The porcine epidemic diarrhea virus (PEDV)

The porcine epidemic diarrhea virus (PEDV) suddenly emerged in the United States in May of 2013 and is now posing significant economic and public health concerns (2,3).

Cumulative number of swine samples in the affected US states testing positive for PEDv

States affected by PEDV

Timeline of Porcine Epidemic Diarrhea virus  in US June to Dec 2013

Timeline of samples testing positive for PEDv per week spanning June 16 to Dec 22, 2013

The virus responsible for the outbreak is distantly related to human coronaviruses (CoV). In pigs, porcine epidemic diarrhea virus causes non-life-threatening gastrointestinal disease in adult animals but is fatal in newborn animals. PEDV infects gastrointestinal tissues in newborn piglets to cause acute vomiting, anorexia, and watery diarrhea and eventual death.

Coronaviruses were first discovered in 1965 using cell cultures of human embryonic trachea, nasal epithelium or primary human kidney cells (7). The name coronavirus (Latin corona or crown) stems from the appearance of crown-forming spike-like projections seen on the surface of purified virus as viewed by electron microscopy. Coronaviruses are classified according to the species they infect or their associated disease. Although most human CoVs infections are responsible for 5–30% of the common colds or mild gastroenteritis that occur seasonally, sudden outbreaks do occur. The recent severe acute respiratory syndrome (SARS) epidemic of 2002 and the current spread of the Middle East respiratory syndrome coronavirus (MERS-CoV) are the two most recent examples of severe and sudden CoV outbreaks. The SARS coronavirus which originated in the Chinese province of Guangdong was responsible for at least 8000 cases of severe atypical pneumonia worldwide and for 750 related deaths (1), SARS CoV originated from a cross-species coronavirus infection between the masked palm civet and the Chinese horseshoe bat (Rhinolophus spp). Subsequently infected civet sold in markets infected humans. While the animal source(s) or mode of transmission for the current MERS-CoV remain unanswered, as of January 2014 the WHO has noted a total of 178 laboratory-confirmed cases of MERS-CoV infection and 75 related deaths (8).

Porcine CoV virus, like other coronavirus, is ~ 100 nm in size and comprised of a 28 kilobase length (+) strand RNA that is encased in virus nucleocapsid protein (N). This RNA-nucleocapsid complex is surrounded by a lipid envelope that contains virally-encoded membrane glycoprotein (M), virus envelope protein (E), and a trimer complex comprised of virus spike glycoprotein (S).

Structure of porcine coronavirus

Structure of Porcine Epidemic Diarrhea virus (PEDv) coronavirus

The overall structure of the PEDV genome is like that of other coronaviruses. Its RNA gemome is capped at the 5’ end and contains a conserved short leader sequence, leftward open reading frames (ORFs) that encode virus replication proteins or virus proteins that contribute to disease pathogenesis or tissue tropism. The virus structural proteins are encoded at the rightward end of the genome and always found in the order of S–E–M–N.

Porcine epidemic diarrhea virus genome map

Structure of PEDv genome

Molecular analysis which traced the evolution and temporal patterns of US PEDV indicate that the PED virus responsible for the current US epidemic resembles the PEDV G2a, isolate, AH2012 and likely originated from the Chinese province of Anhui. The Chinese PEDV isolate is the same virus that caused an epidemic in 2010 that resulted in a 50 to 90% death rate in new born piglets (4). Trace analysis also suggests that the China PEDV came to the US at or near 18 months ago (5) and that this virus originated from a recent cross-species virus transmission between an infected bat and a domestic pig.

Recommendations (6) from the American Association of Swine Veterinarians include:

Immediate actions
• Farmers should be attentive to anything sourced internationally be it personnel and visitors and feed supplies or ingredients that are of international origin. Additional security measures should include:
• Limiting traffic (people and equipment) onto the farm,
• Thoroughly cleaning and disinfecting anything coming onto the farm,
• Enforcing downtime requirements and maintaining a log of visitors,
• Taking care when disposing of dead stock particularly if using a communal disposal method,
• Isolating newly arriving animals and continuing vet to vet discussions about animal health at the herd of origin, and
• Showering into the facility where practical and changing or disinfecting boots and coveralls with veterinarians especially vigilant not to carry virus between herds on their person, equipment or vehicles.

Preventative actions
Because of the economic importance of preventing coronavirus infection in livestock and domestic animals, a variety of live-attenuated and killed virus vaccines have been tested, including a vaccine to prevent coronavirus transmissible gastroenteritis (TGE) in pigs. Unfortunately, these vaccines did not provide complete protection when animals subsequently exposed to disease-causing, wild-type virus. In some cases, the wild-type virus evolved to escape recognition by vaccine-induced antibodies.
This is in contrast to many other porcine diseases in which the IgG antibody of sow colostrum provides protection to the piglet, serum antibody and colostrum antibody are less important in providing protection to piglets compared than is the IgA antibody found in sow milk. Since maternal IgA antibody found in the milk of recently infected sow could transfer protective immunity to combat the virus in newborn suckling piglet, exposure of the sow nurse to PEDV prior sucking newborns may lessen the chance of virus infection in the newborn piglet. (6).

Reference List

1. A.D.A.M.Health Solutions. 2014. Severe acute respiratory syndrome (SARS). US National library of Medicine. link
2. American Association of Swine Veterinarians. Porcine Epidemic Diarrhea virus (PEDv) Testing Data from NAHLN Laboratories Report. 1-8-2014. link
3. Gee, K. Pig Virus Threatens To Bump Pork Cost. The Wall Street Journal . 1-11-2014. Dow Jones & Company Inc. link
4. Huang, Y. W., A. W. Dickerman, P. Pineyro, L. Li, L. Fang, R. Kiehne, T. Opriessnig, and X. J. Meng. 2013. Origin, evolution, and genotyping of emergent porcine epidemic diarrhea virus strains in the United States. MBio. 4:e00737-13.
5. Li, W., H. Li, Y. Liu, Y. Pan, F. Deng, Y. Song, X. Tang, and Q. He. 2012. New variants of porcine epidemic diarrhea virus, China, 2011. Emerg.Infect.Dis. 18:1350-1353.
6. Schwartz, K., Henry, S., Tokach, L., Potter, M., Davidson, D., and Egnor, C. Infective material, concepts and procedures for intentional sow herd exposure to Porcine Epidemic Diarrhea virus. 12-17-2013. link
7. Tyrrell, D. A. and M. L. Bynoe. 1965. Cultivaton of a novel type of common cold virus in organ cultures. Br.Med.J 1:1467-1470.
8. WHO Global Alert and Response (GAR). 2014. Middle East respiratory syndrome coronavirus (MERS-CoV) – update Jan 9th 2014. World Health Organizaton. link

[starbox]

Leave a Reply