To infect an animal, a virus must first enter the body. Skin provides an effective barrier to infection because the
outer layer of the epidermis is composed of dead, keratinized cells, and these cells cannot in general support virus
replication. The major portals of entry of viruses into animals are outlined below.
I: Pathways of Viral Entry into Animals
1) Lesions (scratches, abrasions) in the skin.
2) Respiratory Tract.
3) Alimentary (digestive) Tract.
4) Urogenital Tract.
5) Arthropod Borne (mosquitos, ticks, etc.).
6) Vertebrate Bites
7) Direct blood contact (also contaminated needles).
8) Conjunctiva (eye).
II: General Characteristics of Viral Infections
1) Infection is cell type specific, meaning most viruses can only infect a limited number of cell types such as
intestinal epithelial cells, lymphocytes, etc. The ability to infect these cell types may also be species specific, such
as canine lymphocytes. Together, the types of cells that a virus can infect is known as the host range, or tissue
tropism of the virus. It is determined by several different factors, including the presence of appropriate receptors
on cells, and the ability of a virus to express and replicate its genome within the cells. The latter is determined by
whether the cell contains appropriate enzymes, such as DNA polymerases. Cells in which a virus can replicate and
produce progeny virus are called permissive, and those where it cannot replicate are called non-permissive.
2) Infection can be either localized, disseminated or generalized (systemic). Generalized infections are usually
associated with lymphatic spread and viremia, and the virus may ultimately localize to specific secondary organs.
3) Infection can be either lytic (cytopathic) or non-lytic (non
cytopathic). That is, infected cells may either be killed by the virus, or may survive. Surviving cells may continue to
shed virus by budding, or may be "activated" to produce virus at some future point.
4) Infection can be either acute (temporary) or persistent. In acute infections, the virus is completely cleared from
the organism, assuming that it survives. In persistent infections, virus persists in the host for an extended period,
and it may not cause severe disease for a long time. However, a persistently infected animal can be shedding
infectious virus. A virus can also be latent. In latency (which is a type of persistant infection) there may be no free
virus present in the animal, and even viral proteins may be absent or undetectable. However latent viruses can be
"reactivated" in individual cells. Therefore an animal can have both latently infected cells and virus producing
cells at the same time. The best known examples viruses that establish latent infections are some members of
theHerpesviridae.
III: Pathways of Virus Spread in the Animal.
1) Localized Spread in Epithelial Cells: Many viruses replicate in epithelial cells at the site of entry (respiratory,
gastrointestinal, etc), and spread from cell to cell in this tissue. This usually occurs by virus being released from
one cell and infecting nearby cells.
2) Invasion of Subepithelial Tissue and Lymphatic System: Many viruses can traverse the basement membrane
that underlies epithelial layers, and invade the underlying connective tissue and lymphatic system. Once there,
viruses will be exposed to macrophages, and may be destroyed. Some viruses, however, can replicate in
macrophages. They can also move from the lymphatic system into the bloodstream, in which case the animal
becomes viremic (presence of virus in the bloodstream).
3) Spread of Virus by the Bloodstream: Once in the bloodstream, a virus rapidly spreads throughout the body.
In this primary viremia, the number of virus in the bloodstream can be very low.
4) Replication in secondary target organs. The virus may then infect and replicate in secondary target organs
(such as liver, spleen, bone marrow, skin, etc). This can then give rise to a secondary viremia, in which there is a
much higher titer of virus in the bloodstream than during primary viremia. In addition, some viruses have the a
bility to cross the blood
brain barrier, and infect cells in the central nervous system. Usually, viremia is relatively short lived because the
host immune system will clear the animal of free virus. However, some viruses can evade the immune system and
persist in specific cells/ tissues for long periods of time. As mentioned above, persistantly infected animals may
continue to shed infectious virus. Viral evasion of the immune system can be due to several factors, which will
discussed subsequently.
4) Spread of Virus to Other Animals: Infectious virus can be shed from many of the same tissues that are involved
in initial infection (ie, skin, respiratory secretions, saliva, genital secretions, urine, blood). In addition, some viruses
can spread via milk, and some blood borne viruses can cross the placenta to reach the fetus. Severe cytolytic infe
tions of the fetus can cause death or abortion. They may also cause teratogenic effects.
A final mechanism of virus spread is via insect vectors. Viruses are commonly passed from animal to animal of the
same species. However, much more complex life cycles are also possible, including interspecies transfer (like rabies
,some Hantaviruses, etc). In some cases, the virus does not cause disease in one host species, but does in another.
Some of the most complex life cycles are those of the so-called arboviruses, which stands for Arthopod
Borne. These viruses , which include members of many different virus families, have the ability to replicate in both
vertebrate and invertebrate hosts. They can have relatively simple, or very complicated, life cycles. One example is
eastern equine encephalitis virus (Family Togaviridae, genus Alphavirus). In its Enzootic cycle, this virus is maintaine
d in wild birds (catbirds, song sparrow, yellow warbler, others) by the mosquito Culiseta melanura. It does not cause
significant disease in these birds. However, under some conditions the virus may be picked up by other species of
mosquito (epizootic vectors) and transferred to new hosts, such as horses and humans. The virus can replicate and
cause disease in these animals. However, it will not be passed on from them to other animals. They are therefore
called dead end hosts.
Source:https://msu.edu/course/mmg/569/animals.htm