Infectious diseases that are transmitted vertically from sow to piglet

Thursday, December 5, 2013

Viruses, like porcine parvovirus and PRRS virus, have been the most successful vertically
transmitted infectious agents, followed in pigs by the 'Suis-cide' bacterial agents

by S. ERNEST SANFORD

When talking about transmission of pig diseases, we are almost always referring to passage and spread of infectious diseases from one pig to another, transmission of genetic or inherited diseases being the lone exception. Disease agents (pathogens) can be transmitted from one animal to another in essentially two ways: vertical transmission from mother (sow) to offspring (piglet); and horizontal transmission from one animal to another.

Dorland's Illustrated Medical Dictionary defines "vertical transmission of infectious agents" as "transmission of infection from one generation to the next, as by milk or through the placenta."

Horizontal transmission is defined as transmission of an infectious agent, such as a bacterium, virus, fungus or parasite, between members of the same species that are not in a parent-child relationship. Spread of an infectious agent occurs from one person or group to another, usually through direct contact or contact with contaminated materials (fomites).

In this article, I shall concentrate primarily on the infectious diseases that are transmitted vertically from the sow to her offspring. This can occur in many different ways such as:

In utero (via the placenta in the uterus before birth); perinatally (in the birth canal during the birthing process); postnatally (via colostrum and/or milk during nursing); or via secretions from the sow onto her piglets.

It's worthwhile noting that the last (postnatal) example above starts to drift into the arena of horizontal transmission. For the purposes of this article, however, I shall use postnatal as an example of vertical transmission.

The infectious agents I'll cover include viruses, bacteria, fungi/ moulds and parasites. I will touch on a few examples of each of these agents.

Viruses. Viruses, as a group, can lay claim to being the most successful infectious agents using vertical transmission as a means of spread of infection from one generation to the next.

Of the viruses that can lay claim to being among the most successful at vertically transmitting infectious agents, Rubella virus, which causes Rubella disease in humans (also known as German measles), provides the classical example of a vertically transmitted disease agent. Rubella virus can cross the placenta during the first trimester of pregnancy and infect the fetus, causing a constellation of devastating congenital abnormalities (birth defects) including deafness, eye abnormalities (blindness), congenital heart defects and mental retardation.

There is no shortage of examples of pig diseases that are (vertically) transmitted in utero via the placenta during pregnancy. Sticking to the virus theme, prime examples are: hog cholera (classical swine fever) virus; Aujeszky disease (pseudorabies) virus; porcine parvovirus and Porcine Reproductive and Respiratory Syndrome virus.

The first two viruses above are exotic to Canada but the third, porcine parvovirus (PPV), is ubiquitous, with virtually every herd in Canada infected with this virus. It thus presents an excellent example of in utero transmission. The fourth is only too familiar for all of us dealing with pigs over the last 25 years. All four viruses cross from the sow to her fetuses in utero via the placenta during pregnancy.

One of several outcomes occurs depending on the age of the fetus when it is infected in utero with PPV – the very early fetus dies and is resorbed. A little older and the fetus dies and is expelled at birth as a mummified fetus. If the fetus is 70 days of age or older, it develops its own immunity to protect itself and is born alive. As most readers already know, all of the above stages, except the resorbed fetuses, can be observed at parturition in a PPV case. Porcine Reproductive and Respiratory Syndrome (PRRS) virus is a case all by itself which I shall leave out of this discussion.

Perinatal transmission of pig diseases. The "Suis-ide" bacterial agents – Streptococcus (Strep) suis, Haemophilus parasuis and Actinobacillus suis – all represent examples of infectious disease agents that the sow passes on to her piglets perinatally, usually within a few hours or at most a few days after birth. Strep suis and H. parasuis can infect piglets as they are passing through the birth canal during the birthing process, so piglets can actually be infected at birth. If not infected during the birthing process, they become infected with these two agents within hours or a few days after birth. A. suis usually infects piglets within days of birth.

Another agent that fits the vertical transmission criterion is Mycoplasma hyopneumoniae (the cause of enzootic pneumonia), also M. hyorhinis and M. hyosynoviae which have become agents of increasing concern over the last decade. I'll get back to M. hyopneumoniae later.

Fungi/ moulds. Ringworm infection could be squeezed in as a possible example of vertical transmission of a fungus in swine. But infection with the ringworm fungus would really be an example of horizontal transmission even if a sow infected her piglet, which could possibly occur early in a pig's life. Actually, I have not seen a case of ringworm in pigs in many decades.

Parasites. We used to have a ready-made vertically transmitted parasite in Eperythrozoon suis, also known as "epi." Unfortunately, in 2002 the nomenclaturists (scientists responsible for naming and classifying organisms) changed the classification of E. suis from a protozoon (a parasite) and reclassified it as a mycoplasma.

Having lost "epi," the next likely parasite seems to be Sarcoptes scabiei, the cause of scabies. I, however, decided to choose cytoisospora suis (this is another name change from Isospora suis), the cause of porcine coccidiosis ("cocci"), as my parasitic example of vertical transmission. It does take a bit of a stretch of the imagination for C. suis to qualify for this role, but it does suffice.

We know pigs usually pick up the coccidia eggs in feces at about five days of age and start a new cycle of "cocci" infection. However, we are dealing with a "chicken and egg" scenario when it comes to coccidial infections. How did the first piglet get infected? It must have come from the sow a long time ago, but we have no clinical or experimental proof of that. Once the first piglet got infected, it set the stage for repeated cycles of infections of other piglets. So the sow was no longer needed for continued coccidial infections. The cycle goes on via horizontal transmission from piglet to piglet.

Why this great interest in vertical transmission of diseases? What is the significance? It goes back to prevention and control of spread of diseases within a herd. A naïve gilt, recently introduced into a new herd is, in most cases, responsible for the transmission to neonatal piglets of disease agents endemic to that particular herd. This happens when gilts, naïve to these organisms, get exposed after introduction into the new herd (ideally during acclimatization) and have not had sufficient time to "cool down." This means they are likely to be still shedding their newly acquired endemic herd organisms. Note that it's not only the gilt that will do this, but most often it is the gilt that is guilty of doing so.

During the acclimatization process, newly entered gilts, intentionally or unintentionally, are exposed to resident herd organisms. Depending on the organism, she might still be actively harbouring and shedding the organism during pregnancy and is capable of passing it in utero or soon after birth to her piglets.

M. hyopneumoniae would be another good example of this phenomenon. The newly infected gilt can harbour M. hyopneumoniae and shed it to her piglets postnatally. The piglets are protected by passive immunity from the mother, but become actively infected after the mother's passive immunity wears off in the late nursery or in the finisher. M. hyopneumoniae is notoriously slow in establishing infection and can take weeks or even months after initial infection before clinical signs appear.

So we can say that viruses, like porcine parvovirus and PRRS virus, have been the most successful vertically transmitted infectious agents, followed in pigs by the "Suis-cide" agents, Strep suis, H. parasuis and A. suis, as well as the swine-associated mycoplasmas, M. hyopneumoniae, M. hyorhinis and M. hyosynoviae. Interest in vertical transmission of infectious agents originates from the knowledge that newly introduced naïve gilts are often the source of infection by vertical transmission of these agents to their piglets as they shed them, in utero, or postnatally after having acquired them during acclimatization into the new herd. BP

S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email: ernest.sanford@boehringer-ingelheim.com