig farms or slaughterhouses that had an infection with Trichinella. On Trichinella negative farms the rats were also negative, only when on a negative farm positive rats are found the rat can be called a reservoir. So they are not a reservoir that can infect new herds. But if a rats gets infected with Trichinella it can be a vector and spread it to other pigs on the farm. (Stojcevic et al. 2004, 666-670)
On the other side Meerburg investigated the presence of Campylobacter and Salmonella on farms. And what they found was that there were no campylobacter genotypes that could be found in both pigs and rodents. They didn’t rule out the possibility that there could be horizontal transmission, but it was not a conclusion they could make. Possibilities they did give for infection of the pigs were consumption of infected rodents by the pig and infecting each other after that. The rodents could possibly be infected by ingesting infected faeces from the pigs.(Meerburg et al. 2006, 960-962)
A study done on transmission of Toxoplasma gondii found that the rat is a part of the transmission route. They found that the direct transmission to pigs is rare, however rats are a reservoir of Toxoplasma. The biggest factor in transmission of Toxoplasma to pigs are the cats living on the farm, they get infected by hunting the with Toxoplasma infected rats, and in turn infect pigs with Toxoplasma oocysten. (Weigel et al. 1995, 736-741) In addition to this study Kijlstra found that rodents are actually a part of the transmission of Toxoplasma to pigs that can be used as control or prevention group. Several farms participated in a rodent control campaign after which a drop in T. gondii seroprevalence in pigs was found. When the campaign was stopped the numbers of infected pigs went up again. (Kijlstra et al. 2008, 183-190)
Discussion
Although a lot of articles that were found simply said that the rat or more generally rodents are a problem when we talk about disease transmission in most cases it was not proven if this was actually a problem. Most often the assumption that if a rat was present with or without the disease it was the source of it was made with a citation of another article which also didn’t have the proper research for such conclusions.
In several of the found studies had small groups of caught rats or mice on different kind of farms. This ranged from poultry dairy to pig farms, and numbers from ten rats to a hundred. So the sample groups were relatively small, and often the brown rat and the house mouse where thrown together in one group, the rodents. So not all studies can be used as relevant information for the research question, because the proper conclusion could not be made.
Conclusion
Although the claim that the rat plays an important part in the transmission of diseases is often made there is still a lot of research to be done. It is an assumption that is generally made based on other articles that made exactly the same assumption.
The brown rat can be a reservoir of a disease and infecting negative herds with a new pathogen. They can be carriers of a disease which means they are not the source of a pathogen in a herd, but if it is already there they can spread it from pig to pig within the herd. And lastly they can be vectors, bringing pathogens from one place to the next by carrying them on their fur or paws.
Most studies found that the rat can be a carrier of diseases. But just a few studies have tried to find if the brown rat is not only a carrier of diseases but also plays a part in the transmission of these diseases to pigs from farm to farm.
Although about a lot of diseases it could not be said with certainty that the rat plays a role in transmission route to pigs it could also not completely be ruled out. A lot of studies have been done on Salmonella, and the transmission route. While others like studies on Staphylococci are small and need more extensive study before assumptions or conclusions can be mad.
A lot of other pathogens still have to be studied.