Reservoirs

This page collects observations, interpretations, and consequences for action about the reservoirs of SARS-CoV2 in general. Please observe the structure of the page, when you add your content. Please use references where possible. Remember to find the relevant page. For example, if your observation is about transmission routes, please use that page, instead of posting your content here.

What is already known

• Exposure to animals is associated with the first cases of COVID19 disease in humans.
• Humans developed clinical symptoms in December 2019 attributed to SARS-CoV-2 infection after visiting a live animal market in Wuhan, China, which suggested animals from the live market as main source of COVID19 disease.
• Later it was suggested that other animals also played a role in transmission, because 13 patients of the first patient cluster involving 41 patients did not visit the live animal market, which would imply that other animals than only those present on the live animal market play a role in the transmission.
• The causative virus for COVID19 disease; SARS-CoV-2, clusters genetically in lineage B of Sarbecovirus with 2 bat-derived SARS-CoV strains, suggestive for bats as virus reservoir , .
• However, no clear evidence is available to date on the animal reservoir(s) of SARS-CoV-2.
• The virus has now established itself well in the human population, humans get infected easily and spread the virus.
• On the role of animals in transmission: SARS-CoV-2 turns out to be very effective in being transmitted between humans, and as far as is known to date, human to human interactions contribute much more to the risk of getting infected than human to animal contact .
• Several types of coronaviruses can infect animals and can be transmitted to other animals and people. There is no evidence that animals pose a risk of infection to humans.

Animals as reservoirs

• Viruses do occur abundantly in humans and in animals, and humans and animals have many viruses in common. However, because viruses need to enter body cells in order to multiply, they need to adapt themselves very precisely to particular hosts, and even are adapted to very specific proteins present on the surface of specific host tissues, such as lungs, to survive. That means that although viruses constantly mutate and change, only seldom such a change would allow a virus to infect another tissue, or even other host. Viruses often need a very specific protein, that can attach to a tissue cell, to enter it and start multiplying. These tissue proteins vary widely between tissues and between animal species, including humans. Generally, viruses do not easily “jump” from animals to humans to multiply abundantly, cause disease and form a new pathogen for humans.

• However, in the last decades there are several examples in which viruses did jump from animals to humans causing disease.

• As example, previous outbreaks of coronaviruses in humans involved direct exposure to civet cats (SARS) and camels (MERS), that carry viruses that are genetically very similar to SARS-CoV-1 or MERS-CoV, respectively. Small mutations of the viruses in these animals made these viruses successful in infecting human tissues, multiply, and cause disease.

• However, experience from MERS and SARS has shown that humans in fact were infected by an intermediate animal host. So, although in the case of SARS humans were infected by civet cats and in the case of MERS by infected camels as intermediate animal hosts, civet cats and camels in their turn appeared to be infected by bats as the primary virus reservoir. Therefore, distinction must be made between primary animal species in which coronaviruses normally circulate and the animal species in which the coronavirus variant emerges that transmitted the virus to humans: as secondary, intermediate host between the primary animal reservoir and humans. The transmission route of the SARS-CoV-2 to humans may therefore also involve intermediate animal hosts .

Bats

Observations

• Bats do harbour many viruses, including coronaviruses. Bats are implicated in many virus diseases, including severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), porcine epidemic diarrhea (PED) and severe acute diarrhea syndrome (SADS) . Similarity of SARS-CoV-2 to bat SARS-like CoVs has been demonstrated particularly for virus RaTG13 sampled from a Rhinolophus affinis bat, which suggests that bats can serve as reservoir hosts for SARS-CoV-2 . However, it is not known if bats are the primary reservoir and if they infected other animals more in contact with humans that caused the virus to so efficiently enter humans as a new host species.

Malayan pangolins

Observations

• Malayan pangolins ( Manis javanica ) are the most illegally trafficked mammals: they are used as both a food source and their scales are utilized in traditional Chinese medicine. A number of pangolin species now are regarded as critically endangered on the International Union for Conservation of Nature Red List of Threatened Species. Based on available data from Parties in their responses to Notifications to the Parties 2017/035 and 2014/059 and data from UNODC’s World WISE database, illegal trade in pangolins between 1999 and

2017 involved an estimated 192,576 pangolins, far exceeding levels of legal trade in this period. This illegal trade involved all eight species and is based on 1,557 seizures that have taken place in Asian and African range States and non-range States. The majority of these seizures took place in 2006 or later and these analyses can therefore be considered an evaluation of pangolin trafficking in the last decade. .

• In Malayan pangolins ( Manis javanica ) that were illegally imported into Guangdong province a coronavirus (CoV) has been found that was similar to SARS-CoV-2. However, no pangolin CoV has yet been identified that is sufficiently similar to SARS-CoV-2 across its entire genome to support direct human infection.The study suggested that another intermediate animal host is involved. As for the bats, it is not known if pangolins are the primary reservoir and if they infected other animals more in contact with humans that caused the virus to so efficiently enter humans as a new host species.. However, two sublinages of the SARS-CoV-2-related coronavirus were identified in Malayan pangolins seized in anti-smuggling operations in southern China, of which one showed 96% homology with the human receptor-binding domain (RBD). This RBD protein is crucial for the virus to attach to host cell proteins, such as to the human ACE2 protein present in lung epithelial tissue

Pets

• Although COVID19 was found in a dog owned by an infected person in HongKong , there is no evidence that companion animals can spread the disease. Based on this observation, when a pet contracts the virus from a human infected with SARS-CoV-2, the infection seems to die out quickly in most cases without further spread (abortive infection). Coronaviruses do occur in cats and dogs but these are different viruses than SARS-CoV-2 and they do not cause disease in humans.

Observations

• People ask "Can my Dog or Cat be infected?" and "Can my pet infect me?"

One report is known to date on detection of SARS-CoV-2 in a dog.

One March 27, 2020, in Belgium a cat was reported to be infected by coronavirus by the faculty for veterinary medicine in Luik. The owner was infected with coronavirus. The cat became ill one week after the owner was ill. However, no further evidence of further spread or other cases in cats were reported. .

Analysis and interpretation

• From the isolated report on the infected dog in HongKong from an infected owner, it can be speculated that the dog was infected by its owner rather than vice versa. However, even this would not be a cause for grave concern because there is no evidence that the virus rapidly multiplies in dogs, but suggests that an abortive infection took place and that the virus does not effectively infects dogs.

Consequences for action

• There is no evidence that companion animals (e.g. dogs or cats) pose a risk of infection to humans. As a general precaution, it is always wise to observe basic principles of hygiene when in contact with animals .

Questions about live animal markets

Observations

• Live animal markets are common in Asia, but also in Africa. There are "classical" live markets with mammals (sheep and goats, cattle, pigs), birds (poultry, geese, ducks) but also markets with wild life, bush meat (monkeys, snakes, reptiles, etc), and, mostly in Asia, also the "wet" markets with mostly seafood. Many of the consumers who buy there appreciate buying living animals considering them "fresh" and "healthy" when killed on the market, or taken home and killed there for consumption,.

• Are Fish, crustaceans, shellfish, amphibians related to spread of COVID19 disease?
  • The Wuhan live market with seafood present was suspected, but no evidence of animal sources for SARS-CoV-2 among fish, crustaceans, shellfish, amphibians have been published to date. Other animal species (see Bats) are considered more important in the epidemiology of SARS-CoV-2.